EDC Complete

DEPT. OF ELECTRONICS AND COMMUNICATION ENGINEERING SUB: EC 2151 ELECTRIC CIRCUITS AND ELECTRON DEVICES

(Common to ECE, CSE and IT Branches)SEM / YEAR: II / I

UNIT I CIRCUIT ANALYSIS TECHNIQUES

Kirchoff’s current and voltage laws – series and parallel connection of independent sources – R, L and C – Network Theorems – Thevenin, Superposition, Norton, Maximum power transfer and duality – Star-delta conversion.

UNIT II TRANSIENT RESONANCE IN RLC CIRCUITS

Basic RL, RC and RLC circuits and their responses to pulse and sinusoidal inputs –Frequency response – Parallel and series resonances – Q factor – single tuned and Double tuned circuits.

UNIT III SEMICONDUCTOR DIODES

Review of intrinsic & extrinsic semiconductors – Theory of PN junction diode – Energy band structure – current equation – space charge and diffusion capacitances – Effect of temperature and breakdown mechanism – Zener diode and its Characteristics.

UNIT IV TRANSISTORS

Principle of operation of PNP and NPN transistors – study of CE, CB and CC Configurations and comparison of their characteristics – Breakdown in transistors – Operation and comparison of N-Channel and P-Channel JFET – drain current Equation – MOSFET – Enhancement and depletion types – structure and operation Comparison of BJT with MOSFET – thermal effect on MOSFET.

UNIT V SPECIAL SEMICONDUCTOR DEVICES

Tunnel diodes – PIN diode, varactor diode – SCR characteristics and two transistor equivalent model – UJT – Diac and Triac – Laser, CCD, Photodiode, Phototransistor, Photoconductive and Photovoltaic cells – LED, LCD.

TEXT BOOKS:1. Joseph A. Edminister, Mahmood, Nahri, “Electric circuits” – Shaum series, Tata Mc Graw Hill, (2001)2. S.Salivahanan, N. Suresh Kumar and A. Vallavanraj, “Electronic devices & Circuits” Tata Mc Graw Hill, 2nd edition (2008)3. David A. Bell, "Electronic Devices and circuits", Oxford University Press, 5th Edition, (2008)

REFERENCES:1. William H.Hayt, J.V.Jack, E.Kemmebly & Steven M. Durbin, “Engineering circuit analysis” Tata Mc Graw Hill, 6nd edition (2002)2. J. Millman and Halkins, Satyebranta jit, “Electronic devices & Circuits” Tata Mc Graw Hill, 2nd edition (2008)

1

UNIT-1

CIRCUIT ANALYSIS TECHNIQUES

PART-A1. Define current & voltage?

The flow of free electrons in a metal is electric current and its unit measure is in amperes. The ability of the charged body to do work is called as electric potential and unit is volts.

2. State ohm’s law.

The ratio of potential difference b/w any two points of a conductor to the current flowing, provided the physical conditions do not change.

3. State the limitations of ohm’s law.

Doesn’t apply to non metallic conductors. Doesn’t apply to non linear devices.

4. State Kirchhoff’s voltage law.

The algebraic sum of the potential around a closed loop is zero

5. State two salient points for series combinations.

The current is same to all the elements The total resistance is greater than the greatest resistance in the circuit

6. State Super position theorem.

The Responses in circuit in multiple source is given by the algebraic sum of the responses due to the individual sources acting alone

7. State Thevenin’s theorem.

Any linear network with output terminal can be replaced by a single voltage source in series with the impedance (Thevenin resistance)

8. State Norton’s theorem.

Any linear network with output terminal can be replaced by a single current source in parallel with impedance

9. When superposition theorem can be applied?

When there are more than one source acting in a network.

2

10. Explain the uses of Thevenin’s theorem.

Applicable to all linear circuits This Theorem is useful when it is desired to know the effect of responses in the network

PART B1. Calculate the mesh current i1 and i2 in the following circuit

Equation for Mesh 1

Equation for Mesh 2

Now we have to solve the two equations:

2. Explain about Krichoff's current and voltage laws with suitable examples. KIRCHOFF’S VOLTAGE LAW (KVL)

The sum of the voltage drops around any closed loop is zero.

MAJOR IMPLICATION

KVL tells us that any set of elements which are connected at both ends carry the same voltage.

We say these elements are in parallel.

KVL clockwise, start at top:

Vb – Va = 0

Va = Vb

3

KVL EXAMPLE

Path 1: 0vvv b2a Path 2: 0vvv c3b Path 3: 0vvvv c32a

vcva

+

+

3

21

+

vb

v3v2

+

+

-

Examples of three closed paths:

a b c

KIRCHOFF’S CURRENT LAW(Sum of currents entering node) (Sum of currents leaving node) = 0

MAJOR IMPLICATION

KCL tells us that all of the elements in a single branch carry the same current.

We say these elements are in series.

Current entering node = Current leaving node

i1 = i2

KIRCHHOFF’S CURRENT LAW EXAMPLE

Currents entering the node: 24 A

Currents leaving the node: 4 A + 10 A + i

Three formulations of KCL:

A 18i 0i10424 :3

A 18i 0i10)4(24 :2

A 18i i10424 :1

i 10 A

24 A -4 A

Page no. 64-67, R.S. Sedha, ‘A text book of Applied Electronics’ S.CHAND pub 2005.

3. Use nodal analysis to find v1 in this circuit.

4

4. Using superposition theorem find the voltage drop across each resistor

Since we have two sources of power in this circuit, we will have to calculate two sets of values for voltage drops and/or currents, one for the circuit with only the 28 volt battery in effect.

and one for the circuit with only the 7 volt battery in effect:

When re-drawing the circuit for series/parallel analysis with one source, all other voltage sources are replaced by wires (shorts), and all current sources with open circuits (breaks). Since we only have voltage sources (batteries) in our example circuit, we will replace every inactive source during analysis with a wire.

Analyzing the circuit with only the 28 volt battery, we obtain the following values for voltage and current:

5

Analyzing the circuit with only the 7 volt battery, we obtain another set of values for voltage and current:

5. Determine the voltage drop across each one (applying ohm’s law in its proper context)

6

6. Find the voltage across AB using thevenin's theorem and hence draw its equivalent circuit

UNIT-2

TRANSIENT RESONANCE IN RLC CIRCUITSPART-A

1. Define waveform

Instantaneous values of voltage or current plotted against time.

2. Define time period

Time taken to complete one full cycle.

T=2π/ω

3. Define peak factor

Ratio of peak value of the wave to the rms value of the wave

Peak factor = Vp/Vrms

4. Define form factor

Ratio of rms value to the average value of the wave.

Form factor = Vrms /Vavg

7

5. Calculate the period for the given frequency 100 KHz?

T=1/f

= 1/100x103 = 10µs

6. A sinusoidal voltage is applied to a capacitor. The frequency of the sine wave is 2KHz.

Determine the capacitive reactance.

Xc = 1/ = 1/2πfc

= 1/2xπx2x103x0.01x10-6 = 7.96KΩ

7. For the circuit having inductance of 50mH and a voltage source of Vrms 10V and

frequency 10KHz connected in series determine the rms current in the circuit?

XL = 2πfL = 3.141KΩ

Irms = Vrms / XL

= 10/3.141x103 = 3.18mA

8. Define quality factor

Quality factor for the circuit is defined as

9. Define resonant circuit.

The circuit that treats a narrow range of frequencies very differently than all other

frequencies is called a resonant circuit.

10. Give the expression for quality factor of series RLC circuit.

Q=

11. Give the expression for quality factor of parallel RLC circuit.

Q=

PART B

1. The RL circuit has an emf of 5 V, a resistance of 50 W, an inductance of 1 H, and no initial current. Find the current in the circuit at any time t. Distinguish between the transient and steady-state current.

Answer: The formula is:

8

After substituting:

Solving:

I.F. =

When , so

This gives us:

The transient current is: A

A

2. A series RL circuit with R = 50 W and L = 10 H has a constant voltage V = 100 V applied at t = 0 by the closing of a switch.

Find

(a) The equation for i (you may use the formula rather than DE),

(b) The current at t = 0.5 s

(c) The expressions for VR and VL

(d) The time at which VR = VL

Answer

We solve it using the formula:

We have:

9

(b) At A

(c)

(d)

V

TC for this example is:

3. A series RC circuit with R = 5 W and C = 0.02 F is connected with a battery of E = 100 V. At t = 0, the voltage across the capacitor is zero.

(a) Obtain the subsequent voltage across the capacitor.

(b) As t → ∞, find the charge in the capacitor.

Answers

We will solve this 3 ways, since it has a constant voltage source:

1 and 2: Solving the DE in q, as:

a linear DE and variables separable

3. Using the formulae and

10

Method 1 - Solving the DE in q

From the formula: , we obtain:

On substituting, we have:

Solving this differential equation as a linear DE, we have:

IF =

So

So

Now, since , (that is, when , ) this gives:

So

As , C.

Now,

For comparison, here is the solution of the DE using variables separable:

11

Since , we have

Method 2: We use the formulae and .

Now

So:

Now

From here, we use and obtain:

So , as before. Also, as , C.

12

4. Find the charge and the current for t > 0 in a series RC circuit where R = 10 W, C = 4 × 10-3 F and E = 85 cos 150t V.

Assume that when the switch is closed at t = 0, the charge on the capacitor is -0.05 C.

Answer:

[We cannot use the formulae and , since the voltage source is not constant.]

From the formula: , we obtain:

Since , and , we have:

Now, we can solve this differential equation in q using the linear DE process as follows:

IF = ,

Then we use the integration formula (found in a standard integral table):

We obtain:

So, dividing throughout by gives:

We now need to find K13

Means

So this gives us:

We are also asked to find the current. We simply differentiate the expression for q:

5. In the RC circuit shown below, the switch is closed on position 1 at t = 0 and after 1 τ is moved to position 2. Find the complete current transient.

Answer:

At t = 0, the switch is at Position 1.

We note that

Using SNB to solve this differential equation, we have:

NOTE: By differentiating, this gives us:

14

We need to find :

.

Now, at , the charge will be:

At , switch at Position 2:

Applying the formula again:

NOTE: The negative voltage is because the current will flow in the opposite direction through the resistor and capacitor.

Exact solution is:

So the current transient will be:

This expression assumes that time starts at . However, we moved the switch to Position 2 at

, so we need:

So the complete current transient is:

For

For

15

UNIT –III

SEMICONDUCTOR DIODES

PART -A

1. What are Semiconductors? Give examples?

The materials whose electrical property lies between those of conductors and insulators are known as Semiconductors. Ex germanium, silicon.

2. What are Intrinsic and Extrinsic Semiconductors?

Pure form of semiconductors are said to be intrinsic semiconductor. Ex: germanium, silicon.If certain amount of impurity atom is added to intrinsic semiconductor the resulting semiconductor is Extrinsic or impure Semiconductor.

3. What is forward bias and reverse bias in a PN junction?

When positive of the supply is connected to P type and negative to N type then it is forward bias. When positive of the supply is connected to N type and negative to P type then it is reverse bias.

4. What is Reverse saturation current?

The current due to the minority carriers in reverse bias is said to be reverse saturation current.

5. Give two applications of PN junction diode. As rectifier in power supplies. As switch in logic circuits

6. Define Transition or space charge capacitance.

The parallel layers of oppositely charged immobile ions on the two sides of the junction form the Transition or space charge capacitance.

CT =εA/W Where ε = permitivity of the material, A=cross sectional area of the junction and W=width of the depletion region

7. Define Diffusion capacitance.

The capacitance that exists in the forward biased junction is called a diffusion or storage capacitance. It is also defined as the rate of change of injected charge with applied voltage.

CD=dQ/dV

8. What is peak inverse voltage? It is the maximum reverse voltage that can be applied to the pn junction without damaging the junction

16

9. What is Zener breakdown?

The breakdown potential can be brought to lower levels by increasing the doping levels in the p-type and n-type materials of the diode. In this case breakdown is initiated through a direct rupture of the covalent bonds due to the existence of the strong electric field at the junction.

10. Draw the V-I characteristics of a pn diode

PART B

1. Explain the theory of pn junction diode.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 86

2. Explain about Zener diode characteristics.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 122

3. Explain about the energy band structure.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 93

4. Discuss the diode current equation.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 98

5. Explain briefly about the VI Characteristics of pn junction diode.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 103

UNIT-IV

TRANSISTORS

Part: A

1. What are PNP and NPN transistors?

PNP Transistor: The figure below shows a PNP Transistor.

17

A block representation of a layer of n-type material between two layers of p-type material is known as a pnp transistor.

NPN Transistor: The figure below shows an NPN Transistor.

A block representation of a p-type material between two layers of an n-type material is known as npn transistor.

2. What is a CE configuration?

CE Configuration-In CE Configuration, the input voltage is applied between the B and E terminals and the output is taken at the C and E terminals.

3. What is a CB Configuration?

In CB Configuration- the base terminal is common to both the input (EB) voltage and output (CB) Voltage.

4. What is a CC Configuration?

In CC Configuration- the collector terminal is common to both input CB Voltage and CE Voltage.

5. What are the characteristics of CE configuration?

The various characteristics of CE Configuration are:

Input Characteristics: To determine the input characteristics, Vce is held constant and IB levels are recorded for several values of VBE.

Output Characteristics: IB is maintained constant at several convenient levels. At each fixed level of IB, VCE is adjusted in steps, and the corresponding values of IC are recorded.

Current gain Characteristics: These characteristics are simply Ic plotted versus Ib for various fixed values of VCE.

6. What are the characteristics of CB configuration?

The various characteristics of CB Configuration are:

18

Input Characteristics: To determine the input characteristics, the output (CB) voltage is maintained constant, and the input (EB) voltage is set to several convenient levels. For each level of input voltage the input current IE is recorded.

Output Characteristics: The emitter current IE is held constant at each of several fixed levels. For each fixed level of IE, the output voltage VCB is adjusted in steps and the corresponding levels of collector current IC are recorded.

Current gain Characteristics: The current gain characteristics are obtained experimentally by holding VCB fixed at a constant level and measuring the IC levels measured for various settings of IE.

7. What are the characteristics of CC configuration?

The various characteristics of CC Configuration are:

Input Characteristics: The common collector input characteristics are quite different from either CB or CE Configurations

Output Characteristics: The CC output characteristics are practically identical to those of the CE circuit.

Current gain Characteristics: The CC current gain characteristics are obtained by plotting IE vs. IB.

8. Define breakdown in Transistors.

If the reverse- bias voltage on the CB junction is allowed to exceed the maximum safety limit specified by the manufacturer then breakdown may occur in transistors. This is also known as transistor breakdown.

9. What is an N-Channel JFET?

The n-channel JFET consists of an n-type channel with two p-type gate regions on either side.

10. What is a P-Channel JFET?

In p-channel JFET the channel is a p-type material and the gate regions are n-type. The drain source potential is applied positive to the source and negative to the drain.

11. What is a MOSFET?

The MOSFET is an abbreviation for Metal Oxide Semiconductor Field Effect Transistors. Like the JFET, it has got5 t a gate source and a drain.

However unlike JFET, the gate of a MOSFET is insulated from the channel .Because of this the MOSFET is sometimes known as IGFET.

12. What are the types of MOSFET?

19

There are two main types of MOSFET: a. Enhancement mode MOSFET b. Depletion mode MOSFET

13. What is an Enhancement mode MOSFET?

The Enhancement mode MOSFET has no Depletion mode and it operates only in the enhancement mode.

14. How does an Enhancement mode MOSFET differ from a depletion mode MOSFET construction wise?

An Enhancement mode MOSFET differs from a depletion mode MOSFET construction wise in the sense that the Enhancement mode MOSFET has no “physical channel”.

PART-B

1. Explain with neat diagram the operation of PNP and NPN transistors. Refer (Electronic devices and circuits by S.Salivahanan) Page no: 141,142

2. Explain the structure and operation of CE, CB and configurations with neat diagram.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 146,148

3. Explain with neat diagram the operation of Enhancement mode and Depletion mode MOSFETS.

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 184,185

4. Explain with neat diagram the operation of N-channel and P-channel JFET.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 175-178

5. Derive the drain current Characteristics.Refer (Electronic devices and circuits by S.Salivahanan) Page no: 180

UNIT: V

SPECIAL SEMICONDUCTOR DEVICES

PART- A

1. What is a tunnel Diode? Give its symbol.

If the impurity concentration is greatly increased by 1000 times or more in a normal PN junction its characters get completely changed and it gives rise to a new type of diode called tunnel diode.

20

2. What is tunneling?

The phenomenon of penetrating the charge carriers directly through the potential barrier instead of climbing over it is called as tunneling.

3. What is a PIN Diode?

PIN Diode is a 3 layer Semiconductor device, two heavily doped P & N materials separated by an intrinsic semiconductor.

4. What are the advantages of PIN diode over the normal PN junction diodes?

1. The capacitance between P & N regions decreases because of increased separation between P and N regions. This advantage allows the PIN diode to have fast response time.

2. There is a greater electron-hole pair generation because of the increased electric field between the P & N regions. This advantage allows the PIN diode to process even weak signals.

5. What is a Varactor Diode?

Varactor Diode is basically a reverse biased PN junction, which utilizes the inherent capacitance of the depletion layer. It is also called as varicap, voltcap diodes. It can be used as a voltage variable capacitor.

6. What is an SCR?

SCR is the abbreviation of Silicon Controlled Rectifier. It is a 4 layer semiconductor device forming a PNPN structure as shown. It is used as a controlled switch to perform rectification.

21

7. What is an UJT? Give its Symbol.

UJT is a three terminal silicon semiconductor device. It has only one PN junction like the ordinary diode.

8. What is a Diac?

Diac is a two terminal device, which can pass current in either direction when the break over voltage is reached in either polarity across the two terminals.

9. What is a Triac?

Triac is a 3 terminal device, which can conduct in either direction, when triggered either by a positive or negative pulse irrespective of the polarity of the voltage across its main terminals.

10. What is a Laser Diode?

LASER stands for ‘Light Amplification by Stimulated Emission of Radiation’. Laser diodes are semiconductors that convert the electrical signal to light. They are typically constructed from GaAlAs for short wavelength devices and In GaAsP for longer wavelength devices.

22

11. Define population Inversion?

If the number of electrons in excited state is more than the number of electrons in ground state, then this state is called population Inversion.

12. What is CCD?

Charge Coupled Device is abbreviated as CCD. CCD is an array of MOS capacitive structures with a common semiconductor substrate. An electrical contact is made to each of the metal gates. By proper biasing the MOS capacitors are made to operate in depletion or inversion modes.

13. What is a photodiode? Give its symbol.

Photodiode is a reverse biased PN junction whose operation depends on the intensity of light fall on it. It is embedded in a clear plastic casing and light is allowed to fall upon one surface across the junction. The remaining sides of the plastic are embedded in a metal casing or painted black.

A K

14. What is a phototransistor? Give its symbol.

Phototransistor is similar to a BJT. It can be either a 2 lead or a 3 lead device. In a 2 lead phototransistor base terminal does not exist. Instead light intensity is applied as input to the transistor. In a 3 terminal device base terminal is provided, so that it can act as a normal BJT. The symbol is as shown.

15. Define photoconductive effect?

The conductivity of certain semiconductor materials increases when they are exposed to radiations. This effect is called photoconductive effect.

16. What is a photoconductive cell? Give its Symbol?

Photoconductive cell is a device in the form of a slab of semiconductor deposited on an insulating substrate with ohmic contacts at opposite ends. It is generally made up of cadmium compounds like cadmium sulphide (CdS) and cadmium selenide (CdSe).

17. Define Photovoltaic effect?

Photovoltaic effect is the effect that converts light energy into electric energy.

23

18. What is an LED?

A PN junction diode that emits light when forward biased is known as LED. The Light emitted may be visible or invisible. The amount of light output is directly proportional to the forward current.

19. What is an LCD?

LCD is the abbreviation of Liquid Crystal Display. It is an electronic display device that operates by applying a varying electric voltage to a layer of liquid crystal, thereby inducing changes in its optical properties.

PART- B

1. What is a tunnel diode? Explain its characteristics with the help of V-I Curve? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 128

2. Explain the construction of a PIN Diode? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 134

3. Explain the working of a varactor diode? Also give its specification? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 124

4. In detail show the V-I characteristics of an SCR? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 201

5. With suitable diagram explain the two transistor equivalent model of SCR? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 202

.6. Explain the construction, working and V-I characteristics of UJT?

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 212

7. Explain the construction and characteristic of Diac? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 211

8. Explain the construction and characteristic of TRIAC? Refer (Electronic devices and circuits by S.Salivahanan) Page no: 209

BE/B.Tech DEGREE EXAMINATION, MAY/JUNE 2009SECOND SEMESTER

ELECTRONICS AND COMMUNICATION ENGINEERING

EC2151-ELECTRIC CIRCUITS AND ELECTRON DEVICES

(Common to ECE, CSE and IT)

(Regulation 2008)

Time: three hours Answer all questions. Maximum: 100 marks

PART-A—(10*2=20 marks)

1. State maximum power transfer theorem

24

Maximum power transfer theorem states that maximum power is delivered from a source to a load when the load resistance is equal to the source resistance.

2. Draw the equivalent circuit with the voltage source between terminals A and B.

3. A 50Hz sinusoidal voltage V=311 sinωt is applied to a RL series circuit. If the magnitude of resistance is 5Ω and that of the inductance is 0.02 H; calculate the effective value of steady state current and relative phase angle.

Given V=311 sinωt

F=50Hz; R=5 Ω; L=0.02H

Phase angle θ = tan-1VL/VR , θ = tan-1(I*XL)/ (I*R)

= tan-1XL/ R

XL=ω*L ω=2Πf

= tan- (2*Π*50*0.02)/5 =51.488˚

4. What is selectivity?

It is defined as the ratio of fo (resonance frequency) to the bandwidth

Selectivity = fo / (f2 – f1)

5. Define transition capacitance.

The parallel layers of oppositely charged immobile ions on the two sides of the junction

form the Transition or space charge capacitance.

CT =εA/W Where ε = permittivity of the material, A=cross sectional area of the

junction and W=width of the depletion region

6. What is clipper? List the various types of clipper.

The circuit with which the waveform is shaped by removing a certain portion of the i/p signal voltage

above or below a certain level is called as clipper.

7. What is large signal current gain?

The ratio of o/p current to the i/p current is called current gain.

In CB configuration the large signal current gain is defined as the ratio of collector current to the

emitter current. In CB ά = IC/IE;

In CE β = IC/IB;

In CC γ = IE/IB;

25

8. Compare any four salient features of BJT with JFET.

Sl.no BJT FET

1 Input impedance is low High input impedance

2 Voltage controlled device Current controlled device

3 Poor thermal stability Better thermal stability

4 More noise when compare with FET Less noisy

9. Draw the equivalent circuit of tunnel diode.

10. What is intrinsic stand-off ratio?

It is defined as the ratio of base1 resistance to the total base resistance.

ή = RBI/ (RB1+RB2)

PART-B—(5*16=80 marks)

11. (a) Use Delta- star conversion to find resistance between terminals ‘AB’ of the circuit shown: (16)

26

(Or)

(b) (i) State and explain Kirchhoff’s voltage laws. Determine the current supplied by the voltage source in the given circuit. (10)

27

(ii) Find the currents in all the resistors by superposition theorem in the circuit shown, calculate the power consumed. (6)

28

12. (a) Explain the parallel resonance of RLC circuits and derive the Q factor of the same. (16)

29

Ref Page no: 8.24 (Circuit theory by J.Jebastine)

(Or)

(b) Discuss in detail the transient response of series RC circuit with sinusoidal excitation. Derive

the steady state current for the same. (16)

Ref Page no: 6.61 (Circuit theory by J.Jebastine)

13. (a) Derive the PN current equation from the quantitative theory of diode currents. (16)

For Ge, Io = K1 T2 e –VGo/VT

For Si, Io = K2 T3/2 e –VGo/2VT

Diode current equation I= Io [e – (V/ήVT ) -1]

Ref Page no: 95- 98 (Electronic devices & circuits S.Salivahanan)

(Or)

(b) (i) With a neat circuit diagram explain the operation of zener voltage regulator. (8)

Ref Page no: 489 (Electronic devices & circuits S.Salivahanan)

(ii) Define and derive the drift and diffusion currents. (8)

Drift current density due to electrons Jn = qnμnE A/cm2

Drift current density due to holes Jp = qpμpE A/cm2

Diffusion current density due to electrons Jn = qDn dn/dx A/cm2

Diffusion current density due to holes Jp = -qDp dp/dx A/cm2

Ref Page no: 80 (Electronic devices & circuits S.Salivahanan)

30

14. (a) Explain the construction and operation and also the characteristics of enhancement MOSFET

differ from depletion MOSFET. (16)

CONSTRUCTION:

CHARACTERISTICS:

Ref Page no: 184 (Electronic devices & circuits S.Salivahanan)

(Or)

(b) (i) Explain the CE configuration of BJT in detail with required diagrams. (10)

Ref Page no: 148 (Electronic devices & circuits S.Salivahanan)

(ii) Define and compare ά, β and γ. (6)

In CB configuration current amplification factor ά = ∆ Ic/∆ IE

In CE configuration current amplification factor β = ∆ Ic/∆ IB

In CC configuration current amplification factor γ = ∆ IE/∆ IB

Ref Page no: 152 (Electronic devices & circuits S.Salivahanan)

31

15. (a) Derive Explain the construction and operation of LCD. (16)

Ref Page no: 559 (Electronic devices & circuits S.Salivahanan)

(or)

(b) (i) Draw and explain the two transistor equivalent model of SCR. (10)

Ref Page no: 202 (Electronic devices & circuits S.Salivahanan)

(ii)Draw and explain the VI characteristics of TRIAC. (6)

Ref Page no: 210 (Electronic devices & circuits S.Salivahanan)

32

B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2009.

SECOND SEMESTER

ELECTRIC CIRCUITS AND ELECTRON DEVICES

(Common to CSE, IT and ECE)

Time: Three hours Maximum: 100 Marks

Answer all questions

PART-A (10*2= 20 Marks)

1. Define superposition theorem The Responses in circuit in multiple source is given by the algebraic sum of the responses due to the individual sources acting alone

2. Determine the equivalent resistance of the given circuit across AB

3. Draw the structure of a double tuned circuit

4. Write the expression used for Q-factor of parallel circuit

Q=

5. What is the significance of diffusion capacitance in a PN diode?

The capacitance that exists in the forward biased junction is called a diffusion or storage capacitance. It is also defined as the rate of change of injected charge with applied voltage. CD=dQ/dV

Its value is much larger than CT. the value of CD range from 10 to 1000 pF. The effect of CD is negligible for reverse bias PN junction; it is inversely proportional to frequency.

6. List out the four main applications of zener diode

Voltage regulator, stabilizer, power supply and battery charger.

33

7. Compare the output characteristics of CE&CB mode PNP transistor

Sl no property CE CB

1 Input resistance Moderate(abt 750Ω) Low (abt100 Ω)

2 Output resistance Moderate(abt 45KΩ) High (abt450K Ω)

3 Current gain high 1

4 Voltage gain Abt 500 Abt 150

5 Phase shift 180 o 0 o or 360 o

8. What is the working principle of MOSFET?

By applying a transverse electric field across an insulator deposited on the semiconductor material, the thickness and hence the resistance of a conducting channel of a semi conducting material can be controlled. o

9. Write the significance of varactor diodeThe transition capacitance inversely with the reverse voltage. They are used in FM radio and TV receivers, AFC circuits, self adjusting bridge circuit and adjustable band pass filter.

10. In what way photo diode differs from photo transistor?

Sl. no Photo diode Photo transistor

1 Amplification is not done Amplification is done

2 Less sensitive More sensitive

3 Current produced is very low Greater flow of current

4 I= Is +Io(1-e (v/ήvt)) Ic= (β+1) (Ico+I L )

PART-B (5*16=80 Marks)

11. (a) (i) Calculate the equivalent resistance and current flow through AB of fig1 (10)

34

(ii) Briefly discuss about maximum power transfer theorem (6)

Maximum power transfer theorem states that maximum power is delivered from a source to a load when the load resistance is equal to the source resistance.

(OR)

(b) (i) Obtain the Norton's equivalent circuit across AB of the network shown (10)

35

(ii) Write short notes about duality (6)

In an electrical circuit there are pair of terms which can be interchanged to get new circuits. Such pair of dual terms is given below.

Current- voltage, open-short, L-C, R-G, series-parallel, voltage source-current source, KCL-KVL

Consider a n/w containing R-L-C elements connected in series, and excited by a voltage source as shown.

The integro_differential equation for the above n/w is

Similarly, consider a n/w containing R-L-C elements connected in parallel and driven by a current source as shown.

The integrodifferential equation for the above n/w is

If we observe both the equations, the solutions of these two equations are same. Thus these two n/w's are called duals.

12. (a) (i) A RLC series circuit having R=120Ω, L=120mH and C=25μF is fed from a 100V 50Hz supply. Find the value of power (10)

36

(ii) Briefly describe about single tuned circuit (6)

(OR)

(b) (i) Obtain the frequency response and Q factor of a RLC series circuit (6)

Refer (Circuit theory by J.Jebastine) Page no: 8.1-8.10.

(ii) In a series RC circuit shown the capacitor has an initial charge q0 =2500 μC. At t=0, the

switch is closed. Find the current (10)

Equation for the ckt is Ri + 1/c ∫ idt + Qo /C =E

Refer (Circuit theory by J.Jebastine) Page no: 6.32.

13. (a) (i) Derive the expression for the conductivity of N type and P type semiconductors (8)

For N type as n>>p conductivity σ = q.n.μn

For P type as p>>n conductivity σ = q.p.μp

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 66

(ii) Discuss briefly about the breakdown in PN junction diodes (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 105

37

(OR)(b) (i) Explain about the energy band structure of open circuited PN junction (8)

Refer (Electronic devices and circuits by S.Salivahanan Page no: 93

(ii) Write short notes about Zener diode and its characteristics (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 122

14. (a) (i) Describe about the working principle and characteristics of NPN transistors (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 141, 145-151.

(ii) Explain about enhancement and depletion type MOSFET (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 184-186.

(OR)

38

(b) (i) Explain how breakdown occurs in a transistor and draw the CB, CC and CE characteristics

of BJT (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 141,142, 145-151

(ii) Briefly describe about thermal effect of MOSFET (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 188,189

15. (a) (i) Explain the working principle and characteristics of Tunnel diode (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 128

(ii) Compare DIAC and TRIAC (8)

Sl.no DIAC TRIAC

1 Two terminal device Three terminal device

2 Triggering device for TRIAC Control device

3 Gate terminal is absent Gate terminal present

4 Five layer device Six layer device

5 Firing angle can not be changed Firing angle can be changed

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 209,211

(OR)

(b) (i) Briefly describe about UJT (8)

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 212

39

(ii) Write short notes about LED and LCD (8)

LED: It is a PN junction device which emits light when forward biased, by a phenomenon called

electroluminescence. It consumes more power- requires 10-250 mW power per digit. It has good

brightness level.

LCD: liquid crystal displays are used for display of numeric and alphanumeric character in dot matrix

and segmental displays. It consumes very less power- requires 10-200μW power per digit. It has moderate

brightness level.

Types: 1) Nematic liquid crystal (NLC)

2) Cholesteric

Refer (Electronic devices and circuits by S.Salivahanan) Page no: 557,559.

40