I'm trying to determine the common mode gain and differential mode gain of the follow amplifier.
Now I understand what the common mode voltage and differential mode voltages are, but I'm unclear as to what the common mode gain is. My best guess is the output voltage / common mode voltage.
Also how would I go about determining these given the current mirror connected to the emitter of the amplifier?
Appreciate any help.
Best Answer
One way to find the gains is to pick a operating point, analyze at that, then perturb it a little and analyze at that. It takes separate pertubations for a common mode change and a differential mode change.
For example, analyze everything at 0 V on both inputs. For common mode gain, raise each input 1 V and analyze what happens to the output. The change in output divided by the change in input (1 V in this example) is the common mode gain.
Similarly, starting with the previously analyzed case of both inputs at 0, raise the positive input 1 mV and see what you get. The differential mode gain is then the resulting output change divided by the differential input voltage change (1 mV in this example). If the output clips, then the input change was too large. In that case, make the input change smaller and try again.