Differential Amplifier - Common mode input gain vs. Single ended input gain

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Joined Jun 7, 2020
What differences can I expect to see between the common mode input gain and single ended input gain for a differential amplifier with the same parameters?(i.e. The only difference is that one input is grounded.)

Also, is it possible to approximate the differential gain of an amplifier using the single ended input gain? If so, what are the limitations of this?


Joined Jun 13, 2013
The best way is simply to compare both gain formulas:
* Single-ended input gain: Ad=(+-) gm*Rc/2 with gm=transconductance. The sign depends on what output is used.
* Common mode gain: Ac=gm*Rc/(1+2gm*ro) with ro=diff. resistance of the current source in the common emitter lag.
For an ohmic resistor ro=Ro.
* It is, however, noted that also during single-ended operation, we have a certain amount of common-mode output.
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