[FONT="]I have fully explained how RF transmitters work on my website. I have used very simple language that will not confuse the beginner.
In exactly the same way I will describe how to work out the value of C3.
C3 is present to hold the base of the RF transistor steady (fixed) so that the base does not move up and down. This allows the transistor to operate as a common-base stage.
This means the collector of the transistor must be able to move up and down and also the emitter.
In the collector circuit is a coil and capacitor. These two components form a circuit called a resonant circuit and when we talk about transmitters, we call the two components a TANK CIRCUIT.
When we bias the transistor, by putting a voltage on the base and connecting the emitter to the 0v rail via a resistor, the transistor is turned on and it produces a small amount of noise on the collector. This is called junction noise and the resonant circuit in the collector receives this noise and the two components create something that is amazing.
Even though these two components are classified as passive (non amplifying) items, they work together to create a waveform that is a sinewave and can have an amplitude that is higher than the noise or voltage that is being delivered to it.
This is exactly what happens and this waveform is picked up by the 5p6 capacitor connected to the collector and passed to the emitter. If this 5p6 is a larger value, it will “suck away” nearly all the waveform and the circuit will not gradually increase in amplitude to a point where it will start to “really work.”
The small waveform that passes through the 5p6 moves the emitter up and down and this is detected by the transistor.
Now we come to another point.
There are two ways of getting a transistor to amplify – to reproduce a waveform. One is to hold the emitter steady (fixed) and deliver the waveform to the base. The other is to hold the base steady and move the emitter up and down.
That’s what we have done.
The 5p6 is just like a shock-absorber.
Imagine I am holding one end and you are holding the other.
If I push towards you, you will move backwards a small amount and the shock absorber will reduce in length, when I pull, the shock absorber will extend and you will move slightly forward.
This is what the 5p6 does.
When a rising voltage is present on the collector of the transistor, this rising voltage is passed through the 5p6 and raises the emitter.
When the emitter voltage rises, the difference between the emitter voltage and the base voltage is LESS and the transistor turns off slightly.
This makes the collector voltage rise and the action increases.
The transistor could actually do this very quickly but the capacitor in the collector-circuit has a period of time in which is will charge and discharge and this determines the timing of the whole circuit. The energy from the capacitor is passed to the coil and the coil has a certain period of time for it to absorb the energy and turn it into flux.
When the energy passes from the capacitor to the coil, the coil produces magnetic flux and when the capacitor cannot deliver any more energy, the flux collapses and cuts the turns of the coil to produce a voltage in the coil of OPPOSITE POLARITY.
This opposite voltage is detected by the 5p6 to turn the transistor off slightly and this is how the stage operates (oscillates).
This means the TANK CIRCUIT is producing its opposite polarity waveform when the transistor is completely tuned off and that why it can produce a waveform that is higher than the supply voltage.
The value of the capacitor on the base can be almost any value as long as it holds the base steady at 100MHz.
If you want to inject the stage will audio, the capacitor must allow the audio waveform to rise and fall.
But audio is say 5kHz and the oscillator 100MHz so the reactance (resistance) of the capacitor at these two frequencies is 20:1 and it will have a very big effect at 100MHz and very little effect at 5Hz.
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