01/03/2020

Transistor Biasing Interview Short Question Answer - 1

1
Explain the term : Biasing of the transistor


Biasing of the transistor
  • The proper flow of zero signal collector current, maintain input circuit always forward biased and output circuit always reversed biased in all the conditions is known as the biasing of the transistor.


2
Explain the term : Faithful amplification of the transistor signal


Faithful amplification
  • When a transistor is used as an amplifier, a weak signal is given to the base of the transistor and amplified output is obtained from collector circuit. 
  • The process of obtaining amplified output signal without change of its shape is called as faithful amplification.


3
What is meaning of the ‘zero signal collector current’?


Zero signal collector current
  • The zero signals means that the ac signal is not given to the transistor. The flow of collector current when only DC biasing signal is given to the transistor is called as zero signal collector current.


4
What are the conditions to be satisfied for faithful amplification of the transistor?
OR
What are the essential conditions for biasing of the transistor?


Conditions for faithful amplification of the transistor
  • Proper flow of zero signal collector current
  • Emitter – base circuit remains in forward biased
  • Collector – base circuit remains in reverse biased


5
What is the DC voltage level to be maintaining at the input and output circuit of the transistor for faithful amplification?


In order to achieve faithful amplification in CE transistor  
Emitter – base voltage (VBE) >
0.5 V Germanium transistor
0.7 V Silicon transistor
Collector – base voltage (VCE)  >
0.5 V Germanium transistor
          1.0 V Silicon transistor
  • At the emitter – base circuit once the potential barrier ( Si - 0.7 V and Ge  -  0.5 V ) overcomes, the input circuit should remain in forward biased. 
  • The collector – emitter voltage always greater than the knee voltage ( Ge - 0.5 V and Si - 1.0 V ) otherwise the output circuit should not be properly reverse biased.                                 


6
Explain the term : Stabilization of the transistor


Stabilization
  • It is process of making the transistor operating point independent of temperature changes and variation of the transistor inherent parameters.

7
Describe the different method of Transistor biasing


Transistor biasing method
  • Base resistor
  • Emitter bias
  • Collector feedback resistor
  • Voltage divider ( Universal biasing )


8
Explain the term : Stability factor


Stability factor
  • It is defined as the rate of change of collector current with respect to collector leakage current at constant base current and current amplification factor β in the common emitter transistor.


9
Describe the disadvantages of base resistor bias.


Disadvantages of Base resistor bias
  • Poor stabilization and
  • Possibility of thermal run away due to high stability factor


10
Which biasing method is also known as fixed bias method? Why?


The base resistor method is known as fixed bias method because the base current does not depends upon the collector current.

11
Why the fixed bias method has poor thermal run away?


The fixed bias method has poor thermal run away due to large value of stability factor.  The stability factor in this method is
S = β + 1

12
What should be ideal value of stability factor?


One

13
Give reason : The base bias resistor method of transistor is unstable.


Base bias method
  • The Q point of the transistor in the base bias method depends upon current amplification factor β therefore it is unstable.

.
14
Give reason : The emitter bias method of the transistor is stable.


Emitter bias method
  • The Q point in the emitter bias method does not depend upon current amplification factor β and variation of base – emitter voltage. 
  • Therefore the operating point Q is stable in this method.


15
Describe the disadvantages of collector feedback resistor biasing method.


Disadvantages of collector feedback resistor biasing
  • Gain of the amplifier reduces due to negative feedback circuit. 
  • High stability factor
  • The operating point some extent depend upon temperature.


16
Describe the effect of temperature on current amplification factor β and base emitter voltage.


As the temperature increases the current amplification factor β increase whereas the base emitter voltage decreases.

17
Which biasing method is known as universal biasing method?


Voltage divider method

18
What is reason behind the name voltage divider bias?


Voltage divider bias
  • There are two resistances R1 and R2 are connected across the supply voltage and the voltage divider is done by these resistors R1 and R2. 
  • These resistances provide biasing therefore it is called as voltage divider bias.


19
Which biasing method provides highest thermal stability?


Voltage divider method

20
Explain the term : Mid - point biasing


Mid – point biasing
  • The transistor amplifier circuit is designed such that its operating point lies at the center of the DC load line. 
  • As the operating point lies on the center of load line, it is called as mid-point biasing.


21
Describe the importance of the midpoint biasing.


Importance of midpoint biasing
  • When the AC signal is given to the base of the transistor, the magnitude of the output current and output voltage of the transistor varies according to the position of the operating point. 
  • If the operating point lies above the midpoint on the DC load line, the transistor is operated in the saturation region and part of the output signal will be clipped off. 
  • If the operating point lies below the midpoint on the DC load line, the transistor is operated in the cur-off region resulting portion of the output signal will be clipped. 
  • The mid – point biasing is best amplification method because there is no cut of output signal and transistor is operated in the active region.


22
Explain the term : Thermal runaway


Thermal runaway
  • The collector leakage current in the transistor greatly depends upon the temperature. 
  • The flow of the collector current produces heat at the collector junction. 
  • This will increase the temperature of the transistor and if stabilization is not done properly, collector leakage current is further increased.

IC = βIB + ( β +1 )ICBO
Where ICBO = Collector leakage current
  • If the ICBO increases, the IC also increases and this process is cumulative resulting collector current increases rapidly to burn out transistor. 
  • The burn out of the sterilized transistor is known as thermal runaway.





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