help me about the analog cmos of razavi

[china_xiong]

hello,everybody, I have a question when i read chap10.5.1 on slewing in two_stage op amp , <<design of analog cmos integrted circuits>>by razavi.chap10.5.1 . Fig 10.34(b) ,the node x is said to be a virtual ground,the reason? It is said that thegain of the second stage make it a virtual ground, i can't
get it througeh.

 

[naalald]

Hi,
The small signal voltage change of the node X is ΔVout/Av in which Av is the gain of the second stage. So, it's changes is negligible comparing with the Vout changes and can be considered zero. For large signal behaviour, you may say so: Since the current of M5 is the addition of the currents I1 and Iss (assuming that M5 can provide two currents), the current of M5 is almost constant and therefore Vgs of M5 will remain constant and finally the node X voltage is almost constant.

 

[gpwu]

Vx = I/gm; if gm is large enough, Vx is very small.

let gm=100u; I=10uA, then Vx=0.1V