How to bias a MOSFET amp?

I read in sci.electronics.design that Paul Burridge wrote (in ) about 'How to bias a MOSFET amp?', on Sat, 28 Aug 2004:

Oh, thank you, Paul. Remind me to invite you to explain something one day.

People are using 'linear' in two different senses. For audio, in fact for any amplifying stage with an **untuned load**, linearity requires linearity of output current with respect to input voltage, (Class A single ended or push-pull, Class B push-pull).

But with a **tuned load**, 'linearity' can be achieved even with Class C biasing. This is why linearity in this case is defined as output power being proportional to input power.

With a **tuned load**, the output power also depends more or less linearly on the supply voltage, so amplitude modulation can be achieved by varying the supply voltage.

Aha! very amusing. Most of them have been intentional, yes, but who here can say they haven't ended up at some point with something they hadn't bargained for?

I won't bore you with explanations you're already well acquainted with. But I'm still reeling from the revelation that you confused AM with Class C. :-/

I'm sure you have. But even class A isn't perfect. The pitfalls of large-signal handling and all that. Do you know of an active device with a *perfectly* linear transconductance between say 0 and 20V? No? I thought not...

No kidding? Only two? ;-)

Okay. I'm quite happy with that. Any not?

I am not. For a normal ham amp to be linear it can not be biased class C. Class C will not reproduce a SSB or AM signal. It only works with constant signal levesl such as FM or CW. The tunes circuit "rings" and reproduces the missing portion of the sine wave of a single frequency. It can not do this for signasl where the amplitude is constantly changing such as SSB or AM. As a circuit is baised from A to B to C portions of the waveform is clipped out. Class B can be used for audio or rf if it is in a push pull circuit so that as one device (tube or transistor) is cut off the other is conducting on the other portion of the cycle.

The term linear is now being used incorrectly for almost any RF amp even if the amp is biased class C. While it is not linear many use the term linear when the word amplifier or class B or C ampifier should be used.

Any class ( A, B , C ) of amp can be plate modulated for AM. It is then not really an amplifier.

high

low

Yeah, exactly what I was wondering about, how do they combine the outputs? They imply that they are combining power, but I suspect they are adding voltages. BTW, the modules are not all the same. As I recall fom the WLW web site, 60 are 1-2 KW, 1 is 500W, 2 are 200W, and 1 is 100W. They claim something like 90% efficiency from 3 phase 60~ in to RF out. Also, there must be some huge bandpass filter on the output to suppress the sampling frequency.

Tam

There is also a fractional stage - a 64 stage AM modulation would sound quite nasty, so an analogue signal is added to make up.

There is also a 'spare stage' dthat can be switched in if one of the stages fails.

Nice stuff, but the only really interesting thing IMO is the output combiner. The rest is just 'how do we make this digital'.

Thomas

I read in sci.electronics.design that Active8 wrote (in ) about 'How to bias a MOSFET amp?', on Sat, 28 Aug 2004:

You should use a plain text reader, as Manitou intended.(;-)

In article , John Woodgate wrote: [...]

Small quibble:

you need to add "for a large enough drive" to the above. For small drive levels the drive and the device gm control the output power.

In article , John Woodgate wrote: [...]

I think this is right:

If this isn't the slow modulation case, for a single device RF output stage to work as a linear you need to bias it to about the point where gm is 1/2 the "mid current" gm. This makes the stage class AB.

If you want to go towards class B or C there are things you can do in the slow modulation case. Basically as the RF input increases, you move into class C and as it decreases you move back towards AB. Moving to class C lowers the output power for large signals a bit so the AB class's standing current can be reduced by some amount.

In article , Ralph Mowery wrote: [...]

I disagree with this. If the stage puts out more RF than it takes in, it is an amplifier even if the purpose of the stage is to be a modulator.

I'd always understood that it refered to an operating frequency range with a wide bandwidth, therefore suited to FM or frequency hopping.

RL

I read in sci.electronics.design that Tim Wescott wrote (in ) about 'How to bias a MOSFET amp?', on Sat, 28 Aug 2004:

The point that the discussion has reached is that the term 'linear' IS used in a different sense that includes Class C, whether we like it or not.

These are the 'electronic engineering professionals' who have 49 different definitions of 'level' in the International Electrotechnical Vocabulary? (;-)

Tube class-C amps often self-bias (via grid rectification) when operating normally. Without drive, they often have high, sometimes dangerous, idle currents.

John

What makes an amp Class C is a low conduction angle while operating. When it's not operating, is has no class at all.

John

By that definition, it could also be an oscillator!

Can't agree. If you're running any amount of standing current, then it ain't class C.

Curious. I have a book (by an author whose name I can't recall off hand) called "High-Linearity RF Amplifier Design" in which said author claims that a linear RF amplifier is one in which the transmitted signal, regardless of however non-linear it may be, is passed through sufficient filtering to clean it up! Work that one out if you can!

If you are putting RF in and getting RF out at the same frequency, in any reasonable case the circuit is acting as an amplifier. The circuit may well oscillate when no input is applied. This sort of amplifier was very common in the past and still is somewhat common.

A super-regen receiver is the most obvious example. Many tube based FM receiver designs had a FM detector that would oscillate with no input signal. The "burst lock oscillator" in a TV is in fact a very narrow pass filter and amplifier when there is a burst to lock to. With no burst it oscillates.