i have narrow pulse of width 10ns repeating at 20us rate to be processed coming from a photo diode, so i have used a very high GBWP opamp like below,as my interest is to work at 100Mhz as pulse is 10ns, i have managed to get a pole at 100MHz using 1K resistor and 1.6p capacitor in the first stage while having a gain of 1K
simulate this circuit – Schematic created using CircuitLab
but when it comes to the second stage my gain curve is flat when i use a 100femto capacitor and also 1.6p, with a different phase response here
one can see that first graph is a 100fF response and second is a 1.6p response,
how to calculate the feedback capacitor of the second stage,
when i observe the pulse response, the 100fF response is quite sharp while the 1.6p response quite expanded pulse output
why a 100fF is a better suit here
more importantly is a 100f capacitor, how practical it is on a PCB board
Best Answer
All you need to know :-)
From the late great Jim Williams- 132 pages ! AN47
http://cds.linear.com/docs/en/application-note/an47fa.pdf