discuss mathematically,the equivalent circuit of UJT
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Pulse Circuits - Unijunction Transistor
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Unijunction Transistor is such a transistor that has a single PN junction, but still not a diode. Unijunction Transistor, or simply UJT has an emitter and two bases, unlike a normal transistor. This component is especially famous for its negative resistance property and also for its application as a relaxation oscillator.
Construction of UJT
A bar of highly resistive n-type silicon, is considered to form the base structure. Two Ohmic contacts are drawn at both the ends being both the bases. An aluminum rod like structure is attached to it which becomes the emitter. This emitter lies near to the base 2 and a bit far to the base1. Both of these join to form a PN junction. As single PN junction is present, this component is called as a Unijunction transistor.
An internal resistance called as intrinsic resistance is present inside the bar whose resistance value depends upon the doping concentration of the bar. The construction and symbol of UJT are as shown below.
Construction UJT
In the symbol, the emitter is indicated by an inclined arrow and the remaining two ends indicate the bases. As the UJT is understood as a combination of diode and some resistance, the internal structure of UJT can be indicated by an equivalent diagram to explain the working of UJT.
Working of UJT
The working of UJT can be understood by its equivalent circuit. The voltage applied at the emitter is indicated as VE and the internal resistances are indicated as RB1 and RB2 at bases 1 and 2 respectively. Both resistances present internally are together called as intrinsic resistance, indicated as RBB. The voltage across RB1 can be denoted as V1. The dc voltage applied for the circuit to function is VBB.
The UJT equivalent circuit is as given below.
Equivalent Circuit
Initially when no voltage is applied,
VE=0
Then the voltage VBB is applied through RB2. The diode D will be in reverse bias. The voltage across the diode will be VB which is the barrier voltage of the emitter diode. Due to the application of VBB, some voltage appears at point A. So, the total voltage will be VA + VB.
Now if the emitter voltage VE is increased, the current IE flows through the diode D. This current makes the diode forward biased. The carriers get induced and the resistance RB1 goes on decreasing. Therefore, the potential across RB1 which means VB1 also decreases.
VB1=(RB1RB1+RB2)VBB
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