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NIJUNCTION TRANSISTOR (UJT)
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Physical structure
NIJUNCTION TRANSISTOR (UJT)
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Equivalent circuit
• The equivalent circuit comprised of two
resistors, one fixed ( R B2) and one variable
( R B1) and a single diode ( D).
• R B1 varies with I E .
NIJUNCTION TRANSISTOR (UJT)
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Equivalent circuit
NIJUNCTION TRANSISTOR (UJT)
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Equivalent circuit
• R BB is the interbase resistance when I E = 0
i.e.
( )021 =
+= E I B B BB
R R R
• Typical range of R BB ! "Ω # $0 "Ω
• The position of the aluminum rod
determine the relative values of R B1 and
R B2.
NIJUNCTION TRANSISTOR (UJT)
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021
1
1
=
=+
= E
B
I
BB BB
B B
B R V V
R R
RV η
NIJUNCTION TRANSISTOR (UJT)
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021
1
=+
= E
I B B
B
R R
Rη
NIJUNCTION TRANSISTOR (UJT)
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%or V E > V RB1 by V D (0.&' → 0.0 ), the
diode will fire and I E will begin to flow
through R B1.
NIJUNCTION TRANSISTOR (UJT)
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The emitter potential V P is given by
D BB P V V V +=η
NIJUNCTION TRANSISTOR (UJT)
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*haracteristics of representative +T
NIJUNCTION TRANSISTOR (UJT)
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Programmable unijunction Transistor (PUT)
Although it has the same name as a UJT the programmable
unijunction transistor’s structure is not the same. It is
actually more similar to an SCR.
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Programmable unijunction Transistor
The PUT can be “programmed” to turn on at a certain voltage
by an eternal voltage divider. This yields a curve similar to a
UJT.
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Programmable unijunction Transistor (PUT)
!ternal "UT resistors R# and R$ replace uni%unction
transistor internal resistors R&# and R&$' respectively. These
resistors allo( the calculation o) the intrinsic stando)) ratio *.
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Programmable unijunction Transistor (PUT)
VR is voltage divider (R1 and R2 can be specified)
Vc capacitor voltage
Wen Vc ! VR te PUT "ill conduct
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UJT RELAXATION OSCILLATORS
&asic UJT relaation oscillator
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UJT RELAXATION OSCILLATORS
Assume that the initial
capacitor voltage' V C
is +ero. ,hen thesupply voltage V BB is
)irst applied' the UJT
is in the - state. I E
is +ero and C charges
eponentially through
R to(ards V .
The operation
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UJT RELAXATION OSCILLATORS
,hen the supply
voltage V C /= V E 0
reaches the )iring
potential' V P ' the UJT
)ires and C discharges
eponentially through
R2 until V E reaches
the valley potential V V .
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UJT RELAXATION OSCILLATORS
,hen V E reaches the valley potential V V the
UJT turns -' I E goes to +ero and the
capacitor is recharged.This process repeats itsel) to produce the
(ave)orms )or vC and v R2 as sho(n belo(1
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UJT RELAXATION OSCILLATORS
The waveform, vC
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UJT RELAXATION OSCILLATORS
The waveform, v R2
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UJT RELAXATION OSCILLATORS
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UJT RELAXATION OSCILLATORS
Cn!itin "r s#itchin$%ON
To s(itch2on a UJT'
the emitter current I E
must be able to reach
the pea3 current I P i.e.
11 R I V P I I R
P E
>=
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UJT RELAXATION OSCILLATORS
Cn!itin "r s#itchin$%ON
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UJT RELAXATION OSCILLATORS
In other (ords' R1 must
be small enough such
that I E is not limited to a
value less than I P (hen
V C = V P .
Cn!itin "r s#itchin$%ON
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UJT RELAXATION OSCILLATORS
Thus' to )ire the UJT1
P P BB V R I V +> 1
1 R I V V
P P BB >−
P
P BB
I
V V R
−<1
Cn!itin "r s#itchin$%ON
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UJT RELAXATION OSCILLATORS
Cn!itin "r s#itchin$%O&&
To s(itch2o)) a UJT'
the emitter current I E
must drop belo( I V
(hen V C = V V .
4ence1
V V BB V R I V +<
1
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UJT RELAXATION OSCILLATORS
Thus' to the UJT1
1 R I V V V V BB <−
V
V BB
I V V R −>1
Cn!itin "r s#itchin$%O&&
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UJT RELAXATION OSCILLATORS
Thus' to ensure the s(itching -5 and -' the)ollo(ing condition must be met1
V
V BB
P
P BB
I V V R
I V V −>>− 1
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UJT RELAXATION OSCILLATORS
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UJT RELAXATION OSCILLATORS
-t can be shown that
−
−=
P BB
V BB
V V
V V C Rt ln
11
and
( )
+=
V
P
B
V
V C R Rt ln
212
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UJT RELAXATION OSCILLATORS
The periodic time
21 t t T +=
-n many cases, t 1 >> t 2, therefore;
−−=≅
P BB
V BB
V V
V V C Rt T ln
11
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UJT RELAXATION OSCILLATORS
/hen V BB and V P are much greater than V V ,then
−≅
P BB
BB
V V
V
C RT ln1
nd if η V BB
>> V pn
i.e. V P
V BB
, then
−
= BB BB
BB
V V
V C RT
η
ln1
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UJT RELAXATION OSCILLATORS
or
−
=η 1
1ln1C RT
The frequency
==
η1
1ln
11
1C R
T f
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