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Old 28th April 2014, 05:56 AM   #1
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Default Very simple quasi complimentary MOSFET amplifier

By way of background, I've been intrigued by some of the very simple amplifier circuits presented here, and have wanted to explore quasi-complimentary designs for several months now.

Inspiration for this project has come from Gareth (TGM lineup), Hugh (AKSA-55) and Carlos (original DX amplifier). I had originally planned to assemble Quasi's Nmos200; however, difficulties procuring boards has left me with a bag of IRF840 MOSFETs and nowhere to turn!

The schematic I present is as simple as I can possibly make it, without comprosing it to the point where it is just a giant oscillator. If you can think of any way to reduce the parts count further please let me know. Likewise, if I've omitted something essential for function or stability I'd like to know too.

I know there are 101 ways I can make this 'better': CCS on the LTP, degen resistors, bootstrap or CSS on the VAS, RC filters on the rails, etc. For this project I want to keep it as simple as possible and then my next project will be to embellish this circuit with some extra features.

Please let me know if there's anything I've missed (noting my Keep it Simple design ethos) and check my R values.

Next step will be to prepare a board layout and send the Gerber files to my board house.
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Last edited by Ranchu32; 28th April 2014 at 05:59 AM.
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Old 28th April 2014, 06:09 AM   #2
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two initial comments:
biasing for Q5 looks dodgy - missing a resistor i think.
Q3 needs to be fed from junction of Q1 and R5 - looks like a typo.

Last edited by kasey197; 28th April 2014 at 06:10 AM. Reason: spelling
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Old 28th April 2014, 06:10 AM   #3
Mooly is offline Mooly  United Kingdom
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FET's are voltage driven rather than current drive which means you can not connect the gates as shown. Q5 is wrongly connected too. It should be from "gate to output", not gate to source. As it is now it can only pull the gate down, not turn it on. Also you need "gate stopper" resistors in series with the FET gates mounted as close to the FET's as possible.
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Old 28th April 2014, 06:11 AM   #4
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Well spotted kasey

Q3 actually needs to be an NPN.
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Old 28th April 2014, 07:09 AM   #5
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Thanks kasey and mooly, I'll update the schematic shortly. Are gate stoppers strictly required or just recommended for stability?
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Old 28th April 2014, 07:25 AM   #6
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Gate stoppers are essential, not an option. Use something like 270 or 330 ohm on a design like this.
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Old 28th April 2014, 07:58 AM   #7
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Mooly, would you please explain the function of the gate stops and your reasoning for the large value? The stoppers on quasi's nmos design are only 27R. Thanks
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Old 28th April 2014, 08:01 AM   #8
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Quote:
Originally Posted by Mooly View Post
Q5 is wrongly connected too. It should be from "gate to output", not gate to source.
Q5 emitter to OUT, collector to mosfet gate, R12 gate to source like R9.

Also if you increase the value of R9 enough you might be able to eliminate Q4.

Also put R11 in Q7 source lead, not drain.
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Old 28th April 2014, 08:30 AM   #9
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Google MOSFET Citation 12 this is about as simple you will get.
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Old 28th April 2014, 10:20 AM   #10
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Bone - thanks for the reference to Nelson Pass' project. Reading this article and studying several other schematics from this vintage, I believe the performance will be inadequate without a bootstrap on the VAS.

All - updated schematic attached. Is this workable? How to I go about optimizing the values of R10, R12 and R13?

Any thoughts on how best do I determine the required bias voltage?

thanks
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