On Sat, 28 Aug 2004 19:28:03 +0100, Paul Burridge
wrote:

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.

---
On the contrary, I'd like to hear why you think class AB or B isn't
(or can't be) linear, input-to-output.
---

But I'm still reeling from the revelation that you confused AM
with Class C. :-/

---
Go back and read it again in the context of "is that a smart thing to
do?" with your tongue-in-cheek detector energized.
---


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...

---
We weren't talking about components with perfectly linear transfer
functions, _you_ were alluding to deterioration of input-to-output
linearity in systems using different driver biasing schemes. Or so I
thought, when you said:

"You will be aware than linearity starts to go out of the window when
Class A slides into Class AB and beyond."

And I'll repeat:

"I'd _love_ to hear your explanation for why that "happens".".


But never mind, now that Woodgate's cleared it up there's no need for
you to embarrass yourself further.

--
John Fields

On Sat, 28 Aug 2004 19:20:06 +0100, John Woodgate wrote:

snip

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.

Just an obsevation... when you enclose a word or words in double
asterisks, my reader no longer renders them in bold, thus removing
the intended emphasis. IIRC it's the double underscore enclosure
that causes my reader to underline the text.

We now return you to our regularly scheduled terminology debate.

--
Best Regards,
Mike

I read in sci.electronics.design that Active8
wrote (in ) about 'How to bias a
MOSFET amp?', on Sat, 28 Aug 2004:

Just an obsevation... when you enclose a word or words in double
asterisks, my reader no longer renders them in bold, thus removing the
intended emphasis. IIRC it's the double underscore enclosure that causes
my reader to underline the text.

You should use a plain text reader, as Manitou intended.(;-)
--
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

Dad just bought a 2kW FET amplifier. It's ex-industrial use, but has MRF
line transistors, and we expect no trouble putting it online for 160-10
meter use.
It's a dozen MRF-150s

Look up the specs (and the prices (OUCH)) on the MRF-154.

--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR

In article ,
John Woodgate wrote:
[...]
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.


Small quibble:

you need to add "for a large enough drive" to the above. For small drive
levels the drive and the device gm control the output power.

--
--
forging knowledge

In article ,
John Woodgate wrote:
[...]
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.

I think this is right:

If this isn't the slow modulation case, for a single device RF output
stage to work as a linear you need to bias it to about the point where gm
is 1/2 the "mid current" gm. This makes the stage class AB.

If you want to go towards class B or C there are things you can do in the
slow modulation case. Basically as the RF input increases, you move into
class C and as it decreases you move back towards AB. Moving to class C
lowers the output power for large signals a bit so the AB class's standing
current can be reduced by some amount.


--
--
forging knowledge

In article . net,
Ralph Mowery wrote:
[...]
Any class ( A, B , C ) of amp can be plate modulated for AM. It is then
not really an amplifier.

I disagree with this. If the stage puts out more RF than it takes in, it
is an amplifier even if the purpose of the stage is to be a modulator.


--
--
forging knowledge

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. :-/

From basic systems theory:

A system S is linear if and only if for any two input signals x1 and x2
that generate the output signals y1 = S(x1) and y2 = S(x2), and for any
two real constants A1 and A2, the output signal y = S(A1*x1 + A2*x2) is
equal to A1*y1 + A2*y2.

This condition is approached with a properly adjusted RF linear
amplifier, even one operated class AB or B. It is _not_ approached with
a class C amplifier.

So if you define "linear" the way electronics engineering professionals
define "linear" a class A, AB or B amplifier can be made to act
linearly, more or less, and a class C amplifier cannot. So the term
isn't a misnomer, and its use is obvious.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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. :-/

I'd always understood that it refered to an operating frequency range
with a wide bandwidth, therefore suited to FM or frequency hopping.

RL

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

A system S is linear if and only if for any two input signals x1 and x2
that generate the output signals y1 = S(x1) and y2 = S(x2), and for any
two real constants A1 and A2, the output signal y = S(A1*x1 + A2*x2) is
equal to A1*y1 + A2*y2.

This condition is approached with a properly adjusted RF linear
amplifier, even one operated class AB or B. It is _not_ approached with
a class C amplifier.

The point that the discussion has reached is that the term 'linear' IS
used in a different sense that includes Class C, whether we like it or
not.

So if you define "linear" the way electronics engineering professionals
define "linear" a class A, AB or B amplifier can be made to act
linearly, more or less, and a class C amplifier cannot. So the term
isn't a misnomer, and its use is obvious.

These are the 'electronic engineering professionals' who have 49
different definitions of 'level' in the International Electrotechnical
Vocabulary? (;-)
--
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