How to achieve efficiency of more than 70% and decrease the distortion of the circuit to 0.1% in THD, without affecting my Vpp=30 with the circuit below with both in 1k and 6k frequency of Vs?

Update Also, I only have this component available, and I can only make this kind of design in the project that was taught in university. I am also working with 10W speaker.

I meant by efficiency=(Pout/Pin)100, for this circuit, I get 63.82%=(9.97/(7.812))*100

If not I would like to theory behind it

Updated Circuit enter image description here enter image description here enter image description here

  • 3
    \$\begingroup\$ Title asks about how to increase distortion but the body asks the opposite. The theoretical limit of a Class-AB amp is around 75%. So you shouldn't expect more than 70% in practice. \$\endgroup\$ Jun 9 at 13:55
  • 1
    \$\begingroup\$ What is "6f frequency"? You want to decrease the distortion "by 0.1"...does that mean reduce the distortion by 10%, to about 1.7% distortion? \$\endgroup\$ Jun 9 at 13:57
  • 4
    \$\begingroup\$ If you want to decrease distortion this answer to a previous question of yours explained that moving the feedback resistor (R4 above) from the op-amp output to the main power output does this. Choosing a better/faster op-amp will also give lower distortion (when implemented as I stated in that answer). \$\endgroup\$
    – Andy aka
    Jun 9 at 14:40
  • \$\begingroup\$ @ElliotAlderson I meant k frequency of Vss, and by 0.1 I meant my THD 0.1% instead of 7.4% \$\endgroup\$ Jun 9 at 17:54
  • \$\begingroup\$ is there a way to fix the clipping in the sine wave? \$\endgroup\$ Jun 9 at 18:52

5 Answers 5


How to achieve efficiency of more than 70%

The efficiency of a class-AB amp doesn't really depend on the schematic. With power supply +/-Vcc and output voltage Vout, then the amp will burn a power Iout.(Vcc-Vout) as heat in the transistors and source resistors, and Iout.Vout is used as useful output power. So if you want higher efficiency, use a lower power supply voltage.

Another way to increase efficiency is to design the amp better, so it delivers less distortion at low bias current, so the transistors burn less power when they do nothing. However this is only relevant when the output signal is small enough that the amp stays in the class A zone of class AB. It becomes irrelevant at mid-high power, because it goes to class B, and at this point it spends little time in the class A zone.

In conclusion, efficiency depends on power supply voltage across the whole range of power, and on bias current at low power (or idle).

and decrease the distortion of the circuit to 0.1% in

I'll try.

enter image description here

First thing is to add a Vbe multiplier to bias those MOSFETs. Without it, they'll go into thermal runaway, so it is absolutely necessary. This is because the threshold voltage goes down when they get hot (negative tempco) which increases idle bias current, which makes them hotter, and this goes in a loop until the idle bias current goes out of control and smoke gets out. This is especially important with small TO-220 transistors, which get hot really fast.

Unfortunately it is not possible to know the Vgs_th tempco without measuring it. For example IRFP140/240 have -4-6 mV/°C total, P and N channel combined. Assuming this will be the same here, I add a Vbe multiplier.

This multiplies the transistor Q1's Vbe by a ratio R4/R3. Note R4 and R3 should be a potentiometer. Since Q1's Vbe has a tempco around -2mV/°C, once it is multiplied by about 10 to get the correct gate-to-gate voltage to bias the MOSFETs, this will have -20mV/°C, which is too much.

However, even if it is placed between the power devices on the heatsink, the transistor gets much less temperature rise than the MOSFETs themselves, because the heatsink is doing its job and dumping that heat into the air. That somewhat compensates for it, but be warned this circuit will overcorrect.

It is possible to get a much more accurate bias current by putting two Vbe multipliers in series, one of which is not on the hear sink (Q2).

enter image description here

In this case, R15 is fixed, and R3/R4 is a pot.

Next, we have DC. It is not a good idea to AC couple the output of the opamp to the gates, as you have done, because then the DC operating point of the output stage is not fixed, and you need an output capacitor to get rid of it.

So in the first schematic of this answer, R7-R8 do this job. In the second schematic, the Vbe multiplier transistors do it as a side job.

Then feedback wraps around the whole thing, and we get a bit more than 0.1% distortion at 1kHz.

It will be difficult to go lower by using an opamp, because the opamp does not offer flexibility in custom compensation scheme, which are the secret sauce for low distortion, as they allow a much higher open loop gain to be used for correcting distortion in the output stage.

I set the source resistors to 0R22 and idle bias current to 20mA.

  • \$\begingroup\$ Is it not possible to the circuit with inverting op-amp? From what I see you using non inverting terminal for Vs. \$\endgroup\$ Jun 10 at 3:07
  • \$\begingroup\$ Yes you can make it inverting \$\endgroup\$
    – bobflux
    Jun 10 at 7:54
  • \$\begingroup\$ My efficiency is worse both in 1k and 6k Circuit, did I do something wrong? and is there a way to increase to approximately 10W efficiency or a bit higher? \$\endgroup\$ Jun 10 at 10:24
  • \$\begingroup\$ 10W isn't efficiency, watts are power, so I have no idea what you mean \$\endgroup\$
    – bobflux
    Jun 10 at 12:57
  • 1
    \$\begingroup\$ What I meant is that I used to get 63.82% with 10W at RL from the original circuit, but now my efficiency is 0.84% with 8.41mW at RL. so, I wanted to know did I made any mistakes while following your circuit \$\endgroup\$ Jun 10 at 17:47

You have no control of the Vgs and resulting current of MOSFETs. Some will be with a massive current and in class-A, some will be turned off and producing extreme crossover distortion in class-B and maybe a few will be in class-AB with low distortion and efficiency as high as 65% at high power. All amplifiers have negative feedback that reduces distortion but yours has none. Your schematic has its parts so far apart that everything looks tiny and hard to see. Mosfets

  • \$\begingroup\$ I forgot to say that an LM358 is never used for audio because it produces crossover distortion, noise and poor high frequencies slew rate. \$\endgroup\$
    – Audioguru
    Jun 9 at 15:20
  • \$\begingroup\$ This qualifies as the second worst amp schematics I've ever seen (first place being the one with BJTs posted previously)... \$\endgroup\$
    – bobflux
    Jun 9 at 15:36
  • \$\begingroup\$ These circuits are 50 years old as were taught by very old teachers. \$\endgroup\$
    – Audioguru
    Jun 9 at 16:43
  • \$\begingroup\$ Because of the limited components, my professors are not allowing us to design better. instead, using the design that was thought in the presentation \$\endgroup\$ Jun 9 at 16:49

A couple of ways that you can reduce the distortion:

  • Use complementary MOSFETs connected as source followers.
  • Include the output transistors in the feedback loop. This means that instead of connecting the feedback resistor to the output of the opamp, connect it to the output of the amplifier where the speaker is connected.
  • Use an audio grade opamp.
  • Add a resistor between the non-inverting opamp input and ground to reduce the output opamp's output offset voltage, you'll need to experiment with different values.
  • Use a larger capacitor on the output. You've been told this multiple times before but you never seem to actually do it. Try 1000uF.
  • Allow for headroom in your power supplies. If you need 30 Vpp output a \$\pm\$ 15 V supply isn't going to be enough.

Doing this should get the distortion under 1% in simulation. I just threw together a quick circuit in LTspice and got 0.99% distortion at 30Vpp, 6KHz using \$\pm\$18V for the output supply and a TL084 type opamp. I biased the MOSFETS at around 800uA.

I'm sure there are other things that could be improved, but try some of these to start.

  • \$\begingroup\$ Thank you, sorry my professor is giving us a hard time I seem to have forgotten the capacitors. as for the op-amp I want to use one that is audio grade but unfortunately only this one is available at uni. because this project was designed in the last 2 weeks before the finals \$\endgroup\$ Jun 9 at 16:53
  • \$\begingroup\$ I Updated my circuit. I didn't understand the "Use complementary MOSFETs connected as source followers." and is the resistor is ok at the non-inverting? \$\endgroup\$ Jun 9 at 17:11
  • \$\begingroup\$ Okay, I had to make some changes to my answer. I had used your bias resistor values which put the MOSFETs I used in class A. Rebiasing @800uA (maybe not true class B but close), around 3W per transistor, brought the distortion up to around 1%. I don't know what your requirements are, you haven't really laid out what all is allowed in your design, but low distortion and class B tend to be at odds with each other. \$\endgroup\$
    – GodJihyo
    Jun 9 at 17:12
  • \$\begingroup\$ Honestly, at this I don't know, he said class b push-pull audio amplifier using the components we have at uni. and in uni, I only have this op-amp available. and I only have this week left before final starts \$\endgroup\$ Jun 9 at 17:14
  • \$\begingroup\$ @Sabretooth2438 In the new schematic I can see the transistors better, it looks like you're using complementary ones, so that's okay (I thought you had 2 of the same kind because the probe symbol was covering the part number). You seem to have lost one of the capacitors (C2). The resistor should be okay I think, you can try different values to get the DC on the opamp output as low as possible, but it's not all that critical. \$\endgroup\$
    – GodJihyo
    Jun 9 at 17:22

Are you simply simulating the circuit with perfectly matched transistors or are you building the circuit with random spec'd transistors? The important DC voltages on the waveform will show you what is causing the asymmetrical clipping.clipping

  • \$\begingroup\$ I am building a circuit based on the components that were given to me and available at the university. there is no other op-amp available besides this nor I can replace any transistor beside this \$\endgroup\$ Jun 10 at 17:51

You removed C2 that totally messed up the symmetrical biasing of the Mosfets. Put C2 back. amplifier


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