With the help of circuit diagram, explain the working of transistor as a oscillator.

Explanation

Feedback: Transfer of small amount of energy from output to the input of the system is known as feedback.
Oscillator: It is a device which delivers a.c. output waveform of desired frequency from d.c. power without any excitation of input signal. Its essential components are:
(i) Tank circuit: It consists of an inductor of inductance L connected in parallel to the capacitor of capacitance C. This circuit produces oscillations of decreasing amplitude. It is due to loss of energy in the form of heat produced by resistance offered by the circuit.
(ii) Transistor amplifier: The oscillations occurring in the tank circuit are applied to transistor amplifier, and we get increased output of these oscillations.
(iii) Feedback circuit: it transfers a part of output energy to tank circuit.
Working: When the tapping key (T.K) is pressed, a very small collector current flows through the coil L.. As the coil L is inductively coupled to the coil L., due to the change of current and hence change of magnetic flux through the coil L., a small voltage is induced in the coil L. This small voltage produces a small current to flow in the emitter-base circuit. If due to the induced voltage so produced, the emitter gets forward biased, then the small emitter current causes a corresponding increase in the collector current (in a transistor, `I_(e)= I_(b)+ I_(c)`). As the collector current rises, the growing magnetic flux linked with the coil L. increases the induced voltage across the coil L., which in turn will further increase the forward bias of the emitter. This increases the emitter current and hence the collector current increases still further. In this way, the collector current through the coil L. goes on increasing, till the induced e.m.f across the coil L attains a saturation value.
As the current reaches saturation value and stops changing, the magnetic flux also stops changing. The emitter current which was being sustained by the induced voltage in the coil L begins to decrease, causing the collector current to decrease as well. The decreasing collector current through the coil L. will induce voltage in the coil L in the reverse direction, which will further decrease the emitter current. This results in further decrease of the collector current. The .inertia. of the collapsing flux carries the collector current below its normal value (in the absence of any emitter current). As the current becomes minimum and stops changing, the magnetic flux also stops changing. In the absence of induced voltage, the collector current starts increasing to its normal value again, causing the induced emitter current once again. This marks the beginning of the next cycle and the process is repeated giving oscillations of constant amplitude because of the feedback energy to the L-C circuit in proper phase. The frequency of the oscillations produced is given by
`f= (1)/(2pi sqrt(LC))`
The frequency can be varied by changing the capacitance of capacitor C by the variable capacitors.