1. UNIJUNCTION TRANSISTOR (UJT)
Physical structure
One lightly-doped (high resistivity) silicon slab.
Two base contacts (B1 and B2) at both end of the slab.
The p-n junction is form by alloying an aluminum rod to the slab.
The aluminum rod is closer to B2 contact than B1 contact.
B2 is made positive with respect to B1.

2. UNIJUNCTION TRANSISTOR (UJT)
Physical structure

3. UNIJUNCTION TRANSISTOR (UJT)
Equivalent circuit
The equivalent circuit comprised of two resistors, one fixed (RB2) and one variable (RB1) and a single diode (D).
RB1 varies with IE.
Variation of RB1 : 5 k to 50  for the corresponding variation of 0 A to 50 A in IE.

4. UNIJUNCTION TRANSISTOR (UJT)
Equivalent circuit

5. UNIJUNCTION TRANSISTOR (UJT)
Equivalent circuit
RBB is the interbase resistance when IE = 0 i.e.
Typical range of RBB : 4 k - 10 k
The position of the aluminum rod determine the ralative values of RB1 and RB2.

6. UNIJUNCTION TRANSISTOR (UJT)

7. UNIJUNCTION TRANSISTOR (UJT)

8. UNIJUNCTION TRANSISTOR (UJT)
For VE > VRB1 by VD (0.35  0.70 V), the diode will fire and IE will begin to flow through RB1.

9. UNIJUNCTION TRANSISTOR (UJT)
The emitter firing potential VP is given by:

10. UNIJUNCTION TRANSISTOR (UJT)
Characteristics of representative UJT:

11. UNIJUNCTION TRANSISTOR (UJT)
The emitter characteristics:
For fixed values of  and VD, VP varies with VBB.

12. UJT RELAXATION OSCILLATORS
Basic UJT relaxation oscilator

13. UJT RELAXATION OSCILLATORS
The operation
Assume that the initial capacitor voltage, VC is zero. When the supply voltage VBB is first applied, the UJT is in the OFF state. IE is zero and C charges exponentially through R1 towards VBB.

14. UJT RELAXATION OSCILLATORS
When the supply voltage VC (= VE) reaches the firing potential, VP, the UJT fires and C discharges exponentially through R2 until VE reaches the valley potential VV.

15. UJT RELAXATION OSCILLATORS
When VE reaches the valley potential VV the UJT turns OFF, IE goes to zero and the capacitor is recharged.
This process repeats itself to produce the waveforms for vC and vR2 as shown below;

16. UJT RELAXATION OSCILLATORS
The waveform, vC

17. UJT RELAXATION OSCILLATORS
The waveform, vR2

18. UJT RELAXATION OSCILLATORS

19. UJT RELAXATION OSCILLATORS
Condition for switching-ON
To switch-on a UJT, the emitter current IE must be able to reach the peak current IP i.e.

20. UJT RELAXATION OSCILLATORS
Condition for switching-ON

21. UJT RELAXATION OSCILLATORS
Condition for switching-ON
In other words, R1 must be small enough such that IE is not limited to a value less than IP when VC = VP.

22. UJT RELAXATION OSCILLATORS
Condition for switching-ON
Thus, to fire the UJT;

23. UJT RELAXATION OSCILLATORS
Condition for switching-OFF
To switch-off a UJT, the emitter current IE must drop below IV when VC = VV. Hence;

24. UJT RELAXATION OSCILLATORS
Condition for switching-OFF
Thus, to fire the UJT;

25. UJT RELAXATION OSCILLATORS
Thus, to ensure the switching ON and OFF, the following condition must be met;

26. UJT RELAXATION OSCILLATORS

27. UJT RELAXATION OSCILLATORS

28. UJT RELAXATION OSCILLATORS
It can be shown that;
and;

29. UJT RELAXATION OSCILLATORS
The periodic time;
In many cases, t1 >> t2, therefore;

30. UJT RELAXATION OSCILLATORS
When VBB and VP are much greater than VV, then;
And if VBB >> Vpn i.e. VP  VBB, then

31. UJT RELAXATION OSCILLATORS
The frequency;

32. UJT RELAXATION OSCILLATORS
Example
For the UJT relaxation oscillator in the following figure, it is known that;

33. UJT RELAXATION OSCILLATORS
Example (cont’d)

34. UJT RELAXATION OSCILLATORS
Example (cont’d)
Determine;
The value of VP to switch-on the UJT;
The range of R1 to switch-on and switch-off the UJT;
Frequency of oscillation if RB1 = 100  during discharge phase of the capacitor C;
b) Sketch the wave shape of VC and VR2.

35. UJT RELAXATION OSCILLATORS
Example – SOLUTION a)
Substituting values;

36. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) a)
The value VP to switch-on the UJT when vC = VP which corresponds to IE = IP = 10 A  0 A may be calculated as follows;

37. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) a)
Substituting values;

38. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) b)
Substituting values;

39. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) b) c)

40. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) c)
Substituting values;

41. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) c)
Substituting values;

42. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) c)

43. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) d)

44. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) d)
While C is charging, the UJT is inactive.

45. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) d)
While VC = VP, the UJT is active.

46. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d) d)
Substituting values;

47. UJT RELAXATION OSCILLATORS
Example – SOLUTION (cont’d)