Hi,
I've been trying to optimise the frequency response of a CMOS differential amplifier, but I'm running into difficulty - my first-order hand calculations give completely different values from what I'm getting from simulation, so I don't know which node in my circuit corresponds to which poles.
I was hoping that someone could just run through my thinking and confirm/deny whether I've got the right idea for the first-order analysis.
The circuit in question is here:
The first significant pole is at the drain of M3, and consists of the capacitances: Cgs3 + Cdb3 + Cgd1 + Cdb1 + Cgs4 + (1+A)Cgd4, where A=(gm4)(rds4||rds2), ie. miller effect, and the resistance: 1/gm3
And second significant pole is at the drain of M4 and consists of capacitances Cgd4 + Cdb4 + Cgd2 + Cdb2 + (1+A)Cgd5 + Cgs5 (with A = (gm5 || ro)), and resistance (1/gm4)
Finally, a third significant pole is at the drain of M5, consisting of Cl (the load capacitance) + Cdb5, with resistance ((1/gm5) || Rload).
I don't have any numbers to hand right now, but roughly from memory, with the above first-order analysis my values were just wrong - they would predict a pole at say 1Mhz but looking at the pole/zero plots from a simulation, there were poles at 300kHz and 40MHz, but nothing at
1MHz.Is my theory right? If so, why the large discrepancy? Second order effects? Or have I gone wrong somewhere?
Thanks for any help,
Bill