Hwo to calulate the reference input voltage?

[wjx197733]

I read the book DESIGN OF ANALOG CMOS INTEGRATED CIRCUIT writed by Razavi, in chapter 13, I have a problem: in figure 13.11, Razavi said that the reference input voltage of figure13.11(b) is greater than figure13.11(a), but I think their reference input voltage is equal.

Who can help me ?

 

[gajen]

It does not say that reference input voltage is higher. It says that input referred noise voltage is higher in 11b.
As in 11a current source drain is common to both differential inputs so noise will be same for both input transistors. So effectively no noise for the differential circuit.

But in figure 11b both differential input transistors will see different noise voltages as these 2 current sources will be different and uncorrelated so noise will be In^2 +In^2 and this multiplied with sq of resisotr.

So input referred noise voltgae is more on 11b