Uni Junction Transistor (UJT) | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

UNI JUNCTION TRANSISTOR (UJT)

Another device whose construction is similar to that of the FET is shown in the figure.

It is a three terminal device, having two layers.It consists of a slab of lightly doped N-type silicon material. The two base contacts are attached to both ends of this N-type surface. These are denoted as B1 and B2 respectively.

    A P-type material is used to form a pn junction at the boundary of the aluminium rod and N-type slab. The N-type is lightly doped while P-type is heavily doped. That is N-type provides high resistivity and P-type provides low resistivity.

  This device was originally described in the literature as the double-base diode, but is now commercially available under the designation Uni Junction transistor (UJT).

 The standard symbol for this device is shown in the fig.

Here the emitter arrow is inclined and points toward B1 and This emitter arrow which is at an angle to the vertical line representing N-type material indicates the direction of flow of conventional current when UJT is forward biased.

EQUIVALENT CIRCUIT OF UJT:

The circuit of UJT is shown in the fig.

In the construction ,the emitter terminal is closer to B₂ as compared to B_1.

If we see the equivalent circuit of UJT, the internal resistances of the two bases are represented as R_b1 and R_b2. Hence R_b1 is greater than R_b2. The pn junction is represented by a normal diode with V_d  as the drop across it. When the emitter diode is not conducting then the resistance between the two bases B₁ and B₂ is called interbase resistance denoted by R_bb.

                          R_bb=R_b1+R_b2

Its value ranges from 4kΩ to 12kΩ.

Intrinsic stand off ratio (η)

Consider UJT as shown in the fig. to which supply V_bb is connected. With I_e=0, That is emitter diode is not conducting ,

             R_bb=R_b1+R_b2

Then the voltage drop across R_b1  can be obtained by using potential divider rule:

           Replacing with its equivalent circuit

R_bb=R_b1+R_b2

When I_e =0

  = η.V_BB  When I_e=0

η = Intrinsic stand off ratio =  () when Ie=0

η = ( ) when I_e=0

The value of η is from 0.5 to 0.8 the voltage V_rb1 is called intrinsic stand off voltage, because it keeps the emitter diode reverse biased for all emitter voltages less than V_rb1.

Operation:

1.The supply V_bb is applied between B₂ and B₁ .

2. Variable emitter voltage V_e  is applied across the emitter terminals.

3. If V_e  is varied , potential of A is decided by η and is equal to ηV_bb

                    V_a=V_rb1= ηV_bb

Case 1:  If V_e< V_a

As long as V_e < V_a, the pn junction is reverse biased. Hence emitter current I_e will not flow. Thus, UJT is said to be off.

Case 2: if V_e>V_p

The diode drop V_d  is generally between 0.3 to 0.7v.

We can write V_p=V_a+V_d = ηV_bb+V_d

When V_e becomes equal to or greater than V_p, the pn junction becomes forward biased and current I_e flows. Thus, UJT  is said to be  ON.

 VI Characteristics:

1.The graph of I_E against emitter voltage plotted for a particular value of V_bb is called the characteristics of UJT.

The characteristics can be divided in to three main regions.

1.cut off     2.  Negative resistance region   3.  Saturation region.

1.cut off:   When V_e< V_p and the pn junction is reverse biased. A small amount of reverse saturation current I_EO flows through the device which is negligibly small of the order of micro amps.  This condition remains till the peak point.

2. Negative resistance region: When emitter voltage V_E  becomes equal to V_p then pn junction becomes forward biased and I_E starts flowing. The voltage across the device decreases in this region though the current through the device increases. Hence the region is called negative resistance region. This decreases the resistance R_b1. This region is stable and used in many applications. This region continues till valley point.

3. Saturation region:   The region to the right of the valley point is called saturation region. In the valley point, the resistance changes from negative to positive. The resistance remains positive in the saturation region.

     As V_bb increases, the potential V_p corresponding to peak point will increase.

 Applications of UJT:

The  UJT is mainly used in the triggering of other devices such as SCR. It is also used in the sawtooth wave generators and some timing circuits. The most popular application of UJT is as a relaxation oscillator to obtain short pulses for triggering of SCRs.

The relaxation oscillator using UJT which is meant for generating sawtooth waveform consists of a UJT and a capacitor, which is charged through emitter resistor as the supply voltage V_bb is switched ON.