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I had an oscillation problem when testing my fabricated chip. As shown in these figures, the transient waveforms of the op amp outputs got severe oscillations. Ch1 and Ch2 are the two output nodes and the red curve is differential output after math operation.

transient output

Transient zoom-in

the oscillation frequency for the two channels seem to be different, the lower one is around 20KHz and 133KHz for the higher one.

More about the op amp, it is a fully-differential class AB power amplifier designed for audio applications, it is connected with a closed-loop gain of 2. It is driving a 16 Ohm + 200 pF load and is supplied by a power supply of 1.2V

I suspects the amp is not stable. But during the pre-layout simulation, I checked the stability conditions of the amplifier and ensured it has good phase margin (>60 Deg) and gain margin (<-20 dB)

Please advice me on any possible source for this problem or any diagnostic test needs to done. Thanks in advance!

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    \$\begingroup\$ It's pretty hard to make suggestions without seeing your schematic diagram. Please add the schematic to your question. \$\endgroup\$
    – Dwayne Reid
    Commented Apr 11, 2016 at 4:26
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    \$\begingroup\$ Check your power supply decoupling. \$\endgroup\$
    – jp314
    Commented Apr 11, 2016 at 4:28
  • \$\begingroup\$ Have you by chance built this thing of yours on one of those white proto boards with all the holes for sticking leads into? \$\endgroup\$
    – Michael Karas
    Commented Apr 11, 2016 at 5:18

1 Answer 1

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Try removing the capacitive load as a first step. Capacitive load will create a pole in the feedback loop in combination with the amplifier output resistance which will degrade the phase margin. Also ensure input signals don't contain the ringing at the same time you are probing the output lines. Check supply bypassing and stray inductance in feedback path. Try adding feedback capacitance to roll off higher frequency amplifier gain (although the 20kHz ringing is unfortunately in band if your device is for audio).

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