This is a
low-impedance (Z = 50 to 200 ohms) microphone input circuit or pre-amplifier that employs
low-cost, low-noise precision operational amplifiers such as the OP27
and the OP37.
The simple circuit above
amplifies
differential signals from low-impedance microphones by 50 dB.
Because of the high working gain of the circuit, use of the OP37 (which
is a
high-speed op
amp) is recommended if bandwidth is important to the application.
To ensure stability, a dummy resistor Rp must be placed between the OP37
inputs. This will prevent amplifier oscillation due to 100% feedback
from the open input in case the microphone is unplugged.
Primary
Reference: Analog Devices Datasheets for OP27/OP37
See
also: Operational Amplifier
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