September 19, 2009

Unijunction Transistor

The basic structure of a unijunction transistor (UJT) is shown in Fig.1. It is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. Contacts are then made to the device as shown; these are referred to as the emitter, base 1 and base 2 respectively. Fig.2 shows the schematic symbol used to denote a UJT in circuit diagrams. For ease of manufacture alternative methods of making contact with the bar have been developed, giving rise to the two types of structure - bar and cube.

The equivalent circuit shown in Fig.4 has been developed to explain how the device works, and it is necessary to define the terms used in this explanation.

RBB is known as the interbase resistance, and is the sum of RB1 and RB2:

RBB = RB1 + RB2

N.B. This is only true when the emitter is open circuit.

VRB1 is the voltage developed across RB1; this is given by the voltage divider rule:

VRB1 =
RB1 + RB2
Since the denominator of equation 2 is equal to equation 1, the former can be rewritten as:
VRB1 = x VBB
The ratio RB1 / RBB is referred to as the intrinsic standoff ratio and is denoted by Eta (the Greek letter eta).

If an external voltage Ve is connected to the emitter, the equivalent circuit can be redrawn as shown in Fig..

If Ve is less than VRB1, the diode is reverse biased and the circuit behaves as though the emitter was open circuit. If however Ve is increased so that it exceeds VRB1 by at least 0.7V, the diode becomes forward biased and emitter current Ie flows into the base 1 region. Because of this, the value of RB1 decreases. It has been suggested that this is due to the presence of additional charge carriers (holes) in the bar. Further increase in Ve causes the emitter current to increase which in turn reduces RB1 and this causes a further increase in current. This runaway effect is termed regeneration. The value of emitter voltage at which this occurs is known as the peak voltage VP and is given by: VP = Eta AVVBB + VD

The characteristics of the UJT are illustrated by the graph of emitter voltage against emitter current.

As the emitter voltage is increased, the current is very small - just a few microamps. When the peak point is reached, the current rises rapidly, until at the valley point the device runs into saturation. At this point RB1 is at its lowest value, which is known as the saturation resistance.

The simplest application of a UJT is as a relaxation oscillator, which is defined as one in which a capacitor is charged gradually and then discharged rapidly. The basic circuit is shown in Fig.7; in the practical circuit of Fig.8 R3 limits the emitter current and provides a voltage pulse, while R2 provides a measure of temperature compensation. Fig. 9 shows the waveforms occurring at the emitter and base 1; the first is an approximation to a sawtooth and the second is a pulse of short duration.

The operation of the circuit is as follows: C1 charges through R1 until the voltage across it reaches the peak point. The emitter current then rises rapidly, discharging C1 through the base 1 region and R3. The sudden rise of current through R3 produces the voltage pulse. When the current falls to IV the UJT switches off and the cycle is repeated.

It can be shown that the time t between successive pulses is given by:

           VBB - VV
t + R1C ln secs (5) Megaohms. C in µF.

The oscillator uses a 2N2646 UJT, which is the most readily available device, and is to operate from a 10V D.C. power supply.

From the relevant data sheet the specifications for the 2N2646 are:

VEB2O IE(peak) PTOT(max) IP(max) IV(max)      Eta      Case style TO18
30V 2A 300mw 5µA 4ma 0.56 - 0.75
It is important that the value of R1 is small enough to allow the emitter current to reach IP when the capacitor voltage reaches VP and large enough so that the emitter current is less than IV when the capacitor discharges to VV. The limiting values for R1 are given by:
          VBB - VP               VBB - VV
R1(max) = and R2(min) =
From the specifications for the 2N2646 the average value of Eta is 0.56 + 0.75/2 = 0.655. Substituting this value in equation (4) and assuming VD = 0/7V: VP = 0.655 x 10 + 0.7 = 7.25V.
So R1(max) = 10 - 7.25/5µA = 550K, and if VV = approx VBB/10, 
R1(min) = 10 - 1/4mA = 2.25K.
If we choose a value for R1 somewhere between these limits, e.g. lOK, the value of C can be calculated from equation.

If f = 1MHz, t = 1/f = 1msec. VBB - VP = 10 - 7.25 = 2.75 and VBB - VV = 10 - 1 = 9


Rearranging equation(5) to make C the subject: C = VBB - VV
R1 ln

so C = = approx 84nF.
104 ln (9/2.75)
Because of component and UJT tolerances it is sufficient in most circumstances to use an approximate formula: f = 1/CR, which assumes that Eta is 0.63 - well within 5% of the average value for the 2N2646. In practice one would use a variable resistance (or a variable resistance in series with a fixed resistance) for R1 so that the frequency of oscillation could be adjusted to give the required value.

R2 is not essential; if it is included, a value of 470 ohms is appropriate for the 2N2646. The value of R3 should be small in comparison with RBB, with which it is in series, so as to prevent it from affecting the value of the peak voltage. A value of 47 ohms or thereabouts is satisfactory.



The COMMON-EMITTER CONFIGURATION (CE) is the most frequently used configuration
in practical amplifier circuits, since it provides good voltage, current, and power gain. The input to the CE is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, making the emitter the element "common" to both input and output. The CE is set apart from the other configurations,
because it is the only configuration that provides a phase reversal between input and output signals.


The COMMON-BASE CONFIGURATION (CB) is mainly used for impedance matching, since it has a low input resistance and a high output resistance. It also has a current gain of less than 1. In the CB, the input is applied to the emitter, the output is taken from the collector, and the base is the element common to both input and output


Electronics is the study and use of electrical that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves. and semiconductors. The pure study of such devices is considered as a branch of physics, while the design and construction electronic circuits to solve practical problems is called electronic engineering.

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