UNIJUNCTION TRANSISTOR UJT Physical structure One lightlydoped high

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UNIJUNCTION TRANSISTOR (UJT) Physical structure • One lightly-doped (high resistivity) silicon slab. • Two

UNIJUNCTION TRANSISTOR (UJT) Physical structure • One lightly-doped (high resistivity) silicon slab. • Two base contacts (B 1 and B 2) 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 B 2 contact than B 1 contact. • B 2 is made positive with respect to B 1.

UNIJUNCTION TRANSISTOR (UJT) Physical structure

UNIJUNCTION TRANSISTOR (UJT) Physical structure

UNIJUNCTION TRANSISTOR (UJT) Equivalent circuit • The equivalent circuit comprised of two resistors, one

UNIJUNCTION TRANSISTOR (UJT) Equivalent circuit • The equivalent circuit comprised of two resistors, one fixed (RB 2) and one variable (RB 1) and a single diode (D). • RB 1 varies with IE. • Variation of RB 1 : 5 k to 50 for the corresponding variation of 0 A to 50 A in IE.

UNIJUNCTION TRANSISTOR (UJT) Equivalent circuit

UNIJUNCTION TRANSISTOR (UJT) Equivalent circuit

UNIJUNCTION TRANSISTOR (UJT) Equivalent circuit • RBB is the interbase resistance when IE =

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 RB 1 and RB 2.

UNIJUNCTION TRANSISTOR (UJT)

UNIJUNCTION TRANSISTOR (UJT)

UNIJUNCTION TRANSISTOR (UJT)

UNIJUNCTION TRANSISTOR (UJT)

UNIJUNCTION TRANSISTOR (UJT) For VE > VRB 1 by VD (0. 35 0. 70

UNIJUNCTION TRANSISTOR (UJT) For VE > VRB 1 by VD (0. 35 0. 70 V), the diode will fire and IE will begin to flow through RB 1.

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

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

UNIJUNCTION TRANSISTOR (UJT) Characteristics of representative UJT:

UNIJUNCTION TRANSISTOR (UJT) Characteristics of representative UJT:

UNIJUNCTION TRANSISTOR (UJT) The emitter characteristics: For fixed values of and VD, VP varies

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

UJT RELAXATION OSCILLATORS Basic UJT relaxation oscilator

UJT RELAXATION OSCILLATORS Basic UJT relaxation oscilator

UJT RELAXATION OSCILLATORS The operation Assume that the initial capacitor voltage, VC is zero.

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 R 1 towards VBB.

UJT RELAXATION OSCILLATORS When the supply voltage VC (= VE) reaches the firing potential,

UJT RELAXATION OSCILLATORS When the supply voltage VC (= VE) reaches the firing potential, VP, the UJT fires and C discharges exponentially through R 2 until VE reaches the valley potential VV.

UJT RELAXATION OSCILLATORS When VE reaches the valley potential VV the UJT turns OFF,

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 v. C and v. R 2 as shown below;

UJT RELAXATION OSCILLATORS The waveform, v. C

UJT RELAXATION OSCILLATORS The waveform, v. C

UJT RELAXATION OSCILLATORS The waveform, v. R 2

UJT RELAXATION OSCILLATORS The waveform, v. R 2

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS Condition for switching-ON To switch-on a UJT, the emitter current IE

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.

UJT RELAXATION OSCILLATORS Condition for switching-ON

UJT RELAXATION OSCILLATORS Condition for switching-ON

UJT RELAXATION OSCILLATORS Condition for switching-ON In other words, R 1 must be small

UJT RELAXATION OSCILLATORS Condition for switching-ON In other words, R 1 must be small enough such that IE is not limited to a value less than IP when VC = V P.

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

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

UJT RELAXATION OSCILLATORS Condition for switching-OFF To switch-off a UJT, the emitter current IE

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

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

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

UJT RELAXATION OSCILLATORS Thus, to ensure the switching ON and OFF, the following condition

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

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS It can be shown that; and;

UJT RELAXATION OSCILLATORS It can be shown that; and;

UJT RELAXATION OSCILLATORS The periodic time; In many cases, t 1 >> t 2,

UJT RELAXATION OSCILLATORS The periodic time; In many cases, t 1 >> t 2, therefore;

UJT RELAXATION OSCILLATORS When VBB and VP are much greater than VV, then; And

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

UJT RELAXATION OSCILLATORS or; The frequency;

UJT RELAXATION OSCILLATORS or; The frequency;

UJT RELAXATION OSCILLATORS Example For the UJT relaxation oscillator in the following figure, it

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

UJT RELAXATION OSCILLATORS Example (cont’d)

UJT RELAXATION OSCILLATORS Example (cont’d)

UJT RELAXATION OSCILLATORS Example (cont’d) a) Determine; i. The value of VP to switch-on

UJT RELAXATION OSCILLATORS Example (cont’d) a) Determine; i. The value of VP to switch-on the UJT; ii. The range of R 1 to switch-on and switchoff the UJT; iii. Frequency of oscillation if RB 1 = 100 during discharge phase of the capacitor C; 1. b) Sketch the wave shape of VC and VR 2.

UJT RELAXATION OSCILLATORS Example – SOLUTION a) Substituting values;

UJT RELAXATION OSCILLATORS Example – SOLUTION a) Substituting values;

UJT RELAXATION OSCILLATORS Example – SOLUTION (cont’d) a) The value VP to switch-on the

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

UJT RELAXATION OSCILLATORS Example – SOLUTION (cont’d) d) While V = V , the

UJT RELAXATION OSCILLATORS Example – SOLUTION (cont’d) d) While V = V , the UJT is active. C P

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

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

UJT RELAXATION OSCILLATORS Example – SOLUTION (cont’d)

UJT RELAXATION OSCILLATORS Example – SOLUTION (cont’d)