On Fri, 27 Aug 2004 08:07:02 -0700, John Larkin
wrote:


Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
^^^^^

Oops, I meant Id/Vg. But you all knew that.

John

On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
wrote:

That means, to get a linear amp, the input signal has to be converted
to PWM gate drive. That's hard to do at high frequencies. At 300 MHz,
a power mosfet doesn't much look like a high-speed switch any more.

Certainly not at that kind of frequency! But for the lower HF bands,
it's *perfectly* feasible.
--

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

On Fri, 27 Aug 2004 15:40:15 -0700, Tim Wescott
wrote:

Don't operate SSB much, do you?

Nope. I'm a CWer. But the use of MOSFETs at RF for Anything other than
SSB (FM & AM in particular are ideally-suited) is as Kosher as Jim
Thompson's Saturday afternoon lunch of salt beef sandwiches with extra
dill.

--

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

On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
wrote:


Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.

---
Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

--
John Fields

On Sat, 28 Aug 2004 00:28:21 +0100, Paul Burridge
wrote:

On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
wrote:

That means, to get a linear amp, the input signal has to be converted
to PWM gate drive. That's hard to do at high frequencies. At 300 MHz,
a power mosfet doesn't much look like a high-speed switch any more.

Certainly not at that kind of frequency! But for the lower HF bands,
it's *perfectly* feasible.

Have you actually built a class C linear RF power amp? Tell us how it
works.

John

On Fri, 27 Aug 2004 19:01:37 -0500, John Fields wrote:

On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
wrote:

Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.

---
Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

It can be. Depends on the biasing. It *is* AM, however.

Class C just means that the conduction angle is less than 180 deg. I
recall seeing it specified as a certain amount less than 180 once,
but I don't recall how much. The idea is to give the output tank
enough kick to get it to swing.

So you amplify the carrier with class C and modulate the plate. I've
seen projects which do this with bipolars. One that comes to mind is
an old Radio Electronics article for a uWave ATV commo system using
gunplexers, but that can't be right since gunnplexers are modulated
at the gunnplexer. I think it was another ATV project. If I ever
find that old article I'll letcha know.
--
Best Regards,
Mike

John Fields wrote:
On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
wrote:



Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.


---
Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

In a tube setup the RF amplifier should be operating in class C and the
power audio amplifier should provide nice linear modulation to the RF
amp's plate supply. In fact* one needs to provide sufficient excitation
to the RF final, too, lest the thing go into a current limited mode on
the modulation peaks.

Presumably you could make a REALLY EFFICIENT setup with transistors by
operating the RF final in class E, but you get that pesky capacitance
problem back...

* So I understand, I'm just a tube wannabe.

--

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

Paul Burridge wrote:

On Fri, 27 Aug 2004 15:40:15 -0700, Tim Wescott
wrote:


Don't operate SSB much, do you?


Nope. I'm a CWer. But the use of MOSFETs at RF for Anything other than
SSB (FM & AM in particular are ideally-suited) is as Kosher as Jim
Thompson's Saturday afternoon lunch of salt beef sandwiches with extra
dill.

Motorola used to list RF MOSFETs for linear amplifier use. I don't know
if they went to On semi or FreeFall.

--

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

On Fri, 27 Aug 2004 18:09:43 -0700, Tim Wescott wrote:

John Fields wrote:
On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
wrote:

Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.

---
Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

In a tube setup the RF amplifier should be operating in class C and the
power audio amplifier should provide nice linear modulation to the RF
amp's plate supply. In fact* one needs to provide sufficient excitation
to the RF final, too, lest the thing go into a current limited mode on
the modulation peaks.

Presumably you could make a REALLY EFFICIENT setup with transistors by
operating the RF final in class E, but you get that pesky capacitance
problem back...

* So I understand, I'm just a tube wannabe.

I don't know how well it works, but I saw a class E schem using a
section of line to take out the odd harmonics.
--
Best Regards,
Mike

"Paul Burridge" wrote in message
...
On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
wrote:

That means, to get a linear amp, the input signal has to be converted
to PWM gate drive. That's hard to do at high frequencies. At 300 MHz,
a power mosfet doesn't much look like a high-speed switch any more.

Certainly not at that kind of frequency! But for the lower HF bands,
it's *perfectly* feasible.

You should know.

SioL