raja.cedt wrote on Aug 24th, 2009, 5:57am:hi,
can any body explain why pole at the cascode node is not affecting much in the frequency response as well as settling..(i slightly increased cap at the cascode node and checked)?
Thanks,
Rajasekhar.
The cascode transistor is an impedance transformer with the source end impedance appearing to be about (1+gm*ro) times smaller than the drain end. That works out to around 30 for older processes and around 10 for the modern ones.
Assuming that the poles due to the cascode device are far far away at the moment (10x > unity gain frequency), then you will see a significant change only if you change the load at the source node significantly, maybe by factor 3x-4x or so.
Regards,
Vivek