This micro-project is intended as an add on to an microprocessor based (Arduino) Scope / Audio Analyser. It has a gain of ~ 100 and uses
a standard opamp.
Inspired by Tobi Delbruck's lecture.
The amplifier is intended for a supply voltage of 5 V. As supply voltages in microprocessor systems are "noisy", the bias for the Electret Condenser
Microphone as well as the Opamp have been extra filtered. As it turned out, that the LM321 has an output voltage range of DC ... 3 V when powered
with 5 V, we adjusted the bias for the opamp to around 1.5 V in order to have a maximum dynamic range. The output is dc-coupled and of low impedance
in order to be connected directly to the A/D converter input. The gain is set by the ratio of R7 / R6. It is set to approx. 100 which is a useful value
for the used microphone (ABM-713-RC). For mamimum resolution, the reference voltage should be set to 3 V - or you may use a microcontroller with 3.3 V
As the used Opamp has a Gain-Bandwidth product of 1 MHz, the max. Bandwidth is 10 kHz, when using a gain of 100. Therefore the capacitor
across R7 (10pF) is not necessary. It may be of interest, when using other Opamps. Especially, when using a rail-to-rail type, the bias of
the positive input mayst be readjusted to half of the supply. The lower cutoff frequency is set by the values of C3, R3, R5. Increase the
capacitor to move it into the sub-Hz range. With the given values, a simulation shows the following amplitude response :
Amplitude Response (Simulation), Gain vs. Frequency.
Real World : Voltage at the Output.
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