How to bias a MOSFET amp?

From: Bill N2CQR MOHBR (meara.london_at_virgin.net)
Date: 08/27/04 

Date: 26 Aug 2004 22:40:36 -0700

I'm trying to improve my limited understanding of how RF amps operate.
I've been studying "Solid State Design for the Radio Amateur" and
Experimental Methods in RF Design"

SSDRA has a very helpful section that asks the reader to calculate max
output power for a Class A RF amp (common emitter). They ask the
reader to consider maximum voltage and current swings in the collector
circuit that will keep the output linear. Basically, my understanding
is that (with an RF choke in the Vcc line) max peak signal voltage on
collector is Vc-Ve. Max peak current is the standing or quiescent
current. In this way when the collector voltage is hitting its peak
collector current is dropping almost to zero.

While the SSDRA example uses BJTs, I'm guessing that essentially the
same restrictions would apply to the drain circuit of a MOSFET amp.

Here's my problem: When I look at MOSFET amp circuits in the
literature, they often have linear amps with 12 volts on the drain,
but with standing currents of only around 40 milliamps. And they
claim 7 watts out. How can that be? Using the analysis outlined
above, 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 SSB
Transceiver
http://farhan.net.co.nr/xcvr1.html

I'm trying to understand the biasing on his IRF510 final, and the RF
output he's getting.

He says he measures 20-24 volts peak RF across a 50 ohm load at the
output. That's about 8 watts peak output.

He's using 12 volt supply, and recommends setting the idle current
through the MOSFET at 80 ma. Can that be right? According to my
reading of Solid State Design for the Radio Amateur (SSDRA)(page 23)
with a 12 volt supply we can expect peak signal voltage at the Drain
of around 12 volts (with an RF choke in Vcc line). Peak current could
be max 80 ma.(maintaining Class A). Under these biasing conditions,
assuming Class A operation, max output power of .96 watts would be
provided by a load of 150 ohms.

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

Relevant Pages

  • How to bias a MOSFET amp?
    ... Max peak current is the standing or quiescent ... same restrictions would apply to the drain circuit of a MOSFET amp. ... claim 7 watts out. ... He says he measures 20-24 volts peak RF across a 50 ohm load at the ...
    (sci.electronics.design)
  • Re: lm317 as current regulator
    ... Is the input limit on the ic still limited ... would that not mean it can handle 55.5 watts. ... My circuit will have a differential of 20 volts at 1 amp, ...
    (sci.electronics.components)
  • Re: Power ratings in older vs newer Mesa SimulClass power sections
    ... the end - even amp techs who know the answer. ... currently own) were all rated at 75 watts when in SimulClass mode. ... various switch settings. ...
    (alt.guitar.amps)
  • Re: How does one add a 12v trigger to an amplifier...?
    ... I wish to power on via the 12v trigger is a Mackie FR2600 high current ... It draws ac line power at 120 volts plugged into a 20 amp ... Those are audiophool watts, or "Peak Intermittent Modulated Power", i.e. ... more than 80% of the branch circuit rating. ...
    (sci.electronics.basics)
  • Re: OK, now *I* need some sound advice
    ... a high end audio system these days will be powered by a secondary ... If you have a couple THOUSAND watts and are doing competitions then they ... So, if you have a 500 watt amp, but are only running it at ... Parked the audio will drain the battery even with his battery charger on it. ...
    (alt.autos.toyota)