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

how to calculate the feedback capacitor of the second stage,

one can see that first graph is a 100fF response and second is a 1.6p response,

when i observe the pulse response the 100fF response is quite sharp while the 1.6p response quite expanded pulse

why a 100fF is a better suit here

more importantly is a 100f capacitor, how practical it is on a PCB board

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

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

but when it comes to the second stage my gain curve is only flat when i use a 100femto capacitor or else i have problem with gain curve it starts falling very early itself, why this happens, how to calculate the feedback capacitor of the second stage,

why 100fF gives a pole at 100Mhz while it should be 2p according to the formula applied for first stage

^{simulate this circuit – Schematic created using CircuitLab}

more importantly is a 100f capacitor, how practical it is on a PCB board

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

how to calculate the feedback capacitor of the second stage,

one can see that first graph is a 100fF response and second is a 1.6p response,

when i observe the pulse response the 100fF response is quite sharp while the 1.6p response quite expanded pulse

why a 100fF is a better suit here

more importantly is a 100f capacitor, how practical it is on a PCB board