Analog Circuits Questions and Answers – Transistor Bias Configuration

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Transistor Bias Configuration”.

1. BJT is biased to _________
a) Work as a switch
b) Prevent thermal runaway
c) Increase DC collector current
d) Operate it in the saturation region
View Answer

Answer: b
Explanation: A BJT is biased to operate in the active region, to work as an amplifier. It is not biased in the cut-off or saturation region to work as a switch. Also, biasing is done to maintain a stable collector current so that the operating point does not change. This also prevents thermal runaway.

2. For the given circuit, β = 150.
Figure 1 having stability factor S1 of base bias circuits
Fig. 1 has stability factor S1
If the above circuit is changed to below circuit
Figure 2 having stability factor S1 of base bias circuits is transformer coupled amplifier
Fig. 2 having stability factor S2
Choose the correct option which is having better stability with S1 and S2 Values.
a) S2 = 151, S1 = 150. Circuit 1 has better stability
b) S2 = 100, S1 = 10 Circuit 2 has better stability
c) S2 = 151, S1 = 10.3 Circuit 1 has better stability
d) S2 = 151, S1 = 10.3 Circuit 2 has better stability
View Answer

Answer: c
Explanation: The above circuits are a collector to base bias circuits, where stability factor
\(S = \frac{1+β}{\frac{1+βR_C}{R_C+R_B}}\)
Thus, in the question, S1 =\( \frac{1+150}{1 + \frac{150*10}{110}} = \frac{151}{14.63}\) = 10.32
However, in circuit 2, which is a transformer coupled amplifier, RC = 0
Thus, S2 = 1+150 = 151.

3. Consider the biasing circuit shown. The β for the circuit is large. R3 = 1kΩ, R4 = 2kΩ. The stability factor varies between 10 and 11. Find the maximum and minimum values of R2.
Find the maximum & minimum values of R2 if β is large, R3 = 1kΩ & R4 = 2kΩ for circuit
a) Minimum = 16.36kΩ, Maximum = 20kΩ
b) Minimum = 16.36kΩ, Maximum = 18kΩ
c) Minimum = 10 kΩ, Maximum = 20kΩ
d) Minimum = 6 kΩ, Maximum = 10kΩ
View Answer

Answer: a
Explanation: Circuit is a self bias circuit.
Base resistance = RB = R1*R2 / (R1+R2)
Since β is large, stability factor, S = 1 + RB/RE = 1 + RB/R3
1 + RB/R3 = 10
RB/R3 = 9 => RB = 9k => R2 = 16.36 kΩ
For S = 11
RB/R3=10 => RB = 10k => R2 = 20kΩ.

4. Choose the incorrect option according to self bias circuit?
a) Voltage gain increases
b) Stability factor is independent of collector resistance
c) BJT can be used in either of the three configurations
d) Excellent stability in collector current is achieved
View Answer

Answer: d
Explanation: In a self bias circuit, due to emitter resistance a negative feedback exists. This decreases voltage gain. Also, stability factor S does not depend on collector resistance, only on base and emitter resistance and β, if required. \(S = \frac{(1+β)(R_B +R_E)}{R_B + (1+β)R_E}\).
S is least in self bias circuit, hence excellent stability is achieved.

5. In the circuit given, the two Si transistors are similar. Given β=50, Vcc=12V, I1=5mA. Find I?
Find I if β is 50, Vcc is 12V, I1 is 5mA & similar two Si transistors in given circuit
a) 5 mA
b) 4.807 mA
c) 4.5 mA
d) 5.2 mA
View Answer

Answer: b
Explanation: The circuit is a current mirror circuit. Both transistors are similar. I1= IREF = 5 mA
Β is not large so 2/β is not negligible.
Thus current I = I1/(1 + 2/β) = 5mA/(1 + 2/50) = 4.807 mA.
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6. Given Vout = 5V, β=100, I1=10mA, R1=100KΩ. Find the output resistance.
Find the output resistance when Vout is 5V, β is 100, I1 is 10mA & R1 is 100KΩ.
Graph representing variation in curve for early voltage VA & IC
a) 8 kΩ
b) 8.163 kΩ
c) 7.582 kΩ
d) 8.4 kΩ
View Answer

Answer: b
Explanation: The circuit is a current mirror, whose output resistance, ROUT = (VA + VCE)/IOUT
Here, IOUT = I1 / (1+ 2/β) = 10mA / (1 + 2/100) = 9.8 mA
VA = Early voltage = 75 V (obtained from the graph)
ROUT = (VA + VCE)/ IOUT = (75 + 5) / 9.8 * 10-3
ROUT = 80,000/9.8 = 8.163 kΩ.

7. Why is self bias circuit not used in IC amplifier?
a) To reduce power losses
b) To reduce area used on the chip
c) Stability factor reduces in the IC
d) Voltage gain is reduced
View Answer

Answer: b
Explanation: Self biased circuits are not preferred in IC amplifiers because they need large resistances R1 and R2, since then S will be smaller and stability will be more. However, using large resistances in ICs means a requirement of larger chip area, so to reduce this area requirement, we use current mirror circuits instead.

8. Considering all transistors to be similar and β is very large, when I1 = 10 mA, find current I2.
Find current I2 for similar transistors with very large β when I1 is 10 mA in the circuit
a) 10 mA
b) 50 mA
c) 25 mA
d) 20 mA
View Answer

Answer: c
Explanation: Let current through the transistor Q1/Q2 be IC. Since both are similar, we can say that,
IC = I1/2
Similarly, current through transistors Q3 to Q7 is assumed to be IC’, where IC’ = I2/5
Since all transistors are similar. IC = IC
I2 = 5IC’ = 5 IC = 2.5*I1.

9. Consider the following circuit, where the transistors are similar Si transistors. Given I1 = 2mA, I2 = 1μA, Vcc = 12 V, find R1 and R2.
Find R1 & R2 for all Si transistors when I1 is 2mA, I2 is 1μA, Vcc is 12 V
a) R1 = 10kΩ, R2 = 6kΩ
b) R1 = 5.6kΩ, R2 = 20kΩ
c) R1 =18kΩ, R2 = any value
d) R1 = 18kΩ, R2 = 5.6kΩ
View Answer

Answer: d
Explanation: R2 = (Vcc – VBE)/I1 = (12 – 0.7)/2m = 5.65 kΩ – Using KVL
Also, I2*R1 = VT ln(I1/I2) = 0.026 * ln(2)
Thus, R1 = 0.026*ln(2)/1μ = 18 kΩ.

10. Widlar current source was introduced to obtain a smaller output current.
a) True
b) False
View Answer

Answer: a
Explanation: In a current mirror circuit, to obtain lower values of output current, the resistance values required to increase a lot, which becomes difficult to manufacture on an IC. Instead, we use a widlar current source, where an emitter resistance is also present, affecting the output current.

Sanfoundry Global Education & Learning Series – Analog Circuits.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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