Construction of tuned collector oscillator
It consists of a tank circuit or LC tuned circuit connected to the Collector of the transistor. Hence, the name of this oscillator is tuned collector oscillator. The stabilized self bias to the amplifier is provided by the arrangement consisting of R1 , R2 , R3 and Ce.
The coil L2 is coupled to the coil L1 of the tank circuit. In fact, L1 is the primary and L2 secondary coil of a transformer. The low resistance path to the oscillation is provided by the capacitor C.
We know, a transformer introduces a phase shift of 180o between its primary coil and secondary coil. Hence a phase shift of 180o is introduced between the voltages of L1 and L2.
When a transistor is in common emitter configuration, a phase shift of 180o is introduced between the input circuit (i.e. base-emitter circuit) and the output circuit (i.e. collector-emitter circuit).
A phase shift of 180o is also introduced in the circuit by the transistor. Therefore, a total phase shift of 360o or 0o exists between the input and output voltages, leading to positive feedback which is essential for sustaining the oscillation in the circuit.
Working of tuned collector oscillator
When key K is closed, collector current begins to flow. As the collector current increases, the capacitor C1 stars getting charged. After the capacitor is fully charged, it begins to discharge through the coil L1 , giving rise to the damped oscillations of a definite frequency.
The voltage in coil L2 is induced by these oscillations. This voltage is applied between the base and emitter and appears in the collector circuit in the amplifier form.
Feedback of energy from the collector circuit to the based circuit is achieved losses taking place in the tank by means of mutual inductance M between L1 and L2. Thus circuit are compensated. Hence undamped oscillations are obtained at the output.
Analysis of tuned collector oscillator
The equivalent circuit of a tuned collector oscillator is shown in the figure 3. Let R be the resistance of the secondary side reflected to the primary.
The voltage across the tank circuit ( L1-C1 ) is given by
and feedback voltage
where M is the mutual inductance between primary and secondary coils of the transformer.
Now, feedback ratio is given by
The impedance of transformer is given by
Using eqns. (1) and (2), we get
Putting this value in eqn. (6) and using eqn. (5) also, we get
Using eqn. (7), we get
According to Barkhausen criterion for sustained oscillations,
Using eqns. (4) and (8), we get
Multiplying both sides by (hoe / jwC1), we get
Comparing real parts of eqn. (9), we get
Which is the required condition for sustained oscillations.
Comparing imaginary parts of eqn. (9), we get
which is the angular frequency of oscillations.
Eqn. (12) tells us that the frequency of oscillations is more than the frequency of resonance of the tuned circuit.
If the resistance R is negligible, then eqn. (12) becomes
Which is the frequency of oscillations in a tuned collector oscillator.