Would be interesting to see which of the bias cases produces the best SNR of the fundamental
If that is only to check if an amplifier is less noisy when driven into saturation, any amplifier should be so, if we think of noise as an input signal that would be amplified by the DUT amplifier. The Vout vs Vin plot looks linear only at small signal. For a bigger signal the plot shape looks like an S, and its slope (slope = DUT amplification) is big near zero and with no amplification at the ends of the S shape.
Considering the signal + noise input as a small noise added on top of a big signal, then the big signal will put the amplifier in saturation and will act like a "bias" on the Vout vs Vin plot. When saturated the amplification is small for everybody, including for noise. A large swing of the useful signal will keep our amplifier mostly in its saturation regions, so the input noise won't be amplified either, and thus on average the saturated amplifier will show less noise at its output.
In real life, this saturation and how it affect smaller signals can be observed as "blinding" (by a strong light source, or by a strong noise, etc.), when we can not see the rest of the image details because our senses were driven into saturation by the very strong "blinding" signal.
I think the noise is "lowered" in a saturated amplifier just like the details are lost when blinded.
The DUT doesn't see the noise any more because the DUT is blinded by the useful signal.
If the F
0/noise ration is needed for something else, then LTspice might help, but I think this requires special models that includes noise.
Didn't play much with it, but LTspice has ".noise" type of analysis. There are also ".four" and ".meas" SPICE directives that can write the results in the log file, and they could be used in combination with a ".step param".
The idea is to run two simulations:
- one for ".noise" with ".step param R1", and a ".meas" to get a RMS noise value for each .step
- one for ".tran" with the same ".step param R1", and a ".four" to get the amplitude of F
0Then grab the data from the .log files (either manually, or with a script if the number of steps for R1 is very big) and calculate A
2(F
0)/Noise
RMS for each step.
On a second thought, I don't think the ".noise" simulation will include the "blinding" caused by a large signal, just like an ".ac" type of simulation doesn't include large signal behavior.
Maybe just adding a noise source somewhere in the oscillator would be easier. Then run a normal ".tran" with ".step param R1", and then postprocess the result with a FFT to compute S/N ration considering only F
0, as requested. On all these, overlap the noise from ".noise" at each R1 to include the internal noise of the circuit. Probably not very accurate, but better than nothing. (Not accurate because at large signal, the gain of the amplifier changes with the instantaneous amplitude of the signal, while in .noise the gain is considered constant no matter the amplitude.)
Side note, a ".noise" with ".step param R1" for the Peltz oscillator will also show the frequency response of the circuit, just like one can measure the frequency response of an RF amplifier using a noise source and a spectrum analyzer.