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Transmitter

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ehs1

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I have a transmitter circuit that I previously built and it works, but I dont understand the schematic well at all. Please feel free to enlighten me about any insights such as biasing or whatever it may be. Please be as detailed as you can. Thanks for your cooperation in advance.

I have attached the circuit. Thanks
 

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The transistor on the left is the heart of the first stage of a two stage design. This first stage has a simple task which is to amplify the voltage coming from the microphone. The 2N3904 transistor is configured as a common emitter amplifier. The 2Mohm resistor on its base provides base bias current while the 22Kohm resistor on the collector is the collector DC load for this configuration. The base bias resistor is arranged to take current from the collector so as to provide some DC feedback to stabilize the bias vs temperature and from one transistor to the next, as Beta varies from one part to another. Without doing any calculations, i would guess that the collector DC quiescent voltage might stabilize at approximately 1.5 volts.

The microphone is likely an electret type. This type requires DC bias voltage to operate and this bias is provided through the 10Kohm resistor attached to the mic. AC voltage at this junction, that is generated when sound strikes the mic, passes through the 0.1 uF capacitor and is applied to the base of the transistor. Since the transistor is biased to act as an amplifier, the AC voltage is amplified at the collector. From there it passes through the 1 uF capacitor to the base of the other transistor.

The second transistor is configured as an oscillator and is somewhat more complicated. It appears to be configured for DC bias in a common emitter configuration, with the 39Kohm base resistor providing base bias, and the 100 ohm emitter resistor providing DC feedback to limit collector current.

In AC operation, the oscillator becomes a bit more complicated, but to put it simply, it is setup with the base at AC ground. The coil and 10 pF capacitor in parallel and connected to the collector do most of the work of determining the oscillation frequency. Since the antenna also has an impedance, and it is tied directly to the collector tank circuit that I just mentioned, it also contributes to the resonant frequency and there is no way to tell how much without knowing more about the antenna.

Anyway, when the oscillator starts up, the 10 pF capacitor connected from the collector to the emitter provides feedback which makes the configuration unstable and causes it to oscillate. Imagine at some instant that the voltage at the collector is going up, this voltage is also passed to the emitter which tends to turn off the transistor, which in turn tends to cause the collector voltage to go even higher. So, it would seem that the feedback is positive, which we would normally expect in an oscillator. At some point the transistor won't be able to push the collector voltage any higher and the process of feedback will reverse direction and force the collector voltage to go back down. When the voltage gets to some low point where the transistor can't turn on any further, the voltage then changes direction and starts going back up again. This process repeats and you get AC at the output.

In this case, the AC should be at a very high frequency because the coil and capacitor values that dtermine frequency are quite small. Perhaps this is designed to work at approximately 100 MHz?

The audio voltage applied to the base of the second transistor causes the transistor collector-base junction capacitance to go up and down, which in turn tends to change the oscillators frequency following the audio AC voltage. So, this tends to frequency modulate the oscillator and you get FM. Incidently, you also get some AM as a result, but then this clearly is not meant to be a high quality transmitter. FM receivers won't care if there is some AM on the signal.
 
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