Electret microphone amplifier

In lab_3 we'll build an amplifier that amplifies the microphone signal enough that the AVR32 ADC can convert the signal with sufficient resolution to be able to measure sound volume. We'll use an LM324 opamp (datasheet here) with some resistors and electrolytic or tantalum polarized capacitors. For the power supply, we'll use the regulated 3.3V supply from the AVR32 board, with additional power supply decoupling to block the digital noise that comes from driving the servo motor.

From the datasheet we see that the LM324 can operate with a supply voltage down to 2.6V so we are OK with a 3.3V Vdd supply.

The amplifier circuit uses the microphone JFET to generate the first voltage Vmic from the 6.8k resistor Rmic. Then a blocking capacitor Cmic=10uF blocks the DC level from Vmic and couples it to the opamp positive input V+. The DC level of V+ is set by the resistive divider from the two 56k resistors to 3.3V/2=1.65V. Since Vmic is a bit lower than 1.65V we orient Cmic so the + side is on the V+ side. The opamp circuit then amplifies Vmic by 100 to form Vaud. The feedback circuit is formed from the resistive divider formed from the 100k and 1k resistors, together with the 10uF Chp. The steady state Vaud will then be Vdd/2. We hope that a 2mV Vmic signal will amplify to a 200mV Vaud signal which should be sufficiently large to quantize.

The illustration below shows a sketch of this amplifier schematic, along with the bypassed power supply for the servo motor.

The microphone is only 5mm across and the pins are a bit short for the protoboard, so it may pop out. The negative (n-JFET source) terminal of the JFET is the one that is on the right when the pins are closer to the top when looking at the bottom of the microphone, as shown in the right illustration below:

By trying different resistor values, we find that we get the largest signal using a 7kOhm resistor in source configuration, which produces a DC output voltage of about 500mV with a 3.3V supply.

The pinout of the quad opamp is below. The power supply pins are in the middle on opposite sides. You can remember these as “low is high” since the positive supply is on pin 4 and negative on pin 11. Each opamp follows the pattern ”+ - out” from the middle to the corner.

And also, here's how it looks after wiring up the microphone preamp:

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