mic preamp

[dragon91]

hello everyone... I have recently made a mic preamp( non-inverting with 2nd order buttersworth filter)... the problem with the circuit is that the output of my opamp is not stable... the signal is constantly varying from 0 and 3V. Can u tell me what the problem is......... here is my circuit diagram.........

 

[dragon91]

better description

Here is a better description with the picture of what I am doing and using....

 

[Nigel Goodwin]

You bias the second opamp, but not the first one.

 

[dragon91]

mic opamp bias

You bias the second opamp, but not the first one.

I cant bias the first one as it is built in within the microphone.... it is a mic integrated with an amplifier.. it comes in one package and there is no way I can provide a dc offset for it..... without taking apart the mems mic.....

 

[Nigel Goodwin]

Presumably you're only simulating this? - you can't just throw a few components together at random to try and replicate an amplified mike. The opamp isn't biased, and it won't work if it isn't.

 

[audioguru]

The second opamp is an oscillator because its gain is 83, not 10.

 

[dragon91]

gain same for both opamps

So the gain needs tp be the same for both amps at both stages???? I really dont understand why I need to dc bias the first stage when it alrady has a dc offsrt of 1.1V and I decoupled the output and input of the opamp.... can u please clarify this for me... can u show me how the circuit should be....

 

[dragon91]

I didnt get the part when u said the 2nd opamp is an oscillator???' that is exactly what is happening at the output of my opamp... It constantly oscillates between 0 to 3V. why does that happen???? because of the dc offset or the different gains at each stage????

 

[dragon91]

I have improved the gain.... but what should I dc bias...... should I dc bias the Vcc to the microphone????????

 

[Nigel Goodwin]

I have improved the gain.... but what should I dc bias...... should I dc bias the Vcc to the microphone????????

The first opamp should be biased just like the second one, and the mike connected via a capacitor. If it's an electret mike capsule, then you need power to it as well.

 

[dragon91]

how can I bias the first opamp.... should I bias the vcc of the opamp... because thats the only thing I can bias without taking apart the mems mic with an integrated amplifier.......

 

[Nigel Goodwin]

how can I bias the first opamp.... should I bias the vcc of the opamp... because thats the only thing I can bias without taking apart the mems mic with an integrated amplifier.......

You keep saying this - if the first opamp is inside the mike, why are you showing it on the circuit? - because the diagram as shown won't work. If there is an opamp in the mike, then it is biased - it won't work otherwise.

 

[audioguru]

The second-order Sallen and Key lowpass filter has a certain amount of max gain to be a Butterworth filter. If the gain is too high then it oscillates.
I make this type of filters using a gain of 1 for the opamp, same value resistors and the capacitor values are 2:1, or use the same value capacitors and a fgain of 1.6.

You can use the Texas Instruments Filter-Pro program to calculate lowpass filters with any amount of gain but the capacitors and resistors have odd values.

 

[dragon91]

ok... so I can only get a gain of 1 from the buttersworth low pass filter..... so the capacitor values should be 2:1 meaning what are we comparing the ratio with??? like 2 being the capacitors and 1 is what?????

The second-order Sallen and Key lowpass filter has a certain amount of max gain to be a Butterworth filter. If the gain is too high then it oscillates.
I make this type of filters using a gain of 1 for the opamp, same value resistors and the capacitor values are 2:1, or use the same value capacitors and a fgain of 1.6.

You can use the Texas Instruments Filter-Pro program to calculate lowpass filters with any amount of gain but the capacitors and resistors have odd values.

 

[dragon91]

DO U MEAN SOMETHING LIKE THIS... I have used a unity gain low pass butersowrth filter and another opamp to amplify the signal.....

 

[dragon91]

well I simulated it and it doesnt seem to work.. my output at the third stage remains at 3V.

 

[dragon91]

I think this works... I forgot to ac couple the third stage.....

 

[Nigel Goodwin]

None of the three opamps are biased now?.

 

[dragon91]

is this better... I have biased all 3.........

 

[Nigel Goodwin]

Just needs an input coupling capacitor now, as the source is shorting the bias out.