AP 3302 Pt. 3
Section 2
CHAPTER 9
Ringing and Blocking Oscillators
Unijunction Transistor as Pulse Generator
The unijunction (single junction) transistor consists of a bar of n-type semiconductor with two non-rectifying contacts (base 1 and base 2), one at each end, as shown in Fig 17a. A third connection (rectifying this time) is made to the other side of the bar near base 1. If bases 1 and 2 are connected together externally, the device behaves as a normal p-n junction diode. However under normal conditions a voltage VB is applied between base 1 and base 2, such that base 2 is biased positively with respect to base 1. A very small inter-base current therefore flows. With no emitter current flowing, the bar behaves as a potential divider with a fraction X of VB appearing at the emitter junction. This voltage effectively reverse-biases the emitter junction and little current flows in the device. However if we apply a positive-going voltage to the emitter greater than XVB the emitter junction is now forward-biased and a large emitter-base 1 current flows.
The unijunction transistor may therefore be used as a pulse generator by connecting it in the circuit shown in Fig 17b. The action is simple: the capacitor C1 charges through R1 towards the supply voltage level +VB. When the capacitor voltage reaches a value greater than +XVB the unijunction 'fires' and C1 quickly discharges into the emitter circuit. The voltage developed across R2 by this discharge current provides the output pulse. When the emitter voltage has fallen to a low value the emitter junction again becomes reversed-biased and the unijunction switches off. The process is then repeated at a p.r.f. determined by the charging time constant C1R1 seconds.
