Richard Harrison wrote:
Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

Is the class-C amp conjugately matched during
the 75% of the cycle when it is off? Is there
any such thing as an instantaneous conjugate
match? Don't we have to move downstream from
non-linear sources for our linear math models
to start working?
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
...
Richard Harrison wrote:
Cecil, W5DXP wrote:
"What is the linear source impedance of a class-C amp?"

A conjugate match is necessary for maximum power transfer.

Is the class-C amp conjugately matched during
the 75% of the cycle when it is off? Is there
any such thing as an instantaneous conjugate
match? Don't we have to move downstream from
non-linear sources for our linear math models
to start working?
--
73, Cecil http://www.w5dxp.com

Richard, its a common myth that Class C amps are non-linear, but the truth of
the matter is that although the condition at the input of the pi-network is
decidedly non-linear, the energy storage in the pi-network tank circuit isolates
the input from the output and the result is a totally linear condition at the
output of the pi-network. Evidence proving this is true is that the output of an
unmodulated signal at the output of the network is an almost pure sine wave.
With a Q of at least 12 the difference between a pure sine wave from a signal
generator and that from the pi-network output can not be seen on a dual trace
scope with the traces overlapping.

I don't know about the energy storage in the filters you mention, but I would
assume that if the filter output is a sine wave then the energy storage required
to produce a linear output is sufficient.

Walt, W2DU

Cecil, W5DXP wrote:
"Is the class-C amp conjugately matched during the 75% of the cycle it
is off?"

We must consider the complete cycle.

Working with spark ignition systems (Hettering) you may have encountered
a "dwell meter". It indicates the % of the time ignition points are
closed. When the points are closed, impedance between the meter and the
battery is insignificant. The meter if left continuously connected
through the points would indicate full-scale. When the points open,
their impedance is infinite. Left continuously open, the meter indicates
zero on the dwell scale.

Dwell is measured while the engine is rotating and the meter is being
connected intermittently to the battery through the ignition points.

Intermittent opening and closing of the points causes the same scale
reading that would be caused by replacing the points with some
particular value of fixed resistance (a resistor).
The main difference is that no dissipation occurs in the open ignition
points and precious little energy is lost in the closed points. Voila!
We have produced a dissipationless resistance.
The Class C amplifier is a switch which operates in the same manner. The
Kettering ignition points have a low-resistance ignition coil primary in
series, and the Class-C amplifier has a tuned plate circuit in series,
but both are being switched on and off repeatedly.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Cecil, W5DXP wrote:
"Is the class-C amp conjugately matched during the 75% of the cycle it
is off?"

Matched across what boundary? from output pi network to load?
From active device to pi network?
I'd venture that between active device and pi network, there isn't a
conjugate match, at any given instant, and perhaps not even considered
over the entire cycle (without resorting to some things like "apparent
impedance" which doesn't have a real clean definition).

In order to provide a true "conjugate match" to a device that is
changing state, the load must also be changing state: i.e. the match is
single valued, for any Zload, there is a single Zmatch that maximizes
power transfer; except perhaps for some trivial cases, like Z=zero or
infinity, but in such cases, the load doesn't dissipate ANY power, so
what is there to maximize.

Furthermore, if one looks at situations where you have, for instance, a
very low source impedance (a stiff voltage bus) or a very high source
impedance (a constant current source), power transfer is maximized to a
given load impedance when the reactive components are conjugate. In such
a case, the source and load resistances are not equal.



One might look at
http://p1k.arrl.org/~ehare/temp/conj...ch_theorum.pdf
http://mysite.orange.co.uk/g3uur/index.html





We must consider the complete cycle.

Working with spark ignition systems (Hettering) you may have encountered
a "dwell meter". It indicates the % of the time ignition points are
closed. When the points are closed, impedance between the meter and the
battery is insignificant. The meter if left continuously connected
through the points would indicate full-scale. When the points open,
their impedance is infinite. Left continuously open, the meter indicates
zero on the dwell scale.

Dwell is measured while the engine is rotating and the meter is being
connected intermittently to the battery through the ignition points.

Intermittent opening and closing of the points causes the same scale
reading that would be caused by replacing the points with some
particular value of fixed resistance (a resistor).
The main difference is that no dissipation occurs in the open ignition
points and precious little energy is lost in the closed points. Voila!
We have produced a dissipationless resistance.

I would say "apparent resistance".. the "conjugate match" and any other
linear circuit analysis can't necessarily be "averaged". Something like
Kirchoff's current law or voltage law (or Ohm's law, for that matter)
has to be true at any instant.

The challenge faced by folks faced with analyzing "real" circuits is
that you have to be careful about how you turn a circuit that is likely
time-varying AND nonlinear into a linearized approximation. For
instance programs like SPICE's transient analysis uses linear circuit
theory (via matrix analysis) in combination with an iterative
differential equation solver, and tries to treat the circuit as linear
at a given instant. (granted, newer versions of SPICE and its ilk are a
bit more sophisticated, since they can handle nonlinear terms in the
matrix).


The Class C amplifier is a switch which operates in the same manner. The
Kettering ignition points have a low-resistance ignition coil primary in
series, and the Class-C amplifier has a tuned plate circuit in series,
but both are being switched on and off repeatedly.

Complicated substantially by the fact that the active device in a RF
amplifier generally doesn't act as an ideal switch. So the piecewise
linearization you describe isn't totally applicable. For instance, a BJT
acts like a constant current source if base drive is fixed, but in RF
circuits, the base drive isn't fixed. In FET circuits you worry about
the gate capacitance.


This is why there are all sorts of variants of SPICE modified for
switching power supplies.



Best regards, Richard Harrison, KB5WZI

Jim Lux wrote:
"Matched across what boundary?"

Where there is a conjugate match in the transmitter-antenna system, it
exists at every pair of terminals. That is, the impedances looking in
opposite directions are conjugates. The resistive parts of the impedance
are equals and the reactances looking in opposite directions are
opposites of each other.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Where there is a conjugate match in the transmitter-antenna system, it
exists at every pair of terminals.

Quoting w2du's web page:
“The Conjugate Theorem also shows that in a sequence of
matching networks it is necessary to match at only one
junction *if the networks are non-dissipative*. In actual
practice, since there is usually some dissipation, it
is frequently desirable to adjust at more than one point.”
--
73, Cecil http://www.w5dxp.com

Cecil, W5DXP wrote:
"Quoting W2DU`s web page:"

Sure hope Walt finds a publisher soon for his latest edition of
"Reflections". Web TV (a Microsoft company) doesn`t allow me to read
pdf.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in news:26007-4851914B-
:

....
The Class C amplifier is a switch ...

If you say it enough times, will it become true?

Owen

Owen Duffy wrote:
"The Class C amplifier is a switch...
If you say it enough times, will it become true?"

True is true no matter what anyone says. I`ve never seen Terman
misspeak.

On page 255 of his 1955 opus Terman wrote:
"---the Class C amplifier is adjusted so the plate current flows in
pulses that last less than half a cycle."

On page 450 he wrote:
"The high efficiency of the Class C amplifier is a result of the fact
that plate current is not allowed to flow except when the instantaneous
voltage drop across the tube is low; i.e. Eb supplies energy to the
amplifier only when he largest portion of the energy will be absorbed by
the tuned circuit."

Sounds like a switch to me. When switched on, voltage drop across the
tube is low. When switched off, voltage drop across the tube is Eb, but
since current is zero, no power is lost at that instant in the tube.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in
:

Owen Duffy wrote:
"The Class C amplifier is a switch...
If you say it enough times, will it become true?"

True is true no matter what anyone says. I`ve never seen Terman
misspeak.

On page 255 of his 1955 opus Terman wrote:
"---the Class C amplifier is adjusted so the plate current flows in
pulses that last less than half a cycle."

On page 450 he wrote:
"The high efficiency of the Class C amplifier is a result of the fact
that plate current is not allowed to flow except when the
instantaneous voltage drop across the tube is low; i.e. Eb supplies
energy to the amplifier only when he largest portion of the energy
will be absorbed by the tuned circuit."

Sounds like a switch to me. When switched on, voltage drop across the
tube is low.

Lets plug some real world numbers in...

Take a DC supply of 1000V, and a valve that saturates at 200V, the RF
approximately sinusoidal voltage swing on the anode is from 200V to
1800V. (For avoidance of doubt, whilst the RF voltage on the anode is
approximately sinusoidal, the anode current waveform is not.)

If the conduction angle is 120 degrees (typical for Class C amplifiers),
the valve starts conducting at about 1000-800*sin((180-120)/2) or 600V
instantaneous anode voltage... and continues conducting as the
instantaneous anode voltage passes through the minimum and rises again,
cutting off when then instantaneous anode voltage again reaches 600V. In
this case, the anode voltage during conduction varies between 400 and
600V, 40% to 60% of the supply voltage.

The switch analogy is not a good one.

When switched off, voltage drop across the tube is Eb, ...

That is wrong. The anode voltage is approximately a sinusoidal voltage
swing of almost (70% to 90%) Eb zero to peak superimposed on the DC
supply voltage (Eb).

You have taken a quote from Terman and weaved your own flawed extensions
(being the switch analogy and the statement about instantaneous anode
voltage).

Owen