What is a Tuned Collector Oscillator | How to Construct a Tuned Collector Oscillator

A Tuned Collector Oscillator is another widely used as a local oscillator as well as beat frequency oscillator in radios. A tuned collector oscillator consists of a parallel combination of L and C placed at the collector of the transistor tuned to a specific frequency. The tuned collector oscillator produces oscillations at a frequency determined by this resonant tuned circuit.

 

Tuned Collector Oscillator Circuit

The amplifier device used here is a BJT transistor Q1. The L1 and C1 forms the LC tank circuit. L1 forms the primary winding of the transformer and L2 the secondary. The resistors R1 and R2 form the biasing resistors of the transistor. The emitter resistor Re forms the current limiting resistor and provides stability. The capacitors C2 and Ce are the bypass capacitors. The winding of the primary and the secondary of the transformer provides 180 degree phase shift. The common emitter configuration of the transistor provides another 180 degree of phase shift between the input and the output. This gives a total of 360 degree phase shift which is needed for sustained oscillations.

Operation of a Tuned Collector Oscillator
A tuned collector oscillator consists of a parallel combination of L and C placed at the collector of the transistor. The tuned collector oscillator produces oscillations at a frequency determined by the resonant tank circuit. When the circuit is powered on, the transistor begins to conduct and the capacitor C1 starts charging. The charge is stored in the capacitor as an electrostatic field between the plates. When the capacitor is fully charged, it tries to discharge through the primary coil L1 of the transformer. The capacitor C1 then discharges into the L1 and the charge is stored in the coil L1 as an electromagnetic field.

As the voltage across C1 decreases, the coil L1 produces a back EMF that opposes the decrease in current flow. This back EMF charges the capacitor C1 and the capacitor gets charged again by the stored charge in the coil L1. These back and forth charging and discharging cycles are repeated to form a series of oscillations in the tank circuit.

The oscillation at the primary winding L1 of the transformer is coupled to the secondary winding L2 through inductive coupling. A signal is induced in the secondary winding of the transformer. The output of the secondary coil L2 is coupled to the input of the transistor Q1. The secondary winding of the transformer produces a 180 degree phase shift with the primary. The common emitter amplifier produces another 180 degree phase shift and thus a total of 360 degree phase shift is produced. This 360 degree phase shift is necessary for the positive feedback and sustenance of oscillations. The amount of feedback is adjusted by the number of turns of the secondary coil L2 of the transformer.

The energy lost in the tank circuit due to damped oscillation is overcome by the amplified signal at the collector. The frequency of oscillations can be varied by changing the parallel capacitor or the inductor.

The frequency of oscillations of the tank circuit is determined by the equation

F = 1/2π√ [(L1C1)]

Where F is the frequency of oscillation, L1 is the inductance of the primary coil of the transformer and C1 is the parallel capacitor across L1.

Applications of a Tuned Collector Oscillator
1. It is used in radio as a local oscillator.
2. It is used in radios as a beat frequency oscillator for resolving SSB signals.
3. It is used as a signal generator.
4. It is used as an RF oscillator.

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