# How to know the gain of my opAmp

I'm having a LM358 opAmp with the following Schematic, as I'm not from Electrical background, can you please help me determining it's gain. I just want to find out it's gain.

simulate this circuit – Schematic created using CircuitLab

I think it is R5/R4=4.7/1=4.7?

• Being pedantic, the gain won't be 4.7, but -4.7. – Brian Drummond Jan 13 '14 at 20:01

You are right about the gain, however there is a small issue with your circuit.

Due to the low values of R4/R5/C2, you will have an undesirable roll off at the low end.

The effective input impedance is set by R4, so the roll off frequency (-3dB) will be:

$1\over{2 \pi R C}$ = $1 \over{2\pi \ 1k\Omega \ 100nF}$ = $1591Hz$ which is way too high for a decent audio amp.

If you change R4/R5 to 10k and 47k, and increase C2 to 1uF or above, then you will have a much lower range. Also don't forget to check the ADC loading at the output also, the same thing can happen there also (see the PIC datasheet for the ADC specs and it's maximum input impedance, as @Andy aka mentions 10k is almost certainly too high, in fact I'm not quite sure what your intentions are with C1 and R6)

Another thing is that if you want to keep the 2.5V offset, then you need to leave C1 out as it will just remove it

One last things is that if you are using a standard electret mic, then 100 Ohms is probably a bit low for R3 (1k - 10k is probably better, but again remember about input/output impedances)

Yes the gain will be approximated by R5/R4.

R2 and R1 provide a common mode offset for maximum dynamic range on the output of your amplifier stage. Ideally, this should be set at 50% of your ADC reference voltage.

• R2 and R1 don't affect the average dc level seen by the ADC because of C1. Also R6 is too high to get accuracy from the PIC's ADC unless you put an extra cap across the PIC ADC input terminal to gnd - then you'll have to consider what bandwidth the circuit will run at. – Andy aka Jan 13 '14 at 9:24
• To that end, it might be worthwhile considering a DC path into the ADC as being a high impedance input, it will have a tendency to float. Agreed the divider does not affect the DC offset seen by the ADC, however it does allow for maximum output swing in the amp stage. – Martin Jan 13 '14 at 9:31