This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Bipolar Amplifiers – Comparison of Biasing Methods”.
1. A sinusoidal voltage of 10mV is applied to the base of the transistor. The collector current is found to be sinusoidal. If we neglect the parasitic capacitances and early effect, how can we increase the magnitude of the collector current to drive a speaker at high volume?
a) Biasing
b) Increasing VCC
c) Increasing R2
d) Increasing R1
View Answer
Explanation: Increasing R1 or R2 doesn’t increase the collector current. Moreover, increasing R2 reduces the base current which further decreases the collector current. Increasing VCC increases the output voltage but doesn’t affect the collector current and this isn’t a very cost-effective way. Biasing the transistor allows us to shift the input voltage to a higher value and this leads to a higher collector current.
2. Which of the following disasters are not evaded by the base-bias method?
a) Temperature dependency
b) Thermal runaway
c) Noise
d) It is not prone to any disaster
View Answer
Explanation: The base bias method is prone to thermal runaway. This biasing method doesn’t protect the collector current from the reverse saturation current, this reverse saturation current increases with temperature which leads to an increase in the collector current. This further leads to an increase in the reverse saturation current and the process leads to damaging the device. A proper biasing method is necessary to evade this phenomenon.
3. If a transistor circuit is arranged using a base-bias method, what happens to the response of the circuit if we change the transistor?
a) The transistor has no current gain
b) The response changes slightly
c) The response remains the same
d)The response changes heavily
View Answer
Explanation: A proper biasing method allows us to design the circuit so that even if the transistor gets changed, the response remains almost same. The base-bias method is heavily affected by the current gain of the transistor. Changing the transistor leads to a change in the current gain and thus the response of the circuit will change heavily.
4. In the following transistor circuit, VBE=0.7V, β=100, VCC=5V and early effect is neglected. Calculate R1 such that IC=8.6mA.
a) 10KΩ
b) 20KΩ
c) 30KΩ
d) 50KΩ
View Answer
Explanation: We find that the base current has to be 86μA since the collector current is 100 times the base current. This leads us to the base resistance which is simply VCC-VBE/IB and this comes out to 50KΩ.
5. In the following transistor circuit, VBE=0.7V, β=100, VCC=5V and early effect is neglected. Calculate R1 such that VCE=0.7V.
a) 40KΩ
b) 20KΩ
c) 100KΩ
d) 10KΩ
View Answer
Explanation: If VCE=0.7V, we conclude that the collector current is 4.3mA. This implies that the base current is 43 μA since it is 1/100 of the collector current. The base resistance can hence be derived by a KVL from the supply voltage through R1 the ground via the base-emitter junction as VCC-VBE/IB and it comes out to 100KΩ.
6. In the following circuit, what happens to the transconductance of the device if R1 changed from 50 KΩ to 100 KΩ?
a) It increases
b) It decreases
c) It remains the same
d) It doubles
View Answer
Explanation: The transconductance is given by the collector current divided by the thermal voltage. We readily note that the base current would decrease if R1 increases by a factor of 2 and hence we see that the collector current will also decrease. We invoke the small signal model and find that the base emitter voltage does get changed a bit but considering that the change is very less, the base current being micro-ampere, we can conclude that the transconductance would get reduced.
7. In the following circuit, what happens to rπ of the device if R1 changed from 50 KΩ to 100 KΩ?
a) It doubles
b) It becomes half
c) It increases
d)It decreases
View Answer
Explanation: Firstly, we note that the transconductance of the device will decrease since the base current is halved and so the collector current would reduce slightly. rπ is inversely proportional to the transconductance and hence it will increase. This suggest that we have increased the input impedance of the device.
8. What kind of feedback is being used in the Self-bias circuit?
a) Positive feedback
b) Negative feedback
c) No feedback happens
d) Neutral feedback
View Answer
Explanation: Negative feedback is used to take the collector current and feed it into the base. The collector voltage decreases with an increase in the collector current. A reduction in the collector voltage will lead to decrease in the amount of current provided as a feedback to the base. Since the base current has reduced, the collector current gets reduced. The collector current is thus regulated by the feedback mechanism.
9. In the following circuit, we want to reduce the sensitivity of the collector current to the current gain. Which condition would allow us to do so?
a) R2 >> β*RF
b) R2 << β*RF
c) β*R2 >> RF
d) β*R2 << RF
View Answer
Explanation: We invoke the method of KVL from the Supply voltage via R2 and Rf to ground. The KVL equation is VCC = IC(R2 + RF/β) + VBE. We observe that if β*R2>>RF, IC becomes almost independent of the current gain.
10. In the following transistor circuit, VBE=0.7V, β=100, VCC=10V and early effect is neglected. Calculate IB if R2=2*RF=1K.
a) 46 μA
b) 50 μA
c) 54 μA
d) 0
View Answer
Explanation: We can write a KVL equation as VCC = IC(R2 + RF/β) + VBE. The collector current turns out to be 4.3mA and hence the base current is 46 μA.
11. In the following transistor circuit, VBE=0.7V, β=100, VCC=10V, R2=2*RF=1K. and early effect is neglected. If the supply voltage increases by 10%, what happens to the load current?
a) It decreases by .5%
b) It increases by .5%
c) It remains the same
d) It increases by 10%
View Answer
Explanation: We need to relate the supply voltage to the collector current. K.V.L. allows us to write VCC = IC(R2 + RF/β) + VBE and use the Δ operator to find that ΔIC = ΔVCC(R2 + RF/β) which leads us to conclude that the collector current would increase by .5%
12. The self-bias biasing method offers a stable collector current with respect to changes in the base-emitter voltage as compared to the base-bias method.
a) True
b) False
View Answer
Explanation: If the self-bias method is implemented properly, the base-emitter bias voltage remains almost the same throughout the operation. This implies that the bias collector current remains much more stable compared to the base biased method.
13. The self-bias biasing method offers a stable collector current with respect to changes in β as compared to the self-bias method.
a) True
b) False
View Answer
Explanation: The self-bias method can eliminate the effect of β to a great extent as compared to the base-biased method. This can be done by manipulating the resistances connected to the transistor and this is done by observing the relation, between the resistances connected to the device, given by KVL.
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