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What is an oscillator ? Discuss the use of a junction transistor as an oscillator. |
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Answer» Solution :Oscillator. It is an electronic device which provides a.c. output waveform of desired from d.c. power withour input signal. It consists of a tank circuit consisting of inductor L and capacitor C connected in parallel. The frequency of the tank circuit is given by `v = 1/(2pi sqrt(LC))` Due to the resistance of inductive coil, there OCCURS al 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 L - C circuit is coupled with transistor in such a way that there is a proper feedback to the L-C circuit at the proper timings so that the energy of L-C circuit remains the same throughout oscillations. When key K is pressed the collector attains positive potential due to which a weak collector current will start rising in `L_1`. The increasing magnetic flux is linked with `L_1` and hence with L. The e.m.f. induced in L will charge upper plate of the capacitor positively and hence support the forward biasing of the E-B circuit. This results in an increase in `I_e` and hence `I_c`. Due to this more increasing magnetic flux is linked with L and hence with L. The process continues till the collector current becomes maximum i.e. saturated. Capacitor C gets discharged through inductor L and support to forward biasing of E-B circuit is withdrawn. Thus `I_e` and hence `I_c` DECREASES. There is a decreasing magnetic field in `L_1` and hence with L. The e.m.f. induced in L charges the lower plate positively. This results in opposition to forward biasing and further decrease of `I_e`. and hence Is This process repeats till `I_c = 0`. Thus oscillations set up in the circuit. |
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