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I have designed a simple amplifier (maybe too simple) that has bad DC oscillation (instability). I attach a picture with the schematic. It's not the entire amplifier but only the final stage, where DC oscillations occur.

The V_out_L or R node should be at 200 mV but it's not stable: the voltage range randomly from 0.02 to 1 V at a frequency that can be observed with a multimeter. The VBE multiplier is set with a trimmer to get 10 mA quiescent current on MOSFET, but oscillations persist also with the MOSFET in the off state!

After a while the VBE multiplier died, so there is a serious problem. This happens also without load. The circuit is made on a PCB with SMD components and a good 4-layer layout.

Do you see any issue in the schematic that can lead to these oscillations?

enter image description here

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  • \$\begingroup\$ Have you simulated the circuit? Does it oscillate in the simulation too? Or is this the simulation? If yes, please show oscillogram. \$\endgroup\$
    – winny
    Commented Oct 27, 2022 at 8:04
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    \$\begingroup\$ For starters, it looks like you got your pmos and nmos swapped. \$\endgroup\$
    – LetterSized
    Commented Oct 27, 2022 at 8:12
  • \$\begingroup\$ @LetterSized this is an amplifier and not a digital H bridge. \$\endgroup\$
    – Andy aka
    Commented Oct 27, 2022 at 9:00
  • \$\begingroup\$ Vbe multiplier is normally supposed to be driven with a constant current source. \$\endgroup\$
    – Rohat Kılıç
    Commented Oct 27, 2022 at 9:34
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    \$\begingroup\$ It should output around VCC/2 so you should first check if the low frequency on the output comes from the power supply itself. Then, I don't see any decoupling or snubber, so check for high frequency oscillations with a scope, because the multimeter can read high frequency as random DC. I'm also interested in seeing the layout, how did you couple the SMD 5551 transistors with the heatsink? \$\endgroup\$
    – bobflux
    Commented Oct 27, 2022 at 9:56

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