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On Sat, 28 Aug 2004 08:43:21 -0700, John Larkin
wrote: On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge wrote: On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin wrote: Have you actually built a class C linear RF power amp? Tell us how it works. It depends on how you define "linear" basically. But the term is a total misnomer in RF amp terminology and very misleading. I can't understand how it got there. :-/ Define "linear"? You must be joking. I'll take that as a "no" to my question. Not surprised. Actually I've built *several* class C RF amps, John. However, I wouldn't call any of them linear. You will be aware than linearity starts to go out of the window when Class A slides into Class AB and beyond. Let's not have an argument over definitions. It's an open invitation to John Woodgate. ;-) -- "What is now proved was once only imagin'd." - William Blake, 1793. |
On Sat, 28 Aug 2004 10:56:03 +0100, Ian Jackson
wrote: In message , Paul Burridge writes On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin wrote: Have you actually built a class C linear RF power amp? Tell us how it works. It depends on how you define "linear" basically. But the term is a total misnomer in RF amp terminology and very misleading. I can't understand how it got there. :-/ Don't forget that you cannot really modulate a 'linear' amplifier by varying the supply rail (which is what 'plate & screen' mod does). The modulated stage has to be non-linear (eg Class C) where the power output varies as the square of the supply volts. --- Nonlinear? Yes. Class "C"? No. From http://sound.westhost.com/class-a.htm : " Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1 Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB) Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1. Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! ** This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF) Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - because of frequency limitations (and the fact that they nearly all seem to sound disgusting), many are only suitable for industrial control of motors and loud but crappy sub-woofers (this may change if transistors with an infinite bandwidth become available soon - yeah, right!) All Class-D amps have a major limitation in the output filter, whose response is highly dependent on the load impedance. " If the PA was biassed in Class A, there wouldn't be any modulation. --- There could be; all that would be required would be for the gain of the stage to vary with the modulating input. I don't believe there's a constraint on class A biasing which inherently precludes a class A stage from being modulated. -- John Fields |
On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
wrote: On Sat, 28 Aug 2004 08:43:21 -0700, John Larkin wrote: On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge wrote: On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin m wrote: Have you actually built a class C linear RF power amp? Tell us how it works. It depends on how you define "linear" basically. But the term is a total misnomer in RF amp terminology and very misleading. I can't understand how it got there. :-/ Define "linear"? You must be joking. I'll take that as a "no" to my question. Not surprised. Actually I've built *several* class C RF amps, John. However, I wouldn't call any of them linear. You will be aware than linearity starts to go out of the window when Class A slides into Class AB and beyond. I am not aware of any such things. So I take it you have not designed any class C linear RF power amplifiers. Let's not have an argument over definitions. It's an open invitation to John Woodgate. ;-) So, let's not have any definitions at all. Then nobody would ever be wrong. John |
On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
wrote: Actually I've built *several* class C RF amps, John. --- Intentionally??? --- However, I wouldn't call any of them linear. You will be aware than linearity starts to go out of the window when Class A slides into Class AB and beyond. --- Really? I'd _love_ to hear your explanation for why that "happens". I've heard a lot of amps that sounded pretty good at both low and high volumes, and in between, and they've almost all had class AB outputs. --- Let's not have an argument over definitions. It's an open invitation to John Woodgate. ;-) --- And you don't like getting your ears pinned back?^) -- John Fields |
On Sat, 28 Aug 2004 10:27:42 -0700, John Larkin
wrote: I am not aware of any such things. So I take it you have not designed any class C linear RF power amplifiers. It depends on what you call "power" (here we go again). Certainly not beyond 500mW, no, if that answers your question. So, let's not have any definitions at all. Then nobody would ever be wrong. I imagine Kevin would be the major beneficiary of that measure. :-) -- "What is now proved was once only imagin'd." - William Blake, 1793. |
On Sat, 28 Aug 2004 12:39:34 -0500, John Fields
wrote: On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge wrote: Actually I've built *several* class C RF amps, John. --- Intentionally??? Aha! very amusing. Most of them have been intentional, yes, but who here can say they haven't ended up at some point with something they hadn't bargained for? However, I wouldn't call any of them linear. You will be aware than linearity starts to go out of the window when Class A slides into Class AB and beyond. --- Really? I'd _love_ to hear your explanation for why that "happens". I won't bore you with explanations you're already well acquainted with. But I'm still reeling from the revelation that you confused AM with Class C. :-/ I've heard a lot of amps that sounded pretty good at both low and high volumes, and in between, and they've almost all had class AB outputs. I'm sure you have. But even class A isn't perfect. The pitfalls of large-signal handling and all that. Do you know of an active device with a *perfectly* linear transconductance between say 0 and 20V? No? I thought not... -- "What is now proved was once only imagin'd." - William Blake, 1793. |
On Sat, 28 Aug 2004 19:20:06 +0100, John Woodgate
wrote: Oh, thank you, Paul. Remind me to invite you to explain something one day. People are using 'linear' in two different senses. No kidding? Only two? ;-) For audio, in fact for any amplifying stage with an **untuned load**, linearity requires linearity of output current with respect to input voltage, (Class A single ended or push-pull, Class B push-pull). But with a **tuned load**, 'linearity' can be achieved even with Class C biasing. This is why linearity in this case is defined as output power being proportional to input power. Okay. I'm quite happy with that. Any not? -- "What is now proved was once only imagin'd." - William Blake, 1793. |
But with a **tuned load**, 'linearity' can be achieved even with Class C
biasing. This is why linearity in this case is defined as output power being proportional to input power. Okay. I'm quite happy with that. Any not? -- I am not. For a normal ham amp to be linear it can not be biased class C. Class C will not reproduce a SSB or AM signal. It only works with constant signal levesl such as FM or CW. The tunes circuit "rings" and reproduces the missing portion of the sine wave of a single frequency. It can not do this for signasl where the amplitude is constantly changing such as SSB or AM. As a circuit is baised from A to B to C portions of the waveform is clipped out. Class B can be used for audio or rf if it is in a push pull circuit so that as one device (tube or transistor) is cut off the other is conducting on the other portion of the cycle. The term linear is now being used incorrectly for almost any RF amp even if the amp is biased class C. While it is not linear many use the term linear when the word amplifier or class B or C ampifier should be used. Any class ( A, B , C ) of amp can be plate modulated for AM. It is then not really an amplifier. |
Tam/WB2TT wrote:
You are only scratching the surface. Check out the Harris DX series of high power AM transmitters. It will blow your mind away. Basically, the instantanous RF output power is synthesized by turning on 0 to 64 fairly low power (~KW) modules. I don't know what the sampling frequency is, but probably 20 KHz.. All modules are driven by a square wave signal at the carrier frequency. There is no modulator. There is also a fractional stage - a 64 stage AM modulation would sound quite nasty, so an analogue signal is added to make up. There is also a 'spare stage' dthat can be switched in if one of the stages fails. Nice stuff, but the only really interesting thing IMO is the output combiner. The rest is just 'how do we make this digital'. Thomas |
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