The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Pole at the cascode node
https://designers-guide.org/forum/YaBB.pl?num=1251118648

Message started by raja.cedt on Aug 24^th, 2009, 5:57am

Title: Pole at the cascode node
Post by raja.cedt on Aug 24^th, 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.

Title: Re: Pole at the cascode node
Post by Mariusz on Aug 24^th, 2009, 7:25am

It is a low impedance node.

Title: Re: Pole at the cascode node
Post by raja.cedt on Aug 24^th, 2009, 8:18pm

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

Thanks

Title: Re: Pole at the cascode node
Post by nobody on Aug 24^th, 2009, 11:02pm

What if you decrease the size of cascoded device, which means you are going to decrease the gm.

Title: Re: Pole at the cascode node
Post by vivkr on Aug 25^th, 2009, 4:03am

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

Title: Re: Pole at the cascode node
Post by raja.cedt on Aug 25^th, 2009, 9:09pm

hi,
thanks for your replies......but what i feel there is some zero which is canceling that pole.

Thanks,
Rajasekhar.

Title: Re: Pole at the cascode node
Post by subgold on Aug 26^th, 2009, 5:07am

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.