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

"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

Paul Burridge wrote:

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


I think the RF guys (I'm not one!) call an amplifier "linear" if the
RF output amplitude follows the input drive amplitude. You can do this
with a transistor that has very low quiescent bias. So "linear" does
not mean "class A" to them. The key here is that an RF amp has a tuned
output, whereas an audio amp doesn't. So the lopsided bias would
normally produce intolerable distortion in something like audio, but
the tuned output circuit changes the pulsey-looking collector/drain
current back into a nice sine wave. So you don't need a lot of idle
current, and the transistor really amplifies half of the incoming sine
cycle.

Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
beyond the initial knee. You could get gobs of watts at zero standing
current, but then you'd have some zero-clipping (no output) for the
smallest drive levels, so a little idle current helps.


The only sensible way to do it AFAICS is to operate the MOSFET in
class C as a high speed switch and reconstruct the pulsed output into
a sine wave carrier by means of a suitable tuned circuit. I wouldn't
consider driving a MOSFET for RF use in any other way. The efficiency
should be pretty darned good, too.

Don't operate SSB much, do you?

--

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

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.

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

John Larkin wrote:
On 26 Aug 2004 22:40:36 -0700, (Bill N2CQR
MOHBR) wrote:



Even if he were to be running this amp Class B (or close to it), I
can't see how he'd get 8 watts out with only 80 milliamps of standing
current

I'm very new to this kind of analysis, and strongly suspect that I'm
misreading either SSDRA or Farhan's excellent article.

Can someone please let me know where this apparent discrepency is
coming from.

Thanks and 73

Bill N2CQR M0HBR CU2JL
http://planeta.clix.pt/n2cqr



I think the RF guys (I'm not one!) call an amplifier "linear" if the
RF output amplitude follows the input drive amplitude. You can do this
with a transistor that has very low quiescent bias. So "linear" does
not mean "class A" to them. The key here is that an RF amp has a tuned
output, whereas an audio amp doesn't. So the lopsided bias would
normally produce intolerable distortion in something like audio, but
the tuned output circuit changes the pulsey-looking collector/drain
current back into a nice sine wave. So you don't need a lot of idle
current, and the transistor really amplifies half of the incoming sine
cycle.

Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
beyond the initial knee. You could get gobs of watts at zero standing
current, but then you'd have some zero-clipping (no output) for the
smallest drive levels, so a little idle current helps.

John

RF guys call the amplifier "linear" if the output, after filtering,
looks like a bigger version of the input -- basically the same criterion
as any other amplifier. The reason that you can get away with half as
many active elements as with an audio amplifier is because if the
modulation is narrow compared to the carrier each half of the waveform
looks the same, so amplifying half of it then filtering reconstructs the
half you didn't play with.

Class A amplifiers (and push-pull class AB or B amplifiers) are used in
RF work, but mostly because they cut down on the harmonics that must be
filtered out.

--

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

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