Tuned Collector Oscillator


Tuned Collector Oscillator





Tuned Collector Oscillator-Fig-1 shows the circuit of tuned collector oscillator. It contains tuned circuit L1-C1 in the collector
and hence the name. 

 

Tuned Collector Oscillator

 





The frequency of oscillations depends upon the values of L1 and C1 and is give
by :









The feedback coil L2 in the base circuit is magnetically couple to the tank circuit coil L1. In
practice, L1 and L2 form the primary and secondary of the transformer respectively. The biasing is
provided by potential divider arrangement. The capacitor C connected in the base circuit provides
low reactance path to the oscillations.





Fig-1




Circuit operation





When switch S is close, collector current starts increasing and charges the
capacitor C1.





When this capacitor is fully charge, it discharges through coil L1, setting up oscillations of frequency determine by exp. (i).





These oscillations induce some voltage in coil L2 by
mutual induction.





The frequency of voltage in coil L2 is the same as that of tank circuit but its
magnitude depends upon the number of turns of L2 and coupling between L1 and L2. The voltage
across L2 is apply between base and emitter and appears in the amplified form in the collector
circuit, thus overcoming the losses occurring in the tank circuit. The number of turns of L2 and
coupling between L1 and L2 are so adjust that oscillations across L2 are amplified to a level just
sufficient to supply losses to the tank circuit.






It may be note that the phase of feedback is correct i.e. energy supply to the tank circuit is in
phase with the generated oscillations. A phase shift of 180º is create between the voltages of L1 and
L2 due to transformer *action. A further phase shift of 180º takes place between base-emitter and
collector circuit due to transistor properties. As a result, the energy fedback to the tank circuit is in
phase with the generated oscillations.





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