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

"Tim Wescott" wrote in message
...
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.


I believe Tyco. Yes, that same Tyco.

Tam
--

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

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