Op-amp building blocks

An electret microphone fed from 5V will supply a signal on the order of 100mV. To boost it up to a few volts (to feed it into an Arduino, for example) it needs amplification. We can do that with the op-amp block.

The microphone board contains the circuitry to generate a signal centered around 2.5V and also capacitor C2 which decouples the op-amp feedback network (R5 and R4) from V-. This makes the op-amp behave like a voltage follower at DC (R4 disconnected from V-) and as an amplifier with gain 19 at AC. So, it will amplify the audio waveform, but not the 2.5V that it rides on.

Top trace is microphone output and bottom one is op-amp output.

The huge open-loop gain of op-amps (at least one billion in the case of the OPA357) makes them virtually unusable as amplifiers without some form of feedback network to lower the gain to a reasonable and predictable value. There is one exception where are used in open-loop mode, and that is as comparators. Any minute difference between IN+ and IN- gets amplified so much that it saturates the output either close to the positive rail or to the negative one.