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I'm designing a class A audio amplifier but am struggling with the efficiency of my circuit.

I know of 'push-pull' Class A designs, but only want to use a single output mosfet. I would like to hit a minimum of 12 watts using a 24V 1.5A supply.

My current circuit: enter image description here

I know I'm limiting the top of the gate threshold by feeding the gate though the drain of the output Mosfet in order to stabilize the operating point. Would the only way to get the full range to be use separate DC feedback? Would I have to use another active component (like a PNP transistor) from the input?

Also have I got the constant current source arrangement correct? I'v noticed there is a current drop as the draw by the output mosfet falls, is this anything to be concerned about?

As I am a beginner at audio amplifier design, the design goals are to be a simple, but high quality audio amplifier.

Thank you ESE community :)

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    \$\begingroup\$ Include Falstad link so I can see dc current, symmetry etc \$\endgroup\$ – Tony Stewart EE75 Jun 25 '19 at 0:31
  • \$\begingroup\$ falstad.com/circuit/circuitjs.html \$\endgroup\$ – G Frank Jun 25 '19 at 0:39
  • \$\begingroup\$ pastebin.com/raw/0LJ9uQEE \$\endgroup\$ – G Frank Jun 25 '19 at 0:41
  • \$\begingroup\$ in future you can export link as well in answer and use shorturl in comments only \$\endgroup\$ – Tony Stewart EE75 Jun 25 '19 at 0:42
  • \$\begingroup\$ oww, yes i see that now. No problem :) \$\endgroup\$ – G Frank Jun 25 '19 at 0:43
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The basic problems with your circuit are extreme sensitivity to DC bias point of the upper FET, interaction between DC bias point and gain of the input transistor and lower FET, and low AC gain.

I put your circuit into LTspice and made a few changes to improve DC stability and AC gain:-

enter image description here

To remove the interaction between M1 and Q1 biasing I AC coupled the signal through C3, with R8 isolating the signal from M1's bias circuit (which I decoupled with C5 so it wouldn't affect the AC gain). I also applied negative feedback to M2's bias with R2, and decoupled it to AC with C4. Finally I added small Source degeneration resistors (R11, R12) to further stabilize the FET bias points.

With these changes the AC gain jumped to 47dB which was more than I wanted, so I applied a small amount of negative feedback to the Emitter of Q1 (via C6 and R9) which brought the gain down to 39dB. This also improved the bandwidth and harmonic distortion figures. Frequency response is now flat within 1dB from 20Hz to 100kHz, and distortion is less than 5% at a peak-to-peak output voltage of 17.2V.

Power supply ripple rejection was extremely poor at +10dB (yes, it was amplifying it!) so I added a filter (R13 and C7) on the supply rail to the input stage, which provided -27dB of ripple rejection.

Even though this circuit looks good in LTspice, in practice it would probably still need trimpots to adjust the bias for individual FETs. I tried changing the bias resistor values by +- 20% and it didn't upset the bias points much. However each FET dissipates 14~16W of quiescent power, so their operating points may drift significantly as they heat up.

I would like to hit a minimum of 12 watts using a 24V 1.5A supply.

This is where we come unstuck. A class A amplifier cannot achieve over 25% efficiency without using a transformer or choke. With an 8 ohm load and 24Vpp sine wave the maximum possible undistorted output power is 9W, with a peak load current of 1.5A. Since in Class A peak load current = supply current, this requires 24V*1.5A = 36W of input power. 9W/36W = 25% efficiency.

In practice it will be even less efficient due to voltage drops in the output transistor(s) etc. In this circuit the maximum output power (with <5% distortion) is only 4.6W, from an input power of 24V*1.38A = 33W. Very clean sound, but an enormous waste of energy and a lot of heat to get rid of.

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  • \$\begingroup\$ thanks Bruce, you have gave me lots to think about. Can I get a download of the LTspice file plz? \$\endgroup\$ – G Frank Jun 25 '19 at 10:09
  • \$\begingroup\$ Even with added improvements 5% THD is not so “clean” that would require much more open loop gain and then stability compensation by an integrator cap, so this is never done in good design practice and a poor design to learn by comparison unless looking for a fuzz tone amp for strings that can keep your coffee warm. \$\endgroup\$ – Tony Stewart EE75 Jun 25 '19 at 13:07
  • \$\begingroup\$ Why use C3 when Q1 can amplify as well as provide correct bias range to the gate of M1? apart from being very sensitive of the change in bias of Q1. @Bruce Abbott \$\endgroup\$ – G Frank Jun 25 '19 at 18:49
  • \$\begingroup\$ @SunnyskyguyEE75 THD figures can be deceptive. 0.1% crossover distortion in a Class B transistor amp sounds disgusting, but 5% of mostly even harmonic distortion in a Class A amp (at maximum output) is barely noticeable. More feedback improves the technical THD spec, but sounds worse when the amp is slightly overloaded because the clipping is harsher. Traditional Class A valve amps only used a small amount of negative feedback (if any), yet sounded better than early transistor amps with much lower distortion figures. \$\endgroup\$ – Bruce Abbott Jun 25 '19 at 22:06
  • \$\begingroup\$ Q1 cannot supply the 'correct' bias range to M1 because FETs do not have a well defined bias voltage. Gate threshold voltage of an IRFP240 can be as little as 2.0V or as much as 4.0V, and is strongly temperature dependent. Bipolar transistors are better, but in this circuit any bias variations on Q1 are amplified at the Collector, making it virtually impossible to provide a sufficiently stable bias to the FET. It might seem OK in simulation, but in practice it would drift like crazy! \$\endgroup\$ – Bruce Abbott Jun 25 '19 at 22:23
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Class A Complementary FETs will always be a struggle with 1 BJT driver

  • low open loop gain, thus >10% THD even with negative feedback since it is very quadratic.
  • only get 16Vpp from a 24V supply.
  • loss of low-frequency response with a single supply requiring large Cap may have poor characteristics.
  • poor supply ripple rejection (hum etc)
  • too many pots that are critical to adjust for Vgs(th) and BJT supply variations

Consider: Class AB complementary drain followers with OA

  • Using bipolar supply can drive 2 Ohm load easily.
  • OA gain reduces THD to 0.1%
  • microamp driver supply +/- 16V or more ( to can gain 8V headroom to get the rail to rail outputs.
  • low idle power
  • DC to 100kHz BW min.
  • excellent supply ripple rejection. enter image description here

Simulation. Fixed SIM

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  • \$\begingroup\$ Your design is good, but not sure OP is ready to incorporate op-amps yet. Yes, the OP's design burns up 24 watts of continuous power, but at least the OP understands why... \$\endgroup\$ – user105652 Jun 25 '19 at 2:29
  • \$\begingroup\$ Note when looking at an AC coupled wave, always enable the property to capture negative peak then compare with V+max. The difference ratio to max pp is roughly your THD in %. \$\endgroup\$ – Tony Stewart EE75 Jun 25 '19 at 5:09
  • \$\begingroup\$ For the moment I want to stick to a purely class A design as I really like the sound they produce. I knew how inefficient this topology is but as it will be for reasonable quite listening though sensitive speakers, I don't mind. I will most likely move on to a separate AB/B design next but will take what I have and will learn from building this amplifier. \$\endgroup\$ – G Frank Jun 26 '19 at 20:45
  • \$\begingroup\$ Ok But class A FET transistor has a very small linear range compared to tubes , so not so “pure” in fact even H bias is really asymmetric full swing. But it is a play on words to consider pure A as pure sound here. The simulator I offered shows all this. But maybe sounds ok on a single stringed instrument which is a matter of perception. \$\endgroup\$ – Tony Stewart EE75 Jun 26 '19 at 21:29

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