Waveform distortion when choosing small time steps

Heisenberg · August 8, 2023, 4:30pm

Hi,
I was playing around with the demo circuits included with QSPICE. In audio demo circuit, everything works fine. As soon as I force the simulator to use 10ns as maxtime step, it gives weird results.

How to trust this simulator?
Regards,
AK

Rhino · August 8, 2023, 8:34pm

Hi
What is “weird results”?
I changed maxtimestep to 10 ns and have not any issues.

Heisenberg · August 9, 2023, 4:29am

bordodynov · August 9, 2023, 5:11am

This indicates that the amplifier is unstable.
The problem is insufficient frequency correction.

Heisenberg · August 9, 2023, 5:23am

Why this only manifesting when we are choosing small time steps?
Also its not showing up in latest version

Rhino · August 9, 2023, 2:06pm

Probably you change schematic and forget.
Or schematic changed with last update…
or…

What QSPICE version you use?

Rhino · August 9, 2023, 2:09pm

I can confirm problem

Rhino · August 9, 2023, 2:14pm

I can confirm problem (have not updated version on second PC)
I check two AudioAmp`s from different QSPICE updates.
Schematics the same.

bordodynov · August 9, 2023, 5:07pm

Here is my method for determining the stability of an amplifier in small signal mode. I determine the output impedance using a current generator. In order to simulate the high current mode, I used an additional voltage source. This is based on the fact that the transistor current is weakly voltage dependent.
I derived Re(V(OUT)). This voltage is equal to the real part of the output impedance. When the phase is 180 degrees, then this part is negative and there are conditions for oscillation to begin.
It can be seen that this amplifier is not stable.