April 1970 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The unijunction transistor (UJT) was originally known as a 'double-base
diode' and explains why to this day the terminals are labeled
'E,' "B1,' and B2." It is commonly modeled as a diode connected
between two resistors, with one resistor being variable. As
the name implies, unlike a bipolar junction transistor (BJT)
that is more familiar to most people and has two semiconductor
junctions connected to the base, a UJT has just one junction.
As is explained in detail in the article, the net effect of
the UJT's arrangement is a region of negative resistance which
makes it good for use as an oscillator. In fact, the relaxation
oscillator was one of the most popular uses of the UJT.
Getting to Know the UJT
An all-round signal generator: saw-tooth, square-wave, sine-wave.
By Frank H. Tooker
Almost everyone who has an interest in electronics is aware
of the existence of a device called the unijunction transistor,
or UJT. (If he remembers it from its very beginning, he might
recall that it was originally referred to as a "double-base
diode.") The UJT is used most often in circuits requiring a
positive-going spike pulse and, occasionally, as a generator
of sawtooth waveforms.
However, the UJT is actually much more versatile than these
two uses would imply. It can also be used to generate square
waves and, believe it or not, sine waves having quite pure waveforms.
It behooves the serious electronics experimenter to learn more
about all of these uses - and to do so, he will need to know
more about the UJT itself.
How the UJT works. The UJT can be represented by a circuit
approximation consisting of two resistances in series, with
a diode connected at their junction as in Fig. 1. (Also shown
in the figure is the accepted schematic symbol and base diagram
for the UJT. Note that in both cases the leads are identified
as E, B1, and B2 for emitter, base-1, and base-2.) The resistance
approximation is a passive representation of the UJT. In simple
terms, this means that a pair of resistors and a diode connected
as shown will not operate as a unijunction transistor. The approximation
is simply a means by which operation of the UJT can be explained.
In the majority of applications, the emitter is the control
electrode of the UJT. The magnitude and polarity of a potential
applied to the emitter determine whether or not the UJT will
fire. With the emitter circuit open (diode nonconducting), resistance
RB1 is maximum, and the sum of RB1 and RB2) called interbase
resistance, is between 5000 and 10,000 ohms for the 2N2646 and
2N2647 (two typical, useful UJT's).
Resistance RB1 is shown variable because current flow in
the emitter circuit causes a decrease in the ohmic value of
this resistance. The greater the current flow, the lower the
resistance. Hence, a UJT exhibits negative resistance, a characteristic
that can be thought of as amplification. What actually happens
inside the UJT is that current flowing into the E-to-B1 circuit
"pulls" current carriers from the B2 area, increasing the circuit's
The supply voltage, usually applied through a series resistor,
is connected across the interbase resistance, B1 to B2, with
B2 positive with respect to B1. To fire the UJT, a positive
potential (called the peak-point voltage) is applied to the
The ratio of RB1 to the interbase resistance is called η
(Greek eta), or the intrinsic standoff ratio. The peak-point
voltage is this ratio times the supply voltage plus the potential
hill of the diode (about 0.5 volt). Thus, the voltage required
for firing the UJT varies as the supply voltage is varied, and
in the same direction.
UJT relaxation oscillators. The schematic diagram in Fig. 2,
or some variation of it, is probably familiar to most experimenters.
It is the one most commonly used circuits for relaxation oscillators
by circuit designers.
Referring to the diagram, capacitor Ct charges up through
resistor Rt at a rate determined by the RC time constant of
these two components. The larger these values, the slower the
charging rate. During the charging interval, the emitter junction
is reverse biased, and the only current flowing in the emitter
circuit is due to leakage (similar to the Ico of
a bipolar transistor). Emitter leakage for the 2N2646 is a maximum
of 2 µA, and for the 2N2647, only 0.2 µA.
When the potential across Ct reaches the value of peak-point
voltage for the particular UJT being used in the circuit, the
emitter junction goes suddenly into conduction. Using the UJT
approximation shown in Fig. 1, RBl promptly drops to a much
lower value and Ct in Fig. 2 discharges abruptly through load
resistor RL, producing a spike pulse of voltage across the output
Capacitor Ct does not discharge to zero potential. Rather,
it is discharged to a value determined by the series resistance
between the emitter and ground and the magnitude of the discharge
current. The actual value to which Ct discharges is termed the
"valley voltage." When Ct discharges to this value, the emitter
junction of the UJT becomes reverse biased again; then Ct begins
to recharge, and the cycle repeats. The charge-discharge action
of Ct produces a sawtooth waveform signal.
When it doesn't work. The operation of a UJT relaxation oscillator
involves more than just raising the potential across timing
capacitor at to the firing level. A certain value of current,
called the "peak-point emitter current," is required to fire
the unijunction transistor. This current must be supplied through
timing resistor Rt (see Fig. 2). If the current through Rt is
too low, capacitor Ct will charge to a value that is below the
peak-point voltage, and operation will cease. The UJT will not
fire. This need for sufficient emitter current becomes important
when Rt must have a large value to operate the UJT at a very
low repetition rate.
The peak-point emitter current for the 2N2646 is about 5
µA; for the 2N2647 it is only 2 µA. It is important to bear
in mind that even though the 2N2647 is 2.5 times better than
the 2N2646, if an electrolytic capacitor is used in the circuit,
the leakage current of the capacitor has the same effect as
an identical increase in the peak-point emitter current of the
UJT. Consequently, care must be exercised in choosing a capacitor
with the lowest leakage or the value of the 2N2647 might be
Characteristics vary from one UJT to another, even for those
with the same type number. Thus, if the relaxation oscillator
is to have a definite repetition rate, the value of timing resistor
Rt should be made adjustable to allow you to "trim" the circuit
to the desired frequency or repetition rate.
The circuit shown in Fig. 3 has trimming facilities. Varying
the resistance of either Ra or Rt2 varies the interbase voltage,
thereby altering the peak-point voltage and, thus, the repetition
rate. The value of potentiometer Ra in such a circuit should
be limited to a maximum of 5000 ohms.
Negative-pulse generator. Pulses obtained at the B1 terminal
of a UJT are positive-going. Negative-going pulses can be obtained
from the B2 terminal when a resistor is connected between B2
and ground. Negative pulses can also be obtained from a resistor
connected in series with the lower end of the timing capacitor.
The circuit shown in Fig. 4 provides a choice of either positive
or negative pulses, depending on the setting of S1. Adjusting
the setting of level control potentiometer R2 adds resistance
to one of the two circuits, while it subtracts an equal amount
of resistance from the other circuit. So, when R2 is set for
maximum amplitude of a negative output pulse, resistance in
the positive side of the circuit is zero, and vice versa. This
gives the circuit maximum efficiency, providing maximum pulse
amplitude in either direction.
The repetition rate of the circuit with the component values
shown is about one pulse in every two seconds. This rate was
selected to provide a useful instrument for checking experimental
hookups of JK flip-flops, SCR's, SCS's, and other pulse-operated
Square-wave generator. The circuit of a square-wave generator
(actually a dual-UJT multivibrator) is shown in Fig. 5. This
circuit generates excellent square waves within the frequency
range of efficient operation of the unijunction transistors.
When the power is applied to the dual-UJT circuit, both emitters
are made positive with respect to ground through resistors Rt1
and Rt2. One UJT fires promptly, bringing both ends of the timing
capacitor, Ct, to a value well below the peak-point voltage.
This UJT remains conducting while Ct charges through the timing
resistor of the other UJT circuit.
As soon as the second UJT's emitter becomes sufficiently
positive with respect to ground, it suddenly conducts, driving
the first UJT negative and causing it to stop conducting. With
the second UJT conducting and the first cut off, Ct starts charging
in the opposite direction, through the timing resistor of the
first UJT. Now, when the emitter of the first UJT becomes sufficiently
positive, it fires, and the second UJT cuts off. The alternate-
stage fire/cutoff cycle is self repeating whenever power is
applied to the circuit, and the output of the system is a train
of rectangular pulses.
Sine-wave generator. Sine waves are produced by allowing
a UJT circuit to charge and discharge a capacitor through an
inductance. When the charge-discharge period is equal to the
resonant frequency of the LC circuit, sine waves are generated
across the capacitor. A schematic diagram of a UJT sine-wave
generator is shown in Fig. 6.
The tuned circuit of the generator is made up of inductor
L and capacitor C. Field effect transistor Q2 operates as a
source follower to prevent loading down the tuned circuit; this
stage is not otherwise essential to the operation of the oscillator
as a sine-wave generator. The circuit shown has operated well
up to 50,000 Hz.
The output of the sine-wave generator is obtained across
Q1's source resistor. The waveform here is cleanest when the
ratio of inductance to capacitance is high.
Modulate a relaxation oscillator. A UJT relaxation oscillator
can be frequency modulated by applying the modulating signal
across a resistor in the B2 circuit as shown in Fig. 7. The
waveform of the modulating signal can be sine, sawtooth, square,
triangular, or irregular.
A practical example of a modulated UJT oscillator is shown
in Fig. 8. This circuit is known as a "bell-tone" oscillator.
In operation, Q2 and its associated components make up a relaxation
oscillator which, when unmodulated, has an operating frequency
of about 700 Hz. Unijunction transistor Q1 and its associated
components make up a low-frequency astable multivibrator. The
wave-form of the Q1 setup is not as good as that of the circuit
in Fig. 6, but it serves the purposes of the bell-tone oscillator
In the multivibrator, C1 charges through R2, while diode
D1 is maintained in a forward conducting state by the charging
current and the current through R1. When Q1 fires, reverse bias
is applied to D1, and the diode appears as an open circuit.
Transistor Q1 remains conducting while capacitor C1 discharges
through resistor R1. At the end of this interval, D1 begins
conducting again, Q1 cuts off, and the cycle repeats. The result
is a rectangular signal across R3.
Since the B2's of Q1 and Q2 are tied together, each time
Q1 fires, its B2 signal decreases the interbase voltage of Q2
and causes an increase in Q2's operating frequency. As Q2 conducts
and cuts off, the pitch of the sound heard from the loudspeaker
rises and falls sharply, giving the sound a distinct bell-like
quality. The speaker is preferably a small one, such as a 3"
replacement type, to provide "tinny" reproduction. When working
with the circuit, adjust potentiometer R3 to obtain the most
Posted May 31, 2013