How to bias a MOSFET amp?

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

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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

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Reply to
Bill N2CQR MOHBR
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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

Reply to
John Larkin

It's a bias voltage, rather than a bias current. The MOSFET enters its linear conducting area after a certain threshold voltage is attained.

Reply to
John Walton

Keep studying.

The static bias is the zero-signal drain current and your calculation is for static DC loss. Signals applied to the gate can add to the drain current, as well as subtract.

RF final amps are unlikely to be biased classA. Class B and C will have the final acting more like a switch. Current is imited by the load impedance - which in this case is a complex impedance that looks like 50 ohms at the output only.

As to the answer for your posting - the fet in this case is biased by varying gate static voltage using the variable resistor in the schematic, as instructed.

RL

Reply to
legg

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.

John

Reply to
John Larkin

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That doesn't make any sense to me.

Unless things have changed pretty drastically from how they were when
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Reply to
John Fields

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.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Don't operate SSB much, do you?

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

-- snip --

Well, AM tube finals were often operated class C with the modulation applied to the plate supply. This is harder to do with silicon because the varying collector voltage modulates the collector-base capacitance and causes weird phase shifts.

And there are linear MOSFET RF amps; they're necessary for single-sideband.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

^^^^^

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

John

Reply to
John Larkin

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

John

Reply to
John Larkin

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.
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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

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

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

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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
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Reply to
John Fields

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.

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

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.

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Best Regards,
Mike
Reply to
Active8

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.

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Best Regards,
Mike
Reply to
Active8

............................................................................. ...........

You are only scratching the surface. Check out the Harris DX series of high power AM transmitters. It will blow your mind away. Basically, the instantanous RF output power is synthesized by turning on 0 to 64 fairly low power (~KW) modules. I don't know what the sampling frequency is, but probably >20 KHz.. All modules are driven by a square wave signal at the carrier frequency. There is no modulator.

Tam

Reply to
Tam/WB2TT

I believe Tyco. Yes, that same Tyco.

Tam

Reply to
Tam/WB2TT

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

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

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