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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 IC 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 IC and VCE constant. There are two techniques to make Q point stable.

stabilization techniques :resistor biasing circuits are used which allow IB 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. stabilization techniques2. compensation techniques

In first, resistor biasing circuits are used which allow IB 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 IC .

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 VCC 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 VCC 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 reverse-bias 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 RC || 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 iC = 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 zin (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 zin 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 RE 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 ( IC) 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 βdc without saturating the transistor.

Collector Feedback Bias:

In this case, the base resistor is returned back to collector as shown in .If temperature increases. βdc increases. This produces more collectors current. As IC increases, collector emitter voltage decreases.

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It means less voltage across RB and causes a decrease in base current this decreasing IC, 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.Characteristics Common Base

Common

EmitterCommon Collector

1.Input Dynamic

Resistance

Very Low(less

than 100 ohm)

Low(less than

1K)Very High(750K)

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2.Output dynamic

resistance

Very High(less

than 1M)

High(less than

45K)Low(50 ohm)

3. Current Gain Less than 1 High(100)Very High(greater

than 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

7.Phase relation b/w

i/p and o/pIn Phase

Out of

Phase(180¤)In Phase

8. ApplicationsFor High Freq.

apps

For Audio Freq.

Apps

For impedence

Matching Apps.

- See more at: http://www.mycircuits9.com/2012/06/comparison-of-three-

transistor.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. βdc 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 IC, 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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