V o F C L (b) Figure . (a) Block diagram of an oscillator (b) Tank circuit iii) Feedback network The circuit used to feed a portion of the output back to the input is called the feedback network. If the portion of the output fed to the input is in phase with the input, then the magnitude of the input signal increases. This process is called positive feedback which is necessary for sustained oscillations.
Working The tank circuit generates electrical oscillations and acts as the AC input source to the transistor amplifier. Amplifier amplifies frequency as shown in Figure . (a). Non- sinusoidal oscillators generate complex, non-sinusoidal waveforms like square- wave, triangular-wave and sawtooth-wave as shown in Figure .
(b), (c), (d). t (d) t (a) t (b) t (C) Figure . (a) Sinusoidal waveform (b) Square waveform (c) Triangular waveform (d)Sawtooth waveform Sinusoidal oscillations are of two types: Damped and undamped. If the amplitude of the electrical oscillations decreases with time due to energy loss, it is called damped oscillations as shown in Figure .
(a). On the other hand, the amplitude of the electrical oscillations remains constant with time in undamped oscillations as shown in Figure . (b). t (a) t (b) Figure .
(a) Damped oscillations (b) Undamped oscillations Transistor oscillator An oscillator circuit consists of three components. They are i) tank circuit ii) amplifier and iii) feedback network. The block diagram is shown in Figure . (a).
- - - - - - - - Unit electronics and Communication · to generate audio tones · to generate clock signal in digital circuits · as sweep circuits in TV sets and CRO EXAMPLE . Calculate the range of the variable capacitor that is to be used in a tuned-collector oscillator which has a fixed inductance of μH. The frequency band is from kHz to kHz. Solution Resonant frequency, LC π On simplifying, we get C f L π i) When frequency = kHz, C = = pF ii) When frequency