On Sun, 29 Aug 2004 00:48:51 +0000 (UTC),
(Ken Smith) wrote:

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

By that definition, it could also be an oscillator!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Sun, 29 Aug 2004 00:46:11 +0000 (UTC),
(Ken Smith) wrote:

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.

Can't agree. If you're running any amount of standing current, then it
ain't class C.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Sun, 29 Aug 2004 02:10:52 GMT, legg 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. :-/

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

Curious. I have a book (by an author whose name I can't recall off
hand) called "High-Linearity RF Amplifier Design" in which said author
claims that a linear RF amplifier is one in which the transmitted
signal, regardless of however non-linear it may be, is passed through
sufficient filtering to clean it up! Work that one out if you can!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

In article ,
Paul Burridge wrote:
On Sun, 29 Aug 2004 00:48:51 +0000 (UTC),
(Ken Smith) wrote:

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

By that definition, it could also be an oscillator!

If you are putting RF in and getting RF out at the same frequency, in any
reasonable case the circuit is acting as an amplifier. The circuit may
well oscillate when no input is applied. This sort of amplifier was very
common in the past and still is somewhat common.

A super-regen receiver is the most obvious example. Many tube based FM
receiver designs had a FM detector that would oscillate with no input
signal. The "burst lock oscillator" in a TV is in fact a very narrow pass
filter and amplifier when there is a burst to lock to. With no burst it
oscillates.


--
--
forging knowledge

In article ,
Paul Burridge wrote:
On Sun, 29 Aug 2004 00:46:11 +0000 (UTC),
(Ken Smith) wrote:

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.

Can't agree. If you're running any amount of standing current, then it
ain't class C.

If the conduction angle is less than 180 degrees it is class C. Note that
I said "you move into class C" this means the amplifier isn't always
running as class C. It only runs in class C with large signals. Since
the large signal case is were the losses really matter, this sort of
design will work quite a bit better than a simple class AB circuit.

--
--
forging knowledge

On Sun, 29 Aug 2004 12:49:46 +0100, Paul Burridge
wrote:


Can't agree. If you're running any amount of standing current, then it
ain't class C.

Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

John

On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
wrote:

Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

Toobz? I was talking of transistors. Many tranny designs for Class C
RF utilize self-bias, too, but there is still no quiescent current!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
wrote:

On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
wrote:

Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

Toobz? I was talking of transistors. Many tranny designs for Class C
RF utilize self-bias, too, but there is still no quiescent current!


What makes an amp Class C is a low conduction angle while operating.
When it's not operating, is has no class at all.

John

On Sun, 29 Aug 2004 12:08:37 -0700, John Larkin
wrote:

On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
wrote:

On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
wrote:

Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

Toobz? I was talking of transistors. Many tranny designs for Class C
RF utilize self-bias, too, but there is still no quiescent current!


What makes an amp Class C is a low conduction angle while operating.
When it's not operating, is has no class at all.

---
I ran into a surgeon like that once...

--
John Fields

On Sun, 29 Aug 2004 15:04:53 -0500, John Fields wrote:

On Sun, 29 Aug 2004 12:08:37 -0700, John Larkin
wrote:

On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
wrote:

On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
m wrote:

Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

Toobz? I was talking of transistors. Many tranny designs for Class C
RF utilize self-bias, too, but there is still no quiescent current!


What makes an amp Class C is a low conduction angle while operating.
When it's not operating, is has no class at all.

---
I ran into a surgeon like that once...

Burridge? shudder

--
Keith