Simplest sinusoidal variable frequency oscillator is amstrong circuit. Simplest sinusoidal constant frequency oscillator is crystal oscillator. Parasitic oscillation is caused by transistor inter electrode capacitance. If you movie towards an oscillating circuit its frequency increases because of hand capacitance. Frequency stability is maximum in wein bridge oscillator. Practically in order to change frequency of oscillation capacitance is changed
No input signal is needed to start an oscillation. For proper oscillation loop gain should be greater than or equal to one. Copitts oscillator also called LC oscillator, tank circuit oscillator and resosnant circuit oscillator. In RC phase shift oscillator phase shift of output is 180 degree, feedback factor = 1/29, loop gain is greater than 29. Three RC sections are there in RC phase shift oscillator, each stage doesn’t provide exactly 60 degree phase shift but total phase shift will be 180 degree
When FET is used in RC phase shift oscillator then minimum gain is 29. In LC and RC oscillators there frequency varies with temperature. Frequency stability of RC oscillator less than LC oscillator. In transistor RC phase shift oscillator voltage shunt feedback is used. Colpitts oscillator provides feedback from the centre of the two capacitors. Colpitts, Hartley and crystal oscillators are tuned oscillators
Hartley oscillator is generally used in radio receiver. Crystal oscillator have great stability, Because of the superior electrical property of quartz it is used in crystal oscillator. Low output is the limitation of crystal oscillator. Frequency range obtained from crystal oscillator is 25 KHz to 5 MHz. Frequency of crystal oscillator depends on thickness of crystal, angle of cut and physical size of crystal
Q of a crystal is in the range of more than 10,000. X axis connects the corner of the crystal. For generating 1KHz signal wien bridge oscillator is used (so it is called audio oscillator). Minimum gain of wien bridge oscillator is 3. No Phase shift is formed in wien bridge oscillator. For generating 1MHz signal colpitts oscillator is used. Oscillators generates square wave if product of loop gain and feedback gain is less than one. Wien bridge oscillator is used in laboratories. Principle of relaxation amplifier is charging and discharging of capacitor. Relaxation oscillator is equivalent to astable multivibrator