Input Impedance of OP AMP

[arunkumar.kunjappan]

Hello,

Can anyone suggest me a method to calculate the input impedance (both common mode and differential) of an OP AMP?

Thanks in advance.

 

[klystron]

Bu design it is very high
Measure input current with a small series resitor ; and measure the input voltage at the same time and then calculate the input impedance
Look at the specification sheet
Analyze the cicuit diagram
Use an impedance vector analyzer

 

[arunkumar.kunjappan]

I would like to know if there is any test circuit that can be used for measurement and/or simulation.

 

[klystron]

Measure input current with a small series resitor ; and measure the input voltage at the same time and then calculate the input impedance

 

[mtwieg]

There are a ton of ways to do it depending on exactly what you want to know. Are you interested in the frequency dependence of the impedance? Or just the DC bias currents?

Also keep in mind that input impedance may vary a lot between components of the same part number, simply due to different fabrication quirks, so don't expect any empirical model to hold true all the time.

 

[pptiger]

do you mean an op amp‘s original input impedance ?they are not calculated but born. generally the input impedance of an op amp made in FET technology is high and in BJT is low about several MΩ