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.

hi,
   thats true..but i increased the cap at the that node..still i didn't see much impact...

Thanks

raja.cedt wrote on Aug 24^th, 2009, 5:57am:


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

the first nondominant pole of the amp is located at about gds/cgs of the cascode transistor, it definitely has some influence on the frequency response and settling. a zero may come into play if you connect the bulk and source of the cascode transistor together.