Hi,
I had a query regarding the displayed AC amplitude of the output voltage of a small circuit of mine. The circuit consists basically of an N channel enhancement MOSFET whose Source terminal is connected to a parallel combination of ~10ohms and 100nF. A sinusoidal voltage riding on some DC of around 7V is applied to the Gate of this MOSFET and the Drain is connected to 10V DC.
The output observed on the source terminal is a somewhat distorted sine wave whose peak to peak amplitude when observed with AC coupling on the virtual oscilloscope tool in the T&M menu is around 50mVpp. But when the amplitude of the output on the same node is checked using the "Calculate nodal voltages" function in the "AC analysis" option in the "Analysis" menu, it shows an amplitude of 12.68mV. Also, when checked using the digital multimeter tool in T&M, an rms amplitude of 8.97mV is displayed, which again corresponds to a 12.68mV peak amplitude. But as per the oscilloscope, the amplitude looks to be around 50mV/2=25mV. Please find the snapshots of the 3 instances below:
I was wondering why this apparent difference is there, and which value is the correct one. Is this an issue caused by the signal not being a pure enough sine wave, ie, does the "calculate nodal voltages" tool give out the amplitude of the fundamental sine tone alone? I'm more interested in the actual peak to peak amplitude of the waveform, and am okay with it not being a very pure sine wave, for my application. So can you please tell if I can safely assume that the peak to peak amplitude of the AC signal generated by this circuit will be 50mVpp itself, as observed in the oscilloscope?
Thanks,
Anoop