The characteristic of UJT was discussed in previous lecture. It is having negative resistance region. The negative dynamic resistance region of UJT can be used to realize an oscillator.

The circuit of UJT relaxation oscillator is shown in **fig. 1**. It includes two resistors R_{1} and R_{2} for taking two outputs R_{2} may be a few hundred ohms and R_{1} should be less than 50 ohms. The dc source V_{CC} supplies the necessary bias. The interbase voltage V_{BB} is the difference between V_{CC} and the voltage drops across R_{1} and R_{2}. Usually R_{BB} is much larger than R_{1} and R_{2} so that V_{BB} approximately equal to V. Note, R_{B1} and R_{B2} are inter-resistance of UJT while R_{1} and R_{2} is the actual resistor. R_{B1} is in series with R_{1} and R_{B2} is in series with R_{2} .

Fig. 1

As soon as power is applied to the circuit capacitor begins to charge toward V. The voltage across C, which is also V_{E} , rises exponentially with a time constant

t = R C

As long as V_{E} < V_{P}, I_{E} = 0. the diode remains reverse biased as long as V_{E} < V_{P} . When the capacitor charges up to V_{P} , the diode conducts and R_{B1} decreases and capacitor starts discharging. The reduction in R _{B1} causes capacitor C voltage to drop very quickly to the valley voltage V_{V} because of the fast time constant due to the low value of R_{B1} and R_{1}. As soon as V_{E} drops below V_{a} + V_{D} the diode is no longer forward biased and it stops conduction. It now reverts to the previous state and C begins to charge once more toward V_{CC} .

The emitter voltage is shown in **fig. 2**, V_{E} rises exponentially toward V_{CC} but drops to a very low value after it reaches V_{P}. The time for the V_{E} to drop from V_{P} to V_{V} is relatively small and usually neglected. The period T can therefore be approximated as follows:

**Fig. 2**

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