Hi everyone, I'm using an instrumentation amplifier to work with sound signals that receive a microphone, this is the circuit

I can choose the gain with the trimmer, it is between 50 and 500.

At lab, I have tested this circuit with a function generator instead the microphone, and It works! , but when I try with the microphone I can't see any amplification, but I don't know why, I have measured that the output signal of the microphone(Vin node) is about 10 mVrms.

Thanks for your answers.

 

1)U1B has no DC route for it's non-inverting input bias current. You need something like 100K to ground.
2) 470 ohms for Rmic seems a bit low. Check the DC level at MK1 and see if it's about +2.5V
3) Cmik could be about 0.1 uf, not 100 uf

 

Your Vin signal is not properly biased.

Why are you using 3-opamp instrumentation amplifier configuration?
A single stage single-ended input op-amp with a gain of 300 is sufficient.

 

It doesn't bother me if you're exploring how instrumentation amps work, but, as MrChips says, it's completely not necessary for this job. There are plenty of cheaper ways to do it well.

 

1)U1B has no DC route for it's non-inverting input bias current. You need something like 100K to ground.
2) 470 ohms for Rmic seems a bit low. Check the DC level at MK1 and see if it's about +2.5V
3) Cmik could be about 0.1 uf, not 100 uf

Ideally the non-inverting and inverting input bias current of all opamps should be 0A because the hight imput impendance... why should I put a 100k resistor to ground?

 

Ideally the non-inverting and inverting input bias current of all opamps should be 0A because the hight imput impendance... why should I put a 100k resistor to ground?

Congratulations. You just found the difference between theory and reality. The bias current is not zero. Add a resistor or it won't work.

 

Ideally the non-inverting and inverting input bias current of all opamps should be 0A because the hight imput impendance... why should I put a 100k resistor to ground?

Because of the high input impedance of the opamp, what will be the input voltage?
Your input is practically floating and the sky is the limit.
That is why you need a 100kΩ resistor to GND.
Also Rmic is too low. Try 2k-10kΩ.
Also Cmic is too high. Try 01.μF.

Did I here someone else say this already?

 

Congratulations. You just found the difference between theory and reality. The bias current is not zero. Add a resistor or it won't work.

Ok but why just to UB1? should I do the same to the others amplifiers?

 

Because of the high input impedance of the opamp, what will be the input voltage?
Your input is practically floating and the sky is the limit.
That is why you need a 100kΩ resistor to GND.
Also Rmic is too low. Try 2k-10kΩ.
Also Cmic is too high. Try 01.μF.

Did I here someone else say this already?

OK, I will do it

 

Ok but why just to UB1? should I do the same to the others amplifiers?

No, because the other opamp inputs are connected to resistive elements.
There is no resistive element on pin-5 to drain away charge.

Edit: I really ought to say "conductive" elements but hopefully you get the idea.

 

Your Vin signal is not properly biased.

Why are you using 3-opamp instrumentation amplifier configuration?
A single stage single-ended input op-amp with a gain of 300 is sufficient.

My thoughts exactly! Way over complicated - and expensive.

 

A tl084 is not specified for +/- 3 volt power supply (6 total). I think minimum is 10. Output is about 2volts less than power supply (on those chips). That leaves about +/-1 volt for output swing if you do try to power With a 6 v supply.