OPA1612 as microphone pramplifier

[Devicet]

Hi, I'm designing an electrect microphone pramplifier that is supplied by 5V. I decided to use OPA1612 because of it's low noise input levels. I heared that I should be aware about Input current noise density - provide low impedance at input and use proper feedback resistors. Here is the circuit I have designed:



Should I remove R3 to lower input impedance and maybe use second chanel for second stage, so I could lower feedback resistors? I was planning to use second channel as 1/2 Vcc voltage buffer. The line_out should provide consumer line level (max 0.89 Vpp). As you can see I used j-fet in common drain mode for low output impedance.

 

[ccaudle]

Using a FET input amp like OPA1642 in a single stage configuration (i.e. no JFET as first stage) would be simpler and should have close to equivalent noise performance. Search for the term OPIC, similar designs have been discussed multiple times on these forums.

 

[Voyager10]

It can't work with R3 = 16K. If you look at the DC operating point of the op-amp, pin 1 can't go high enough to get pin 2 at 1/2 VCC. (If you remove R3 altogether, the DC operating point will be just fine).

 

[ntch]

What electret is that, and why do you use a jfet and a 1Gohm resistor? Isn't there a jfet in the electret body already?
Why not just go straight into the opamp, i.e. as in https://www.ti.com/lit/ug/tidu765/tidu765.pdf ?

 

[jp8]

https://www.ednasia.com/amplify-small-signals-in-low-noise-circuit-with-discrete-jfet/