Folded cascode Sizing Question

[ccw27]

The following folded cascode is taken from Gregorian book. Why is the aspect ratio for the second expression given as (3+2√2)(W/L)11? Does it have to do with mobility of NMOS and PMOS?

Thanks

 

[gevy]

How the choice of cascode bias voltage?
For Q9:
All currents are equal in P-channel current mirror. Let's notice, that Vgs=Vt+Vdsat.
Hence it is necessary to make Vgs (Q9) =Vgs (Q3c or Q4c) +Vdsat=Vt+2Vdsat
The first line factor is 2²=4.

For Q11:
The currents of transistors Q5 and Q6 are double currents concerning other currents. The saturation voltage is equally Vdsat2=√2 Vdsat for a double current
Hence it is necessary to make
Vgs (Q11) =Vgs (Q1c or Q2c) +Vdsat2=Vt + (1+√2) Vdsat
Second line factor is (1+√2)²=1+2*√2+2=3+2*√2.

 

[yeewong_su]

Wonderful! I must be careful to read the book!