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)