UNIJUNCTION TRANSISTOR

Unijunction transistors are three-lead transistors that act exclusively as electrically controlled switches; they are not used as amplifiers.

This differs from other transistors in that general transistors usually provide the ability to act as a switch and also as a an amplifier. But a unijunction transistor does not provide any decent type of amplification because of the way it is constructed. It's simply not designed to provide a sufficient voltage or current boost.

Unijunction Transistor

The basic operation of a UJT is relatively simple. When no potential difference (voltage) exists between its emitter and either of its base leads (B1 or B2), only a very small current flows from B2 to B1. However, if a sufficiently large positive trigger voltage- relative to its base leads- is applied to the emitter, a larger current flows from the emitter and combines with the small B2-to-B1 current, thus giving rise to large B1 output current. Unlike other transistors- where the control leads provide little additional current- the UJT is just the opposite. Its emitter current is the primary source of current for the transistor. The B2 to B1 current is only a very small amount of the total combined current. This means that unijunction transistors are not suitable for amplification purposes, but only for switching. 

Characteristics of Unijunction Transistor

The Unijunction emitter current vs voltage characteristic curve shows that as VE increases, current IE increases up IP at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of RB1, the saturation resistance is lowest at the valley point.

IP and IV, are datasheet parameters; For a 2n2647, IP and IV are 2µA and 4mA, respectively. [AMS] VP is the voltage drop across RB1 plus a 0.7V diode drop. VV is estimated to be approximately 10% of VBB.

(a) emitter characteristic curve, (b) model for VP 

Explanation of Characteristics Curve

Cutoff- This is the region where the unijunction transistor doesn't yet receive enough voltage to turn on. The voltage hasn't yet reached the triggering voltage, so the transistor will not turn on.

Negative Resistance Region- After the transistor has reached the triggering voltage, VTRIG, it now will turn on. After a while if the applied voltage still increases to the emitter lead, it will peak out at VPEAK. From VPEAK to the Valley Point, the applied voltage drops while the current, though, increases. The current increases but the voltage decreases, which is why it's called negative resistance.

Saturation- After the negative resistance region which saw an increase in current comes the saturation region. This is the region where if the applied voltage to the emitter still increases, the current and voltage will rise. 

The most common application of a unijunction transistor is as a triggering device for SCR’s and Triacs but other UJT applications include saw toothed generators, simple oscillators, phase control, and timing circuits. The simplest of all UJT circuits is the Relaxation Oscillator producing non-sinusoidal waveforms.

Unijunction Transistor

Applications

Unijunction Transistor Relaxation Oscillator

When a voltage (Vs) is firstly applied, the unijunction transistor is “OFF” and the capacitor C1 is fully discharged but begins to charge up exponentially through resistor R3. As the Emitter of the UJT is connected to the capacitor, when the charging voltage Vc across the capacitor becomes greater than the diode volt drop value, the p-n junction behaves as a normal diode and becomes forward biased triggering the UJT into conduction. The unijunction transistor is “ON”. At this point the Emitter to B1 impedance collapses as the Emitter goes into a low impedance saturated state with the flow of Emitter current through R1 taking place.

As the ohmic value of resistor R1 is very low, the capacitor discharges rapidly through the UJT and a fast rising voltage pulse appears across R1. Also, because the capacitor discharges more quickly through the UJT than it does charging up through resistor R3, the discharging time is a lot less than the charging time as the capacitor discharges through the low resistance UJT.

When the voltage across the capacitor decreases below the holding point of the p-n junction ( VOFF ), the UJT turns “OFF” and no current flows into the Emitter junction so once again the capacitor charges up through resistor R3 and this charging and discharging process between VON and VOFF is constantly repeated while there is a supply voltage, Vs applied.

One typical application of the unijunction transistor circuit above is to generate a series of pulses to fire and control a thyristor. By using the UJT as a phase control triggering circuit in conjunction with an SCR or Triac, we can adjust the speed of a universal AC or DC motor as shown.

UJT Speed Control Circuit

Using the circuit above, we can control the speed of a universal series motor (or whichever type of load we want, heaters, lamps, etc) by regulating the current flowing through the SCR. To control the motors speed, simply change the frequency of the sawtooth pulse, which is achieved by varying the value of the potentiometer.