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.

Physical structure

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.

UNIJUNCTION TRANSISTOR (UJT)

Equivalent circuit

UNIJUNCTION TRANSISTOR (UJT)

Equivalent circuit

• RBB is the interbase resistance when IE = 0 i.e.

021

EIBBBB RRR

• Typical range of RBB : 4 k - 10 k

• The position of the aluminum rod determine the ralative values of RB1 and

RB2.

UNIJUNCTION TRANSISTOR (UJT)

021

11

E

B

I

BBBBBB

BR VV

RR

RV

UNIJUNCTION TRANSISTOR (UJT)

021

1

EIBB

B

RR

R

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.

UNIJUNCTION TRANSISTOR (UJT)

The emitter firing potential VP is given by:

DBBP VVV

UNIJUNCTION TRANSISTOR (UJT)

Characteristics of representative UJT:

UNIJUNCTION TRANSISTOR (UJT)

The emitter characteristics:

For fixed values of and VD, VP varies with

VBB.

UNIJUNCTION TRANSISTOR (UJT)

UJT RELAXATION OSCILLATORS

Basic UJT relaxation oscilator

UJT RELAXATION OSCILLATORS

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.

The operation

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.

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;

UJT RELAXATION OSCILLATORS

The waveform, vC

UJT RELAXATION OSCILLATORS

The waveform, vR2

UJT RELAXATION OSCILLATORS

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.

11 RIV PIIRPE

UJT RELAXATION OSCILLATORS

Condition for switching-ON

UJT RELAXATION OSCILLATORS

In other words, R1 must

be small enough such that IE is not limited to a

value less than IP when

VC = VP.

Condition for switching-ON

UJT RELAXATION OSCILLATORS

Thus, to fire the UJT;

PPBB VRIV 1

1RIVV PPBB

P

PBB

I

VVR

1

Condition for switching-ON

UJT RELAXATION OSCILLATORS

Condition for switching-OFF

To switch-off a UJT, the emitter current IE

must drop below IV

when VC = VV.

Hence;

VVBB VRIV 1

UJT RELAXATION OSCILLATORS

Thus, to fire the UJT;

1RIVV VVBB

V

VBB

I

VVR

1

Condition for switching-OFF

UJT RELAXATION OSCILLATORS

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

V

VBB

P

PBB

I

VVR

I

VV

1

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS

UJT RELAXATION OSCILLATORS

It can be shown that;

PBB

VBB

VV

VVCRt ln11

and;

V

PB V

VCRRt ln212

UJT RELAXATION OSCILLATORS

The periodic time;

21 ttT

In many cases, t1 >> t2, therefore;

PBB

VBB

VV

VVCRtT ln11

UJT RELAXATION OSCILLATORS

When VBB and VP are much greater than VV, then;

PBB

BB

VV

VCRT ln1

And if VBB >> Vpn i.e. VP VBB, then

BBBB

BB

VV

VCRT

ln1

UJT RELAXATION OSCILLATORS

or;

1

1ln1CRT

The frequency;

11

ln

11

1CRT

f

UJT RELAXATION OSCILLATORS

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

phase

discharge during 100

andμA 10 mA; 10

V; 1 ;6.0 ;k 5

1

B

PV

VBB

R

II

VR

Example

UJT RELAXATION OSCILLATORS

Example (cont’d)

UJT RELAXATION OSCILLATORS

Example (cont’d)

a) Determine;

i. The value of VP to switch-on the UJT;

ii. The range of R1 to switch-on and switch-off the UJT;

iii.Frequency of oscillation if RB1 = 100 during discharge phase of the capacitor C;

b) Sketch the wave shape of VC and VR2.

UJT RELAXATION OSCILLATORS

Example – SOLUTION

21

11

BB

B

BB

B

RR

R

R

R

a)

Substituting values;

k 56.0 1BR k 31BR

UJT RELAXATION OSCILLATORS

k 2k 3k 512 BBBB RRR

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;

BBBB

BpnP V

RRR

RRVV

221

21

Example – SOLUTION (cont’d)

UJT RELAXATION OSCILLATORS

a)

Substituting values;

V 12k 2k 1.0k 3

k 37.0 2

R

VP

V 8PV

Example – SOLUTION (cont’d)

UJT RELAXATION OSCILLATORS

b)

V

VBB

P

PBB

I

VVR

I

VV1

Example – SOLUTION (cont’d)

Substituting values;

m 10

112

μ 10

8121R

UJT RELAXATION OSCILLATORS

b)Example – SOLUTION (cont’d)

k 1.1k 400 1R

c)

PBB

VBB

VV

VVCRt ln11

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

c)

ms 05.5812

112lnμ 1.0k 501

t

Substituting values;

V

PB V

VCRRt ln211

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

c)

Substituting values;

μs 6.41

1

8lnμ 1.0k 1.0k 1.01

t

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

c)

ms 09.5

μ 6.41m 05.511

ttT

Hz 5.196m 09.5

11

Tf

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

d)

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

d) While C is charging, the UJT is inactive.

212

22

BBR RRR

RV

12k3k3k1.0

k1.0

V 235.0

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

d) While VC = VP, the UJT is active.

pnPB

R

VVRR

R

V

12

2

2

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)

d) Substituting values;

V 65.3

7.08k1.0k1.0

k1.0

UJT RELAXATION OSCILLATORS

Example – SOLUTION (cont’d)