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Question-1. What is UJT?
Answer-1: Unijuction transistor has only one p-n junction. It consists of a lightly doped n-type silicon bar. The p- type impurity is diffused into the base producing the pn junction these are referred to as the emitter, base 1 and base 2 respectively. It is also called double based device It is switching device. The single P-N junction accounts for the terminology unijunction. The emitter junction is usually located closer to base-2 (B2) than base-1 (B1).
Question-2. What is inter base resistance?
Answer-2: The resistance of the silicon bar is called inter base resistance RBB represented by the two resistors in series of Rb1 and Rb2. The inter base resistance is given by: Rbb = Rb1 + Rb2
Question-3. What is the mode of operation of UJT?
Answer-3: The operation of UJT may be explained in three different modes. 1) Cut off region; 2) Negative resistance region; 3) Saturation region.
Question-4. Operation of a UJT?
Answer-4: If a progressively rising positive voltage is applied to the emitter the diode will become forward biased when input voltage exceeds ? Vbb by Vd, the forward voltage drop across the silicon diode. Now the emitter current increases regeneratively until it is limited by the emitter power supply. Here we can define the peak point voltage of the UJT, Thus when input positive voltage to the emitter is less then Vp, the pn-junction remains reverse biased and the emitter current is practically zero. When the input voltage exceeds Vp, the diode is forward biased and the emitter current reaches a saturation value limited by Rb1and the forward resistance of pn-junction.
Question-5. Application of UJT?
Answer-5: The device has-a unique characteristic that when it is triggered, its emitter current increases re generatively (due to negative resistance characteristic) until it are restricted by emitter power supply. The low cost per unit, combined with its unique characteristic, have warranted its use in a wide variety of applications. A few include oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, voltage-or current-regulated supplies and one of the most important applications of UJTs is to trigger thyristors (SCR, TRIAC, etc.).
Question-6. What are the features of UJT?
Answer-6: The worth noting points about UJT are given below:
1) The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode as it has three terminals.
2) The structure of a UJT is quite similar to that of an N-channel JFET.
3) The N-type silicon bar has a high resistance and the resistance between emitter and base-1 is larger than that between emitter and base-2. It is because emitter is closer to base-2 than base-1.
4) UJT is operated with emitter junction forward- biased while the JFET is normally operated with the gate junction reverse-biased.
5) UJT does not have ability to amplify but it has the ability to control a large ac power with a small signal. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.
Question-7. What are the characteristics of UJT?
Answer-7: The static emitter characteristic (a curve the relation between emitter voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB.Once conduction is established at VE = VP the emitter potential VE starts decreasing with the increase in emitter current IE. This Corresponds exactly with the decrease in resistance RB for increasing current IE. This device, therefore, has a negative resistance region which is stable enough to be used with a great deal of reliability in the areas of applications listed earlier. Eventually, the valley point reaches, and any further increase in emitter current IE places the device in the saturation region,
Question-8. Explain Ip, VV, and IV ?
Answer-8: Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It represents the rnimrnum current that is required to trigger the device (UJT). It is inversely proportional to the interbase voltage VBB. Valley Point Voltage VV The valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage VBB. Valley Point Current IV The valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.
Question-9. What is the specification of UJT?
Answer-9: The very basic specifications of a UJT are:
(a)Vbb (max) - The maximum interbase voltage that can be applied to the UJT
(b) Rbb -the interbase resistance of the UJT
(c) ? - The intrinsic standoff ratio which defines Vp.
(d) Ip - The peak point emitter current
Question-10. What is relaxation oscillator?
Answer-10: A relaxation oscillator produces a non-sinusoidal output, such as a square wave or sawtooth. The oscillator contains a nonlinear component such as a transistor that periodically discharges the energy stored in a capacitor or inductor, causing abrupt changes in the output waveform.
Question-11. What is UJT relaxation oscillator?
Answer-11: UJT Relaxation Oscillator circuit, mainly used for triggering purposes This circuit is ideally suited for triggering an SCR ? since UJT is capable of generating sharp, high powered pulses of short duration whose peak and average power don?t exceed the power capabilities of the SCR gate for which they are intended.
Question-12. How does a UJT relaxation oscillator works?
Answer-12: When power is applied to the given circuit, capacitor C starts charging exponentially through R to the applied voltage VCC. The voltage across C is the voltage applied to the emitter of UJT. When C is charged to Vp , then UJT turns ON. This greatly reduces the effective resistance between emitter and base1 of UJT. A sharp pulse of current flows from base1 to emitter, discharging C through Rb1. When the capacitor voltage drops below Vp , UJT is brought back to the previous state and the capacitor again begins to charge towards Vbb. This produces a saw tooth wave. Rb1 and Rb2are used to protect UJT from overheating. This intern provides sharp pulses across them: Rb1 produces a positive spike and Rb2produces a negative spike.
Question-13. Practical examples of the use of the relaxation oscillator?
Answer-13: This type of circuit was used as the time base in early oscilloscopes and television receivers. Variants of this circuit find use in stroboscopes used in machine shops and nightclubs. Electronic camera flashes are a monostable version of this circuit, generating one cycle of the saw tooth, the rising edge as the flash capacitor is charged and the rapid falling edge as the capacitor is discharged and the flash is produced upon receiving the firing signal from the shutter button. Use as a time base in oscilloscopes was discontinued when the much more linear Miller Integrator time base circuit using "hard" valves, (vacuum tubes) as a constant current source, was developed.
Question-14. What are oscillators?
Answer-14: Oscillators produce a waveform (mostly sine or square waves) of desired amplitude and frequency. They can take input from the output itself. For a complete oscillator circuit we require a feedback device, amplifier and feedback factor.
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