I have the following circuit, that is used as a signal conditioning circuit in a audio system project. I am trying to calculate the transfer function and the Zin(input impedance) and Zout(output impedance) enter image description here

in Zin, I know how to calculate the impedance of the rc filter but im getting confused because there are one more resistor of 10M that makes the offset of the voltage.

So, how can i calculate the transfer function,Zin and Zout of this circuit?

  • 2
    \$\begingroup\$ The two 10-Meg resistances are in parallel for the small-signal analysis. This is because you consider the 5-V source at a 0-V ac level (imagine a decoupling cap. on this 5-V line). \$\endgroup\$ Sep 24, 2020 at 20:18
  • \$\begingroup\$ 10M resistor values are too high unless you buy and measure many LM324 ICs to pick ones with very low input bias current. The half the supply voltage divider is wrong because the input bias current pulls the inputs to a high enough voltage where they do not work. \$\endgroup\$
    – Audioguru
    Sep 25, 2020 at 1:37

1 Answer 1


Something like this. BUF can be approximated as infinitely high input impedance and zero output impedance, it does isolate left and right circuits. The 5M is the Thevenin/Norton equivalent of two 10M resitors.


simulate this circuit – Schematic created using CircuitLab


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