Principle : To attain self-sustained oscillation, an amplifier is taken. A portion of the output power of this amplifier is returned back (feedback) to the input in phase with the starting power (this process is termed positive feedback) as shown in Fig.
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.
Fig. (a) Principle of a transistor amplifier with positive feedback working as an oscillator.
Circuit : For understanding the oscillator action, we consider the circuit shown in Fig. (b) in which the feedback is accomplished by inductive coupling from one coil winding (T1) to another coil winding (T2). Note that the coils T2 and T1 are wound on the same core and hence are inductively coupled through their mutual inductance. As in an amplifier, the base-emitter junction is forward biased while the base-collector junction is reverse biased.
(c) Rise and fall (or built up) of current Ic and Ie due to the inductive coupling.
Working : Let us try to understand how oscillations are built. Suppose switch S1 is put on to apply proper bias for the first time. Obviously, a surge of collector current flows in the transistor. This current flows through the coil T2 where terminals are numbered 3 and 4 Fig. (b). This current does not reach full amplitude instantaneously but increases from X to Y, as shown in Fig. (c)(i). The inductive coupling between coil T2 and coil T1 now causes a current to flow in the emitter circuit (note that this actually is the ‘feedback’ from input to output). As a result of this positive feedback, this current (in T1; emitter current) also increases from X´ to Y´ [Fig. (c)(ii)]. The current in T2 (collector current) connected in the collector circuit acquires the value Y when the transistor becomes saturated. This means that maximum collector current is flowing and can increase no further. Since there is no further change in collector current, the magnetic field around T2 ceases to grow. As soon as the field becomes static, 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 [Fig. (c)(i)]. 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 (opposite from what it saw when the field was growing at the initial start operation). This causes a further decrease in the emitter current till it reaches Z' when the transistor is cut-off. This means that both IE and IC cease to flow. 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 cutoff, 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). The resonance frequency (ν) of this tuned circuit determines the frequency at which the oscillator will oscillate.
In the circuit of Fig. (b), the tank or tuned circuit is connected in the collector side. Hence, it is known as tuned collector oscillator. If the tuned circuit is on the base side, it will be known as tuned base oscillator.
There are many other types of tank circuits (say RC) or feedback circuits giving different types of oscillators like Colpitt’s oscillator, Hartley oscillator, RC-oscillator.
