Differential and Multistage Amplifiers

Basic BJT Differential Amplifier

For comparison see NMOS Differantial Amplifier

From discussion of bjt physics, we have

which, in turn, allows us to write

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If there is a small resistive mismatch, we can write

or

which allows us to write

For small differential voltage

See an example of a Two stage differential amplifier (source)

Basic NMOS Differential Amplifier

For comparison see BJT Differential Amplifier

From our earlier discussion of mos physics, we have

If we make the two devices identical we can write

and we know that

which allows us to write

When we square the bracket term, we get the quadratic equation

which has the solutions

Finally, we obtain the normalized form for the transfer function for the differential amplifier.

where we have used from earlier analysis the result that

for each transistor in the pair.

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For small differential voltage

Common Mode Rejection Ratio (CMRR):

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Click on image to enlarge