# Electronic Devices and Circuits Questions and Answers – Bias Stability

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Bias Stability”.

1. What is Stability factor?
a) Ratio of change in collector current to change in a current amplification factor
b) Ratio of change in collector current to change in base current
c) Current amplification factor
d) Ratio of base current to collector current

Explanation: Stability factor is defined as the rate at which collector current changes when Base to emitter voltage changes, keeping base current constant. It can also be defined as the ratio of change in collector current to change in base current when temperature changes occur.

2. The base current for a BJT remains constant at 5mA, the collector current changes from 0.2mA to 0.3 mA and beta was changed from 100 to 110, then calculate the value of S.
a) 0.01m
b) 1m
c) 100m
d) 25m

Explanation: Since the current in the above case, remains constant, therefore stability factor is 0.01 as it is defined as the ratio of change in collector current to change in beta.
S=change in collector current/change in beta=0.1mA/10=0.01m.

3. For a n-p-n transistor, the collector current changed from 0.2mA to 0.22mA resulting a change of base emitter voltage from 0.8v to 0.8005V. What is the value of Stability factor?
a) 0
b) 0.25
c) 0.04
d) 0.333

Explanation: Change in Vbe = 0.0005V
Change in collector current = 0.02mA
S = 0.02m/0.0005 = 0.04.

4. There are two transistors A and B having ‘S’ as 25 and 250 respectively, on comparing the value of S, we can say B is more stable than A.
a) True
b) False

Explanation: More the value of S, lesser the stability, since A has lesser S value the change in beta does not affect much on the collector current. When S is high, even if IB changes by a small value, the IC current will drastically vary. Hence stability factor must possess lesser value for the proper working of a transistor.

5. What is the value of Stability factor for an ideal transistor?
a) 100
b) 1000
c) infinite
d) 0

Explanation: For a transistor, the ideal value of S is 0 which interprets that for a change in beta, there should not be changing. In Ideal transistor, the collector current will vary only if either base or emitter current varies or hence for an ideal transistor the value of S is zero.
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6. For a fixed bias circuit having Ic = 0.3mA and In=0.0003mA, S is______________
a) 100
b) 0
c) 11
d) 111

Explanation: For fixed bias S=1+beta
Beta=IC/IB=10
S=1+10=11.

7. For a fixed bias circuit having RC=2Kohm and VCC=60V, IB=0.25mA and S=101, find Vce.
a) 12V
b) 10V
c) 5V
d) 2.5V

Explanation: S = 1 + beta,
=> 100 = IC/IB => Ic = 25mA
Vce = VCC – Ic RC
Vce = 10V.

8. For an ideal transistor having a fixed bias configuration, what will be the value of Beta?
a) 0
b) 2
c) -1
d) 1

Explanation: S = 1 + Beta
S = 0
Beta = -1.

9. The temperature changes do not affect the Stability.
a) True
b) False

Explanation: The temperature changes the value of beta which in turn changes the stability of the transition. The temperature changes affect the mobility of the charge carries which results in a change of the current parameters affecting stability.

10. Comparing fixed and collector to base bias which of the following statement is true?
a) Fixed bias is more stable
b) Collector to base bias is more stable
c) Both are the same in terms of stability
d) Depends on the design

Explanation: For fixed bias circuit, S = 1+beta, more the beta, lesser the stability
For collector to base bias S = (1+beta)/(1+beta(RC/RC+RB))
Hence collector to base bias is more stable.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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