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A BJT class AB audio amplifier has .27 ohm resistors between the push-pull outputs and speaker. There is 55mv and 59mv, respectively, across each resistor meaning a quiescent current of over 200ma. This is the lowest it can set to with the bias pot. Is this much current normal or even acceptable for class AB using BJTs? Is there any way to reduce this or at least have both outputs be the same?

Thanks for the feedback. Not sure what is meant by which class AB design. I can tell you it has a differential input and a voltage amplifier, couple for driver transistors and two output transistors which are 2N4915. Rated output is 42W min into 8 ohms. No bias diodes, uses Vbe multiplier.

Measurements were done after the amplifier warmed up for several hours connected to a speaker set at low volume. I will retake the measurements again after warming up with higher volume to see if there's any change.

What I really want to know if I should be concerned with the 200ma quiescent current with no signal and no load?

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  • \$\begingroup\$ Which class-AB design? What's the nominal power output? What are the IC product numbers of the Power transistors? What are the IC product numbers of the bias diodes (or bjts)? \$\endgroup\$
    – Dave
    Commented Mar 1, 2016 at 3:53
  • \$\begingroup\$ It depends entirely on the design. The one I based the other day ended up around 5mV for minimal crossover distortion. \$\endgroup\$
    – user207421
    Commented Mar 1, 2016 at 10:40
  • \$\begingroup\$ Re your edit, it still seems excessive to me. I would be checking for faults along the bias supply chain, for example wrong trimpot value, or the bias spreader transistor not working. The mismatch is also very curious: can you measure the emitter resistors accurately? Is the amplifier really outputting zero volts when quiescent? Or is there a DC offset? 'Which class-AB design' refers to the topology from about the VAS onwards for the purposes of this question, and also to whether it is really class AB or class B. Much more likely to be the latter. You should really have provided a schematic. \$\endgroup\$
    – user207421
    Commented Apr 2, 2016 at 0:29

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I am not really an audio guy. But I took a look at Douglas Self's "Audio Power Amplifier Design Handbook." Those values, of 55 to 59 mV are ballpark. You might want to warm up the amp (by playing some music) and then check it again when it is warm.

As far as the mismatch goes, it isn't a huge mismatch. But were you measuring it with a load attached or no load? With no load, the two resistors are perfectly in series (or so I assume), so the currents have to be the same. In that case, any voltage mismatch is due to Re mismatch. You can tune Re by adding resistors in parallel with whichever one is higher in voltage until the two voltages match each other. Again, do this with no load attached to the amplifier.

If the two Re's are already matched, but you have different voltages across them when a load is attached, that is due to an output voltage offset. Unless it is large, I wouldn't worry about it.

Ultimately, bias current should be set based on what produces minimum distortion, not on anything else. But it is not very practical for most people to measure distortion while tuning bias current.

Good luck!

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