Amplifier

I designed this circuit to amplify acoustic chirps emitted by a robot. The resulting signals are used to determine the location of the robot. The amplifier provides a maximum of gain of 46 dB, with about 15 dB of automatic gain control. There are two poles of highpass filtering with 500 Hz corner frequencies to help reduce 60 cycle hum. Julien Amelot was kind enough to enter my penciled design into the Eagle schematic capture / PCB layout program. His version of the schematic is shown below.

IC1A acts like a transresistance amplifier. Electret microphones typically have a built-in amplifier composed of a single field effect transistor. Such microphones therefore act like sound pressure controlled "constant" current source in parallel with a resistor. So if you use the inverting input of IC1A (which is a virtual ground) as the positive supply of the microphone, and attach the other end of the microphone to a now noise negative supply voltage, the current through the microphone continues on through R10. The capacitance of the cable between the microphone and IC1A is not very important in this configuration: because of the virtual ground, the voltage does not vary, only the current.

In principle you could double the gain of IC1A by doubling R10... but if you increase it too much, the output of IC1A will bump into the positive supply. You can compensate for that (at the expense of extra noise) by injecting a current of the opposite polarity into the inverting inverting input. To do so, connect a resistor (about 3.3k Ohms) from that inverting input to the output of the LM317. Better yet, attach that resistor to an RC lowpass filtered version of that output (not shown in the schematic).

IC1D acts like a non-inverting amplifier whose gain is determined by the drain-source resistance of T1. R16 swamps the unit-to-unit variations between transistors, at the expense of the amount of AGC. You can short it out if you like, and get some extra gain. The control voltage that determines the resistance of the FET is the voltage at the junction of R18 and R19. Resistors R14 and R15 add to this control voltage another voltage equal to half of the drain-source voltage: this makes the FET resistance appear to be more linear, which reduces distortion.

IC1C acts like an active rectifier that applies the negative peaks of the IC1D output to C13, in order to obtain the AGC control voltage. R18 and R19 bleed the charge off of C13 so that the gain will recover again after a loud sound. They also act as a voltage divider to match the output voltage range of IC1C to the pinch off voltage of T1.

IC1B shifts the voltage up into the 0-5V range, instead of being centered about 0V. That way the output can be fed into a analog-to-digital converter.