Hello everyone,
My circuit is the following simple electret mic + TL071 op amp:
My goal is to amplify a signal from an electret microphone capsule and feed it into an ADC ( NI 6008 USB ). The bandwidth of the AC information is 1Hz - 5Hz aprox, so I have a HPF at the input of the op amp with HPF cuttoff = 0.0339Hz, and an LPF active filter inside the op amp with LPF cuttoff = 3978.9Hz.
As you can see, the output impedance of the mic is about 10kohms and the input impedance of the non inverting amplifier is the DC bias resistor R3=100k, so I have a decent voltage coupling here.
The 3V battery is for the electret mic and the voltage goas through the Rsource resistor = 10kohms.
The problem is the following:
When my source signal is a signal generator ( with the Rsource resistor and the 3V battery removed of course ), everything works great, and the output DC offset is extremely low, which is great.
But when I connect the mic, with Rsource = 2.2kohm, I measure a significant DC voltage at the input of the op amp - about 0.7 volts!
I tried to change the Rsource resistor to the one you see in the circuit diagram, and the DC voltage at the input reduces significally - to about 0.015 volts. But still - it is there.
I also notice something strange - every time the mic is sensing some signal - like me clapping my hands or something like that, the DC voltage at the input drops to 0 Volts, but then it is slowly rising to the value I initialy measured ( 0.015 Volts ). Of cource the voltage at the output is amplified and the DC voltage at the output is 11 times greater ( V_os_input*Gain).
I have no idea why this is happening. Why isn't the DC blocking capacitor blocking the DC voltage from reaching the input terminal of the op amp? And why as I aplly some sort of sound pressure, the circuit behaves normally? I'm extremely confused here.
Thank you,
David.