OP37 low noise microphone preamp circuit design
This microphone preamp schematic electronic circuit project is designed using the OP37 operational amplifier manufactured by Analog Devices and is effective, fixed-gain transformerless microphone preamp which amplifies differential signals from low impedance microphones by 50 dB, and has an input impedance of 2 k. Because of the high working gain of the circuit, an OP37 helps to preserve bandwidth, which will be 110 kHz. As the OP37 is a decompensated device (minimum stable gain of 5), a dummy resistor, RP, may be necessary, if the microphone is to be unplugged. Otherwise the 100% feedback from the open input may cause the amplifier to oscillate.
Noise performance of this circuit is limited more by the input resistors R1 and R2 than by the op amp, as R1 and R2 each generate a 4 nV noise, while the op amp generates a 3.2 nV noise. The rms sum of these predominant noise sources will be about 6 nV, equivalent to 0.9 uV in a 20 kHz noise bandwidth, or nearly 61 dB below a l mV input signal.