BitX20 Microphone

I was reading a discussion on BitX20 yahoo group and Sandeep wrote about a possible typo in the circuit diagram for the microphone preamplifier.

I run a simulation of the original circuit as it is in BitX20 circuit diagram:

As it can be seen, there is absolutely no signal at the output of the circuit.

This happens because the voltage in Q1 collector is virtually equal with the voltage in its emitter and this is because the 10k resistor (R3 in my simulation) is far too low.

Ironically, the microphone preamplifier could work with the values specified in the diagram only if the transistor has very low gain.

A simple but very important rule is that for a transistor operating in common emitter mode, the voltage on collector should be about half of the power supply voltage (in this case, voltage on C3) and it clearly isn’t the case here.

This rule applies to any frequency, from audio amplifiers to microwave amplifiers and beyond.

BitX20 Preamplifier (poor operation)
BitX20 Preamplifier (poor operation – low gain)

It is very simple to find a different value for resistor R3, that will bring the collector voltage to about 5.5V. In this case, the required value should be about 39kohm.

With this resistor, we obtain a gain of 7 times the microphone voltage, without distortion.

BitX20 Preamplifier
BitX20 Preamplifier (low gain corrected)

Sandeep advice was to increase the emitter resistor to 2.2 kOhm instead of the original 220 ohm.

This is the result of his modification:

BitX20 Preamplifier - Sandeep
BitX20 Preamplifier – Sandeep

We can see the circuit works, but the amplifier voltage gain is only 4.5 times the voltage of the microphone.

If people who built the BitX20 radio consider the microphone output signal is too low, they should attempt to increase R3 rather than increasing R2.


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