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Efficiency and maximum power transfer
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Efficiency and maximum power transfer
"Walter Maxwell" wrote in
: "Owen Duffy" wrote in message ... (Richard Harrison) wrote in news:23000- : Jim Lux wrote: "in a linear system" It produces no significant harmonics, so the system is linear. That is a new / unconventional definition of 'linear'. The term is usually used in this context to mean a linear transfer characteristic, ie PowerOut vs PowerIn is linear. Considering a typical valve Class C RF amplifier with a resonant load: Conduction angle will typically be around 120°, and to achieve that, the grid bias would be around twice the cutoff voltage. If you attempted to pass a signal such as SSB though a Class C amplifier that was biased to twice the cutoff value, there would be no output signal when the peak input was less than about 50% max drive voltage, or about 25% power, and for greater drive voltage there would be output. How could such a transfer characteristic be argued to be linear? Owen Owen, 'linear transfer characteristic' isn't the only context for the use of the word 'linear'. Even though the input circuit of a Class C amplifier is non-linear, the output is linear due to the energy storage of the tank circuit that isolates the input from the output, therefore, the output is linear. Proof of this is that the output signal is a sine wave. In addition, the voltage and current at the output terminals of the pi-network are in phase. Furthermore, the ratio E/I = R appearing at the network output indicates that the output source resistance R is non-dissipative, because a ratio cannot dissipate power. This resistance R is not a resistor. Hi Walt, A few issues.... Yes, I understand the context in which you mean linear (though I have issues with your proposition)... but my comment was referring to the assertion that 'no harmonics' relates to linear operation which seems to me to refer to the transfer characteristic linearity context. I do have issue with your stated 'proof'. Firstly, I must qualify that we are talking steady state... the mention of resonant loads means we are in the frequency domain. Whilst it might seem that the tank circuit / pi coupler / whatever is just a network of passive parts and they are all linear, the energy that is supplied to that circuit in each cycle depends on the resonant load impedance and traditional PA design methods suggest that that Eout/Iout relationship is not linear for changes in load Z, although it might be approximately linear over a small range. I recognise a distinction between resistance (the ratio of E/I) and a resistor (one type of component that exhibits resistance)... but I would not claim that resistance is just a 'ratio' because it implies it is a dimensionless ratio. Owen |
Efficiency and maximum power transfer
Walter Maxwell wrote: "Alan Peake" wrote in message Alan, I disagree with you when you say that 'voltage to current' is not a ratio. IMHO, your are definine 'ratio' to narrowly. Below is a quote from Google: ............. Well, someone has redefined "ratio" since I went to school. My old maths text book says "The term ratio is used when we wish to compare the size ofr magnitude of two quantities (or numbers) of the same kind, i.e., expressed in the same units, and is measured by a fraction" All my dictionaries say much the same thing. There is no mention of comparing quantities of different units. That to me would be like comparing apples with oranges. Alan |
Efficiency and maximum power transfer
Richard Harrison wrote: Some people are persuaded that resistance = loss. Not so at all. Resistance is just a name given to the ratio of voltage to current. If you define resistance as simply V/I with no regard to phase, then what you say is true but if V and I aren't in phase then you have impedance consisting of real and imaginary components - resistance AND reactance. Free-space has a lossless Zo of 120 pi (or 377 ohms) according to page 326 of Saveskie`s "Radio Propagation Handbook". This is a ratio which is related to volts and amps but is actually the ratio of the electric field strength to the magnetic field strength in an EM wave. The volts and amps are in phase so it has the units of a pure resistance. I suppose you could also say that a real resistor is also lossless as the heat due to I*I*R is radiated into space and thus is not lost :) Alan |
Efficiency and maximum power transfer
I found a note I intended to post but don`t see it so I suppose it was
lost in cyberspace somewhere. I was responding to Owen Duffy. Owen wrote: "How could such a transfer characteristic be argued to be linear?" I responded: Conditioning. Class C amplifiers are used lawfully in great abundance. That is proof enough that they are relatively free from distortion. Pulses in plate current don`t prevent the output of the Class C amplifier from becoming a pure sinusoid. Just as an internal combustion engine uses an almost endless string of exlosions in its cylinders to produce a smooth uniform rotation of its crankshaft and flywheel, the Class C amplifier uses an almost endless series of pulses to produce a smooth sinusoid. I will quote B. Whitfield Griffith, Jr., Principal Engineer (retired) at Continental Electronics, Dallas Texas, builder of many of the world`s most powerful radio transmitters. Griffith says on page 500 of "Radio-Electronic Transmission Fundamentals", that it is important where you couple the load to the Class C amplifier: "Figure 56-2 shows how the class C amplifier might look in a typical arrangement. Many refinements of the circuit, which are necessary for practical reasons, are omitted here, since we are concerned only with the fundamental principles of its operation at this time. The plate load impedance consists of a tank circuit of a type similar ro that of Fig. 15-5; the difference is that the load resistor is in series with the inductance rather than the capacitance. This is the preferred arrangement, because the harmonic components of the plate current all have frequencies higher than the fundamental and quite naturally tend to follow the capacitive branch of the circuit. By extracting power from the inductive branch, therefore we can expect to find less harmonic energy in the output than would be present if we loaded the capacitive branch. This load resistance may be an actual resistor, if we wish to feed the output of this amplifier into a dummy load for measurement purposes, or it may be the input resistance presented by some type of impedance-matching network so arranged that the loading of the amplifier can readily be varied. Another common method is to couple resistance effectively into the tank inductance by means of the mutual inductance between the tank and a secondary coil which is coupled to it magnetically, where resistive loads appear in the secondary circuit. There is also shown in Fig. 56-2 the r-f waveform of voltage and current which we would expect to find at various points in the amplifier circuit. No allowance is made in these illustrations for the differences in potentials of various portions of the circuit; these diagrams are merely representative of the behavior of the r-f potentials and currents. Notice particularly that the r-f plate voltage is 180 degrees out of phase with the r-f grid voltage. The reason for this is easily understood. When the grid is its at its most positive potential, the plate current is at its maximum. As the plate current is drawn through the load impedance, the increase in plate current causes a corresponding reduction in plate voltage. The plate voltage therefore swings downward at the moment the grid voltage swings upward. We also see that the current in the load resistor is lagging the r-f plate voltage by an angle of a little less than 90 degrees. Correct operation of the tank circuit requires that the resistance of this load resistor be much smaller than the reactance of the coil." Best regards, Richard Harrison, KB5WZI |
Efficiency and maximum power transfer
"Richard Harrison" wrote in message
... I found a note I intended to post but don`t see it so I suppose it was lost in cyberspace somewhere. I was responding to Owen Duffy. Owen wrote: "How could such a transfer characteristic be argued to be linear?" I responded: Conditioning. Class C amplifiers are used lawfully in great abundance. That is proof enough that they are relatively free from distortion. Pulses in plate current don`t prevent the output of the Class C amplifier from becoming a pure sinusoid. Just as an internal combustion engine uses an almost endless string of exlosions in its cylinders to produce a smooth uniform rotation of its crankshaft and flywheel, the Class C amplifier uses an almost endless series of pulses to produce a smooth sinusoid. I will quote B. Whitfield Griffith, Jr., Principal Engineer (retired) at Continental Electronics, Dallas Texas, builder of many of the world`s most powerful radio transmitters. Griffith says on page 500 of "Radio-Electronic Transmission Fundamentals", that it is important where you couple the load to the Class C amplifier: "Figure 56-2 shows how the class C amplifier might look in a typical arrangement. Many refinements of the circuit, which are necessary for practical reasons, are omitted here, since we are concerned only with the fundamental principles of its operation at this time. The plate load impedance consists of a tank circuit of a type similar ro that of Fig. 15-5; the difference is that the load resistor is in series with the inductance rather than the capacitance. This is the preferred arrangement, because the harmonic components of the plate current all have frequencies higher than the fundamental and quite naturally tend to follow the capacitive branch of the circuit. By extracting power from the inductive branch, therefore we can expect to find less harmonic energy in the output than would be present if we loaded the capacitive branch. This load resistance may be an actual resistor, if we wish to feed the output of this amplifier into a dummy load for measurement purposes, or it may be the input resistance presented by some type of impedance-matching network so arranged that the loading of the amplifier can readily be varied. Another common method is to couple resistance effectively into the tank inductance by means of the mutual inductance between the tank and a secondary coil which is coupled to it magnetically, where resistive loads appear in the secondary circuit. There is also shown in Fig. 56-2 the r-f waveform of voltage and current which we would expect to find at various points in the amplifier circuit. No allowance is made in these illustrations for the differences in potentials of various portions of the circuit; these diagrams are merely representative of the behavior of the r-f potentials and currents. Notice particularly that the r-f plate voltage is 180 degrees out of phase with the r-f grid voltage. The reason for this is easily understood. When the grid is its at its most positive potential, the plate current is at its maximum. As the plate current is drawn through the load impedance, the increase in plate current causes a corresponding reduction in plate voltage. The plate voltage therefore swings downward at the moment the grid voltage swings upward. We also see that the current in the load resistor is lagging the r-f plate voltage by an angle of a little less than 90 degrees. Correct operation of the tank circuit requires that the resistance of this load resistor be much smaller than the reactance of the coil." Best regards, Richard Harrison, KB5WZI Richard, I thoroughly enjoyed reading your post above on the analogy between the action of the energy storage of the tank circuit and that of a automobile engine, so I'd like you to read a portion of Chapter 19 from Reflections 2 to see how I approached the same analogy for the book that I quote below: Therefore, the pi-network must be designed to provide the equivalent optimum resistance RL looking into the input for whatever load terminates the output. The current pulses flowing into the network deliver bursts of electrical energy to the network periodically, in the same manner as the spring-loaded escapement mechanism in the pendulum clock delivers mechanical energy periodically to the swing of the pendulum. In a similar manner, after each plate current pulse enters the pi-network tank curcuit, the flywheel effect of the resonant tank circuit stores the electromagnetic energy delivered by the current pulse, and thus maintains a continuous sinusoidal flow of current throughout the tank, in the same manner as the pendulum swings continuously and periodically after each thrust from the escapement mechanism. The continuous swing of the pendulum results from the inertia of the weight at the end of the pendulum, due to the energy stored in the weight. The path inscribed by the motion of the pendulum is a sine wave, the same as at the output of the amplifier. We will continue the discussion of the flywheel effect in the tank circuit with a more in-depth examination later. ..... We now return to conduct a close examination of the vitally important flywheel effect of the tank circuit. The energy storage (Q) in the tank produces the flywheel effect that isolates the nonlinear pulsed energy entering the tank at the input from the smoothed energy delivered at the output. As a result of this isolation the energy delivered at the output is a smooth sine wave, with linear voltage/current characteristics that support the theorems generally restricted to linear operation. We know that the widely varying voltage/current relationship at the tank input results in widely varying impedances, which precludes the possibility of a conjugate match at the input of the tank circuit. However, the energy stored in the tank provides constant impedance at the output that supports both the Conjugate Matching and the Maximum Power-transfer Theorems.1 The acceptance by many engineers and amateurs of the notion that the output of the RF tank is nonlinear is a reason some readers will have difficulty in appreciating that the output of the RF tank circuit is linear, and can thus support the conjugate match. Valid analogies between different disciplines are often helpful in clarifying difficulties in appreciating certain aspects of a particular discipline. Fortunately, energy storage in the mechanical discipline has a valid and rigorous analogous relationship with energy storage in LC circuitry that makes it appropriate to draw upon a mechanical example to clarify the effect of energy storage in the RF tank circuit. (A further convincing analogy involving water appears later in the Chapter, in which the origin of the term 'tank circuit' is revealed.) The smoothing action of the RF energy stored in the tank circuit is rigorously analogous to the smoothing action of the energy stored in the flywheel in the automobile engine. In the automobile engine the flywheel smooths the pulses of energy delivered to the crankshaft by the thrust of the pistons. As in the tank circuit of the amplifier, the automobile flywheel is an energy storage device, and the smoothing of the energy pulses from the pistons is achieved by the energy stored in the flywheel. In effect, it is the flywheel that delivers the energy to the transmission. The energy storage capacity required of the flywheel to deliver smooth energy to the transmission is determined by the number of piston pulses per revolution of the crankshaft. The greater the number of pistons, the less storage capacity is required to achieve a specified level of smoothness in the energy delivered by the flywheel. The storage capacity of the flywheel is determined by its moment of inertia, and the storage capacity of the tank circuit in the RF amplifier is determined by its Q. |
Efficiency and maximum power transfer
"Owen Duffy" wrote in message ... (Richard Harrison) wrote in news:26406- : ... Class C amplifiers are used lawfully in great abundance. That is proof enough that they are relatively free from distortion. Pulses in plate current don`t prevent the output of the Class C amplifier from becoming a pure sinusoid. ... a very long dissertation on Class C amplifiers snipped. Richard, analysis of the Class C amplifier excited with a constant amplitude single frequency sine wave is revealing about their transfer linearity. I do not disagree that a Class C amplifier excited with a constant amplitude single frequency sine wave driving a resonant load produces a low distortion constant amplitude single frequency sine wave output. But the absence of harmonic distortion in such an amplifier is not evidence that the amplifier transfer characteristic is linear. You may be able to use harmonic distortion to detect non-linearity in, for example, audio amplifiers... but not in RF amplifiers with a resonant load... for the reasons set out in your quotation. A Class C amplifier is unsuited to amplfying SSB telephony because it is manifestly non-linear. In fact, a Class C amplifier is so non-linear that it is well suited to use as a relatively efficient harmonic multiplier. Class B and AB RF amplifiers are extremely sensitive to non-linearity in the region near cut-off and must have sufficient idle current in every active device (which means conduction ange is 180°) so that distortion products are sufficiently low. This means that the theoretical conduction angle of 180° for Class B is just not realisable because of distortion, much less 120°. Owen Sorry about the 'long dissertation on Class C amps', Owen, but I thought it appropriate to include it in view of Richard's similar discussion on the automotive engine analogy to the RF tank circuit. I'll try to keep my comments shorter from now on. Walt, W2DU |
Efficiency and maximum power transfer
"Owen Duffy" wrote in message ... (Richard Harrison) wrote in news:26406- : ... Class C amplifiers are used lawfully in great abundance. That is proof enough that they are relatively free from distortion. Pulses in plate current don`t prevent the output of the Class C amplifier from becoming a pure sinusoid. ... a very long dissertation on Class C amplifiers snipped. Richard, analysis of the Class C amplifier excited with a constant amplitude single frequency sine wave is revealing about their transfer linearity. I do not disagree that a Class C amplifier excited with a constant amplitude single frequency sine wave driving a resonant load produces a low distortion constant amplitude single frequency sine wave output. But the absence of harmonic distortion in such an amplifier is not evidence that the amplifier transfer characteristic is linear. You may be able to use harmonic distortion to detect non-linearity in, for example, audio amplifiers... but not in RF amplifiers with a resonant load... for the reasons set out in your quotation. A Class C amplifier is unsuited to amplfying SSB telephony because it is manifestly non-linear. In fact, a Class C amplifier is so non-linear that it is well suited to use as a relatively efficient harmonic multiplier. Class B and AB RF amplifiers are extremely sensitive to non-linearity in the region near cut-off and must have sufficient idle current in every active device (which means conduction ange is 180°) so that distortion products are sufficiently low. This means that the theoretical conduction angle of 180° for Class B is just not realisable because of distortion, much less 120°. Owen Sorry about the 'long dissertation on Class C amps', Owen, but I thought it appropriate to include it in view of Richard's similar discussion on the automotive engine analogy to the RF tank circuit. I'll try to keep my comments shorter from now on. Walt, W2DU |
Efficiency and maximum power transfer
"Walter Maxwell" wrote in
: Sorry about the 'long dissertation on Class C amps', Owen, but I thought it appropriate to include it in view of Richard's similar discussion on the automotive engine analogy to the RF tank circuit. I'll try to keep my comments shorter from now on. Walt, it wasn't so much that it was long, but it was long and for all that was said, it didn't address the linearity issue. I understand your position to be that the behaviour of the tank circuit is independent of the transfer linearity of the active device... but asserting that 'things' are linear because there are no harmonics is wrong and being so, is no support for your argument. I am wary of analogies, the switch analogy that was raised is not a good approximation and I haven't even thought about the car engine. I am genuinely insterested in your argument. I don't accept it (yet?) as you know, and I have spent some time over the last 18 months or so exploring the concept you describe. Fundamentally, I am trying to reconcile what you say with the techniques commonly accepted for designing such a PA. Those design techniques give us a method of predicting power output at different load impedances, and the E/I characteristic for different loads is not always a straight line (as it would be if a Thevenin equivalent circuit exists), though it might appear fairly straight over a narrow domain. Since working from characteristic curves is so prone to error, my modelling has been based on an idealised triode transfer characteristic, but with similar behaviour to an 811A. The analysis is waiting for me to build the analytical equations for the negative feedback due to cathode degeneration in a grounded grid configuration. I need to apply more time to it, and the revived discussion might focus me for a bit! Owen |
Efficiency and maximum power transfer
On Mon, 16 Jun 2008 03:32:23 GMT, Owen Duffy wrote:
The analysis is waiting for me to build the analytical equations for the negative feedback due to cathode degeneration in a grounded grid configuration. Hi Owen, Consult the work of H.W. Bode taken from his lectures at Bell Labs ca. 1939, and then rendered into text as: Network Analysis and Feedback Amplifier Design, Chapter IV Mathematical Definition of Feedback 4.2 Return Voltage and Reduction in Effect of Tube Variations p 46 It attends specifically (grounded grid triode) what you call out above. In a nutshell, Output (or Input) Z can be tailored by what is called the "noise gain" of an amplifier. In today's parlance, that is that portion of Open loop gain that is fed back to the input to create what is called closed loop gain ("noise gain" is simply the difference when all gains are expressed as dB). The higher the "noise gain" the lower the output Z (or higher the input Z) compared to the native (open loop) Z. There are a host of other characteristics improvements that flow from this same boon offered by "noise gain" (dynamic range, noise rejection, linearity, CMRR, PSRR, and so on). This shorthand can be found expanded in discussion in 5.5 Effect of Feedback on Input and Output Impedances of Amplifiers bullets 1. through 4. but it serves the reader to really stick with the first 4 chapters to gain the proficiency to tackle the remaining 15 as the text is heavily cross-referential. The general formula can be found at: 5.11 Exact Formula for External Gain with Feedback (5-30) Bode was not simply a chalk-and-talk theorist wholly ignorant of the practical realities as are evidenced in several chapter headings: Chapter VII Stability and Physical Realizability Chapter IX Physical Representation of Driving Point Impedance Functions Chapter XI Physical Representation of transfer Impedance Functions Chapter XIII General Restrictions on Physical Network Characterizations Ultimately, it takes very little reading applied to the conventional designs found in Amateur class amplifiers to discover there is really very, very little modification of amplifier characteristics offered through negative feedback design (it costs too much). In fact, I would say none whatever - hence the heavy filtering at the outputs and the customers' universal acceptance of barely mediocre performance. It might be said that every transmitter owned by hams is a museum of 1930s performance. And for those who mistake the feedback of stabilization (barely found in those same cheap designs) - this is not negative feedback, it is compensation. It too has scant effect on tailoring (reducing/increasing) impedances. As I am undoubtedly the only copy holder of this book in this group, access can be obtained through: http://books.google.com/books?client...G=Search+Books which will provide a surplus of leads, if not the exact title. Some links might provide a pdf, others full access, yet others limited access, and most have links to copies in the market place. Given the usual confusion over what constitutes a Conjugate match (when most argue an Impedance match in its place) says discussion of "Efficiency and maximum power transfer" without more rigorous resources fails to even reach the level of tepid conjecture. Bottom line is the source presents a real resistance and no appeal to ratios, linearity, load lines, fly-wheels, or partial cycles is necessary to arrive at a definitive value (which, to this point has been notably absent in the face of obviously localized heat and loss). There is plenty of discussion of what it is NOT, but none seem to know what it IS. That the typical Amateur amplifier source Z is demonstrable is embarrassment enough to this shortfall of expertise. (The pile of theories, books and formulas merely support the obvious, not replace it.) 73's Richard Clark, KB7QHC |
Efficiency and maximum power transfer
Richard Clark wrote:
On Mon, 16 Jun 2008 03:32:23 GMT, Owen Duffy wrote: The analysis is waiting for me to build the analytical equations for the negative feedback due to cathode degeneration in a grounded grid configuration. Hi Owen, Consult the work of H.W. Bode taken from his lectures at Bell Labs ca. 1939, and then rendered into text as: Network Analysis and Feedback Amplifier Design, Chapter IV Mathematical Definition of Feedback 4.2 Return Voltage and Reduction in Effect of Tube Variations p 46 It attends specifically (grounded grid triode) what you call out above. In a nutshell, Output (or Input) Z can be tailored by what is called the "noise gain" of an amplifier. In today's parlance, that is that portion of Open loop gain that is fed back to the input to create what is called closed loop gain ("noise gain" is simply the difference when all gains are expressed as dB). The higher the "noise gain" the lower the output Z (or higher the input Z) compared to the native (open loop) Z. There are a host of other characteristics improvements that flow from this same boon offered by "noise gain" (dynamic range, noise rejection, linearity, CMRR, PSRR, and so on). This shorthand can be found expanded in discussion in 5.5 Effect of Feedback on Input and Output Impedances of Amplifiers bullets 1. through 4. but it serves the reader to really stick with the first 4 chapters to gain the proficiency to tackle the remaining 15 as the text is heavily cross-referential. The general formula can be found at: 5.11 Exact Formula for External Gain with Feedback (5-30) Bode was not simply a chalk-and-talk theorist wholly ignorant of the practical realities as are evidenced in several chapter headings: Chapter VII Stability and Physical Realizability Chapter IX Physical Representation of Driving Point Impedance Functions Chapter XI Physical Representation of transfer Impedance Functions Chapter XIII General Restrictions on Physical Network Characterizations Ultimately, it takes very little reading applied to the conventional designs found in Amateur class amplifiers to discover there is really very, very little modification of amplifier characteristics offered through negative feedback design (it costs too much). In fact, I would say none whatever - hence the heavy filtering at the outputs and the customers' universal acceptance of barely mediocre performance. It might be said that every transmitter owned by hams is a museum of 1930s performance. And for those who mistake the feedback of stabilization (barely found in those same cheap designs) - this is not negative feedback, it is compensation. It too has scant effect on tailoring (reducing/increasing) impedances. As I am undoubtedly the only copy holder of this book in this group, access can be obtained through: http://books.google.com/books?client...G=Search+Books which will provide a surplus of leads, if not the exact title. Some links might provide a pdf, others full access, yet others limited access, and most have links to copies in the market place. Given the usual confusion over what constitutes a Conjugate match (when most argue an Impedance match in its place) says discussion of "Efficiency and maximum power transfer" without more rigorous resources fails to even reach the level of tepid conjecture. Bottom line is the source presents a real resistance and no appeal to ratios, linearity, load lines, fly-wheels, or partial cycles is necessary to arrive at a definitive value (which, to this point has been notably absent in the face of obviously localized heat and loss). There is plenty of discussion of what it is NOT, but none seem to know what it IS. That the typical Amateur amplifier source Z is demonstrable is embarrassment enough to this shortfall of expertise. (The pile of theories, books and formulas merely support the obvious, not replace it.) 73's Richard Clark, KB7QHC Hi Richard, A more modern treatment is _High Linearity RF Amplifier Design_ by Peter B. Kenington. ISBN 1-58053-143-1. I think Amazon still carries it. 73, Tom Donaly, KA6RUH |
Efficiency and maximum power transfer
Owen Duffy wrote:
(Richard Harrison) wrote in news:23000- : Jim Lux wrote: "in a linear system" It produces no significant harmonics, so the system is linear. That is a new / unconventional definition of 'linear'. The term is usually used in this context to mean a linear transfer characteristic, ie PowerOut vs PowerIn is linear. Or, as I used it, that superposition holds. One can build an amplifier or other device where the Pout(Pin) =straight line, but is not linear in the formal sense. Say you built a widget that measured the input frequency and amplitude, then drove a synthesizer at that frequency and amplitude = 2*input amplitude. Considering a typical valve Class C RF amplifier with a resonant load: Conduction angle will typically be around 120°, and to achieve that, the grid bias would be around twice the cutoff voltage. If you attempted to pass a signal such as SSB though a Class C amplifier that was biased to twice the cutoff value, there would be no output signal when the peak input was less than about 50% max drive voltage, or about 25% power, and for greater drive voltage there would be output. How could such a transfer characteristic be argued to be linear? It would not be.You're right The active device isn't linear. neither is the whole assembly. I think, though, that sometimes we take a more casual view of linear (e.g. people talk about the linearity of a log detector.. referring to the deviation from a Voltage out=dBm in straight line.) And, some confusion about nonlinear devices in a building block that is, by and large, linear (e.g. a power op amp with an AB2 output stage and a fair amount of negative feedback) with some constraints on frequency and amplitude. Owen |
Efficiency and maximum power transfer
Richard Clark wrote:
Ultimately, it takes very little reading applied to the conventional designs found in Amateur class amplifiers to discover there is really very, very little modification of amplifier characteristics offered through negative feedback design (it costs too much). In fact, I would say none whatever - hence the heavy filtering at the outputs and the customers' universal acceptance of barely mediocre performance. It might be said that every transmitter owned by hams is a museum of 1930s performance. And for those who mistake the feedback of stabilization (barely found in those same cheap designs) - this is not negative feedback, it is compensation. It too has scant effect on tailoring (reducing/increasing) impedances. probably not "every transmitter", but certainly the vast majority of designs, particularly those for HF based on tubes in the ARRL handbook (and by extension, those sold to readers of the handbook). Cost *is* a factor. The Harris PWM modular transmitters are very cool, but beyond the means of most hams as a commercially manufactured item (in that, the NRE for a consumer mfr to get there would be prohibitively high) One should also not neglect that the hobby aspect of ham radio provides an incentive (for some) to preserve fine (or not so fine) examples of past radio art. No more unusual than steam train fans or classic auto collectors. There is a visceral satisfaction of seeing those glowing tubes with the plates changing color, notwithstanding that the RF performance, in objective terms, is horrid. As I am undoubtedly the only copy holder of this book in this group, access can be obtained through: I'll bet not..grin |
Efficiency and maximum power transfer
Owen Duffy wrote:
"... but asserting that things are linear because there are no harmonics is wrong and being so, is no support for your atgument." No one is arguing that an amplitude modulated wave can be amplified by a Class C amplifier stage unimpaired by amplitude distortion. Terman wrote on page 525 0f his 1955 opus: "Amplitude distortion exists when the modulation envelope contains frequency components not present in the modulating signal. Thus if the modulating signal is a sine wave, then amplitude distortion will cause the envelope to contain harmonics of the modulating signal, which in turn denotes the presence of high-order sideband components that differ from the carrier frequency by harmonics of the modulating frequency." I`ve used microwave system performance monitors which alarmed on this principle. If there are no harmonics there is no distortion no matter how lousy the transfer function. It is legal to filter out noise and distortion. Best regards, Richard Harrison, KB5WZI |
Efficiency and maximum power transfer
Richard Clark wrote:
On Mon, 16 Jun 2008 03:32:23 GMT, Owen Duffy wrote: The analysis is waiting for me to build the analytical equations for the negative feedback due to cathode degeneration in a grounded grid configuration. Hi Owen, Consult the work of H.W. Bode taken from his lectures at Bell Labs ca. 1939, and then rendered into text as: Hi Richard, In this group, would not the work of Vaughn Bode be more appropriate? - 73 de Mike N3LI - |
Efficiency and maximum power transfer
"Jim Lux" wrote in message ... Owen Duffy wrote: (Richard Harrison) wrote in news:23000- : Jim Lux wrote: "in a linear system" It produces no significant harmonics, so the system is linear. That is a new / unconventional definition of 'linear'. The term is usually used in this context to mean a linear transfer characteristic, ie PowerOut vs PowerIn is linear. Or, as I used it, that superposition holds. One can build an amplifier or other device where the Pout(Pin) =straight line, but is not linear in the formal sense. Say you built a widget that measured the input frequency and amplitude, then drove a synthesizer at that frequency and amplitude = 2*input amplitude. Considering a typical valve Class C RF amplifier with a resonant load: Conduction angle will typically be around 120°, and to achieve that, the grid bias would be around twice the cutoff voltage. If you attempted to pass a signal such as SSB though a Class C amplifier that was biased to twice the cutoff value, there would be no output signal when the peak input was less than about 50% max drive voltage, or about 25% power, and for greater drive voltage there would be output. How could such a transfer characteristic be argued to be linear? It would not be.You're right The active device isn't linear. neither is the whole assembly. I think, though, that sometimes we take a more casual view of linear (e.g. people talk about the linearity of a log detector.. referring to the deviation from a Voltage out=dBm in straight line.) And, some confusion about nonlinear devices in a building block that is, by and large, linear (e.g. a power op amp with an AB2 output stage and a fair amount of negative feedback) with some constraints on frequency and amplitude. Owen Owen, I didn't realize that this thread was specific to 'linear transfer characteristic'. I thought the thread topic was sufficiently broad so as to include the subject of linearity of the output of the tank circuit that permits the use of theorems that require the output to be linear. Richard H's and my posts were simply reminders that the energy storage in the tank circuit is the reason for the linear relation between voltage and current at the output of both Class B and C amplifiers that results in a sine wave. From that perspective I believed our posts were legitimate to the thread topic. Apparently we were wrong. And Owen, I'm somewhat surprised that you don't agree with the flywheel analogy with respect to the smoothing effect of the energy storage in the tank circuit. This analogy has been around for decades--it's not my invention. IMHO, the periodic energy spurts from the pistons entering the flywheel is precisely an analog of the energy spurts of the periodic current pulses entering the tank citcuit. Why do you not agree? Even the pendulum swing is appropriate, because if you trace the position of the pendulum with respect to time you'll discover the trace is a perfect sine wave, while the short spurt of energy supplied by the spring at the beginning of each cycle is just sufficient to overcome the energy dissipated due to friction at the axis plus the aerodynamic resistance. How could this not be an appropriate analogy? Sorry to have forced you away from the thread topic with questions not pertaining to the thread. I am also curious as to why the subject of 'linear transfer characteristic' with respect to Class C amps was even considered, because the Class C amp has always been known to have a distorted output relative to its input. I would agree that the subject is appropriate when considering Class AB and B amplifiers, but not C. Walt, W2DU |
Efficiency and maximum power transfer
On Mon, 16 Jun 2008 00:08:14 -0700, "Tom Donaly"
wrote: Hi Richard, A more modern treatment is _High Linearity RF Amplifier Design_ by Peter B. Kenington. ISBN 1-58053-143-1. I think Amazon still carries it. 73, Tom Donaly, KA6RUH Thanx Tom. 73's Richard Clark, KB7QHC |
Efficiency and maximum power transfer
Jim Lux wrote in
: .... That is a new / unconventional definition of 'linear'. The term is usually used in this context to mean a linear transfer characteristic, ie PowerOut vs PowerIn is linear. Or, as I used it, that superposition holds. One can build an amplifier or other device where the Pout(Pin) =straight line, but is not linear in the formal sense. Say you built a widget that measured the input frequency and amplitude, then drove a synthesizer at that frequency and amplitude = 2*input amplitude. Yes Jim, I should have written Vout/Vin is linear, that Vout(Vin) has no significant terms higher than first order. Noting that a single ended Class B or AB amplifier can only be linear when a resonant load or suitable filter is included as part of the system. Elsewhere it was suggested that I do not accept the 'flywheel' explanation of the tank circuit. That is not true, but it is a limited explanation, simple, and appealing, but limited. Another explanation is to view the anode current waveform as containing a DC component, a fundamental component and harmonic components and a filter that adequately reduces the undesired components provides the solution to a single ended Class B or AB linear amplifier. The filter is not restricted to a resonant 'tank' circuit. I have modelled the operating characteristics of my HF linear using 4 572B in AB2. An FFT of the anode current reveals the spectral content, it is plotted at http://www.vk1od.net/lost/572BIaSpectrum.png . Of course, the output filter must only select the fundamental component for linear operation, selection of a harmonic would not be acceptable for a complex input waveform because it would destroy the absolute relationship between different frequency components of the input. Owen |
Efficiency and maximum power transfer
"Walter Maxwell" wrote in
: "Jim Lux" wrote in message ... Owen Duffy wrote: (Richard Harrison) wrote in news:23000- : Jim Lux wrote: "in a linear system" It produces no significant harmonics, so the system is linear. That is a new / unconventional definition of 'linear'. The term is usually used in this context to mean a linear transfer characteristic, ie PowerOut vs PowerIn is linear. Or, as I used it, that superposition holds. One can build an amplifier or other device where the Pout(Pin) =straight line, but is not linear in the formal sense. Say you built a widget that measured the input frequency and amplitude, then drove a synthesizer at that frequency and amplitude = 2*input amplitude. Considering a typical valve Class C RF amplifier with a resonant load: Conduction angle will typically be around 120°, and to achieve that, the grid bias would be around twice the cutoff voltage. If you attempted to pass a signal such as SSB though a Class C amplifier that was biased to twice the cutoff value, there would be no output signal when the peak input was less than about 50% max drive voltage, or about 25% power, and for greater drive voltage there would be output. How could such a transfer characteristic be argued to be linear? It would not be.You're right The active device isn't linear. neither is the whole assembly. I think, though, that sometimes we take a more casual view of linear (e.g. people talk about the linearity of a log detector.. referring to the deviation from a Voltage out=dBm in straight line.) And, some confusion about nonlinear devices in a building block that is, by and large, linear (e.g. a power op amp with an AB2 output stage and a fair amount of negative feedback) with some constraints on frequency and amplitude. Owen Owen, I didn't realize that this thread was specific to 'linear transfer characteristic'. I thought the thread topic was sufficiently Richard stated "It produces no significant harmonics, so the system is linear." It is that with which I disagree. .... And Owen, I'm somewhat surprised that you don't agree with the flywheel analogy with respect to the smoothing effect of the energy storage in the tank circuit. ... I have not disagreed with that in anything that I wrote. ... I am also curious as to why the subject of 'linear transfer characteristic' with respect to Class C amps was even considered, because the Class C amp has always been known to have a distorted output relative to its input. I would agree that the subject is appropriate when considering Class AB and B amplifiers, but not C. Because Richards statement quoted above (which must be about transfer linearity) is being used to support your assertion that the PA is linear in its terminal V/I response with changing load. Walt, the thread has become muddled with helpers muddying the water. Your proposition needs to be argued with a single logically developed sound argument. Your Chapter 19 tries to do that. I have already stated that (as yet?) I am unconvinced, and I make the observation that I am not alone. I will work through resolving the apparent inconsistencies in my own time and without the confusion of whether or not harmonics exist or more correctly the extent to which they exist, and what that might mean. Owen |
Efficiency and maximum power transfer
Cecil Moore wrote: wrote: R is by definition a physical "property of conductors which depends on dimensions, material, and temperature". That's only one definition. From "The IEEE Dictionary", the above is definition (A). Definition (B) is simply "the real part of impedance" with the following Note: "Definitions (A) and (B) are not equivalent but are supplementary. In any case where confusion may arise, specify definition being used." Definition (B) covers Walt's non-dissipative resistance. A common example is the characteristic impedance of transmission line. In an ideal matched system V^2/Z0, I^2*Z0, or V*I is the power being transferred under non-dissipative conditions. Yes. I agree with that, Cecil. But that's not the claim to which I responded. 73, ac6xg |
Efficiency and maximum power transfer
Owen Duffy wrote:
"Richard stated "It produces no significant harmonics, so the system is linear." It is that with which I disagree." A clear statement. Congratulations. Too bad it is wrong. Terman wrote: "Amplitude distortion exists when the modulation envelope contains frequency components not present in the modulating signal." It is also true that absence of of harmonics is proof of linearity, as in my microwave monitoring system alarm. No alarm, a linear system. An alarm, a nonlinear system. I agree with Terman. I challenge you to prove a mistake in Terman`s writings. Best regards, Richard Harrison, KB5WZI |
Efficiency and maximum power transfer
(Richard Harrison) wrote in news:20731-
: Owen Duffy wrote: "Richard stated "It produces no significant harmonics, so the system is linear." It is that with which I disagree." A clear statement. Congratulations. Too bad it is wrong. Terman wrote: "Amplitude distortion exists when the modulation envelope contains frequency components not present in the modulating signal." Fine. It is also true that absence of of harmonics is proof of linearity, That is not a logical implication of your quote from Terman, it is entirely your statement, and without splitting hairs over the absolute meaning of 'absence', it is wrong when applied to a Class C amplifier with pure sine wave excitation and a resonant load. The converse is not logically equivalent to the original. ... I challenge you to prove a mistake in Terman`s writings. I have no problems with the statement you attribute to Terman and haven't said anything contrary to that during the discussion. Richard, I accept that you are committed to your view, lets leave it at that. I don't think your statements on the matter support Walt's proposition, rather since they are in my view flawed, I think they weaken the body of evidence. Owen |
Efficiency and maximum power transfer
"Owen Duffy" wrote in message ... (Richard Harrison) wrote in news:20731- : Owen Duffy wrote: "Richard stated "It produces no significant harmonics, so the system is linear." It is that with which I disagree." A clear statement. Congratulations. Too bad it is wrong. Terman wrote: "Amplitude distortion exists when the modulation envelope contains frequency components not present in the modulating signal." Fine. It is also true that absence of of harmonics is proof of linearity, That is not a logical implication of your quote from Terman, it is entirely your statement, and without splitting hairs over the absolute meaning of 'absence', it is wrong when applied to a Class C amplifier with pure sine wave excitation and a resonant load. The converse is not logically equivalent to the original. ... I challenge you to prove a mistake in Terman`s writings. I have no problems with the statement you attribute to Terman and haven't said anything contrary to that during the discussion. Richard, I accept that you are committed to your view, lets leave it at that. I don't think your statements on the matter support Walt's proposition, rather since they are in my view flawed, I think they weaken the body of evidence. Owen Owen, as I view your last paragraph above it seems apparent that you do not believe Richard's and my position that the output of a Class C amplifier can be linear. We're talking about at the 'output', not the 'thruput'. How can you refute the evidence of a nearly pure sine wave at the output terminals of the pi-network? Walt, W2DU |
Efficiency and maximum power transfer
"Owen Duffy" wrote in message ... (Richard Harrison) wrote in news:20731- : Owen Duffy wrote: "Richard stated "It produces no significant harmonics, so the system is linear." It is that with which I disagree." A clear statement. Congratulations. Too bad it is wrong. Terman wrote: "Amplitude distortion exists when the modulation envelope contains frequency components not present in the modulating signal." Fine. It is also true that absence of of harmonics is proof of linearity, That is not a logical implication of your quote from Terman, it is entirely your statement, and without splitting hairs over the absolute meaning of 'absence', it is wrong when applied to a Class C amplifier with pure sine wave excitation and a resonant load. The converse is not logically equivalent to the original. ... I challenge you to prove a mistake in Terman`s writings. I have no problems with the statement you attribute to Terman and haven't said anything contrary to that during the discussion. Richard, I accept that you are committed to your view, lets leave it at that. I don't think your statements on the matter support Walt's proposition, rather since they are in my view flawed, I think they weaken the body of evidence. Owen Owen, as I view your last paragraph above it seems apparent that you do not believe Richard's and my position that the output of a Class C amplifier can be linear. We're talking about at the 'output', not the 'thruput'. How can you refute the evidence of a nearly pure sine wave at the output terminals of the pi-network? Walt, W2DU |
Efficiency and maximum power transfer
"Walter Maxwell" wrote in
: .... Owen, as I view your last paragraph above it seems apparent that you do not believe Richard's and my position that the output of a Class C amplifier can be linear. We're talking about at the 'output', not the 'thruput'. How can you refute the evidence of a nearly pure sine wave at the output terminals of the pi-network? Walt, you have posted this twice. There are subtle word shifts here, you are saying "a Class C amplifier can be linear" rather than is (always) linear. It is true that a Class C amplifier with resonant load and a constant amplitude sine wave drive may appear linear when comparing Vout to Vin. But, as I explained earlier, if you vary the drive amplitude, it is clearly not linear... in typical cases output will cease below about 25% of the drive level required for maximum output. Further, if you drive it with a complex waveform, it is clearly non linear at any drive level. Richard's solution to detecting RF PA distortion by monitoring harmonics is an interesting one, because it suffers the disadvantage of output filtering masking the harmonics (unless the monitor point was prior to filtering). The most widely accepted test for linearity (Vout/Vin) of an RF PA is the 'two tone test', where the drive is a complex waveform (the sum of two equal amplitude sine waves quite close in frequency) and at least some of the distortion products due to third order and fifth order etc transfer terms appears in-band in the output after all output filtering, and where they can be reliably compared in amplitude to the desired signals. A Class C RF PA will not appear to be linear under such a test at any drive level. I suspect that the issue of transfer linearity is a red herring to your proposition about the Thevenin equivalent of an RF PA, but if you do depend on arguing that the transfer characteristic of a Class C RF PA is linear, I think you are on shaky ground. Owen |
Efficiency and maximum power transfer
"Owen Duffy" wrote in message ... "Walter Maxwell" wrote in : ... Owen, as I view your last paragraph above it seems apparent that you do not believe Richard's and my position that the output of a Class C amplifier can be linear. We're talking about at the 'output', not the 'thruput'. How can you refute the evidence of a nearly pure sine wave at the output terminals of the pi-network? Walt, you have posted this twice. There are subtle word shifts here, you are saying "a Class C amplifier can be linear" rather than is (always) linear. It is true that a Class C amplifier with resonant load and a constant amplitude sine wave drive may appear linear when comparing Vout to Vin. Owen, with a Class C amplifier biased beyond cutoff the grid is never going to see a constant amplitude sine wave, even if the constant amplitude sine wave were impressed on the grid. How then can the transfer linearity ever occur under these conditions? I maintain that it cannot. But, as I explained earlier, if you vary the drive amplitude, it is clearly not linear... in typical cases output will cease below about 25% of the drive level required for maximum output. Further, if you drive it with a complex waveform, it is clearly non linear at any drive level. Richard's solution to detecting RF PA distortion by monitoring harmonics is an interesting one, because it suffers the disadvantage of output filtering masking the harmonics (unless the monitor point was prior to filtering). The most widely accepted test for linearity (Vout/Vin) of an RF PA is the 'two tone test', where the drive is a complex waveform (the sum of two equal amplitude sine waves quite close in frequency) and at least some of the distortion products due to third order and fifth order etc transfer terms appears in-band in the output after all output filtering, and where they can be reliably compared in amplitude to the desired signals. A Class C RF PA will not appear to be linear under such a test at any drive level. I suspect that the issue of transfer linearity is a red herring to your proposition about the Thevenin equivalent of an RF PA, but if you do depend on arguing that the transfer characteristic of a Class C RF PA is linear, I think you are on shaky ground. Owen Owen, you are either twisting my words, or you're not listening. I've made it very clear that I'm NOT talking about 'transfer linearity', and never have. My position is only that the OUTPUT of the pi-network is linear. The linearity at the output is irrelevant to the waveform at the input of the tank circuit in Class C amplifiers. I don't even understand why the discussion concerning 'transfer linearity' with respect to Class C amplifiers should have come up. Walt, W2DU PS--I didn't send two identical emails--something must have happened at the server to have caused it. |
Efficiency and maximum power transfer
Owen Duffy wrote:
"Richard, I accept that you are committed to your view. Let`s leave it at that." Owen is "throwing in the towel' but not admitting error. I have no allegiance to a particular view. I am happy to view things from another`s perspective. Owen mught do the same. Owen Duffy also wrote: "I understand your position to be that the behavior of a tank circuit is independent of the transfer linearity of the active device...but asserting that things are linear because there are no harmonics is wrong and saying so is no support for your argument." Owen has it wrong. The final amplifier is linear because its output is an exact replica of its input except for amplitude, or close enough so. When the waveshape of the output signal from an amplifier varies in any respect other than amplitude from the waveshape of the signal feeding the amplifier, the amplifier is distorting the signal. Sinewave a-c is considered the perfect waveform. It consists of a single frequency. Any other waveform consists of more than one frequency, So the presence or absence of harmonics in addition to the fundamental is a clear indication of distortion. Anyone can confirm waveform using an oscilloscope. Best regards, Richard Harrison, KB5WZI |
Efficiency and maximum power transfer
Owen Duffy wrote:
The most widely accepted test for linearity (Vout/Vin) of an RF PA is the 'two tone test', where the drive is a complex waveform (the sum of two equal amplitude sine waves quite close in frequency) and at least some of the distortion products due to third order and fifth order etc transfer terms appears in-band in the output after all output filtering, and where they can be reliably compared in amplitude to the desired signals. A Class C RF PA will not appear to be linear under such a test at any drive level. Actually, in modern systems with very complex signals, there are more meaningful tests like noise power ratio with a notch that look for spectral regrowth. The two tone test has the advantage of being moderately easy to perform for middling performance amplifiers/devices. But if you're looking for very high performance, such things as generating the two tones without one generator interfering with the other get to be challenging. I suspect that the issue of transfer linearity is a red herring to your proposition about the Thevenin equivalent of an RF PA, but if you do depend on arguing that the transfer characteristic of a Class C RF PA is linear, I think you are on shaky ground. I don't know that the concept of a Thevenin equivalent (a linear circuit theory concept) really has applicability to "box level" models, except over a very restricted range, where one can wave one's hands and ignore the nonlinearities as irrelevant to the question at issue. Sure, over a restricted dynamic range and bandwidth and restricted class of input signals, a Class C (or class E or Class F or E/F1, or a fancy EER system) can be adequately modeled as a linear ideal amplifier. The real question is what is the value of that model. If the model provides conceptual understanding of some underlying problem, great. For instance, it might help with a link budget. If the model helps design a better amplifier, great. The model might allow prediction of behavior; so that you can, for instance, detect a fault by the difference between model and actual observation, as Richard mentioned with the harmonic energy detector. Owen |
Efficiency and maximum power transfer
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Efficiency and maximum power transfer
Jim Lux wrote in
: Owen Duffy wrote: .... Actually, in modern systems with very complex signals, there are more meaningful tests like noise power ratio with a notch that look for spectral regrowth. The two tone test has the advantage of being moderately easy to perform for middling performance amplifiers/devices. But if you're looking for very high performance, such things as generating the two tones without one generator interfering with the other get to be challenging. Noted. I suspect that the issue of transfer linearity is a red herring to your proposition about the Thevenin equivalent of an RF PA, but if you do depend on arguing that the transfer characteristic of a Class C RF PA is linear, I think you are on shaky ground. I don't know that the concept of a Thevenin equivalent (a linear circuit theory concept) really has applicability to "box level" models, except over a very restricted range, where one can wave one's hands and ignore the nonlinearities as irrelevant to the question at issue. Sure, over a restricted dynamic range and bandwidth and restricted class of input signals, a Class C (or class E or Class F or E/F1, or a fancy EER system) can be adequately modeled as a linear ideal amplifier. I agree with you. I am not implying that you cannot design a PA with controlled equivalent source impedance, but you don't do they way most ham PAs are designed. As I understand it, Walt's proposition is that the Thevinin equivalent source impedance (at the device terminals) of the PA is equal to the conjugate of Zl (at the device terminals) as a consequence of adjustment of the PA for maximum power output, a twist on the Jacobi MPT theorem. For that model to be generally useful in explaining behaviour of the PA in the presense of 'reflections', it would need to be true for a wide range of load impedances. The real question is what is the value of that model. If the model provides conceptual understanding of some underlying problem, great. For instance, it might help with a link budget. If the model helps design a better amplifier, great. The model might allow prediction of behavior; so that you can, for instance, detect a fault by the difference between model and actual observation, as Richard mentioned with the harmonic energy detector. I think it goes to whether Walt's proposition and observations apply in general, and then a valid explanation for what happens. Owen |
Efficiency and maximum power transfer
Richard Harrison wrote:
Owen Duffy wrote: "Richard, I accept that you are committed to your view. Let`s leave it at that." Owen is "throwing in the towel' but not admitting error. I have no allegiance to a particular view. I am happy to view things from another`s perspective. Owen mught do the same. Owen Duffy also wrote: "I understand your position to be that the behavior of a tank circuit is independent of the transfer linearity of the active device...but asserting that things are linear because there are no harmonics is wrong and saying so is no support for your argument." Owen has it wrong. The final amplifier is linear because its output is an exact replica of its input except for amplitude, or close enough so. When the waveshape of the output signal from an amplifier varies in any respect other than amplitude from the waveshape of the signal feeding the amplifier, the amplifier is distorting the signal. Sinewave a-c is considered the perfect waveform. It consists of a single frequency. Any other waveform consists of more than one frequency, So the presence or absence of harmonics in addition to the fundamental is a clear indication of distortion. Anyone can confirm waveform using an oscilloscope. Best regards, Richard Harrison, KB5WZI From _Filtering in the Time and Frequency Domains_ by Herman J. Blinchikov and Anatol I. Zverev: "A system is linear if the input c1f1(t)+ c2f2(t) produces and output c1g1(t)+ c2g2(t) for all f1(t) and f2(t), when it is known that an input f1(t) produces an output g1(t) and an input f2(t) produces and output g2(t). The c1 and c2 are arbitrary constants but may be complex numbers. This property of superposition is characteristic of linear systems." You're ignoring the addition part of the concept of linearity, Richard. Moreover, the functions f1(t) and f2(t) don't have to be sine waves; the concept is more general than that. Finally, read Richard Clark's post. A sine wave out doesn't prove a sine wave in. 73, Tom Donaly KA6RUH |
Efficiency and maximum power transfer
Richard Clark wrote:
"The presumption (forced or otherwise) is that the output is sinusoidal. In fact, the cathode current of the amplifier proves quite positively that only a pulse in, 180 degrees of sinewave, or even less, is sufficient to generate a remarkably clean sinewave at the final`s output." That is a remarkably clear statement of the behavior of a Class C amplifier. The amplifier acts as a generator of a sinewave which is synchronized by its input signal instead of being an accurate reproducer of the waveform at its input. The less than half wave of current flow of the Class C amplifier allows an efficiency exceeding 50%. Walt Maxwell`s tests show that the Class C amplifier sticks to the parameters of a Thevenin source. The question of "what is the source impedance" presented to a load by the amplifier? is answered, not by magic, but by the maximum power transfer theorem. The amplifier must be adjusted to deliver all its available power. Then, the output impedance of the amplifier is simply the conjugate of the load impedance which is easily measured. Best regards, Richard Harrison, KB5WZI |
Efficiency and maximum power transfer
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Efficiency and maximum power transfer
"Owen Duffy" wrote in message ... Jim Lux wrote in : Owen Duffy wrote: ... Actually, in modern systems with very complex signals, there are more meaningful tests like noise power ratio with a notch that look for spectral regrowth. The two tone test has the advantage of being moderately easy to perform for middling performance amplifiers/devices. But if you're looking for very high performance, such things as generating the two tones without one generator interfering with the other get to be challenging. Noted. I suspect that the issue of transfer linearity is a red herring to your proposition about the Thevenin equivalent of an RF PA, but if you do depend on arguing that the transfer characteristic of a Class C RF PA is linear, I think you are on shaky ground. I don't know that the concept of a Thevenin equivalent (a linear circuit theory concept) really has applicability to "box level" models, except over a very restricted range, where one can wave one's hands and ignore the nonlinearities as irrelevant to the question at issue. Sure, over a restricted dynamic range and bandwidth and restricted class of input signals, a Class C (or class E or Class F or E/F1, or a fancy EER system) can be adequately modeled as a linear ideal amplifier. I agree with you. I am not implying that you cannot design a PA with controlled equivalent source impedance, but you don't do they way most ham PAs are designed. As I understand it, Walt's proposition is that the Thevinin equivalent source impedance (at the device terminals) of the PA is equal to the conjugate of Zl (at the device terminals) as a consequence of adjustment of the PA for maximum power output, a twist on the Jacobi MPT theorem. For that model to be generally useful in explaining behaviour of the PA in the presense of 'reflections', it would need to be true for a wide range of load impedances. The real question is what is the value of that model. If the model provides conceptual understanding of some underlying problem, great. For instance, it might help with a link budget. If the model helps design a better amplifier, great. The model might allow prediction of behavior; so that you can, for instance, detect a fault by the difference between model and actual observation, as Richard mentioned with the harmonic energy detector. I think it goes to whether Walt's proposition and observations apply in general, and then a valid explanation for what happens. Owen Owen, on whether my observations apply in general, if you re-read the summarizing paragraph on my Chapter 19A you'll see that I've made measurements of the source impedance of two different xmtrs with several different complex impedance loads. All measurements showed the source impedance equal to the load impedance when all available power is delivered to the load. As to the explanation, Richard H said it well. When all available power is delivered, according to the maximum power transfer theorem the source impedance equals the load impedance. My measurements have proved this to be true in determining the source impedance of the xmtrs I measured. Walt, W2DU |
Efficiency and maximum power transfer
"Richard Clark" wrote in message ... On Wed, 18 Jun 2008 03:39:54 -0500, (Richard Harrison) wrote: The question of "what is the source impedance" presented to a load by the amplifier? is answered, not by magic, but by the maximum power transfer theorem. The amplifier must be adjusted to deliver all its available power. Then, the output impedance of the amplifier is simply the conjugate of the load impedance which is easily measured. Hi Richard, MPT Conjugate Match Z Match You can NOT achieve the COMBINATION of any two, much less all three with a Class C amplifier. This is like checking all three possible answers on a multiple choice exam. It follows that source impedance has not been answered here as a qualifiable (which is certainly not what I was looking for). I will take it that you don't know what the source impedance is as a quantifiable either. That is, unless you unwind all the confounding statements and remove those in error. There cannot be three, simultaneous quantifiables of differing values. 73's Richard Clark, KB7QHC Hi Richard C, Am I hearing you correctly? Are you disagreeing with Richard H? Are you saying that maximum power transfer, conjugate match at the output, and Z match cannot occur simultaneously? Are you serious? As I understand Everitt's statement of the maximum-power-transfer theorem, when the maximum available power is being transferred to the load there is a conjugate match. Does this not also mean there is a 'Z' match? Can't 'Z' be assumed to be the impedance of the source as well as the load? Walt, W2DU |
Efficiency and maximum power transfer
Richard Clark wrote:
"I will take it that you don`t know what the source impedance is as a quantifiable either." Terman wrote on page 76 of his 1955 opus: "Alternatively, a load impedance may be matched to a source of power in such a way as to make the power delivered to the load a maximum. (The power delivered under these conditions is termed the "available power" of the power source.) This is accomplished by making the load impedance the conjugate of the generator as defined by Thevenin`s theorem. That is, the load impedance must have the same magnitude as the generator impedance, but the phase angle of the load is the negative of the phase angle of the generator impedance." Best regards, Richard Harrison, KB5WZI |
Efficiency and maximum power transfer
On Wed, 18 Jun 2008 11:59:40 -0400, "Walter Maxwell"
wrote: Hi Richard C, Am I hearing you correctly? Are you disagreeing with Richard H? Are you saying that maximum power transfer, conjugate match at the output, and Z match cannot occur simultaneously? Hi Walt, For a Class C tube amplifier. All descriptions of tune-up for a Class C tube amplifier describe a qualitative MPT as this classic method offers absolutely no information about the quantitative degree of initial mismatch, nor subsequent proximate match. In other words, there are no quantitative values of load impedance revealed by this method. It may even be said that the classic tune-up only describes "an attempt" at MPT; as it may, in fact, not even achieve anything more than Mediocre Power Transfer. After peaking the grid and dipping the plate, I have observed many different peaks and dips for many various loads to know that not all loads obtained all available power. The classic description of a tune-up is based on qualitative assumptions and the amplifier is brought into its best attempt, which is not demonstrably efficient, nor even proven to be "matched" conjugately or by impedance. This takes more information (so far unrevealed) obtained by current into the known load (unrevealed), and power into the source (unrevealed). No one other than myself has expressed the loss of the source because no one else has ever enumerated its resistance (a topic commonly hedged and avoided) Hence discussion of efficiency is lost in the woods and correlation to MPT/Z/Conjugation is equally doomed to ambiguity. Are you serious? As I understand Everitt's statement of Everitt notwithstanding, Lord Kelvin trumps him with "when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind" This thread has suffered from a lack of measurables that are not that difficult to obtain. So, to return to my very specific question: What is the source resistance of any power amplifier? I will further loosen constraints (if that isn't loose enough) For any match? One complex number is sufficient, and certainly that value will resolve all imponderabilities is what I am asking for. 73's Richard Clark, KB7QHC |
Efficiency and maximum power transfer
"Richard Harrison" wrote in message ... Richard Clark wrote: "I will take it that you don`t know what the source impedance is as a quantifiable either." Terman wrote on page 76 of his 1955 opus: "Alternatively, a load impedance may be matched to a source of power in such a way as to make the power delivered to the load a maximum. (The power delivered under these conditions is termed the "available power" of the power source.) This is accomplished by making the load impedance the conjugate of the generator as defined by Thevenin`s theorem. That is, the load impedance must have the same magnitude as the generator impedance, but the phase angle of the load is the negative of the phase angle of the generator impedance." Best regards, Richard Harrison, KB5WZI That's the way I understand it too, Richard. But also works in the opposite direction too, by adjusting the source impedance to have the same magnitude and opposite phase of the load impedance. This is the procedure I used to prove the value of the source impedance. Walt, W2DU |
Efficiency and maximum power transfer
"Richard Clark" wrote in message ... On Wed, 18 Jun 2008 11:59:40 -0400, "Walter Maxwell" wrote: Hi Richard C, Am I hearing you correctly? Are you disagreeing with Richard H? Are you saying that maximum power transfer, conjugate match at the output, and Z match cannot occur simultaneously? Hi Walt, For a Class C tube amplifier. All descriptions of tune-up for a Class C tube amplifier describe a qualitative MPT as this classic method offers absolutely no information about the quantitative degree of initial mismatch, nor subsequent proximate match. In other words, there are no quantitative values of load impedance revealed by this method. It may even be said that the classic tune-up only describes "an attempt" at MPT; as it may, in fact, not even achieve anything more than Mediocre Power Transfer. After peaking the grid and dipping the plate, I have observed many different peaks and dips for many various loads to know that not all loads obtained all available power. The classic description of a tune-up is based on qualitative assumptions and the amplifier is brought into its best attempt, which is not demonstrably efficient, nor even proven to be "matched" conjugately or by impedance. This takes more information (so far unrevealed) obtained by current into the known load (unrevealed), and power into the source (unrevealed). No one other than myself has expressed the loss of the source because no one else has ever enumerated its resistance (a topic commonly hedged and avoided) Hence discussion of efficiency is lost in the woods and correlation to MPT/Z/Conjugation is equally doomed to ambiguity. Are you serious? As I understand Everitt's statement of Everitt notwithstanding, Lord Kelvin trumps him with "when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind" This thread has suffered from a lack of measurables that are not that difficult to obtain. Richard, are you inferring that I have not submitted the measurables required to determine the source impedances of the xmtrs I measured? What additional measureables that I haven't already submitted are you asking for to prove the source impedances that I've already submitted are valid? So, to return to my very specific question: What is the source resistance of any power amplifier? Richard, the source impedance of one of the xmtrs I measured with load impedance of 17.98 + j8.77 ohms measured 18 - j8 ohms. Considering measurement error, wouldn't you agree that these two impedances qualify for a conjugate match, and that this value of source impedance is valid at least within the realm of possibility? For any match? One complex number is sufficient, and certainly that value will resolve all imponderabilities is what I am asking for. OK, Richard, is impedance 18 - j8 ohms sufficient? Richard Clark, KB7QHC Walt,W2DU |
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