CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
Feedback amplifier and transistor oscillator
In an oscillator, we get ac output without any external input signal. To attain this, a portion of theoutput power is returned back (feedback) to the input in phase with the starting power. This process is termed positive feedback
The feedback can be achieved by inductive coupling (through mutual inductance) or LC or RC networks. Different types of oscillators essentially use different methods of coupling the output to the input (feedback network), apart from the resonant circuit for obtaining oscillation at a particular frequency.
Working of feedback amplifier
Consider the circuit as shown in which the feedback is accomplished by inductive coupling from one coil winding (T1) to another coil winding (T2). The coils T2 and T1 are wound on the same core and hence are inductively coupled through their mutual inductance.
When switch S1 is put on to apply proper bias for the first time, a surge of collector current flows in the transistor. This current flows through the coil T2. This current increases from X to Y, as shown in the graph.
The inductive coupling between coil T2 and coil T1 causes a current to flow in the emitter circuit, the ‘feedback’ from input to output. As a result of this positive feedback, The current in T1 (emitter current) increases from X´ to Y´.
The current in T2 (collector current) connected in the collector circuit acquires the value Y when the transistor becomes saturated. The collector current can increase no further. Since there is no further change in collector current, the magnetic field around T2 stops to grow and there will be no further feedback from T2 to T1. Without continued feedback, the emitter current begins to fall. Consequently, collector current decreases from Y towards Z. However, a decrease of collector current causes the magnetic field to decay around the coil T2. Thus, T1 is now seeing a decaying field in T2. This causes a further decrease in the emitter current till it reaches Z′ when the transistor is cut-off. Both IE and IC are zero. Therefore, the transistor has reverted back to its original state (when the power was first switched on).The whole process now repeats itself. That is, the transistor is driven to saturation, then to cut-off, and then back to saturation. The time for change from saturation to cut-off and back is determined by the constants of the tank circuit or tuned circuit (inductance L of coil T2 and C connected in parallel to it).
[A tank circuit or tuned circuit is a parallel combination of a capacitor and inductor]The resonance frequency (ν) of this tuned circuit determines the frequency at which the oscillator will oscillate.
If the tank or tuned circuit is connected in the collector side, it is known as tuned collector oscillator. If the tuned circuit is on the base side, it will be known as tuned base oscillator.