This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “BJT Construction and Operation”.
1. Which of the following is not a valid type of a BJT?
Explanation: A BJT is a device with a layer of semiconductor sandwiched between 2 unlike types of semiconductors and hence, PPN is not a valid type of a BJT.
2. In a BJT, the outer layers are much more thick as compared to the middle layer.
Explanation: In a BJT, the thickness of the middle layer or the base is thin as compared to the collector and emitter.
3. In a BJT, which of the following layers is heavily doped?
Explanation: The emitter is the most heavily doped and contain the maximum amount of charge carriers. It is the emitter’s task to inject carriers into the base. These bases are thin and lightly doped. For npn B JT, emitter injects electrons, and for pnp, it injects holes.
4. Considering the resistances of emitter, collector and base to be Re, Rc and Rb respectively, which of the following is the correct statements?
a) Re > Rb > Rc
b) Rc > Rb > Re
c) Rb > Rc > Re
d) Rb = Rc > Re
Explanation: As the base is lightly doped, the number of free charge carriers are less and hence the resistance is high and as the emitter is the most highly doped, its resistance is low.
5. In a pnp-BJT, when the E-B junction is forward biased and no voltage is applied across C-B junction, what happens to the width of the depletion region in the E-B junction?
c) Remains same
d) Can’t be determined
Explanation: On application of a forward bias voltage across E-B junction, the width of the depletion region decreases.
6. Which of the following statements is true about proper functioning of a BJT?
a) One junction is forward biased and one is reverse biased.
b) Both junctions are forward biased
c) Both junctions are reverse biased
d) Can’t be determined
Explanation: In a BJT, depending upon the biasing of the two junctions, the BJT behaves differently. The BJT may be in saturation, wherein it acts like a short circuit, or it may be in cut-off, i.e an open circuit. The BJT can be either in forward active or reverse active mode. Active mode is the common mode, used in BJTs and obtained by one forward biased and one reverse biased junction.
7. What is the typical order of magnitude of the base current for a BJT?
a) 10 -8
b) 10 -9
c) 10 -6
d) 10 -3
Explanation: As the base current is quite lower as compared to the collector and emitter current, it is usually in the order of microamperes.
8. The collector current in a BJT is temparature-independent.
Explanation: The collector current I C in a BJT is made up of two components – one due to majority carriers and the other due to minority carriers. The component of IC due to minority carriers i.e ICO is temparature sensitive.
9. Which of the following currents in a BJT is also called leakage current?
Explanation: Leakage current in BJT is represented by ICO, which is due to the flow of minority carriers in the transistor. It consists of ICBO and ICEO. ICO depends on temperature, doubling with 10° rise in temperature. It thus effects total collector current, IC, and hence affects the power dissipation.
10. Which of the following relations are correct?
a) IE + IB = IC
b) IC + IB = IE
c) IE + IC = IB
d) IB + IE = IB
Explanation: On applying KCL to the BJT, we get IC + IB = IE.
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