Digital Frequency Division Bat Detector: Alternative preamplifiers

Additional input stage

The circuit works well with a piezo transducer as the microphone, but lacks sensitivity for use with the more desirable electret or MEMS microphones. We examined some possibilities for input pre-amplifiers.

JFet preamp

This simple JFet preamplifier only adds 4 components but only gives a gain of 6 - not enough to use an Electret or MEMS microphone, but a useful boost in sensitivity when used with the piezo microphone. Due to its high input resistance it does not significantly load the sensor.

We next looked at pre-amplifiers using bipolar junction transistors. (common transistors)

BJT preamplifiers


This single stage amplifier using a BJT also adds only 4 components and gives enough gain to allow electrets or MEMS to be used. However the input resistance is very low so it can not be used with Piezo sensors.

Due to the low component count either of the above circuits can be built into the case as a "birds nest".

Next we examined a two-stage BJT amplifier.


With a component count of 10 its not a complicated circuit to build, but will need its own board. None of the values are too critical.  Even different transistors can be substituted - e.g. BC108. The output, which connects to C1 of our detector, is taken from the collector of Q2.


This preamplifier gives a gain of around 9000. You could add a small resistor in the emitter of Q1 (around 100 ohms) which would increase the input resistance to 10k or so - a good match for electret or MEMS microphones. Of course it would also reduce the gain somewhat.