# Determining value of capacitor, in a opamp ampifier circuit

I am working on the follwing circuit, and have found all values for the components, except for C3 which i have a hard time finding.

My requirements are that it should be able to handle 40-16k Hz with a flat frequency response, and that i can change the gain of the amplifier.

Because of this i would preferably have a cut off frequency of 160k Hz in the circuit:

where R3 = 7,3k ohm R4 = 1k ohm P1 = 0-242k ohm

The thing is that i see the last part of the circuit as a low pass filter, with the output on the negative terminal of the opamp. The problem is that R4 is a part of it, and i will always have a gain lover than 1/sqrt(2) and therefor a cut off frequency does not seem to make sense.

How can i determine a value for C3?

• You can always play with it in a simulation Commented Mar 20, 2017 at 16:06
• Yes, ive got it work in a simulation but it is a school project and i therefor need to solve it in an other way Commented Mar 20, 2017 at 16:08
• "Because of this i would preferably have a cut off frequency of 160k Hz" No, you want C3 to give you the lower frequency cut off of your amplifier - 40Hz. You only need to consider R4 and make the impedance of C3 equal to that at 40Hz. Commented Mar 20, 2017 at 16:23