THD & IMD
All single stage amplifiers have some non-linear large signal effects which produces distortion or a harmonic noise or a mix of input frequencies called intermodulation noise with the aliasing or sum and difference products of non-linear gain usually near the tip of one polarity decreasing gain.
Would you like to see a simulation of negative feedback, NFB for a high gain single stage NPN amp?
Input =10mVpp, Output =604mVpp Gain= 60.4 depends on collector current
To see the asymmetry in small signal gain distortion the AC coupled compare the peaks Vpk+ + Vpk-
- subtract and divide by Vpp *100% = asymmetric distortion
- which is similar to and proportional to THD (which takes the ratio of rms of harmonics to fundamental)
- using the positive difference 302.1mV-301.6mV = 0.5mV/301 = 0.17% distortion
This is excellent, but if Rfb is increased to get a gain =100 then distortion increases to 1% and with no NFB 10%.
Collector Q DC bias point also changes with NFB.
If the noise is in the signal input, unless the noise and signal occupy a different spectrum or can be limited by the GBW of an amplifier, -ve FB (neg. feedback) cannot affect the Signal to Noise Ratio.
They can be improved by filtering by suppressing the spectrum of unwanted noise using active filters with neg. FB.