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 R1 and R2 for taking two outputs R2 may be a few hundred ohms and R1 should be less than 50 ohms. The dc source VCC supplies the necessary bias. The interbase voltage VBB is the difference between VCC and the voltage drops across R1 and R2. Usually RBB is much larger than R1 and R2 so that VBBapproximately equal to V. Note, RB1 and RB2 are inter-resistance of UJT while R1 and R2 is the actual resistor. RB1 is in series with R1 and RB2 is in series with R2 .
Fig. 1
As soon as power is applied to the circuit capacitor begins to charge toward V. The voltage across C, which is also VE , rises exponentially with a time constant
t = R C
As long as VE < VP, IE = 0. the diode remains reverse biased as long as VE < VP . When the capacitor charges up to VP , the diode conducts and RB1 decreases and capacitor starts discharging. The reduction in R B1 causes capacitor C voltage to drop very quickly to the valley voltage VV because of the fast time constant due to the low value of RB1 and R1. As soon as VE drops below Va + VD 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 VCC .
The emitter voltage is shown in fig. 2, VE rises exponentially toward VCC but drops to a very low value after it reaches VP. The time for the VE to drop from VP to VV is relatively small and usually neglected. The period T can therefore be approximated as follows:
Fig. 2
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