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
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).