Undamped Oscillations from Tank Circuit

A tank circuit produces damped oscillations. In practice we need continuous undamped oscillations for the successful operation of electronics equipment. In order to make the oscillations in the tank circuit undamped it is necessary to supply correct amount of energy to the tank circuit at the proper time intervals to meet the losses.

The following conditions must be fulfilled;

1.      The amount of energy supplied be such so as to meet the losses in the tank and the a.c. energy removed from the circuit by the load. For example if losses in LC circuit amount ot 5 mW and a.c. output being taken is 100 mW, then power of 105mW should be continuously supplied to the circuit.

2. The applied energy should have the same frequency as the of the oscillations in the tank circuit.
3. The applied energy should be in phase with the oscillations set up in the tank circuit.

Positive feedback Amplifier-Oscillator

1. A transistor amplifier with proper +ve feedback can act as an oscillator.

1. The circuit needs only a quick trigger signal to start the oscillations. Once the oscillations have started, no external signal source is necessary.
2. In order to get continuous undamped output from the circuit, the following condition must be met;

             m_vA_V =1

          where A_V = voltage gain of amplifier without feedback.

                     m_v = feedback fraction.

        This relation is also called Barkhausen criterion