# Unsure of how to find the equivalent resistance of the following circuit due to the input resistance of the LM386 amp

My questions:

• How do i find the voltage across R3 for diagram 1? Do i have to take into account the input impedance of the LM386 amp (The second amp in the circuit schematics) or can i ignore it and just use the voltage divider rule (V=Vin*R/(R+C)), where Req is the equivalent resistance of R2 and R3. If i can ignore it is the equivalent resistance just going to be R2 + R3.

• Is the amp always going to have a 50 k ohm impedance or is it going to change depending on the input voltage going through the amp?

• For my high pass filter to remove the dc component, will my cutoff frequency change due to the input impedance of the lm386 amp or not significantly? I.e do i have to take this value into account when finding the cutoff?

• For diagram 3, how would you find the equivalent resistances. Also so for the 1st circuit in diagram 3 is R3 parallel to R2, then in series to R1? Then for the second circuit in diagram 3, to find the equivalent resistance is it just in series so they add together? Note R3 is connected to an amp in this case. ## 1 Answer

I think as a first approach I would consider the $50\;k\Omega$ input impedance constant coming in parallel with $R_3$. Your circuit shows a low-pass filter buffered then driving a high-pass filter. The transfer function from source $B_1$ to $R_3$ is thus:

$H(s)=\frac{1}{1+sR_4C_3}\frac{sC_2(R_3||R_{in386})}{1+s(R_2+R_3||R_{in386})C_2}$

The calculations are given below

You can rearrange this expression in a low-entropy format as shown here

How to find Vout for the following band pass filter 