Electronic – Crossover distortion returns when load is applied

amplifieraudiodistortionmosfet

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

Best Answer

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.