EC8351 / Electronics Circuits – I

TWO MARKS QUESTIONS AND ANSWERS

UNIT – I Biasing of Discrete BJT, JFET and MOSFET

1. What is Biasing?
 The process of giving proper supply voltages and resistances for obtaining the desired Q-Point is
called Biasing.
 For proper operation, emitter-base junction should be forward biased and Base-collector Junction
should be reverse biased.

2. What is operating point? (Nov 2016, Nov 2017)

The voltages (VCEQ) and current (ICQ) which set to operate the transistor in the active region are called
quiescent values. These quiescent values determine the operating point or Q-Point for the transistor.

3. What are the techniques used to establish the operating point in active region?
Compensation and stabilization techniques are needed to establish the operation point in active region.

4. What is the effect of temperature on Q-Point?

Q-Point tends to shift its position due to any (or) all of the following three main factors.
a) Reverse Saturation current ICO which doubles for every 100c increase in temperature.
b) Base-Emitter Voltage VBE decrease by 2.5mV/0C
c) Transistor current Gain, β increase with temperature.

5. What is reverse Saturation Current [ICO (or) ICBO]?

Reverse saturation current is the current flowing through reverse biased collector-base junction that is the
collector to base leakage current with emitter open.

6. What is Thermal Runway? (Nov 2017, MAY 2014, NOV 2011)

How thermal runaway occurs in a transistor? (NOV 2018)
The problem with increasing temperature causing increasing collector current is that more current increase
power dissipated by the transistor which, in turn, increases its temperature. This self reinforcing cycle is
known as thermal runaway.
7. Define Stability Factor. (NOV 2012, MAY 2012, NOV 2011, MAY 2010, MAY 2009, Nov 2015,
May 2017, May 2019)
Stability factor, S is defined as the rate of change of collector current, IC with respect to the collector –base
leakage current, ICO Keeping both the current, IB and Current gain as constant.
S=∆Ic/∆Ico Keeping β and IB as constant

8. What are Stability Factor S’ and S”? (NOV 2012, May 2017)
Stability Factor S‟ is the rate of change of IC with VBE, Keep ICO and β as constant.
S‟=∆Ic/∆Vbe Keeping ICO and β as constant
Stability factor S” is the rate of change of IC with respect to β, Keeping ICO and VBE as constant.
S”= ∆Ic/∆β Keeping ICO and VBE as constant
9. List out the three stability factors. (May 2017)
S=∆Ic/∆Ico Keeping β and IB as constant
S‟=∆Ic/∆Vbe Keeping ICO and β as constant
S”= ∆Ic/∆β Keeping ICO and VBE as constant

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10. What are the types of BJT biasing? (NOV 2019)
1) Fixed Bias (or) Base Resistor Method 2) Collector to Base
Bias (or) Biasing with Feedback Resistor 3) Self-Bias (or)
Emitter Bias (or) Voltage Divider Bias

11. What are the advantages and disadvantages of Fixed Bias?

Advantages:
1. Simplicity
2. Small Numbers of Components Required
3. If supply voltage is very large compared to VBE than IB is independent to VBE.
Disadvantages:
1. Stability Factor S = 1+β
Since is large quantity, this is very poor stable circuit and so not widely used for biasing the base.

12. Write the disadvantages of collector to base bias? (MAY 2006)

1. Stability is very poor if collector resistance is very small.
2. Hence, it is not used for amplifier circuits like transformer coupled amplifiers.

13. How self-bias circuit used as a constant current circuit?

How stability is maintained in self-bias circuit?
If collector current increases also emitter resistance increases. Hence the voltages drop across RE
increases thereby decreasing the base current IB. As a result IC is maintained almost constant.

14. Compare all the biasing circuits with respect to stability factor?
How the Self-bias circuit over comes the other types?
Write the advantages of Self- Bias.
In Fixed Bias and Collector to Base Bias, stability factor value is very large which leads to poor stability.
Hence Fixed Bias and Collector to Base Bias are not preferred for Biasing.
In Self- Bias method, when RB/RE is Very small, S=1 (Approx.) which provides good stability. Hence Self-
Bias method is the best one over other types of biasing.

15. What are the types of Bias Compensation Techniques?

1. Diode Compensation
2. Thermistor Compensation
3. Sensistor Compensation

16. What is diode compensation? (April 2018)

Diode compensation is a technique that is used to reduce the Q-point variations by selecting a diode that
has temperature characteristics similar to the transistor.

17. When does a transistor act as a switch? (NOV 2018)

When a transistor works as a switch it works in cutoff and saturation regions.

18. Why silicon is most widely used semiconductor material?(APRIL/MAY 2010)

1. Silicon has low leakage current, ICO 1. Germanium has high leakage current
0.01 µA to 1 µA ICO 2 µA to 15 µA
0
2. Greater working temperature 150 C 2. lower working temperature 700C

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19. What is early effect (or) base width modulation? (NOV 2019)
A greater reverse bias across the collector-base junction increases the collector-base depletion
width, thus decreasing the width of the charge neutral portion of the base.

20. What is reach through (or) punch through? (MAY 2014)

An emitter-to-collector breakdown which can occur in a junction transistor with very narrow base
region at sufficiently high collector voltage when the space-charge layer extends completely across
the base region.

21. What do you understand by DC & AC load line? What is an ac load line? (Nov 2016)
What is the slope of AC load line? (NOV 2018)
DC Load Line
It is the line on the output characteristics of a transistor circuit which gives the values of IC & VCE
corresponding to zero signal (or) DC Conditions.
AC Load Line
This is the line on the output characteristics of a transistor circuit which gives the values of IC &
VCE when signal is applied. The slope of AC load is -1/Rac. where Rac=Rc paralel with load
resistance.
22. What are the factors that affect the Q point of the circuit?(NOV 2009, May 2019)
β, ICO, VBE are the parameters varying with the temperature. These are the factors that affect the Q
point of the circuit.

23. Why do you fix Q point in the middle of dc load line? (Nov 2015, Nov 2005)
What is the function of the Q point? (NOV 2013)
The operating point of a transistor is kept fixed usually at the center of the active region in order
that the input signal is well amplified.
If the point is fixed in the saturation region or the cut off region the positive and negative half cycle
gets clipped off respectively.

24. What is the need for biasing in transistor amplifier?(APRIL 2011, NOV 2008)(APRIL 2004)
When a transistor is biased properly, it works efficiently and Produces no distortion in the output
signal of the amplifier and thus operating point can be maintained stable.

25. What are the transistor parameters that vary with the temperature?
β, ICO, VBE are the parameters varying with the temperature.

26. Why is it necessary to stabilize the operating point of transistor?(NOV/DEC 2005)

List the importance of selecting the proper operating point. (MAY 2015)
 For faithful amplification it is necessary to stabilize the operating point of transistor.
 To avoid distortion.

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27. What are all the factors that affect the stability of the operating point?
What are the factors against which an amplifier needs to be stabilized? (DEC 2014)
The following are the factors that affect the stability of the operating point,
a. Change of Q due to replacement of transistors (change of Q due to change of transistors
parameters (VBE, ICO,β)).
b. Thermal variations.

28. How can collector current be stabilized with respect to ICO variations?(NOV/DEC 2010)
The collector current IC with respect to the reverse saturation collector current ICO variation is
stabilized by, keeping „VBE‟ and „β‟ constant.
Collector current can be stabilized with respect to ICO variations using stabilization and
compensation techniques.

29. Draw the various bias circuits.

Draw the fixed and self bias circuits. (NOV 2008)

Fixed bias Collector to base bias Self bias

30. Define fixed bias, collector to base bias.

Fixed bias: The circuit is called as a “fixed current bias circuit”, because the transistors base
current, IB remains constant for given values of VCC, and therefore the transistors operating point must
also remain fixed.
Collector to base bias:
In this circuit, the base bias resistor, RB is connected to the transistor‟s collector, instead of to the
supply voltage rail, VCC.
31. Why voltage divider bias is commonly used in amplifier circuit?
The voltage divider bias has the following advantages
a. The operating point will be in stable position.
b. The stability will be considerably improved.
c. IC can be reduced to the collector leakage current ICO.

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32. What is meant by stabilization?
The operating point can be made stable by keeping IC and VCE constant. There are two techniques to
make Q point stable.
Stabilization Techniques Compensation Techniques

Resistor biasing circuits are used which allow IB Temperature sensitive devices such as diodes,
to vary so as to keep IC relatively constant with transistors are used which provide
variations in hfe, ICO and VBE. compensating voltages and currents to maintain
the operating point constant.

33. What are thermistor and sensistor devices?

 Thermistor-Negative temperature coefficient device
 Sensistor- positive temperature coefficient device

34. What is thermal stability? (NOV 2013)

IC and VCE remain stable when the circuit temperature change is known as thermal stability.

35. How to avoid thermal run away?

• Collector is made larger than emitter
• Proper design to keep IB constant, so IC kept constant
• Use of heat sink to dissipate heat.

36. Why the input impedance of FET is more than that of a BJT?
 The input impendence of FET is more than that of a BJT because the input circuit of FET is reverse
biased whereas the input circuit of BJT is forward biased.

37. What is pinch off voltage?

Pinch off voltage is the drain to source voltage after which the drain to source current becomes
almost constant and JFET enters into saturation region and is defined only when gate to source
voltage is zero.

38. Write the different types of FET biasing circuits. (MAY 2012)
The FET biasing circuits are classified as,
a. Gate bias, b. Self bias c. Voltage divider bias,
d. Current source bias e. Drain feedback bias f. Zero bias.

39. Why input impedance of MOSFET is higher than that of FET?

Input impedance of MOSFET is higher than JFET because of gate is insulated from the channel by
a thin layer of silicon dioxide.

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40. What are the function adding emitter resistor and source resistor in BJT and FET biasing
circuit respectively? (Nov 2016)
Adding emitter resistor and source resistor tends to stabilize the gain due to change in temperature and
transistor‟s parameters replacement. These resistors provide negative feedback in the circuits.

41. What are the operating regions of N-channel MOSFET and how do you identify the
operating region? (NOV 2014)
There are three operating regions in N-channel MOSFET
 Cut off region
 Linear region
 Saturation region
a. Cut off region
Here the current flow is essentially zero (accumulation mode)
b. Linear region
It is also called weak inversion region where the drain current is dependent on the gate and the
drain voltage with respect to the substrate.
c. Saturation region
Channel is strongly inverted and the drain current flow is ideally independent of the drain-
source voltage (strong-inversion region).

42. Sketch the fixed biasing circuit of JFET. (April 2018)

43. Derive for the stability factor S for a Fixed Bias circuit. (NOV 2010)
 1
Stability factor, S =
 dI 
1   B 
 dI C 
dI B dI
In fixed-bias method of biasing, IB is independent of IC so that =0. Putting the value of B =0
dI C dI C
in the above expression, we have, stability factor, S=β+1.

44. What is the impact of temperature on drain current of MOSFET? (Nov 2016)
The drain current decreases with increasing temperature due to decrease the mobility of the charge
carriers in the channel.

45. In a CB connection emitter current is 6.28 mA and the collector current is 6.20 mA
.Determine the CB dc current gain.(NOV 2011)
Given: IE = 6.28 mA and IC = 6.20 mA
Solution: DC current gain of the CB is IC/IE= 6.20 mA/6.28 mA=0.98
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May 2019
(NOV 2018)
With neat diagrams, explain any two bias compensation techniques. (NOV 2018) (May 2019)
Q: Find the operating region of n-channel MOSFET with VGS=1V, Vt=0.8V and VDS=1.2V. (NOV
2019)
VGS > Vt and (VGS – Vt) < VDS, So This n-channel MOSFET operated in the saturation region.

Q: For the circuit in Fig.1 draw DC load line and find the Q-point. hfe=150, VCC=15V, RC=2100Ω,
R1=566K Ω, R2=120K Ω , VBE-on=0.7V, IB=10µA. (NOV 2019)
Solution:
Collector to emitter voltage, VCE =VCC - ICRC ------1
To draw dc load line:
Put VCE=0 in equation 1
IC= VCC/ RC =15/2100=7.14mA
Put IC=0 in equation 1
VCE= VCC=15V
To find Q-point:
Collector current , ICQ= hfe x IB =150 x 10µA=1.5mA
Collector to emitter voltage, VCE Q=VCC - ICRC =15 –(1.5mA x2100)=11.85V
Q-point = (VCEQ, ICQ) = (11.85V, 1.5mA)

DC load line

Q: For the circuit in Fig.2, find the drain current and drain to source voltage. MOSFET is
operating in saturation region. (NOV 2019)
R1=30KΩ, R2 =20KΩ, VDD=10V, VTN=1V, RD=20KΩ, Kn=0.1mA/V2

Solution:
R2 20 K
Gate to source voltage is VG  VGS  ( )VDD = ( )10 =4V
R1  R2 30 K  20 K
The drain current is ID = Kn(VGS − VT N)2 = (0.1)(4 – 1)2 = 0.9mA
and the drain-to-source voltage is VDS = VDD − ID RD = 10 − (0.9mA)(20KΩ) = -8V
 V.R.S. College of Engineering & Technology, Arasur – 607 107.
(Accredited by NAAC and an ISO 9001:2008 Recertified Institution)
Department of Electronics and communication Engineering

Subject Code / Name: EC8351 / Electronics Circuits – I Year/ Sem: II/III

Unit – I: Power supplies and Biasing of Discrete BJT and MOSFET
Part –A
1. What is meant by biasing?
2. What is Thermal Runway?
3. Define Stability Factor. Also list the three Stability factors.
4. What do you understand by DC & AC load line?
5. Define Operating point and Why do you fix Q point in the middle of dc load line?
6. What is the need for biasing in transistor amplifier?
7. Why is it necessary to stabilize the operating point of transistor?
8. What are the factors that affect the stability of the operating point?
9. Define thermal stability and thermal instability.
10. What is compensation and diode compensation?
11. What are the operating regions of N-channel MOSFET and how do you identify the operating region?
12. Draw a DC load line and find operating point of the circuit shown in figure1.

13. Find the Q – point of the circuit shown in figure 1.

14. What is the impact of temperature on drain current of MOSFET?
15. Give advantages of self bias over fixed bias in transistor amplifier.
Part – B & Part – C
1. Explain the voltage divider bias method and derive an expression for the stability factor.
2. Explain the collector to Base bias & fixed bias methods and derive an expression for the stability factor.
3. Explain the operation of diode compensation, sensistor compensation and thermistor compensation.
4. Explain the different types JFET biasing circuits. Or With neat diagram, explain the working of self bias
and voltage divider bias for common source amplifier.
5. Explain the biasing circuit of MOSFET. Or With a neat diagram explain the source and drain resistance
biasing of MOSFET.
6. Why biasing is necessary in BJT amplifier? Explain the concept of DC & AC load line with neat
diagram. How will you select the operating point, explain it using CE amplifier characteristics?
7. Briefly explain the reason for keeping the operating point of a transistor as fixed.
8. Design a voltage divider bias circuit for a silicon transistor for the specified conditions. VCC=10V,
VCE=5V, IC=2mA, S=30, VE= 1Vand β=150. Determine the values of R 1,R2,RE and RC.
9. Determine the value of RB and RC for a collector to base bias circuit for the specified condition:
VCC=14V, VCE=7V, IC = 4mA and β=125.
10. Calculate the Q point of the self biased JFET having the V DD=20V, IDSS=10mA and VGS=-2.4V at
5.2mA. Also determine the values of resistors RD & RS.