Important Questions of Biasing of Transistor
In this post, some of the important questions of biasing of transistor is given. It includes meaning of biasing of transistor, faithful amplification of transistor signal, conditions for faithful amplifications, zero signal collector current, stabilization of transistor, stability factor, methods of biasing, disadvantages of base resistor bias method, fixed bias method, ideal value of stability factor, emitter bias method, collector feedback resistor bias, voltage divider bias, universal biasing method and thermal runaway.
Biasing of Transistor
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.
Faithful Amplification of Transistor
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.
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.
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
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 - 0.7V and Ge - 0.5V ) overcomes, the input circuit should remain in forward biased. The collector – emitter voltage always greater than the knee voltage ( Ge -0.5V and Si - 1.0V ) otherwise the output circuit should not be properly reverse biased.
Stabilization of Transistor
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
Methods of Biasing
Describe the different method of Transistor biasing
Transistor
biasing method
- Base resistor
- Emitter bias
- Collector feedback resistor
- Voltage divider (Universal biasing)
Stability Factor
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.
Base Resistor Bias
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
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 depend upon the collector current.
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
What should be ideal value of stability factor?
- One
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.
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.
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.
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.
Voltage Divider Bias
Which biasing method is known as universal biasing method?
- Voltage divider method
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.
Which biasing method provides highest thermal stability?
- Voltage divider method
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 unsterilized transistor is known as thermal runaway.
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