Mon February 04, 2013 By: Thomas Albin

Can u please explain the working of a transistor as an oscillator clearly.?

Expert Reply
Tue February 05, 2013

Oscillator: A device which is used for providing the alternating output from a d.c source is called as an oscillator. For maintaining and starting of oscillator need not any external signal.It can produces an output continuous. A simple oscillator circuit is similar to the amplifier circuit. But the difference is an oscillator can provides its own input signal.

Essentials of a Simple Oscillator Circuit:

Essentials of an oscillator:


  • Tank circuit
  • Transistor amplifier
  • Feedback circuit

Tank circuit:

A tank circuit contains inductance L and capacitance C in parallel.L and C are used for finding the frequency of oscillations.

Transistor amplifier:

Transistor amplifier is used for amplifying the input energy and make losses in resistive component.

Feedback circuit:

Feedback circuit is for providing the positive feedback.

Simple Oscillator Circuit:

Simple oscillator circuit:


The above simple circuit has a CE configuration.

This simple circuit is called as collector oscillator because it consists of a capacitor and inductor connected in the collector.The feed back coil L’ is attached to the common base circuit and it is parallel to the coil L. the mutual inductance of the coils L and L’ is M. These coils are acted as a primary and secondary of the transformer.

The emitter resistance RE is for providing the biasing and other two resistances R1 and R2 are available. The capacitor C1 is attached to the base circuit which gives the low resistance where as the capacitor connected parallel to the emitter resistance has a high resistance.Hence the resistance RE has no effect on ac operation of the circuit.

Operation of simple oscillator circuit:

when the switch is closed the collector current starts to increase causes charging of capacitor. After the capacitor reaches the maximum charge, it starts to discharge the charges to the coil L .

Due to mutual induction this oscillations induced a low voltage to the coil L’. The frequency of this is same as the frequency of LC circuit but it is an induced voltage. Its magnitude is depends of the number of coils in L’ and the connecting between L and L’. Now this feedback voltage is passing between the base and emitter which is in the form of amplified form. This energy of this amplifier is used to meet the losses and keeping oscillations in tank circuit.

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