# Microelectronics Questions and Answers – Models of a BJT – Set 2

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Models of a BJT – Set 2”.

1. The following circuit is present at room temperature while the base voltage is 8V. If VCC=10V, find the mode of operation. Assume large signal characteristics with VBE=0.7 and hfe=100.

a) Cut-off region
b) Inverse-active region
c) Active region
d) Saturation region

Explanation: The emitter voltage is 7.3V while the base voltage is 10V. The emitter current is 7.3mA while the collector current can be approximated to 7.3mA since the common base current gain is approximately 1. The collector voltage required for such current is 2.7V and hence we find that the base-emitter junction is forward biased while the base-collector junction is also forward biased. The device lies in the saturation region.

2. The following circuit is present at room temperature while the base voltage is 4V. If VCC=10V, find the base current if the base voltage source has a non-ideal internal resistance of 10Ω. Assume large signal characteristics with VBE=0.7, VCB=0.2v and neglect early effect while hfe=1

a) 1μA
b) 2μA
c) 3μA
d) 5μA

Explanation: We can write two KVL equations for the collector and base current. One starts from the base voltage and goes through the base-emitter junction to the emitter and then to ground. The other starts from the supply voltage and goes through the collector-base junction and into the base voltage. This leads to the base current which is 5μA.

3. The slope in the output characteristics of a npn transistor ____ due to early effect.
a) Remains unaffected
b) Decreases
c) Increases
d) Becomes 0

Explanation: The early effect leads to an increase in the slope of the output characteristics. The transistor is made to behave as a non-ideal current source.

4. By how much does the resistance due to early effect change if the emitter area is doubled?
a) Twice
b) Half
c) 4 times
d) 8 times

Explanation: The emitter area is inversely proportional to this resistance. If it increases, the collector current increases and the resistance decreases.

5. What is the number of independent parameters in Ebers-moll model?
a) 4
b) 3
c) 2
d) 1

Explanation: There are 3 independent parameters but there are 4 parameters in together. The Ebers-model represent the collector and emitter currents as a function of diode current and a controlled current source. This leads to 4 variables while only 3 of them are independent.

6. In the Ebers-Moll model, if the diode from the collector to the base-emitter junction is off, what is the current of the current source connected from the base-emitter junction to the collector?
a) Cannot be determined
b) IC
c) IE
d) IE

Explanation: The controlled source returns the forward biased base-emitter junction current multiplied by the common base current gain. We note that the above situation only implies VCB < 0 which means that the diode is either on or off. If it’s off, the answer will be 0 but we cannot determine that.

7. Which effect is mostly used while representing the parasitic capacitors within a transistor?
a) Miller’s effect
b) Theveni’s effect
c) Norton’s effect
d) Reciprocity effect

Explanation: Often, we have to distribute the capacitance between two terminals as two individual capacitors from each of the terminal to ground. This effect is called the miller’s effect and is very prominently used while representing capacitance is transistors.

8. Apart from the depletion region capacitance, we have another capacitance in the base-emitter region. This is due to the _______ charge profile in the base region.
a) Uniform
b) Non-uniform
c) Increasing
d) Decreasing

Explanation: Two capacitances arise in between the base and the emitter region. One is due to the base-emitter depletion region while the other is due to the non-uniform charge profile in the base region.

9. As we increase VCB, the base width decreases. What happens to the concentration gradient of the carriers within the base?
a) It increases
b) It decreases
c) It remains constant
d) It decreases heavily

Explanation: The concentration gradient increases. This is because the gradient extends primarily from the base-emitter region to the collector-base region and hence if this distance decreases, the concentration falls down faster.

10. Suppose that a transistor is biased at a good operating point. Now, if the transconductance is doubled, find the change in the base width to make the current return to the previous state.
a) Reduced by a factor of 4
b) Reduce by a factor of 2
c) Increase by a factor of 2
d) increase by a factor of 5

Explanation: The base width is inversely proportional to the transconductance of the B.J.T. and this helps us to get more transconductance by reducing the base width. In this case, it should be increased by a factor of 2 to get the current before the transconductance was doubled.

11. For a very high collector current, the power dissipation is increasing. If now the power dissipation is to be reduced by a factor of 2, what should be the change in the emitter area of the device?
a) Reduce by 50%
b) Reduce by 20%
c) Reduce by 10%
d) Doesn’t influence the power dissipated

Explanation: The emitter area directly influences the collector current which in turn brings changes to the total power dissipated. A reduction in the emitter area by a factor of 2 reduces the power dissipated by the device over time since now it generates less current.

Sanfoundry Global Education & Learning Series – Microelectronics.

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