Conventional Electret Microphone Interface

To interface an electret microphone with a microcontroller (MCU), it has conventionally been necessary to use one or two op amp integrated circuits and some resistors to amplify the signal.

Figure 1. Schematic Diagram of Conventional Microphone Interface

The schematic diagram of a conventional electret microphone interface is shown in the figure above. An electret microphone requires a pullup resistor to VDD to bias it – this function is provided by resistor R3. The millivolt audio output signal appears at pin 1 of the electret microphone, but it is superimposed on a DC voltage. Capacitor C1 blocks the DC voltage from the rest of the circuit to prevent saturation, and the capacitor value is sized to allow audio frequencies to pass through it. The MCP6001 op amp is wired in an inverting amplifier configuration, with the gain determined by the ratio -R2/R1. A voltage divider consisting of R4 and R5 biases the non-inverting (+) input of the op amp at VDD/2. This causes the amplified audio output at pin 1 of the op amp to be centered around a DC voltage of VDD/2. This output is suitable for direct connection to either an analog comparator input or analog-to-digital converter (ADC) input of an MCU.

The conventional electret microphone interface has a few drawbacks. Not only does it require several passive and active components to be added to the hardware design, but it also does not provide a mechanism for the MCU to adjust the gain of the amplifier under firmware control.