questions about CMFB from Razavi's textbook

[airboss]

Hi,

I have questions about 2 examples on Razavi's book. They're on p.321 and p.322 and I attach the figures here.

In Figure 9.39, Razavi illustrates that if both Vout1 & Vout2 rise, the drain currents of M3 & M4 increases and Vout,CM decreases. What I don't understand is, why the increase of ID3 and ID4 causes the decrease of Vout,CM?

Also, in Figure 9.40, I know when Vout1 and Vout2 rise, the drain currents of M5, M6 decrease but why then Vout,CM is also lowered?

Thanks for any help!

 

[aomeen]

Hello,

you can think of it in two ways:

1. As voltage, forget about the currents now, and check the path of small signal from CM sense and back to it, check wether the increase at some point will lead to increase or decrease on another "for common source it's inverting , for common gate it's non and so on."

2. As current, remember that as I increase for same W/L , Vgs must increase, but also Vds will change, Remember the Id vs Vds family of curves, at diff. Vgs, the curves had slope of 1/ro , for short channel ro is not so large, now draw the load line, you will find that as I goes up, so does Vgs but Vds will lower..