Design Description

This application note describes how to adjust the gain and calibration of a microphone amplifier to adjust for the variation in sensitivity of electret microphones. Microphones are a primary component of any voice- or speech-based system and electret microphones are a common choice in applications for their small size, cost, and performance. The following figure shows a basic electret microphone amplifier circuit. Most electret microphones are internally buffered with a junction field-effect transistor (JFET), which is biased with the 3.3-kΩ (R1) pullup resistor. Sound waves move the microphone element, causing current flow into the JFET drain. The JFET drain current creates a voltage differential across R1 which is AC coupled and biased to mid-supply and connected to the inverting input of the op amp. The op amp can be configured as a simple inverting amplifier or configured as a band pass-filtered, non-inverting amplifier circuit. With the expected input signal levels and the desired output magnitude and response, the gain and frequency response can be calculated.

One of the main drawbacks of the previous circuit is the fixed gain. Most of the electret microphones have sensitivity variation in the order of ±3dB which will result in different output magnitude which can be a problem in production, leading to yield issues. This can be solved by adding a buffered a digital potentiometer (digipot) like the TPL1401 in the feedback path of the op amp, which will enable programmability of the gain to adjust for sensitivity variation of the microphone in production. The following image shows the typical implementation diagram.