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Two-stage audio amplifier as implemented

Hi, part of my project requires me to amplify an audio signal coming from an electret condenser microphone which outputs an average of 10mVpp to approximately 2.5Vpp. I Used two cascaded non-inverting TL071 OP-amps since one Op-amp doesn't have enough GBW. The output of the mic is low-pass filtered with a cut-off frequency of around 13kHz.

It works perfectly if I input a signal from the signal generator, the output is as required, however with the signal from the microphone there is no output, just a DC offset which is coupled by the capacitor at the end. I also tried buffering the output form the mic. Any help would be much appreciated.

EDIT: Didn't realize the error in the schematic, resistor 1k should be between mic and 5V.

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  • \$\begingroup\$ Is the 1 kohm part at the left of the diagram part of your circuit or part of your model for the microphone? Is it also used when you connect the function generator? \$\endgroup\$ – The Photon Apr 2 at 19:40
  • \$\begingroup\$ JRE's answer was the solution. You're right about the 1k, my mistake. I edited it \$\endgroup\$ – Simon Apr 2 at 19:56
  • \$\begingroup\$ Shows why it's useful to use the internal schematics editor - could have edited it. \$\endgroup\$ – pipe Apr 3 at 7:28
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Put a large resistor from the + input of the left opamp to ground. Say, like 470kohm.

The DC bias from the microphone is getting through to the opamp. The TL071 has very high impedance inputs. The tiny bit of DC that leaks through the first capacitor is enough to push that input away from 0V. The amplification then drives the output to one of the rails.


Just noticed something else.

The microphone bias is messed up.

You need that 1k resistor between the microphone and 5V

As you've got it, the microphone is trying to pull the 5V source up and down in response to the sound.

Best case, it doesn't work.

Worst case, you've killed the microphone.

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  • 2
    \$\begingroup\$ Could be bias current from the op amp itself that saturates the caps -- there is no DC path for the bias current. This is less likely, as it works with a function generator. \$\endgroup\$ – Scott Seidman Apr 2 at 19:36
  • \$\begingroup\$ Thank you so much! I realized there was a problem with the small dc leak from the beginning, I couldn't figure out a way to eliminate it completely. Now it works !! \$\endgroup\$ – Simon Apr 2 at 19:52
  • \$\begingroup\$ Thanks for pointing out the error on the schematic, it was correctly implemented on the breadboard \$\endgroup\$ – Simon Apr 2 at 19:54
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Of course the + input of the first opamp must have a DC bias of 0V.

IF the mic has an output impedance of ZERO ohms then the 120 ohms resistor and the second 0.1uF capacitor produce a cutoff frequency of about 13kHz. But the mic output impedance is about 2.7k ohms and is in parallel with the 1k resistor powering it so the cutoff frequency (without the 120 ohms resistor) is actually 2192Hz which is very muffled.

The value of the 1k resistor powering the mic should be half the supply voltage (2.5V) divided by the mic current (0.5mA)= 5k ohms. Use 5.1k then re-calculate the lowpass filter capacitor value with the 2.7k resistance of the mic parallel with the 5.1k resistor.

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Just add a resistor between mic and +5v

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  • 1
    \$\begingroup\$ Can you explain why this would fix it so that OP can learn? \$\endgroup\$ – Puffafish Apr 3 at 8:15

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