(a) LC oscillations: An LC circuit can be used to produce oscillations of desired frequency. It consists of a tank circuit containing an inductor L and capacitor C connected in parallel.
The frequency of the tank circuit is given by
v = \(\frac{1}{2\pi \sqrt{LC}}\)
Due to the resistance of inductive coil, there occurs a small but constant energy loss and oscillations thus produced are damped. To transmit speech or music, we require undamped electromagnetic waves called carrier waves. To do so an LC circuit is coupled with transistor in such a way that there is a proper feedback to the LC circuit at the time so that the energy of LC circuit remains the same throughout oscillations.
(b) Transistor as an oscillator: An oscillator is an electronic device which converts direct current into alternating current of high frequency and constant amplitude.
Circuit diagram
Fig. shows the circuit diagram for a n-p-n transistor as an oscillator in common-emitter configuration. Common-emitter circuit is reverse biased by battery E. A coil L' is inserted in collector-emitter circuit and it is coupled with L which is connected in the emitter-base circuit.
Working: When key K is closed, a small collector current start rising through L' and a change in electric current hence change in magnetic flux in L' is produced so a small induced e.m.f. is produced in L which is coupled with L'. Thus a small current flows in the emitter-base circuit. If it produces forward biasinig in this circuit, then small emitter current causes a corresponding increase in the collector current and upper plate of capacitor C gets positively charged and more magnetic flux is linked with L' and hence with L, so emitter current is further increased. This process continues till collector current becomes saturated.
As the current reaches saturation value, the mutual inductance stops playing its part. The capacitor C gets discharged through inductance L, and emitter current falls, so collector current also falls and a decreasing magnetic flux is linked with coil L' and hence with L. The decreasing collector current induces voltage in L its reverse direction which decreases the emitter current further. Now the lower plate of the capacitor is positively charged. This process continues till collector current becomes zero. Again mutual inductance stops playing its part. The capacitor gets discharged through inductance L and emitter current increases. The whole process will be repeated again and again.
The frequency of oscillations produced will be given by
V = \(\frac{1}{2\pi \sqrt{LC}}\)
The frequency v can be varied by changing the value of capacitor C by a variable capacitor.
