Ujt relaxation oscillators

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.

Equivalent circuit

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 relative values of RB1 and
RB2.

021
1
1




E
B
I
BBBB
BB
B
R VV
RR
R
V 

021
1



EIBB
B
RR
R


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

The emitter potential VP is given by:
DBBP VVV 

Characteristics of representative UJT:

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.

Programmable unijunction Transistor
The PUT can be “programmed” to turn on at a certain voltage
by an external voltage divider. This yields a curve similar to a
UJT.

External PUT resistors R1 and R2 replace unijunction
transistor internal resistors RB1 and RB2, respectively. These
resistors allow the calculation of the intrinsic standoff ratio η.

VR is voltage divider (R1 and R2 can be specified)
Vc capacitor voltage
When Vc > VR the PUT will conduct

UJT RELAXATION OSCILLATORS
Basic UJT relaxation oscillator

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

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.

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;

The waveform, vC

The waveform, vR2

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 PIIR
PE


In other words, R1 must
be small enough such
that IE is not limited to a
value less than IP when
VC = VP.

Thus, to fire the UJT;
PPBB VRIV  1
1RIVV PPBB 
P
PBB
I
VV
R

1

Condition for switching-OFF
To switch-off a UJT,
the emitter current IE
must drop below IV
when VC = VV.
Hence;
VVBB VRIV  1

Thus, to the UJT;
1RIVV VVBB 
V
VBB
I
VV
R

1
Condition for switching-OFF

Thus, to ensure the switching ON and OFF, the
following condition must be met;
V
VBB
P
PBB
I
VV
R
I
VV 


1

It can be shown that;









PBB
VBB
VV
VV
CRt ln11
and;
  






V
P
B
V
V
CRRt ln212

The periodic time;
21 ttT 
In many cases, t1 >> t2, therefore;









PBB
VBB
VV
VV
CRtT ln11

When VBB and VP are much greater than VV,
then;








PBB
BB
VV
V
CRT ln1
And if VBB >> Vpn i.e. VP  VBB, then








BBBB
BB
VV
V
CRT

ln1

or;








1
1
ln1CRT
The frequency;








1
1
ln
11
1CR
T
f

Ujt relaxation oscillators

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