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?

  • \$\begingroup\$ You can always play with it in a simulation \$\endgroup\$
    – PlasmaHH
    Commented Mar 20, 2017 at 16:06
  • \$\begingroup\$ 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 \$\endgroup\$
    – keffe
    Commented Mar 20, 2017 at 16:08
  • \$\begingroup\$ "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. \$\endgroup\$ Commented Mar 20, 2017 at 16:23

1 Answer 1


C3 is working against R4 to form a high pass filter, not a low pass. You therefore want to adjust C3 relative to R4 for the lowest frequency of interest.

To see how capacitors work as low or high pass, think what happens when you replace them with shorts (high frequency) or open (low frequency).

In this case, the overall opamp stage gain is 1 when C3 is open. When C3 is small compared to R4, the gain is (R3 + P1 + R4)/R4.


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