Tuned Collector Oscillator

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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.

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.

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 180^º is create between the voltages of L₁ and
L₂ due to transformer *action. A further phase shift of 180^º 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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