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Question
Explain briefly the principle on which a transistor-amplifier works as an oscillator. Draw the necessary circuit diagram and explain its working ?
Solution
Transistor amplifier as an oscillator: In an oscillator, the output at a desired frequency is obtained without applying any external input voltage.
The common emitter n-p-n transistor as an oscillator is shown in the following figure.
A variable capacitor C of suitable range is connected in parallel to coil T2 to give the variation in frequency.
Oscillator action:
As in an amplifier, the base-emitter junction is forward biased while the base collector junction is reverse biased. When the switch S is put on, a surge of collector flows in the coil T2. The inductive coupling between coil T2 and T1 cause a current to flow in the emitter circuit i.e., feedback from input to output. As a result of positive feedback, the collector current reaches at maximum. When there is no further feedback from T2 to T1, the emitter current begins to fall and collector current decreases. Therefore, the transistor has reverted back to its original state. The whole process now repeats itself.
The resonance frequency (f) of the oscillator is given by:
`f=1/(2pisqrt(LC))`
The tank of tuned circuit is connected in the oscillator side. Hence, it is known as tuned collector oscillator.
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