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

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. Relaxation Oscillator

8. Operation

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

13. A Square-wave Oscillator