9.58 It can be shown that if the drain resistors of a MOS differential amplifier have a mismatch ?RD and if simultaneously the transconductances of Q1:
and Q2:
have a Is match ?gm, the common-mode gain is given by
![949_kk.jpg](https://secure.tutorsglobe.com/CMSImages/949_kk.jpg)
Note that this equation indicates that RD can be deliberately varied to compensate for the initial variability in gm and RD, that is, to minimize Acm. In a MOS differential amplifier for which RD =5 kΩ and RSS =25 kΩ, the common-mode gain is measured and found to be 0.002 V/V. Find the percentage change required in one of the two drain resistors so as to reduce Acm to zero (or close to zero).