Chapter 5 – Oscillators – Part 3 Relaxation Oscillator Square wave Generators

Relaxation Oscillator Phase-Shift Wien-Bridge RC Oscillators LC Oscillators Hartley Colpitts Relaxation Oscillators Oscillator circuits which have highly non–linear active elements are termed relaxation oscillators. Relaxation oscillators produce nonsinisoidal output signals. Oscillator circuits can be analyzed in terms of linear elements.

Current-voltage characteristics of unijunction transistor Ohmic contacts ground Unijunction device has two ohmic contacts and a single, small-area emitter pn junction located between them.

Operation Principle of unijunction transistor 1. reversed bias condition Vb = 5, 15, 30V and Ve = 0V. Under this condition, pn junction is reversed bias and only reverse current present in the emitter circuit.

2. Forward bias condition 3. If the junction voltage Ve is increased: (i) emitter becomes forward bias, (ii) the emitter current increases, and (iii) carriers are injected (from emitter) into the semiconductor in the region between B1 and E. 4. Resistance of this portion (between B1 and E) decreases. 5. The voltage at E reduces ( or R1/ (R1 + R2) Vb reduces). 6. Thus, the emitter current increases even if the emitter voltage decreases. 7. This results in a very stable negative resistance characteristic.

Unijunction transistor exhibits a negative resistance Current-voltage characteristics Unijunction transistor exhibits a negative resistance region between the peak voltage, Vp and valley point, Vv.

Relaxation Oscillator

Operation

waveform of the output terminal 1 Charging of C discharging waveform at the terminal 2

A Square-wave Oscillator