1
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Explain
the term : Biasing of the transistor
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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.
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2
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Explain the term : Faithful amplification
of the transistor signal
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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.
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3
|
What is meaning of the ‘zero signal
collector current’?
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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.
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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?
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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
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5
|
What is the DC voltage level to be maintaining
at the input and output circuit of the transistor for faithful amplification?
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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.
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6
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Explain the term : Stabilization of
the transistor
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Stabilization
- It is process of making the transistor operating
point independent of temperature changes and variation of the transistor inherent parameters.
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7
|
Describe the different method of
Transistor biasing
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Transistor biasing method
- Base resistor
- Emitter bias
- Collector feedback resistor
- Voltage divider ( Universal biasing )
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8
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Explain the term : Stability factor
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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.
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9
|
Describe the disadvantages of base
resistor bias.
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Disadvantages of Base resistor bias
- Poor stabilization and
- Possibility of thermal run away due to high stability
factor
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10
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Which biasing method is also known as
fixed bias method? Why?
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The base resistor method is known as fixed bias
method because the base current does not depends upon the collector current.
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11
|
Why the fixed bias method has poor
thermal run away?
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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
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12
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What should be ideal value of
stability factor?
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One
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13
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Give reason : The base bias resistor
method of transistor is unstable.
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Base
bias method
- The Q point of the
transistor in the base bias method depends upon current amplification factor
β therefore it is unstable.
.
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14
|
Give reason : The emitter bias method
of the transistor is stable.
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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.
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15
|
Describe the disadvantages of collector
feedback resistor biasing method.
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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.
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16
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Describe the effect of temperature on
current amplification factor β and base emitter voltage.
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As the temperature
increases the current amplification factor β increase whereas the base
emitter voltage decreases.
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17
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Which biasing method is known as universal
biasing method?
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Voltage divider method
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18
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What is reason behind the name voltage
divider bias?
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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.
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19
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Which biasing method provides highest
thermal stability?
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Voltage divider method
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20
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Explain the term : Mid - point biasing
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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.
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21
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Describe the importance of the midpoint
biasing.
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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.
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22
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Explain the term : Thermal runaway
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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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