BW of Transimpedance Amplifier

[aryajur]

Hello,
I am trying to implement a Transimpedance Amplifier using a Common Source Stage. I tried a simple CS stage with a feedback resistor as in image CS.jpg, I get a bandwidth of around 85MHz. I figured that I can increase the Bandwidth by adding a cascode transistor above the input transistor since that would reduce the miller capacitance at the input node. But when I do that as shown in image cascode.JPG, I get a even worse bandwidth of 54MHz.
Why am I getting a lower Bandwidth?? I don't understand this. Any suggestions would help a lot !

 

[Puppet1]

Get Razavi's Integrated Circuits for Optical Communications.

It's on the board.

Whole Chapter on TIA's.

It's good.

 

[pengboy]

it could be because:

at the cascoded node, you reduce the milled effect cap for the input. but at the same time you isolate the feedback resistor with that node. it means the output resistance is increase to (gm//gds).

i am assuming the feedback resistor is much less than gm//gds.. then the bandwidth at the cascoded node will be reduced..

so the bandwidth reduction could happen, eventhough you try to reduce the miller effect.