# Analog Circuits Questions and Answers – Collector Feedback Configuration

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

1. The Collector feedback configuration is better than __________
a) Fixed Bias Configuration
b) Voltage divider configuration
c) C.E. configuration
d) C.B. configuration

Explanation: The fixed bias circuit has been seen to offer low stability with respect to change in ICO. The Voltage divider bias provides the most stable biasing mechanism. Hence, the collector feedback configuration is better than the fixed bias configuration while C.E. and C.B. are not biasing stages.

2. The Collector feedback is done by connecting a resistor from the collector to the __________
a) Emitter
b) Base
c) Supply voltage
d) Bias voltage

Explanation: The collector feedback configuration is done by connecting a resistor from the collector to the base voltage. This is done to stabilize the biasing voltage against thermal runaway.

3. The Collector feedback configuration helps to stabilize __________
a) Bias voltage
b) Collector voltage
c) Bias current
d) Collector current

Explanation: The collector feedback configuration is used to stabilize the Collector current. The Collector current is seen to increase at a sincere rate which can harm the transistor by thermal runaway. Stabilizing this current is necessary during biasing a transistor for proper application.

4. The Collector feedback helps to evade __________
a) Inverse Active mode
b) Pinch Off
c) Thermal Runaway
d) Breakdown

Explanation: The increase in the Collector current is primarily due to ICO. A sudden increase in ICO can increase the Collector current and this will increase the temperature of the device. This is called thermal runaway and due to the high current gain of the transistor, the transistor can get destroyed due to thermal runaway. The Collector feedback configuration helps to evade this phenomenon.

5. Due to the Collector feedback mechanism, the transistor remains always remains in the __________
a) Active mode
b) Saturation mode
c) Inverse Active
d) Cut-off

Explanation: The collector feedback configuration helps to keep the transistor in the active region. This is done because the bias voltage can get changed if the input and the bias voltage is superposed. With the introduction of this feedback mechanism, the transistor always stays biased in the active region.
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6. What are the effects on the output voltage if the Collector resistance increases in a Collector feedback configuration?
a) Not much effect
b) Bias voltage reduces
c) Bias voltage increases
d) Bias voltage doubles

Explanation: The increase in collector resistance doesn’t have much impact on the output voltage since the bias voltage is kept stable by the feedback operation. After all, one of the important applications of feedback is increasing stability and the output voltage is kept stable by this configuration.

7. What kind of configuration is this?

a) Collector feedback
b) Base Bias
c) Self Bias
d) No bias

Explanation: The transistor isn’t biased since the voltage drop from the base to the collector is 0. The transistor action won’t get be manifested.

8. If the current gain of the transistor is β, what is the stability factor pertaining to IC and ICO?

a) β+1/{1 + β * R1/(R1 + β)}
b) β+1
c) β+1/{1 + β * R2/(R1 + β)}
d) β+1/{1 + (R1 + β)}

Explanation: The stability factor is determined by calculating the change in IC with respect to the change in ICO. Hence, we can simply apply the method of K.V.L. and derive a relation between these two currents. After differentiation, we’ll get the stability factor as β+1/{1 + β * R1/(R1 + β)}.

9. To keep the B.J.T. in the active region, what should be the relation between R1 and R2?

a) R1 >> R2
b) R1 << R2
c) R1 = R2
d) No such relation

Explanation: The feedback resistance should be much smaller than the collector resistance since we need to reduce the sensitivity of the collector current to the current gain. Typically, R2 should be lower than R1 by a factor of β.

10. From the base bias to the collector feedback configuration, the stability facto S reduces by a factor of __________
a) 1 + R1/(R1 + β)
b) 1 + β * R1/(R1 + β)
c) β * R1/(R1 + β)
d) R1/(R1 + β)

Explanation: This can be simply observed from the stability factors of both cases. The correct factor is 1 + β * R1/(R1 + β).

11. What is the stability factor against VBE for the collector feedback configuration?
a) β/(Rc * (1 + β))
b) -β/(RB + Rc * (1 + β))
c) β/(RB + Rc * (1 + β))
d) -β/(Rc * (1 + β))

Explanation: This is easily derived from the Collector feedback configuration by using the method of K.V.L. deriving a relation between the collector current and the base emitter voltage. It should be noted that the stability against VBE increases in comparison to the base bias comparison.

12. What is the condition of stability of the following circuit?

a) Highly stable
b) Poorly stable
c) Marginally stable
d) Unstable

Explanation: The Collector feedback configuration does make the biasing stable but here, the collector and the base resistances are same. Hence, we conclude that the circuit is poorly stable. The base resistance should be substantially lower than the collector resistance.

13. If β = 100, what should be the ratio of the collector to base resistance for achieving insensitivity to β?

a) 110
b) 90
c) 20
d) 140

Explanation: The Base resistance should be substantially lower than the Collector resistance by a factor of β. 110 is a good choice but provided 140 is present as an option, a better choice is a factor of 140.

14. Why are we worried about β during the Collector feedback configuration?
a) To maintain a stable q-point irrespective of β
b) To increase the gain
c) To decrease the output impedance
d) To maintain a stable input impedance

Explanation: If we change the device, β changes. But we want to keep the q-point stable so that circuit if represented as a black box, would provide the same characteristics and not be highly dependent on the transistor. Hence, we want to make the circuit insensitive to β.

15. The stability factors change from npn to pnp transistor.
a) True
b) False

Explanation: The stability factors are independent of whether the transistor is of npn or pnp type. It is only dependent on β and the impedance connected to the terminals of the transistor of a hyperbola.

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