Met a question and need help

[rats]

I design a diff amp, I measure -3dB BW @ one of the outputthe is about 7GHz, about when I check the transient response, the rasie-fall time is about 27ps, this is indicate the BW is more than 10GHz, I don't know the explaination of this? Could you please help me.

 

[jorgito]

Hello rats
How do you calculate rise time?
The most common equation used to determine BW from rise time (the same used by scope manufacturers to give rise time) is only valid for first order systems.
A second or higher order system cuold have a faster rise time than BW indicates.

Hope this helps

 

[rats]

Thank you for you reply. The system is 5 identical 2nd order cherry-hooper stage. Do you have any suggestion about the 2nd BW? Is there any forumla for 2nd order system.
I do have other question. How to measure the BW of diff Amp? Is the that correct, if I just measure one of diff output? The design tool is Cadence. I am forward to hearing your reply. I urgently need help

 

[mike_bihan]

The diff mode gain can be measured @ single end. You have 5 stage cascaded so that the common mode gain are almost zero. You measure single end output and plus 6dB to get diff gain.

Also, you can measure it in transient simulation.

bandwidth is easy to measure @ single end.

Rise/fall time indicate your single peak frequency, however, this frequency component can be attenuated a lot by your system.