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A presentation On ujt using transistor circuut
- 1. Unijunction Transistor SUBMITTED BY MAHFUZ HOSSAINANTOR 24524203158 CSE-BUP SUBMITTED TO Md. Sazzadur Rahman, Ph.D Professor IIT-JU
- 2. 2 The UJT isa three-terminal, semiconductor device which exhibits negative resistance and switching characteristics for use as a relaxation oscillator in phase control applications. UJT SYMBOL
- 3. 3 The structure ofa UJT consists of a single bar of lightly doped N-type silicon with two ohmic contacts, Base1 (B1) and Base2 (B2), at its ends. A heavily doped P-type region is diffused near the bar’s middle, forming the Emitter (E). The emitter is connected through a P-N junction to the N-type silicon. This arrangement gives the UJT its unique properties, including negative resistance, making it ideal for triggering and oscillation. BASIC STRUCTURE EQUIVALENT CIRCUIT
- 4. 4 RB2 RB1 VBB 0 = E I BB V h +VD - IE - B2 B1 + - When thevoltage VBB is applied with emitter open. A potential gradient is established along the n-type silicon bar. As the emitter is located close to the base B2, thus a major part of VBB appears between the emitter and base B1. The voltage V1 between emitter and B1, establishes a reverse bias on the pn-junction and the emitter current is cut off, but a small leakage current flows from B2 to emitter due to minority charge carriers. Thus, the device is said to be in OFF state. NO VOLTAGE WITH AMITTER OPEN :
- 5. 5 RB2 RB1 VBB 0 = E I BB V h +VD - IE + VE - B 2 B1 + - With Emitterat Positive Potential : When a positive voltage is applied to the emitter terminal of a UJT, the P-N junction remains reverse- biased until the emitter voltage ( ) exceeds the 𝑉𝐸 Peak Voltage ( ).The peak voltage is given by the 𝑉𝑃 equation: VP=ηVB+VT Where: 1. is the intrinsic standoff ratio (a constant for the 𝜂 UJT). 2. V B is the voltage across Base1 (B1) and Base2 𝑉𝐵 (B2). 3. V T is the threshold voltage (typically 0.6-0.7V 𝑉𝑇 for silicon).
- 6. 6 RB2 RB1 VBB 0 = E I BB V h +VD - IE + VE - B 2 B1 + - With Emitterat Positive Potential : When > , the P-N junction becomes 𝑉𝐸 𝑉𝑃 forward-biased, injecting holes from the emitter into the N-type bar. These holes reduce the resistance of the emitter-to-Base1 region, causing a decrease in the voltage drop across it. As the resistance decreases, the emitter current ( ) increases, leading to 𝐼𝐸 a condition of negative resistance. The device conducts fully at saturation, with the current limited by the emitter power supply.
- 7. 7 The curve betweenemitter voltage (VE) and emitter current (IE) of UJT, at a given value of VBB is known as emitter characteristics of UJT. • At first, in the cut off region, when the emitter voltage increases from zero, due to the minority charge carriers, a small current flows from terminal B2 to emitter. This is called as leakage current. • Above the definite value of VE, the emitter current (IE) starts to flow and increases until the peak (VP and IP) is reached at point P.
- 8. 8 • After pointP, an increase in VE causes a sudden increase in IE with a corresponding decrease in VE. This is the Negative Resistance Region of the curve as with the increase in IE, VE decreases. • The negative resistance region of the curve ends at the valley-point (V), having valley- point voltage VV and current IV. After the valley-point the device is driven to saturation.
- 9. 9 •Simple Construction •Low PowerConsumption •Reliable Triggering •Negative Resistance Property •Wide Operating Range •High Efficiency • Oscillators • Trigger Circuits • Saw tooth generator • Bi-stable networks • Pulse and voltage sensing circuits • UJT relaxation oscillators • Over voltage detectors
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