Re: Single transistor class A audio amplifier

Save up for power supply and heat sinks!

John

the transistor is ~300$ so maybe he should start there ;)

-Lasse

If you're going to use a MOSFET out of spec for this, why not a cheap switching MOSFET (or a bunch of them) instead of one gawdaful expensive RF MOSFET?

True, not exactly 'novel'.

Nelson Pass popularized the "single transistor" audio-amp approach back in the mid-1990s, with his "Zen" amplifier... it's sorta the MOSFET equivalent to a single-ended triode.

A slightly different approach can be seen on the page at

I'd be concerned that you would need to be *very* careful with your layout and bypassing when using a MOSFET that's as fast as this... otherwise you might run into some really severe problems with parasitic oscillation. "Amplifiers oscillate; oscillator's won't."

The IRF "hexfet" parts, and some of the few remaining lateral-MOSFET parts (which seem better designed for Class A and other linear applications) seem to be popular for these sorts of designs.

Be careful about using MOSFETs intended for Class C and switching applications, in a Class A circuit. In particular, look at the SOA curve carefully, and be *very* aware about the need to de-rate the power-handling spec. Parts which may be suitable for a 1000-watt Class C RF amplifier may handle a lot less in linear service.

A choke would make more sense

Beware of power ratings. Major derating is often required, or the device fries.

Why do you want class A though? With an opamp there, Class B makes more sense.

At least with a kilowatt amp your psu should be cheap: rectifier, cap, and a 3w transformer for the LV side. And lots of fun and games geting the 2 to talk to each other without frying anyone.

NT

For something that has a fixed DC _voltage_???

Run the drain (or collector) current thru a choke, then cap couple to the speaker. Did that when I was 18, with one of those Delco door-knob Germanium devices, for my car radio. I was fortunate enough that my father-in-law-to-be ran a machine shop, and milled me a fabulous heatsink ;-) ...Jim Thompson

rote:

in

on

manium

w-to-be

=A0 =A0...Jim Thompson

Yep, thats the way.

I got the impression the OP wanted to keep it ultrasimple, in which case one could just omit the output cap. Not ideal, but quite workable.

NT

The OP also wanted to run the thing as an emitter follower, which kind of nixes the choke idea (although a choke would be the "simple" thing to do if it were common emitter).

You could make a "current sink" with another transistor, biased like a common collector but with no input. Then you'd need two heat sinks.

When I made the simplest circuit in the 80's, I thought,well why not. I used a fairly small hexfet,, but many wanted to select higher current devices, but gate capacitance starts to become a problem. If you use a driver, have the driver create the bias, or even use dual polarity supplies.

Greg

Oh, so _that's_ why they're called "Pass" transistors!

I'm not sure where you're getting the 20W from -- you should be able to get somewhat more than that if I'm getting my back-of-the-envelope math right, and if you can really dissipate 150W from the FET (which is questionable in linear operation).

But yes, that's about it. It'll be a good amplifier to use on cold winter mornings.

Ummm... I suppose I have to say "Fail!" at this point? :-)

nob

=A0 =A0 =A0...Jim

=A0 =A0 =A0Thompson

I didnt see the OP mention emitter follower anywhere. It would seem a doubly poor choice of topology. Not only does one have the downsidse of class A, but also you're stuck with either dc through the load, or even worse efficiency. To the OP I'd suggest starting with an opamp driven class B output, use a quick opamp to iron out crossover distortion.

NT

Well, he talks about using an op-amp for voltage amplification, with a giant FET for current amplification. That says source-follower to me, and when I'm not thinking the phrase that dribbles off my finger-tips is "emitter follower".

He specifically wants to use class A, probably because he's heard that it's low distortion. It's certainly _an_ approach to use, and if you have an educated ear it's probably a worthwhile thing to try at least once, just to get an idea of how well/not-well it works.

As a room heater - once you try to drive anything much more than a pair of headphones.

Regards, Martin Brown

Well, yes. I did point out in another part of this whole thread that it'll be an excellent amplifier to use on cold winter mornings.

Or you could go class H. One could use this source-follower idea, but embedded within a class H amplifier. You'd still have many of the advantages of the source follower, with much lower heat dissipation in your final amplifier stage.

Your supply rail voltages would, inevitably, bleed into your signal to its detriment. But they may not bleed in _much_, particularly as you allow more and more standing voltage across your final amplifier components.

=A0 =A0 =A0...Jim

=A0 =A0 =A0Thompson

it also describes an ordinary class a or b amp.

It can be low distortion. It can also provide a fair bit of distortion. The opamp should iron out any wobbles in the output stage, but it can certainly go wrong.

NT

Have you looked into maintaining the heatsink at a sensible temperature while keeping the junction temperature within spec? Do you intend to plumb the heatsink into your central heating system?

Agreed!

I'd go for the equivalent of Class AB1 or AB2 as a compromise where at low audio levels the amp would be close to class A.