OP37 low noise microphone preamp circuit design electronic project

Description: This microphone preamp schematic is an electronic circuit project utilizing the OP37 operational amplifier from Analog Devices. It functions as a fixed-gain transformerless microphone preamp, amplifying differential signals from low-impedance microphones by 50 dB, with an input impedance of 2 kΩ. The circuit's high working gain allows it to maintain a bandwidth of 110 kHz. As the OP37 is a decompensated device (minimum stable gain of 5), a dummy resistor, RP, may be required if the microphone is disconnected. Otherwise, the 100% feedback from the open input could lead to amplifier oscillation. The noise performance of this circuit is primarily influenced by the input resistors R1 and R2, which each generate 4 nV of noise, compared to the op-amp's 3.2 nV noise. The root mean square (rms) sum of these significant noise sources results in about 6 nV, equivalent to 0.9 µV in a 20 kHz noise bandwidth, or nearly 61 dB below a 1 mV input signal.

This microphone preamp circuit is designed to efficiently amplify audio signals from low-impedance microphones while minimizing noise interference. The choice of the OP37 operational amplifier is crucial due to its high gain and bandwidth characteristics, making it suitable for high-fidelity audio applications. The fixed gain of 50 dB is achieved through careful circuit design, ensuring that the output signal remains within the optimal range for subsequent processing stages.

The input impedance of 2 kΩ is selected to match the characteristics of typical low-impedance microphones, allowing for effective signal transfer without significant loading effects. The bandwidth of 110 kHz ensures that the circuit can handle a wide range of audio frequencies, preserving the integrity of the original sound.

In addition to the basic circuit design, the inclusion of a dummy resistor, RP, is an important consideration when dealing with potential disconnection of the microphone. This resistor helps to stabilize the amplifier by preventing oscillation caused by the open input, which can occur due to the feedback loop characteristics of the OP37 when not properly terminated.

Noise performance is a critical aspect of any audio circuit, and in this design, the contribution of the input resistors R1 and R2 is significant. Their individual noise contributions of 4 nV are relatively higher than that of the OP37, which generates 3.2 nV. The total noise figure of approximately 6 nV is acceptable for high-quality audio applications, ensuring that the preamp remains effective at amplifying desired signals while keeping unwanted noise to a minimum.

Overall, this microphone preamp circuit is a well-engineered solution for audio amplification, offering robust performance, low noise, and high bandwidth, making it suitable for professional audio applications.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.