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UNIT I
TRANSISTOR BIAS STABILITY
PART –A (2 MARK QUESTIONS)
1. Define stability factor (NOV/DEC 2009) (APR/MAY 2010)
Total change in the quiescent collector current ICQ with reference to change in different
variables such as ICO, β, VBE, VCC, RE, etc.
2. Draw the fixed bias single stage transistor circuit (NOV/DEC 2009)
3. How can collector current be stabilized with respect to Ico variations? (NOV/DEC 2010)
If temperature increases. βdc increases. This produces more collectors current. As I C increases,
collector emitter voltage decreases.
4.
Derive for the stability factor S for a Fixed Bias circuit. (NOV/DEC 2010)
5. What are the types of transistor biasing? (NOV/DEC 2011)
Fixed bias, Voltage divider bias, Self bias.
6. Define thermal resistance.
Thermal resistance is a heat property and a measurement of a temperature difference by
which an object or material resists a heat flow (heat per time unit or thermal resistance).
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Thermal resistance is the reciprocal of thermal conductance.Thermal resistance R has the
units (m2K)/W.
7. What are the advantage and disadvantage of fixed bias circuit
It is simple but is not suitable for the transformer of high input current.
8. What is need for biasing? List out the different types of biasing.(APR/MAY11)
To keep the transistor in its active region it is necessary to keep the operating point in the
middle of dc load line.
Types: Fixed bias, Voltage divider bias, Self bias.
9. Draw the circuit that bias a FET amplifier
10. Write the difference between stabilization techniques compensation techniques
The operating point can be made stable by keeping I C and VCE constant. There are two
techniques to make Q point stable.
stabilization techniques :resistor biasing circuits are used which allow I B to vary so as to keep
IC relatively constant with variations in bdc , ICO and VBE.
compensation techniques: temperature sensitive devices such as diodes, transistors are used
which provide compensating voltages and currents to maintain the operating point constant.
11. When does a transistor act as a switch?
When the transistor is allowed to operate between saturation and cut off region, it is used as
switch
12. What is an operating point?
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The operating point is a specific point within the operation characteristic of a technical
device.
13. What is d.c load line?
It is a graph drawn between Vce and Ic of the transistor. In a BJT circuit, the BJT has a
different current-voltage (IC-VCE) characteristic depending on the base current. Placing a
series of these curves on the graph shows how the base current will affect the operating point
of the circuit.
14. Why capacitor coupling used to connect a signal source to an amplifier?
It is used to block dc and allow ac.
15. Draw the single stage self-biased circuit using PNP transistor.(APR/MAY11)
PART- B (16 MARK QUESTIONS)
1. Calculate the stability factor for a fixed bias circuit. (NOV/DEC 2009)
Stability Factor:
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The operating point can be made stable by keeping IC and VCE constant. There are two techniques
to make Q point stable.
1.
2.
stabilization techniques
compensation techniques
In first, resistor biasing circuits are used which allow I B to vary so as to keep IC relatively constant
with variations in bdc , ICO and VBE.
In second, temperature sensitive devices such as diodes, transistors are used which provide
compensating voltages and currents to maintain the operating point constant.
To compare different biasing circuits, stability factor S is defined as the rate of change of collector
current with respect to the ICO, keeping bdc and VCE constant
S = ¶ IC / ¶ ICO
If S is large, then circuit is thermally instable. S cannot be less than unity. The other stability
factors are, ¶ IC / ¶ bdc and ¶ IC / ¶ VBE. The bias circuit, which provide stability with ICO, also
show stability even if b and VBEchanges.
IC = b dcIB + (I + b dc ) ICO
Differentiating with respect to IC,
In fixed bias circuit, IB & IC are independent. Therefore
and S =
1 + bdc. If b dc=100, S = 101, which means ICincreases 101 times as fast as ICO. Such a large
change definitely operate the transistor in saturation.
2.
Explain the voltage divider bias method and derive an expression
for the stability factor. (APR/MAY 2010)
Voltage Divider Bias:
If the load resistance RC is very small, e.g. in a transformer coupled circuit, then there is no
improvement in stabilization in the collector to base bias circuit over fixed bias circuit. A circuit
which can be used even if there is no dc resistance in series with the collector, is the voltage
divider bias or self bias.
The current in the resistance RE in the emitter lead causes a voltage drop which is in the
direction to reverse bias the emitter junction. Since this junction must be forward biased, the
base voltage is obtained from the supply through R1, R2 network. If Rb = R1 || R2 equivalent
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resistance is very – very small, then VBE voltage is independent of ICO and ¶ IC / ¶ ICO ® 0. For
best stability R1 & R2 must be kept small.
If IC tends to increase, because of ICO, then the current in RC increases, hence base current is
decreased because of more reverse biasing and it reduces I C .
Thevenin's voltage is
Rb is the effective resistance seen back from the base terminal.
If VBE is considered to be independent of IC, then
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The smaller the value of Rb, the better is the stabilization but S cannot be reduced be unity.
Hence IC always increases more than ICO. If Rb is reduced, then current drawn from the supply
increases. Also if RE is increased then to operate at same Q-point, the magnitude of V CC must be
increased. In both the cases the power loss increased and reduced h.
3. (i) Explain the circuit which uses a diode to compensate for changes in
VBE and in ICO. (ii) Discuss the operation of thermistor compensation. (APR/MAY
2010)
 The resistance of Thermistor devices changes with temperature. They use ceramic-like
semiconductors with high thermal coefficients of resistance having high sensitivity to
temperature variations.
 Thermistor has a negative temperature coefficient, where the resistance RT of the device
decreases exponentially with increase in temperature. Thermistor is connected in the CE
potential-divider-bias circuit between positive VCC and the Emitter point of the Transistor.
 As the temperature T rises, the resistance RT of the Thermistor (due to the negative
temperature coefficient property of the Thermistor) decreases and the current fed
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through RT into the Emitter resistor RE increases. Since the voltage drop across RE is in the
direction to reverse-bias the Transistor Emitter–base junction and reduces the Collector
current to the previous designed value.
4. Draw a Self (voltage divider) Bias and derive all the stability factors S, S’ and S”. (16)
(NOV/DEC 2010)
The current in the resistance RE in the emitter lead causes a voltage drop which is in the direction to
reverse bias the emitter junction. Since this junction must be forward biased, the base voltage is
obtained from the supply through R1, R2 network. If Rb = R1 || R2 equivalent resistance is very – very
small, then VBE voltage is independent of ICO and ¶ IC / ¶ ICO ® 0. For best stability R1 & R2 must be
kept small.
If IC tends to increase, because of ICO, then the current in RC increases, hence base current is decreased
because of more reverse biasing and it reduces IC .
To analysis this circuit, the base circuit is replaced by its thevenin's equivalent as shown
Thevenin's voltage is
Rb is the effective resistance seen back from the base terminal.
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If VBE is considered to be independent of IC, then
The smaller the value of Rb, the better is the stabilization but S cannot be reduced be unity.
Hence IC always increases more than ICO. If Rb is reduced, then current drawn from the supply
increases. Also if RE is increased then to operate at same Q-point, the magnitude of V CC must be
increased. In both the cases the power loss increased and reduced h.
5. With the help of neat diagram, explain methods used in biasing the FET and MOSFET.
(16) (NOV/DEC 2011)
SELF-BIASING CIRCUIT FOR FET
Self-Biasing circuit of FET is analogous to that of BJT. It differs from earlier Fixed-Bias circuit as
shown in Fig. 5.33. Resistor RS is connected between the Source and Common terminals. It eliminates
the need for separate VGS. When VDD is applied to the circuit, Drain current ID flows through RS.
Circuit is designed so that voltage drop (ID · RS), across RS, provides required magnitude of reversebias voltage for Gate-to-Source junction. Hence, this type of biasing circuit is known as ‘Self-Biasing
circuit’.
VOLTAGE-DIVIDER-BIAS CIRCUIT FOR FET
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When using a BJT, Voltage-Divider Bias makes the circuit to behave independent of beta (β) changes
and Transistor characteristics, so that the Amplifier with Voltage-Divider-Bias Circuit functions more
stable (Fig. 5.35). In a similar way, FET Amplifier with Voltage-Divider-Bias Circuit works more
stable. By using the Voltage-Divider rule,
DRAIN FEEDBACK BIAS CIRCUIT(MOSFET)
Drain voltage VD is fed back to Gate Terminal through feedback resistor RF. The resistor does
not carry any current. Therefore, drain voltage VD = VG.(Gate voltage).
POTENTIAL-DIVIDER-BIASING CIRCUIT FOR EMOSFET
Potential-Divider-Bias for Enhancement MOSFET without Source resistor
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6. How is dc and ac load lines draw on the output characteristic curves of on the output
characteristics curves of an amplifier sketch them &discuss (16)
AC Load line:
Consider the dc equivalent circuit
Assuming IC = IC(approx), the output circuit voltage equation can be written as
The slop of the d.c load line is
.
When considering the ac equivalent circuit, the output impedance becomes R C || RL which is less than
(RC +RE).
In the absence of ac signal, this load line passes through Q point. Therefore ac load line is a line of
slope (-1 / ( RC || RL) ) passing through Q point. Therefore, the output voltage fluctuations will now be
corresponding to ac load line as shown in fig. 2. Under this condition, Q-point is not in the middle of
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load line, therefore Q-point is selected slightly upward, means slightly shifted to saturation side.
To find the voltage gain, consider an unloaded CE amplifier. The transistor can be replaced by its
collector equivalent model i.e. a current source and emitter diode which offers ac resistance r' e.
The input voltage appears directly across the emitter diode.
Therefore emitter current ie = Vin / r'e.
Since, collector current approximately equals emitter current and i C = ie and vout = - ie RC (The minus
sign is used here to indicate phase inversion)
Further vout = - (Vin RC) / r'e
Therefore voltage gain A = vout / vin = -RC / r'e
The ac source driving an amplifier has to supply alternating current to the amplifier. The input
impedance of an amplifier determines how much current the amplifier takes from the ac source.
In a normal frequency range of an amplifier, where all capacitors look like ac shorts and other
reactance are negligible, the ac input impedance is defined as
zin= vin/ iin
Where vin, iin are peak to peak values or rms values
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The impedance looking directly into the base is symbolized z in (base) and is given by
Z in(base) = vin / ib ,
Since,v in = ie r'e
» bi b r'e
zin (base) = b r'e.
From the ac equivalent circuit, the input impedance z in is the parallel combination of R1 , R2 and b r'e.
Zin = R1 || R2 || b r'e
The Thevenin voltage appearing at the output is
vout = A vin
The Thevenin impedance is the parallel combination of RC and the internal impedance of the current
source. The collector current source is an ideal source, therefore it has an infinite internal impedance.
zout = RC.
The simplified ac equivalent circuit is
7. Derive expression for the various stability factors (16)
Emitter Feedback Bias:
In this circuit, the voltage across resistor R E is used to offset the changes in bdc. If bdc increases, the
collector current increases. This increases the emitter voltage which decrease the voltage across base
resistor and reduces base current. The reduced base current result in less collector current, which
partially offsets the original increase in bdc. The feedback term is used because output current ( I C)
produces a change in input current ( IB ). RE is common in input and output circuits.
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In this case
Since IE = IC + IB
Therefore,
In this case, S is less compared to fixed bias circuit. Thus the stability of the Q point is better.
Further,
If IC is to be made insensitive to βdc than
RE cannot be made large enough to swamp out the effects of β dc without saturating the transistor.
Collector Feedback Bias:
In this case, the base resistor is returned back to collector as shown in .If temperature increases. β dc
increases. This produces more collectors current. As I C increases, collector emitter voltage decreases.
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It means less voltage across RB and causes a decrease in base current this decreasing I C, and
compensating the effect of bdc.
In this circuit, the voltage equation is given by
Circuit is stiff sensitive to changes in βdc. The advantage is only two resistors are used.
Then,
Therefore,
It is better as compared to fixed bias circuit.
Further,
Circuit is still sensitive to changes in βdc. The advantage is only two resistors are used.
8. Compare different types of transistor configuration.
sl
no.
1.
Common
Characteristics
Common Base
Input Dynamic
Very Low(less
Low(less than
Resistance
than 100 ohm)
1K)
Emitter
Common Collector
Very High(750K)
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Output dynamic
Very High(less High(less than
resistance
than 1M)
45K)
3.
Current Gain
Less than 1
High(100)
4.
Leakage Current
Very Small
Very Large
Very Large
5.
Voltage Gain
About 150
About 500
Less than 1
6.
Power Gain
Medium
Highest
Medium
2.
7.
8.
Phase relation b/w
i/p and o/p
Applications
In Phase
Out of
Phase(180¤)
Low(50 ohm)
Very High(greater
than 100)
In Phase
For High Freq.
For Audio Freq. For impedence
apps
Apps
Matching Apps.
- See more at: http://www.mycircuits9.com/2012/06/comparison-of-threetransistor.html#sthash.wfdCiup5.dpuf
9. Derive the expression of stability factor for collector feedback amplifier. (APR/MAY11)
Collector Feedback Bias:
In this case, the base resistor is returned back to collector. If temperature increases. β dc increases. This
produces more collectors current. As IC increases, collector emitter voltage decreases. It means less
voltage across RB and causes a decrease in base current this decreasing I C, and compensating the effect
of bdc.
In this circuit, the voltage equation is given by
Circuit is stiff sensitive to changes in βdc. The advantage is only two resistors are used.
Then,
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Therefore,
It is better as compared to fixed bias circuit.
Further,
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