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Re: Single transistor class A audio amplifier

Hi,

> > I'm thinking about building my own audio power amplifier and the > 'novel' idea I came up with was to use one GIANT transistor biased in > Class-A operation to do the current amplification (the voltage > amplification will be done with OPA134 OpAmps). > > I'm thinking of using a BLF578XR MOSFET for the current amplification > but I'm not sure if it's suitable for LF (audio) signals as it's > designed for HF applications. The device is rated at 1200W (!) so it > should be more than capable of handling the power requirements. > > Do you guys think the BLF578XR can be used for this purpose?

Going back to Class A transformer output stages and assuming you're talking about a transformerless amp: Do speakers commonly handle much standing DC current?

Even if you shunt an 8 ohm speaker with a 1 ohm resistor as your "pulldown", you still have 1/8 of the DC current flowing through the speaker.

It seems likely if you wanted to experiment you'd choose a part with at least a published transfer curve (ID vs VDS), never seen one for a NXP RF MOSFET yet. Most non-RF power MOSFET's publish transfer curves.

Tim.

Reply to
Tim Shoppa
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Save up for power supply and heat sinks!

John

Reply to
John Larkin

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

-Lasse

Reply to
langwadt

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?

--
www.wescottdesign.com
Reply to
Tim Wescott

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

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and I think this may be closer to what you are thinking about... a "voltage gain of 1, current gain of lots" MOSFET follower with a bipolar current source. Needs to be fed from a high-voltage preamplifier circuit.

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.

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

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

Reply to
NT

For something that has a fixed DC _voltage_???

--
www.wescottdesign.com
Reply to
Tim Wescott

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

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Reply to
Jim Thompson

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=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

Reply to
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.

--
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Reply to
Tim Wescott

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

Reply to
gregz

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

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Reply to
Tim Wescott

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.

--
www.wescottdesign.com
Reply to
Tim Wescott

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

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
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Reply to
Dave Platt

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=A0 =A0 =A0...Jim

=A0 =A0 =A0Thompson

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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

Reply to
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.

--
www.wescottdesign.com
Reply to
Tim Wescott

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

Regards, Martin Brown

Reply to
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.

--
www.wescottdesign.com
Reply to
Tim Wescott

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=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

Reply to
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.

--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk
Reply to
Mike Perkins

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