I'm trying to improve my limited understanding of how RF amps operate.I've been studying "Solid State Design for the Radio Amateur" andExperimental Methods in RF Design"SSDRA has a very helpful section that asks the reader to calculate maxoutput power for a Class A RF amp (common emitter). They ask thereader to consider maximum voltage and current swings in the collectorcircuit that will keep the output linear. Basically, my understandingis that (with an RF choke in the Vcc line) max peak signal voltage oncollector is Vc-Ve. Max peak current is the standing or quiescentcurrent. In this way when the collector voltage is hitting its peakcollector current is dropping almost to zero.While the SSDRA example uses BJTs, I'm guessing that essentially thesame restrictions would apply to the drain circuit of a MOSFET amp.Here's my problem: When I look at MOSFET amp circuits in theliterature, they often have linear amps with 12 volts on the drain,but with standing currents of only around 40 milliamps. And theyclaim 7 watts out. How can that be? Using the analysis outlinedabove, I'd think that you'd get max output of 12*.040 = .48 watts.Here's an example: I've been looking at Farhan's very FB SSBTransceiverI'm trying to understand the biasing on his IRF510 final, and the RFoutput he's getting.He says he measures 20-24 volts peak RF across a 50 ohm load at theoutput. That's about 8 watts peak output.He's using 12 volt supply, and recommends setting the idle currentthrough the MOSFET at 80 ma. Can that be right? According to myreading of Solid State Design for the Radio Amateur (SSDRA)(page 23)with a 12 volt supply we can expect peak signal voltage at the Drainof around 12 volts (with an RF choke in Vcc line). Peak current couldbe max 80 ma.(maintaining Class A). Under these biasing conditions,assuming Class A operation, max output power of .96 watts would beprovided by a load of 150 ohms.Even if he were to be running this amp Class B (or close to it), Ican't see how he'd...

I think the RF guys (I'm not one!) call an amplifier "linear" if theRF output amplitude follows the input drive amplitude. You can do thiswith a transistor that has very low quiescent bias. So "linear" doesnot mean "class A" to them. The key here is that an RF amp has a tunedoutput, whereas an audio amp doesn't. So the lopsided bias wouldnormally produce intolerable distortion in something like audio, butthe tuned output circuit changes the pulsey-looking collector/draincurrent back into a nice sine wave. So you don't need a lot of idlecurrent, and the transistor really amplifies half of the incoming sinecycle.Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)beyond the initial knee. You could get gobs of watts at zero standingcurrent, but then you'd have some zero-clipping (no output) for thesmallest drive levels, so a little idle current helps.John

---But most of what he sources _is_ fishy.-- John Fields

How to bias a MOSFET amp?

Aug 27, 2004 69 Replies

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Ken Smith 21 years ago

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.

-- kensmith@rahul.net forging knowledge

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Paul Burridge 21 years ago

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.

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keith 21 years ago

Burridge?

Keith

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John Larkin 21 years ago

He said surgeon, not sturgeon.

John

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John Fields 21 years ago

But most of what he sources _is_ fishy.

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John Woodgate 21 years ago

I read in sci.electronics.design that John Larkin wrote (in ) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:

But, being male, PB is not even a potential source of caviar.

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

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Paul Burridge 21 years ago

Hey ladies, gimme a break will ya? Don't make your PMS my problem. ;-)

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

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keith 21 years ago

OTOH, he's a good source of other slime. If you want ot go there, be my guest!

Keith

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keith 21 years ago

Paul's Miserable S*it?

Keith

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Rich Grise 21 years ago

Psychotic Male Syndrome.

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