COMPONENTS AND EQUIPMENTS REQUIRED
6. Power supply
EQUIVALENT CIRCUIT OF UJT
UJT is the Uni Junction Transistor. It is a three terminal device. They are: a) emitter b) base1 c)base2.The equalent circuit is shown with the circuit diagram. So there are two resistors. One is a variable resistor and other is a fixed resistor. The ratio of internal resistances is referred as intrinsic standoff ratio (η).It is defined as the ratio of the variable resistance to the total resistance. Due to the existing pn junction, there will be a voltage drop. If we apply a voltage to the emitter, the device will not turn on until the input voltage is less than the drop across the diode plus the drop at the variable resistance R1.When the device is turned on holes moves from emitter to base resulting in a current flow. Due to this sudden increase in charge concentration in base1 region conductivity increases. This causes a drop at base1.This region in the graph is known as negative resistance region. If we further increase the emitter voltage the device undergoes saturation. So a UJT has 3 operating regions:
- cut off region
- negative resistance region
- saturation region
1. Set up the circuit as shown in the circuit diagram
2. Give the power supply.
3. By varying the input voltage, take the values of voltage and current values
4. Plot the graph
5. Find the in intrinsic standoff ratio from graph.
For VBB=12 V For VBB=6 V
VP= Vd + ηvbb
Find the value of Vp from graph.
η = Vp – Vd/VBB
Similarly find the value of intrinsic stand off ratio for all values of VBB.
Characteristics of UJT were plotted and intrinsic stand off ratio was found.
η = ………………..
SAMPLE VIVA QUESTIONS
1. Name a UJT
2. Define intrinsic standoff ratio-See theory
3. Mention any application of UJT
In triggering circuits