Analyzing a Two Transistor Spy Circuit

 

Analyzing a Two Transistor Spy Circuit

Adding a transistor stage to the above discussed single transistor FM transmitters could enable the designs with extreme sensitivity.

An electret MIC itself has a built in FET which makes it very efficient and makes it a stand alone vibration amplifier device. Adding another transistor stage with it enhances the sensitivity of the device to overwhelming limits.

As may be witnessed in the following diagram, the involvement of an extra transistor stage adds up to the gain of the MIC making the entire unit highly sensitive such that it now picks even the sound as low as a pin dropping on the floor .

The extra transistor prevents excessive loading of the MIC thereby ensuring better efficiency to the sensitivity.

Five things that that make the circuit extremely good with it reception are:

1. The use of a fix capacitor in the tank circuit along with a adjustable trimmer.
2. A low value coupling capacitor with the MIC sufficient to handle the capacitive reactance of the MIC which may be around 4k at 3kHz.
3. A 1u coupler is included between the oscillator and the audio amplifier in order to make up for the low impedance rendered by the 47k base resistor.
4. The coil used is wound practically using super enameled copper wire which ensures higher efficiency than PCB etched type of coil.
5. The entire circuit could be compactly constructed over a small sized PCB for acquiring better stability and a drift free frequency response.