Horror Vacui
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Posts: 84
Germany

The differential amplifier relies on two identical amplifying devices (=transistors). If both device have the same imperfection, they will cancel out in the differential signal path. On the other hand, common mode current will flow through these capacitors, and it will alter the common mode at the output, but this is at a very high frequency and therefore it can be neglected. The transit frequency which is often used as a metric for transistor speed is approximated as gm/Cg, where Cg is the effective capacitance at the gate. The zero you are talking about lies at gm/Cgd, which frequency is higher than f_T by more than a factor of 2! Therefore there is no signal amplification here. If we take into account that diff amps without reactive matching, inductive peaking or reactive resonance operate up to lets say f_T/20 at best, the zero due to Cgd of the diff pair is not important at all.
