Uni-junction transistor

UJT Relaxation Oscillator:

The characteristic of UJT was discussed in earlier. It is has negative resistance region. The negative dynamic resistance region of UJT can be taken in use to realize an oscillator.

The circuit of UJT relaxation oscillator is shown in the figure 1. It includes 2 resistors R₁ and R₂ for taking 2 outputs R2 may be some hundred ohms and R1 should be under 50 ohms. The DC source V_CC supplies the essential bias. The interbase voltage V_BB is difference between V_CC and voltage drops across R₁ and R2. Generally R_BB is much larger than R₁ and R₂ so that V_BB roughly equal to V. Note, R_B1 and R_B2 are inter-resistance of UJT while R1 and R2  is actual resistor. R_B1 is in series with R₁ and R_B2 is in series with R₂ .

Figure 1

As the power is applied to circuit capacitor begins to charge toward V. The voltage across C, which is V_E , rises exponentially with the time constant

As V_E  < V_P, I_E  = 0 diode remains reverse biased as long as V_E  < V_P . When capacitor  charges  up  to  V_P ,  the  diode  conducts  and  R_B1   decreases  and  capacitor  begins to discharge. The reduction in R _B1 causes capacitor C voltage to drop quickly to the valley voltage VV due to the fast time constant because of the low value of R_B1 and R₁. As V_E drops below V_a  + V_D  the diode is no longer forward biased and it thenstops conduction. It now reverts to earlier state and C begins to charge once again toward V_CC

The emitter voltage is illustrated in figure2, V_E rises exponentially toward V_CC but drops to a low value after it reaches VP. The time for V_E  to drop from V_P  to V_V  is small relatively and usually neglected. The period T can thus be approximated as shown below:

Figure 2

Uni-junction transistor

There are 2 additional outputs possible for UJT oscillation one of these is voltage developed at B₁ because of capacitor discharge while the other is voltage developed at B₂ as shown in the figure 3.

When UJT fires Va  drops, causing the corresponding voltage drop at B2. The duration of outputs at B₁ and B₂ can be determined by C discharge time.

If R₁  is small, C discharges quickly and narrow pulse is produced at the output. If R₁ = 0, apparently no pulses appear at B1.

If R₂ = 0, then no pulse can be generated at B₂. If R1  is too large, its positive resistance can swamp negative resistance  and  prevent  UJT  from switching  back when it has fired.

R₂, in addition to giving a source of pulse at B₂, is useful for stabilization of temperature of the UJT's peak point voltage.

Figure 3

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