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.

I read in sci.electronics.design that Paul Burridge
wrote (in ooe1j0hk07082mnusi6nr5vqq0sntl0
) about 'How to bias a MOSFET amp?', on Sat, 28 Aug 2004:

It's an open invitation to John
Woodgate. ;-)

Oh, thank you, Paul. Remind me to invite you to explain something one
day.

People are using 'linear' in two different senses. 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.

With a **tuned load**, the output power also depends more or less
linearly on the supply voltage, so amplitude modulation can be achieved
by varying the supply voltage.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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