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To study and plot the Characteristics of UJT.


Aim: To study and plot the Characteristics of UJT.

Apparatus:

Connecting wires, 2 voltmeters, 1 ammeter,2 power sources, 1 UJT, 2 resistors etc.


Theory:

The UJT- junction is a 3 - terminal solid-state device (emitter and the two bases)
The device has only one PN junction and hence it is known as UNI-JUNCTION transistor. The PN emitter to the base junction is shown as diode D1. The inter-base resistance RBB of the N-type Si bar appears as two resistors RB1 and RB2


Circuit Diagram:


Referring to equivalent circuit:
  1. When no voltage is applied between B1 and B2 with emitter open, the inter-base resistance is given by RBB = RB1 + RB2.
  2. When a voltage VBB is applied between B1 and B2 with emitter open, the voltage will divide up across RB1 and RB2.


VRB1 = VBB where  = the intrinsic stand-off ration =  
The VBB  across RB1 reverse biased diode thereby dropping the emitter current to zero.
  1. When supply is connected at the emitter, the diode is forward biased making the input voltage to exceed by VD
%u200BVp = VBB + Vb
Since the diode is conducting, the resistance between emitter and base (B1) reduces and hence the internal drop from emitter to B1 decreases.
The emitter conductivity characteristics are such that as IE increases the emitter to base (B1) voltage decreases. At a peak point, Vp and the valley point Vv, the slope of the emitter characteristics is 0.
 At points to the left of VB to E-B1 is forward biased and IE exists. Between Vp and Vv increase in IE is accompanied by a relation in emitter voltage VE
This is the negative resistance region of UJT. Beyond the valley point, Vv an increase in IE is accomplished by an increase in VE. This region is known as the saturation region.
Procedure:
  1. Connect the circuit as shown in the circuit diagram.
  2. Set output voltage VBB1 = 5V by varying VBB.
  3. Varying VEE gradually, note down both emitter current IE and emitter voltage (VE).
  4. Step size is not fixed because of non-linear curve. Initially Vary VEE in steps of 1V. Current IE remains zero. As voltage is varied further, current starts increasing while voltage VE drops. Note down the readings VE and IE.
  5. Repeat above procedure (step 3) for VBB1 = 10V.
Observation table:
At constant voltage VBB=7,5,9v


Precautions:
  1. While performing the experiment do not exceed the ratings of the UJT. This may lead to damage of the UJT.
  2. Connect voltmeter and ammeter in correct polarities as shown in the circuit diagram.
  3. Do not switch ON the power supply unless you have checked the circuit connections as per the circuit diagram.
  4. Make sure while selecting the emitter, base-1, base-2 terminals of UJT.
Result:
 I-V characteristics of UJT is studied.

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