CBSE Class 12-science Answered

A transistor as an oscillator:

The circuit of a tuned collector in CE configuration is shown in fig.

The circuit contains a tuned circuit made of variable capacitor C and an inductor L in the collector circuit and hence is named as tuned collector oscillator

The feed back coil L' connected to base circuit is mutually coupled with coil L (due to phenomenon of mutual induction ); the mutual inductance of L and L' being M. In practice L and L' form the primary and secondary coil of the transformer

The biasing is provided by emitter resistance R_E and potential divider arrangement consisting of resistances R₁ and R₂. The capacitor C₁ connected in the base circuit provides low reactance path to the oscillations and the capacitor C_E is the emitter by-pass capacitor so that the resistor R_E has no effect on the ac operation of the circuit.

Circuit Operation:

When the collector supply voltage is switched on by closing switch S, collector current starts increasing and the capacitor C is charged. When the capacitor attains maximum charge it discharges through coil L, setting up oscillations of natural frequency 

These oscillations induce a small voltage in coil L' by mutual induction. This induced voltage is the feed back voltage, its frequency is same as that of resonant LC circuit but its magnitude depends on the number of turns in L' and coupling between L and L' The feedback voltage is applied between the base and emitter and appears in the amplified form in the collector circuit. A part of this amplifier energy is used to meet losses taking place in oscillatory circuit to maintain oscillations in tank circuit and the balance is radiated out in the form of electromagnetic waves.

Positive Feed Back-

The feed back applied in tuned collector oscillator circuit is positive. This may be seen as follows : A phase shift of 180º is created between the voltage of L and L' due to transformer action. A further phase shift of 180º arises between base emitter and collector circuit due to transistor action in CE configuration. Thus the net phase becomes 360º (or zero) which is the required condition for a positive feed back. Due to positive feed back the energy fed back to the tank circuit is in phase with the generated oscillations, thus maintaining oscillations.