# Gain Value For Modified Differential Amplifier Circuit

I am trying to analyze the following differential amplifier circuit and am wondering how to calculate overall gain (it will be fed with two copies of the same audio waveform 180 degrees out of phase with one another):

I know how to calculate gain for a basic differential amplifier circuit like this:

But the modifications in order to add filters to remove high frequencies makes the circuit I am working with more difficult to analyze. For example does R53 count as a feedback resistor and an input resistor at the same time? It's in both the feedback path and the input path. Does R54 factor into the voltage divider before the non-inverting input? Please advise.

I am working with AC signals in the audible range, from 20hZ to 20kHZ. For the purposes of this analysis I am not overly concerned about the frequency response of the filters, I want a general gain coefficient I can calculate for the passband. Thanks!

Step by step:

1. Combine C29 and R49+C37 by parallelism (A is the new impedance name)
2. Then you can use a PI-T transform between A, R54 and C41
3. Parallelism combination between C33 and (C49+R65+R69)
4. PI-T transform again between (R61, R53, C45)

In the end calculate the gain between Vin and V1, by Vout I mean the point 1, then you will have the potential in 1 and by making V1/Z = I, you will have the current passing through R69, and you will finally have Vout by multiplying this current by R69.

Tell me if something is wrong or not clear !

• And then don't forget to check your result in simulation :) Jan 4 at 20:33
• Is it normal that people never up my answers ? Jan 5 at 17:55