Difference between simple cascode current source and modified Wilson current source

[sathi.ec]

mos wilson current mirror output resistance

hi all
i read that wilson current offers stable current values for wide voltage swings and enhanced output impedance.

 

[avinash]

cascode mirrors bjt

wilson current mirror is used over cascode mirror in order to increase the o/p swing.the trade off is imperfect mirroring in wilson as vds is not same for the lower transistors.

 

[Alles Gute]

wilson current mirror improved

wilson current mirror is used over cascode mirror in order to increase the o/p swing.the trade off is imperfect mirroring in wilson as vds is not same for the lower transistors.

hi, avinash
I do not get why you said for wilson vds is not the same. I think for both cascode and wilson, voltage in the gate of M3 and M4 is 2Vt+2Vsat, and the vds of the lower transistors M1 and M2 in both case is Vt+Vsat. Isn't it?

 

[farhad]

improved wilson current mirror analysis

hi
,i think for compare two current source you focus in output impedance & minimum voltage requires to operate as current source in same bias conditions.
the minimum voltage requires to operate a current source for both is vT+2.VDSAT.
the output impedance for cascode current source is
RO=(rds(down).(1+gm(up).rds(up)))+rds(up)
the output impedance for wilson current mirror is
Ro=((1/gm(down)).(1+gm(up).rds(up)))+rds(up)
as result :
the output impedance of cascode current source is greater than output impedance of wilson current source .
thanks
farhad