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I built a small audio amplifier using a OPA2134PA as a pre-amp to drive a complementary pair of Mosfets (IRF9540NPBF and IRF540NPBF). I've biased my Mosfets using two 10k resistors and 6 diodes. I've also fed some of my combined Mosfet output back into the inverting input of the OpAmp. The output looks great all across the audible range of 20Hz to 20,000Hz until I place a low impedance (8-10 Ohm) load on it.

My question is: Is the return of crossover distortion expected when a load is applied and is there a way to mitigate it?

Yellow: Input signal from generator
Blue : Combined output signal from Mosfets.

Without load, the output looks good: enter image description here

With a 10 ohm load, the distortion returns: enter image description here

Pretty accurate representation of my output stage:

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ More biasing needed or a much faster opamp or both. \$\endgroup\$ – Andy aka May 2 '18 at 20:23
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That's not completely unreasonable. Your distortion seems to last for about 3 usec.

First, your bias voltage (3 diode drops) is about 2 volts. Note that the data sheet give 2 volts as the minimum gate threshold, and it could be as much as 4. At high load impedances the FETs are close enough to turn-on that you don't see anything, but that's not enough at lower impedances.

An obvious thing to try is to add a couple more diodes. This will possibly have a serious drawback - excess power dissipation. YMMV.

Another issue to consider is the op amp. Your unit would seem to have adequate speed and slew rate, but remember that it's feeding a fairly large (~4000 pf) combined MOSFET gate capacitance, so it may be struggling to keep up at the zero transition.

I suggest you rerun your scope measurements, but this time also look at the op amp output voltage. You may be surprised by what you see.

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It is best to address your open loop performance before the opamp goes in .Increased idle current is good .I run about 600mA on my car radio circuits .This means that operation is mainly class A .Place a 1 microfarad cap between the gates for reduced distortion at reduced idle currents .

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  • \$\begingroup\$ Do you mean between the gates themselves or from each gate to ground? \$\endgroup\$ – NTDLS May 3 '18 at 0:02
  • \$\begingroup\$ I have always placed the cap gate to gate . \$\endgroup\$ – Autistic May 3 '18 at 2:06
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A lot of amplifiers will use this technique to overcome the cross-over distortion: -

enter image description here

The driver stage to the MOSFETs also feeds the output but via something like 100 ohms. This means that it does the low level work around the cross-over point and the MOSFETs are only called into play when the output level required is outside the middle couple of volts of the power supply range.

If your driver can produce (say) +/- 15 volts peak then it can, into a 10 ohm load (and via a 100 ohm feed resistor), deliver +/- 15 x 10/110 volts = +/- 1.36 volts i.e. enough to deal with cross over.

This would be my preferred method but more MOSFET biasing is something that should also be considered as well as a faster driver circuit.

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