Strange drive waveform on power supply mosfets

‘Afternoon all.

I’ve been poking around inside an amplifier to learn how it works and have come across an odd thing which I was hoping someone could explain.

The amplifier is a Sony xm-6020 and is beautifully laid out, with the PCB marked as to which section of the board does what.

I was having fun tracing the signal through the audio parts of the circuit when I thought I should have a look at the power supply side too.

I get a reasonable square wave from pins 9 and 10 of the NEC 494c and a nice square wave output from the drain pins of the k1191 mosfets but the drive signal to the gate pins looks very off.

Is this indicative of some sort of issue? The amp powers up and plays just fine.


  • BA9BFCB3-52E2-4487-A405-BC2183B71B52.jpg
    467.8 KB · Views: 49
  • A3D539E4-F648-4C68-8252-42F4269E249F.jpg
    1,006.4 KB · Views: 43

Perry Babin

Paid Member
2003-11-20 11:01 am
There is capacitive coupling in the FETs between the gate and drain. When the FETs switches off and the drain voltage is transitioning back high, it pulls the gate voltage slightly which makes the notch in the gate drive.

For this it didn't really matter but for checking the gate waveform, you must use DC coupling for the waveform to be of any use.