Generate negative voltage

[audioguru]

The output of the opamp that is fed from a microphone is the DC voltage of the output of the opamp modulated by the AC signal. Shorting the ouput of the opamp to +5V with a FET will just cause severe distortion.

A FET has very high distortion if its signal has a peak voltage of about 50mV and more. The distortion of the FET is reduced if the half the signal at its drain is fed to its gate.

My circuit has the FET with a signal level as low as the input to the opamp which is about 10mV so the distortion from the FET is very low. Two 100k resistors feed half the source signal to the gate to further reduce distortion. When the control voltage is 0V then the FET is completely turned on and the gain of the opamp is 31.3. When a negative voltage is applied then the signal at the output is reduced.

 

[prprog]

.....
My circuit has the FET with a signal level as low as the input to the opamp which is about 10mV so the distortion from the FET is very low. Two 100k resistors feed half the source signal to the gate to further reduce distortion. When the control voltage is 0V then the FET is completely turned on and the gain of the opamp is 31.3. When a negative voltage is applied then the signal at the output is reduced.

Great circuit (and I think I understand it). How do I generate a negative voltage for the FET? Remember that I will used another OpAmp (or a section of the same opamp) to amplify my microphone signal. How do I generate "less positive" voltage ? I don't undertand the concept yet.

Thanks a lot.

 

[audioguru]

Great circuit (and I think I understand it). How do I generate a negative voltage for the FET? Remember that I will used another OpAmp (or a section of the same opamp) to amplify my microphone signal. How do I generate "less positive" voltage ? I don't undertand the concept yet.

Notice that the opamp circuit has its inputs grounded so it needs a dual polarity supply.
The circuit was part of an Automatic Volume Control circuit and an opamp rectified the output to make the negative control voltage.
This circuit uses an N-channel FET that needs a negative voltage on its gate to turn it off. If you use a P-channel FET (and reverse the polarity of the capacitors and diodes) then it is turned off by a positive voltage on its gate.

The circuit can have as much gain as you need. Its 100k to 3.3k resistors makes it have a max gain of almost 30. Add a preamp if you want.

EDIT: One capacitor (C3) was backwards.

 

[prprog]

Notice that the opamp circuit has its inputs grounded so it needs a dual polarity supply.
The circuit was part of an Automatic Volume Control circuit and an opamp rectified the output to make the negative control voltage.
This circuit uses an N-channel FET that needs a negative voltage on its gate to turn it off. If you use a P-channel FET (and reverse the polarity of the capacitors and diodes) then it is turned off by a positive voltage on its gate.

The circuit can have as much gain as you need. Its 100k to 3.3k resistors makes it have a max gain of almost 30. Add a preamp if you want.

EDIT: One capacitor (C3) was backwards.

Let say that I want to used the N-Channel FET. How do I generate negative voltage? (less positive) with an OpAmp + microphone.

Thanks,

 

[audioguru]

Let say that I want to used the N-Channel FET. How do I generate negative voltage? (less positive) with an OpAmp + microphone.

Attach the schematic of your mic preamp so we can see if it has a dual polarity power supply.