EDC Lab Manual

Electronic Devices & CircuitsLaboratory Notes

By

Mr.M.Siva Ganga Prasad

Associate professor

Dept. of ECE

KL University

Guntur

2010-2011

1

Acknowledgements

We take this opportunity to thank all those who helped directly and indirectly in bringing this laboratory manual. We thank Dr.P.Siddaiah, Dean P&D, Dr. S.Lakshminarayana, HOD, Dept of ECE , co-authors prof N.Venkat Ram, K. Uday Kumar and all the faculty members, non teaching staff members who are involving in the preparation of laboratory manual.

AUTHOR

2

Preface

Electronic Devices & Circuits (EDC) Laboratory practice is an elementary course taught to ECE, EEE and ECM branches of Engineering students at KL University. Basic concepts of different electronic devices like semi conductor diode, Bipolar junction Transistor(BJT), Junction Field Effect transistor(JFET), Uni-Junction Transistor(UJT), SCR etc.. are taught with their V-I characteristics in theory classes. In practical classes students are given hands on experience on what they have studied in lecture classes by verification and testing in laboratory. Students will be making simple mini projects based on the applications of various electronic devices which are used in day to day life. This will enable the students to get familiarized to use different devices and equipment in different circuits designing process which are useful in various communication fields. Our laboratory has facilities like CROs with high frequency range to test the response of various electronic circuits, Trainer boards and IC tester etc.. with all necessary accessories and attachments.

Learning objective for each experiment is given in the beginning of the experiment in the manual. For each experiment , a list of review questions are included. Answering to these questions ensures learning of the process in terms of its application, using different techniques etc. Students are expected to answer all the review questions and to enter their observations in the manual itself.. Students will be working with each facility on rotation basis weekly twice. At the end of the semester, students are expected to familiarize completely for using all the facilities in the laboratory.

Students can refer to appendix in the text book “integrated electronics” for additional information. Students are hereby informed ensure “Safety First experiment next” policy. We hope that this manual is admirably suited for students of second year first semester to have hands on experience in using various electronic devices.

Author

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Table of Contents

Expt.No Name of the experiment Page No

1. V-I Characteristics of PN junction Diode. 5

2. V-I Characteristics of Zener junction Diode. 10

3. Half wave rectifier without and with capacitor filter. 14

4. Full wave rectifier without and with capacitor filter. 19

5. Input & output Characteristics of Transistor in common Base configuration.

25

6. Input & Output Characteristics of Transistor in common Emitter configuration.

30

7. Input & Output Characteristics of Transistor in common Collector configuration.

35

8. Transistor biasing circuit 39

9. Characteristics of junction Field Effect Transistor 42

10. Static Emitter Characteristics of Uni -junction Transistor 48

11. V-I Characteristics of Silicon Controlled Rectifier. 53

12. Transistor switching. 56

13. Emitter Follower

14

EXPERIMENT NO:1

4

V-I Characteristics of PN junction Diode

Learning Objectives:

To learn about

Properties of Diode 1N 4001

Cut-in voltage of Ge diode

Static & dynamic resistance

Of Ge diode

Properties of Diode OA 79

Cut-in voltage of Si diode

Static & dynamic resistance

Of Si diode

Aim:

1. To plot V-I characteristics of Silicon and Germanium diodes.

2. Compare the forward characteristics of ge diode with that of Si diode.

3. Calculate the forward static and Dynamic resistance of the diodes at a particular operating point.

Apparatus Required:

1. Experimental board with Si and Ge diodes.

2. Regulated power supply.

3. Multi meters ( as volt meter and ammeter)

5

Circuit Diagram:

1KΩ

Circuit for Diode forward bias

1KΩ

Circuit for Diode reverse bias

6

mA

V0-30 mA

0-10 V 1N 4001

µA

V0-500 µA

0-20 V 1N 4001

Procedure:

1. Find the type number of the diodes connected in the experimental board.

2. Trace the circuits and identify different components used in the circuit. Read the value of the resistor using the color code.

3. Connect the millimeter and voltmeter of suitable ranges, say 0-to-25mA for ammeter and 0-to-2v for voltmeter in forward bias and 0-100 micro A for ammeter and 0-to-30v for voltmeter in reverse bias.

4. Switch on the power supply. Increase the voltage range from 0 to 30v in proper steps.

5. Note the ammeter and voltmeter readings for each input voltage. Tabulate the observations

Tabular forms:

Forward Bias: Reverse Bias:

7

S.no Vd volts Id mA

S.no Vr volts Ir µA

Model Graph:

8

Forward current Id

Result:

V-I characteristics of both Ge and Si diodes are verified.

Date: Signature of Instructor

Review Questions

1. What is the cut-in voltage of Ge and Si diodes?

2. List out the important properties of PN junction diode.

3. What is static and dynamic resistance of diode?

4. What is reverse saturation current?

5. What are majority and minority charge carriers?

9

Cut-in voltage

Forward voltage VdReverse voltage Vr

Reverse current Ir

EXPERIMENT NO:2

V-I Characteristics of Zener Diode


To learn about

Properties of Zener diode

Break down voltage

Voltage regulation

Zener diode is never used as voltage regulator as the current of zener diode is less and there is a maximum dissipation across zener diode when the current is zero and this is generally as a voltage reference.

. Reverse bias characteristics of Zener diode

Dynamic resistance of Zener diode

Aim:

1. To plot V-I characteristics of Zener diode under reverse biased condition.

2. Compare the forward characteristics with reverse characteristics.

3. Calculate the Dynamic resistance under reverse bias of Zener diode.

Apparatus Required:

1. Experimental board with Zener diode 0-30V, 1A dc.


3. Multi meters ( as volt meter and ammeter)

10

Circuit Diagram:

1KΩ

Zener Diode forward bias

1KΩ

Zener Diode reverse bias

11

A

V0-30 mA

0-10 V 6Z8v

A

V0-30 mA

0-20 V 6Z8v

Procedure:

1. Connect the circuit as shown in figure with zener diode in forward bias.

2. Switch on the power supply

3. Set the voltage across the Zener diode at 0.0 V and observe the current.

4. Increase the Zener voltage in steps of 0.1 V upto 0.7 V and note down the readings of the milli ammeter for each voltage.

5. Connect the circuit for the Zener diode in reverse bias.

6. Set the voltage across the Zener diode at 0.0 V and observe the current.

7. Increase the Zener voltage in steps of 0.2 V until the diode goes into breakdown and note down the reading of the ammeter for each voltage.

8. After the diode goes into breakdown take two or three readings of current at the breakdown voltage.

9. Supply voltage must be greater than Zener break down voltage.

Tabular forms:

Forward Bias: Reverse Bias:

12

S.no Vz volts Iz mA

S.no Vz volts Iz mA

Model Graph:

Result:

V-I characteristics of Zener diode are verified.


Review Questions

1. What is Zener diode and at what range of voltage, it shows breakdown characteristics?

13

2. What is the difference between Zener and avalanche breakdowns which is greater of the two?.

3. Distinguish between an ordinary diode and Zener diode w.r.t their operational characteristics.

4. Mention the Practical applications of a Zener diode.

5. Is the Zener voltage temperature dependent, if yes how?

EXPERIMENT NO:3

Half wave rectifier without and with capacitor filter


To learn about

Diode as a rectifier

Ripple contents in Half wave rectifier

Ripple factor of Half wave rectifier

Efficiency of Half wave rectifier

PIV of Half wave rectifier

Generally a 0.1 µF capacitor is shunted across the diodes in commercially available power supplies to reduce HF noise caused by the reverse recovery current being switched off when the current carries in the diodes are depleted.

Aim:

1.To find ripple factor of HWR

2. Verify the efficiency and percentage regulation of HWR without and with capacitor filter

Apparatus Required:

1. Transformer

2. Diode -1N 4001.

3. Variable resistor 0-6K ohms.

14

4. Capacitor 100 µF

5. Capacitor 0.1 µF



8. DC voltmeter 0-50V

9. DC ammeter 0-50mA

10. AC voltmeter 0-50V

11. 10 v of D.C ,1 Amp power supply.

Circuit Diagram:

i)Half wave rectifier without filter

ii) Hii) Half wave rectifier with capacitor filter

15

Procedure:

1. Connect the rectifier circuit without capacitor filter.

2. Under open circuit condition, take Vdc, Vac . These are no load values.

3. Connect 1 KΩ resistance at the load and take Idc, Vdc, Vac and tabulate.

4. Vary the load in steps of 1 KΩ upto 6 KΩ and tabulate the values .

5. Calculate ripple factor, % of regulation and efficiency of the rectifier for different loads.

6. Plot the graphs between Idc Vs ripple factor, Vdc Vs % of regulation.

7. Connect the filter capacitor and repeat the above steps.

8. The minimum current drawn should be 500 mA and go on decreasing 100 mA steps to study the load regulation and variation of ripple factor with load current.

Tabular forms:

16

Without capacitor:

Vdc (No-load) =

S.No Load(KΩ) Idc mA Vdcvolts Vac volts Ripple factor(γ)

% regulation Efficiency(η)

Without capacitor:

Vdc (No-load) =

S.No Load(KΩ) Idc mA Vdc volts Vac

voltsRipple factor(γ)


17

Model Graphs:

On X-axis: load current Vdc, on Y-axis: % of regulation On X-axis: Load curren Idc, on Y-axis: ripple factor(γ)

Characteristics of half wave rectifier without filter

Result:

Various characteristics of HWR are verified.


Review Questions

1. How the diode acts as HWR rectifier ?

2. What is PIV of HWR?

18

3. What is ripple factor and how much it is in HWR?

4. Mention the Practical applications of a PN junction diode.

5. How the capacitor act as a filter and how it reduces the ripple contents?

EXPERIMENT NO:4

Full Wave Rectifier without and with capacitor filter


To learn about

Diode as a rectifier

Ripple contents in full wave rectifier

Ripple factor of full wave rectifier

Generally a 0.1 µF capacitor is shunted across the diodes in commercially available power supplies to reduce HF noise caused by the reverse recovery current being switched off when the current carries in the diodes are depleted.

19

Efficiency of full wave rectifier

PIV of full wave rectifier

Aim:

1.To find ripple factor of FWR

2. Verifi the efficiency and percentage regulation of FWR without with capacitor filter

Apparatus Required:

1. Transformer

2. Diodes -1N 4001.

3. Variable resistor 0-6K ohms.


5. Capacitor 0.1 µF



8. DC voltmeter 0-50V

9. DC ammeter 0-50mA

10. AC voltmeter 0-50V

Circuit diagrams:

i) Full wave rectifier without filter:

20

ii) Full wave rectifier with capacitor filter:

21

Procedure:

1. Connect the rectifier circuit without capacitor filter.

2. Under open circuit condition, take Idc, Vdc, Vac . These are no load values.

3. Connect 1 KΩ resistance at the load and take Idc, Vdc, Vac and tabulate.

4. Vary the load in steps of 1 KΩ upto 6 KΩ and tabulate the values .

5. Calculate ripple factor, % of regulation and efficiency of the rectifier for different loads.

6. Plot the graphs between Idc Vs ripple factor, Idc Vs % of regulation and Idc Vs efficiency.

7. Connect the filter capacitor and repeat the above steps.

Tabular forms:

Without capacitor:

Vdc (No-load) =

S.No Load(KΩ) Idc mA Vdcvolts Vac volts Ripple factor(γ)


22

Without capacitor:

Vdc (No-load) =

S.No Load(KΩ) Idc mA Vdc volts Vac

voltsRipple factor(γ)


Model Graphs:

On X-axis: D.C current(Idc), on Y-axis: % of regulation On X-axis: D.C current(Idc), on Y-axis: ripple factor

Characteristics of half wave rectifier without filter

23

Result:

Various characteristics of FWR are verified.


Review Questions

1. How the two diodes acts as FWR ?

2. What PIV of FWR?

3. What is ripple factor how much it is in FWR?

4. Mention the Practical applications of a PN junction diode.

5. Compare the performance of HWR and FWR.

EXPERIMENT NO:5

24

Input & output Characteristics of Transistor in common Base configuration


To learn about

Properties of BJT BC 107

Current Gain

Dynamic input resistance

Properties of BJT SL 100

Voltage Gain

Dynamic output resistance

Aim:

To observe the input and output characteristics of a common base Amplifier circuit and find

(a) Dynamic input resistance.

(b) Dynamic output resistance.

(c) Current gain.

Apparatus Required:

1. Transistor BC 107.


3. Multi meters ( as volt meter and ammeter).

4. Bread board and connecting wires.

Circuit diagram:

25

100 Ω + E C + 5KΩ

R.P.S B

VEB +

_ _ VCB R.P.S

Procedure:

1. Note down the type number of the transistor used in the experimental board. Find the important specifications of the transistor from the data sheet. Identify the the terminals of the transistor and trace the circuit.

2. Make the connections as shown in the circuit diagram.

3. For input characteristics, first fix the voltage VBC (say 0 V) and vary the input voltage VBE

In steps of 0.1 V. Note the value of current IE at each step.

4. Repeat step 3 for different voltages of VBC say 2V, 3V, 4V…..

5. For output characteristics first open the input circuit and vary the voltage V BC in steps of 1V up to 10V and Note the collector current Ic.

6. Now close the input circuit and fix the emitter current IE (say 20 µA, 40 µA). Vary the VBC

in steps of 1-10 V. Note the current IC for each step.

7. Plot the input and output characteristics by using the readings taken above.

Tabular forms:

26

mA NPN mA

V V

Ic

Input characteristics:

S.No VCB = 0 Volts VCB = -10 Volts VCB = -15 Volts

Output characteristics:

S.No IE = 0 mA IE = 1mA IE = 2mA

Model graphs:

i) input characteristics:

IE(mA)

27

VEB(V) IE(mA) VEB(V) IE(mA) VEB(V) IE(mA)

VCB(V) IC(mA) VCB(V) IC(mA) VCB(V) IC(mA)

∆ IE

∆ VEB VEB(volts)

ii) output characteristics:

Result:

Input and output characteristics in CB configuration are verified.

28


Review Questions

1. What is the input and output dynamic resistance of BJT in CB mode?

2. List out the applications of BJT in CB mode.

3. What is the input and output Impedance of BJT in CB mode?

EXPERIMENT NO:6

Input & output Characteristics of Transistor in common Emitter configuration


29

To learn about


Current Gain



Voltage Gain


Aim:

To observe the input and output characteristics of a common emitter circuit and find

(a) Dynamic input resistance.

(b) Dynamic output resistance.

(c) Current gain.

Apparatus Required:





Circuit diagram:

0-30mA

330 KΩ + B

BC 107

30

µA NPN mA

Ic

C_

+1KΩ

E

VBB VBE + + VCE VCC 0-30 V 0-1 V _ _ 0-20 v 0-30V

Procedure:



3. For input characteristics, first fix the voltage VCE (say 0 V) and vary the input voltage VBE

In steps of 0.1 V. Note the value of current IB at each step.


5. For output characteristics first open the input circuit and vary the voltage VCE in steps of 1V up to 10V and Note the collector current IC.

6. Now close the input circuit and fix the base current IB (say 20 µA, 40 µA). Vary the VCE in steps of 1-10 V. Note the current IC for each step.


Tabular forms:


31

V V

S.No VCE = 0 Volts VCE = 2 Volts VCE = 4 Volts


S.No IB = 0 µA IB = 20µA IB = 40µA

Model Graphs:

i) input characteristics:

32

VBE(V) VBE(V)IB(mA) VBE(V) IB(mA)IB(mA)

VCE(V) IC(mA) VCE(V) IC(mA) VCE(V) IC(mA)

IB(mA) VCE= 0V VCE = 2V VCE= 4V

VBE(volts)

ii) output characteristics:

Result:

Input and output characteristics in CE configuration are verified.

33


Review Questions

1. What is the input and output dynamic resistance of BJT in CE mode?

2. List out the applications of BJT in CE mode.

3. What are the advantages of CE mode compare to CB and CC?

EXPERIMENT NO:7

Characteristics of Transistor in common Collector configuration


34

To learn about


Current Gain



Voltage Gain


Aim:

To observe the input and output characteristics of a common collector circuit and find

(a) Dynamic input resistance calculation

(b) Dynamic output resistance calculation

(c) Current gain.

Apparatus Required:





Circuit diagram:

0-30mA

+ B

IB BC 107

35

mA NPN mA

IE

E+

__5KΩ

C

VBB VBC + + VEC VEE 0-30 V 0-1 V _ VEC 0-20 V 0-30V

Procedure:



3. For input characteristics, first fix the voltage VCE (say 0 V) and vary the input voltage VBC

In steps of 0.1 V. Note the value of current IE at each step.


5. For output characteristics first open the input circuit and vary the voltage V BC in steps of 1V up to 10V and Note the collector current IB.

6. Now close the input circuit and fix the base current IB (say 20 µA, 40 µA). Vary the VCE in steps of 1-10 V. Note the current IE for each step.


Tabular forms:


36

V V

S.No VCE = 0 Volts VCE = 5 Volts VCE = 10 Volts


S.No IB = 0 mA IB = 5 mA IB= 10 mA

Model graphs:

VCE=0V VCE=5V VCE=10V IE(mA) IB=10 µA

37

VBC (V) IB(mA) VBC(V) IB(mA) VBC(V) IB(mA)

VCE(V) IC(mA) VCE(V) IC(mA) VCE(V) IC(mA)

IB µA

IB=5µA

VBC(V) VCE(V)

Input characteristics

Result:

Input and output characteristics in CC configuration are verified.


Review Questions

1. What is the transistor? What characteristics of this device made to call it as “TRANSISTOR”?

2. Which region is called the saturation region in the output characteristics of BJT.

3. What is the input and output Impedance of BJT in CC mode?

4. Define β and give the relation between α and β.

5. How CC configuration is used for purpose of impedance matching.

EXPERIMENT NO:8

Transistor biasing


To learn about

38

IB= 10µA

Biasing

Self biasing technique

Practical feasibility of self biasing

Need of biasing

Stability of self biasing

Applications of self biasing

Aim:

Design a self bias circuit for the given specifications and test the stability of the operating point.

Apparatus Required:



3. DC Ammeter 0-500 µA, 0-25mA.

4. Multi meters .


6. Resistors RE, RC, R1, R2

Circuit diagram:

VCC

39

R1 RC

BC107

R2 RE

Procedure:

1. Connect the circuit as shown in the diagram.

2. Apply the given value of Vcc.

3. Find the resulting DC values of Ic, VCE.

4. Calculate Icmax from Icmax= Vcc.Rc.

5. Draw the load line and check whether the operating point is at the center of the load line or not.

6. Replace the transistor with another one of the same type.

7. Repeat the steps 3 to 6 and observe whether the operating point is at the same location or not.

Design procedure:

1. VCC , IC , RE , VBE= 0.6V will be given specifications.

2. Find the hfe of the given transistor.

3. S = (1+β)/(1+((βRE)/(RE+RB))), VB = (VCC R2)/(R1+R2), RB = (R1/ R2)/R1+R2)

VB = IBRB + VBE + IERE

VCC = IcRC + VCE + IERE

Using the above formulae find the values of R1 , R2 , RC

40

Result:

Testing the stability of self bias circuit is verified.


Review Questions

1. What are the essentials of BJT to acts as an amplifier?

2. What is the necessity of stability to set the operating point?

3. What is DC load line?

4. What are the three operating regions of BJT?

5. What is operating point?

EXPERIMENT NO:9

Characteristics of junction Field Effect Transistor


To learn about

41

JFET characteristics

Mutual conductance gm

Pinch-off voltage

Dynamic resistance rd

Amplification factor µ

Applications of JFET

Aim:

1. Plot the static drain characteristics of FET.

2. Plot the transfer characteristics of FET.

Apparatus Required:

1. JFET BFW 10.

2. Dual DC Regulated power supply.

3. Experimental board.

4. Multi meters (as volt meters and ammeter) .

Circuit diagram:

_ + 1KΩ

1KΩ D 0 ID

42

mA

0-30mA

G BFW 10

S

VGG VGS 0-10 V VDS 0-20 V VDD +

+ VDS _ _

43

V V

+

__ _ +

Procedure:


2. Fix VGS at 0 V.

3. Set the drain voltage VDS to 0 V. Note drai current. Now vary the value of VDS in steps of 0.5 V and note down the drain current .

4. Repeat the step 3 till the drain current attains the constant value. After the drain current becomes constant take 2 0o 3 readings.

5. Vary the VGS to -1 Vand repeat the steps 3 and 4.

6. Plot the drain characteristics.

7. Fix VDS at 2 V.

8. By varying VGG set VGS to 0 V and note down the value ID . Now vary the VGS in steps of 0.5 V and note down the corresponding values of current ID.

9. Repeat step 8 for VDS= 4 V and 6 V.

10. Plot the transfer characteristics.

11. Calculate rd and gm from the curves and compare them with the data sheet values.

44

Tabular forms:

Drain characteristics:

S.NO

VGS = 0V VGS = -1 V VGS = -2V

Transfer characteristics:

S.NO

VDS = 2V VDS = 4 V VDS = 6V

45

VDS(v) ID(mA) VDS(v) ID(mA VDS(v) ID(mA

VGS(v) ID(mA) VGS(v) ID(mA VGS(v) ID(mA

Model Graphs:

Drain characteristics:

ID(mA) VGS = 0V

VGS = -2V

VGS = -4V

VDS(V)

Transfer characteristics:

IDS(mA)

VDS=4V

VDS = 2V

VGS(V)

46

Result:

Drain and Transfer characteristics of JFET are verified.


Review Questions

1. What are the advantages of FET over BJT?

2. What is pinch off voltage?

3. How MOSFET is different from JFET?

4. Why input impedance is more for FET?

5. What do you understand by a term channel in a JFET?

47

EXPERIMENT NO:10

Static Emitter Characteristics of Uni -junction Transistor


To learn about

UJT characteristics

Input diode concept in UJT

Advantages of UJT

UJT as relaxation oscillator

Negative resistance region

Applications of UJT

Aim:

Plot the static emitter characteristics of UJT.

Apparatus Required:

1. UJT 2N 2646.


3. Experimental board.

4. Multi meters(as volt meters and ammeter) .

5. Resistor 1 KΩ.

48

Circuit diagram:

1KΩ + IE _ B2

E 2N 2646

0-100 mA + B1

VEE VE 0-30 V VDD +

_ _

49

mA

V

+

__

Procedure:


2. Keep VEE and VB at zero before start.

3. Vary VEE slowly such that VE varies in steps of 0.1 V. Note down the corresponding IE

against each voltage in the table.

4. Now adjust VB at 5 V and keep it constant.

5. Now vary VEE slowly. No emitter current will be three until a peak value of 2 to 3 V of VE

is reached. After VE reaches a peak value, it suddenly drops resulting in the flow of emitter current.

6. Note down very accurately the peak value of VE.

7. Now vary VEE for various values of IE in proper steps (10 readings) and note down VE.

8. Now adjust VB at 10 V and repeat steps 5 to 7. Now the peak value of V E will be around 6 V. Tabulate the readings accurately.

9. Draw a graph VE Vs IE against VB as a parameter.

50

Tabular forms:

S.NO

VB = 0V VB = 5 V VB = 10 V

Model graph:

VE(V) peak voltage (Vp)

51

VE(v) IE(mA) VE(v) IE(mA) VE(v) IE(mA

VBB = 10 V

VBB= 5V

VBB= 0V

IE (mA)

Valley voltage ( VV) V(V

Result:

Static emitter characteristics of are UJT are verified.


Review Questions

1. Expand UJT?

2. What is negative resistance?

3. What are the uses of negative resistance devices?

4. What is the purpose of UJT?

52

EXPERIMENT NO:11

V-I Characteristics of Silicon Controlled Rectifier


To learn about

SCR characteristics

SCR as a controlled switch

Advantages of SCR

Two transistor analogy of SCR

Break over voltage

Applications of SCR

Aim:

To verify the V-I characteristics of SCR.

Apparatus Required:

1. SCR2. Dual DC Regulated power supply.

3. Experimental board with SCR.

4. Multi meters(as volt meters and ammeter) .

5. Resistor 1 KΩ.

53

Circuit diagram: SCR

+ _ K 5KΩ

+

+ _ VAK

VGK _ _

Procedure:


2. Keep gate voltage at different values say (0, 1.5, 2, 2.5 volts) and note down SCR break down voltages.

3. Plot the graph between anode voltage and anode current.

Tabular forms:

S.NO

IG = 0 mA IG = 1mA IG = 2 mA

54

IG mA

IA mA

VAK

+

+ _

A

VAK(v) IA(mA) VAK(v) IA(mA) VE(v) IE(mA

Model Graph:

Result:

SCR V-I characteristics are verified.


Review Questions

1. Expand SCR? Why we not calling Germanium control rectifier?2. What is break over voltage in SCR?

3. What are applications of SCR?

4. How the SCR acts as controlled switch?

EXPERIMENT NO:12

55

Common Emitter Amplifier


To learn about

h-parameters

Current Gain

Advantages of CE Amplifier

Operating point

Voltage Gain

r

Aim:

Design the values for Common Emitter Amplifier shown below and verify the frequency response.

Apparatus Required:

1. Transistor BC 107


3. Experimental board with BJT.

4. Multi meters.

5. Resistors and capacitors.

Procedure:

56

A transistor has the following h-parameters.

Vcc=9V. AVmid = -50. S=10.

To begin with, let us choose +9.0 V as Vcc . The load resistor Rc is determined by the a.c. gain required. If we propose to design an amplifier with a midband gain of — 50,

(4.3.1)

or

The emitter resistor RE stabilizes Ic . Owing to the current feedback through RE; the tendency for the collector current to increase with temperature is reduced. The effect of RE on overall temperature stabilization is shown in Fig. An examination of this figure.

57

Fig. Effect of RE on temperature stabilization of Ic

shows that RE should be as large as possible. However, for an amplifier working with a single power supply, there is a limit to which RE can be increased. For a given bias, increase in RE necessarily leads to a decrease in R1 as

Since R1 and R2 are in parallel with ri (the a.c. input resistance of the transistor), too low a value of R1 affects the input impedance. As a

58

rule of thumb, RE is chosen to be equal to ri . For the transistor selected r i = h ie = 1 k . So we choose a value of RE=1k.

In selecting the biasing network (R1 and R2) stability criterion comes in. For an S value of 10, we have Rb= 9RE = 9k since S= 1 + RB/RE for the circuit under discussion. All that remains is to choose R1 and R2.

To select R1 the quiescent collector voltage must be found. Since the voltage at the collector swings from

, the midpoint voltage is average i.e -5.8V. The

quiescent collector current is given by

If we consider the voltage divider network formed by R1 and R2

59

Substituting the values already evaluated, we obtain

or

Since and

R2=12K

The design is now complete except for choosing Cc and C^. The

lower half-power point determines Cc-and CE f1= 100 Hz

For CE, choose a value such that its reactance at the lowest frequency of operation (say at 100 Hz) is small in comparison with RE or

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One need not give importance to the degree of accuracy indicated by the values given above. Standard components close to the values indicated may be used.

The voltage gain Av, the current gain Ai, the input impedance Rin and the output impedance Ro of the CE configuration may be

calculated via the relations

For the design example discussed here, Av = -50; A i=~20; Rin

=1.0 k, and Ro = 60 k. This configuration may be used for voltage, current, or power gain. The input impedance is low and the output impedance is high.

Circuit diagram:

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Tabular forms:

Result:


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Review Questions

EXPERIMENT NO:13

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Emitter Follower


To learn about

Aim:

Design the values for Emitter Follower shown below and verify the frequency response

Apparatus Required:

6. Transistor BC 107


8. Experimental board with BJT.

9. Multi meters.

10. Resistor

THE EMITTER FOLLOWER

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PRELAB

1. What is the approximate value of the voltage gain of an emitter follower? Why may such

a circuit be useful?

2. What are the values of input and output impedances of the emitter follower you expect to

measure? Are they related?

LABORATORY

1. EMITTER FOLLOWER WITH SINGLE AND DOUBLE POLARITY

POWER SUPPLIES.

Wire the circuit shown below. The value of the small protective input base resistor is not critical.

a. Drive it with a symmetrical sine wave at the input (check that the signal from the

waveform generator has no dc offset voltage). Increase the input voltage from

less than 1 V (p-p) to about 5V (do not drive the B-E junction to breakdown on a

negative half cycle). Observe input and output waveforms on a scope and note

their amplitudes and phase relation. Use the scope DC input to see if there is a

dc bias on your signal.

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Next connect VEE to -15 V (see the figure below) and observe the output again. Measure input

and output voltages. Calculate voltage gain.

How large output voltage can you get from the circuit (without distortion)?

b. Measure the input and output impedances, Zin and Zout.

Determine Zin (the impedance looking into the transistor base) by measuring signals on

both sides of the 10k resistor connected in series at the input (Fig.2). You may leave in

place the small input resistor used in the previous circuit.

Load the output with a resistor (1k or less) through a 4.7 mF blocking capacitor. Use a

sinusoidal signal at the input and observe a change in the output signal when the resistor is

connected. Determine the Thevenin equivalent resistance viewing the follower as a voltage

source with Zout in series. Work at a frequency at which the load circuit does not act as a filter.

Hint: Note that the load resistor forms a voltage divider with Zout.

Fig:2

Important points about followers

(Ref: The art of electronics by Paul Horowitz)

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In emitter follower the npn transistor can only "source" current because sourcing is through transistor having low resistance till saturation. Sinking is through R E which is relatively high. For instance, in the loaded circuit shown the output can swing to within a transistor saturation voltage drop of Vcc (about +9.9 V), but it cannot go more negative than -5 volts. That is because on the extreme negative swing, the transistor can do no more than turn off, which it does at -4.4 volts input (-5V output). Further negative swing at the input results in back biasing of the base-emitter junction, but no further change in output. The output, for a 10 volt amplitude sine-wave input, looks as shown in Figure 2.9.

S.No RE Q Point. IE RL -ve Clipping

1. 3.3K 4.2ma 1K 8V

2. 3.3K 4.2ma 680 6V

3. 3.3K 4.2ma 220 1.14V

4. 1K 14ma 1K 17V

5. 1K 14ma 680 14.2

Figure 2.9. Illustrating the asymmetrical current drive capability of the npn emitter follower.

Fig-2.9

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Another way to view the problem is to say that the emitter follower has low small signal output impedance. Its large-signal output impedance is much larger (as large as RE). The output impedance changes over from its small-signal value to its large-signal value at the point where the transistor goes out of the active region (in this case at an output voltage of -5V). The solution is to use complementary symmetry NPN-PNP transistors.

2. EMITTER FOLLOWER WITH COUPLING CAPACITORS.

Can you use a single polarity power supply for a follower operating with a symmetrical input?

Try the circuit shown in the figure below. Measure both ac and dc components of the output

voltage with a scope. Measure the emitter and base dc bias with a DVM.

Explain results. How could you eliminate dc component from the output voltage? Try!

Do you think that the frequency response of the last circuit is different than those in 1?

Determine frequency f-3dB.

_____________________________________________________________________________

The frequency response of Emitter follower with capacitive load:

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The Zo increases with increase of frequency. This phenominon simulates inductance at a particular frequency. So resonance occurs with C & L producing ringing.

Measure Zin at the base of the transistor in the following way.

• Increase the source resistance RS until vout is reduced by about 1/2 (or by some other value).

• Sketch the equivalent circuit (use an equivalent circuit of an amplifier from Chapter 1, not the transistor model) and compute Zin from your measurement.

• From the theoretical result that Zin = (β + 1)[re + RE] and re = VT /IE, estimate the β of your transistor.

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Emitter Follower An emitter follower circuit shown in the figure is widely used in AC amplification circuits. The input and output of the emitter follower are the base and the emitter, respectively, therefore this circuit is also called common-collector circuit.

DC operating point

Solving the second equation, we get :

and we also get:

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AC small-signal equivalent circuit

Voltage gain:

We assume and therefore can be ignored, and have

Now the voltage gain can be found to be:

As , is smaller than but approximately equal to 1.

Input resistance:

The input resistance is in parallel with the resistance of the circuit to its right including the load

, which can be found by as the ratio of the voltage and the current . But as

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we have

Comparing this with the input resistance of the common-emitter circuit , we see that the emitter follower has a very large input resistance.

Output resistance:

The output resistance is in parallel with the resistances of the circuit to its left including the source

, which can be found by , where

and we have

Alternatively, the output resistance of the emitter follower (treated as either a voltage or current

source) can also be found as the ratio of the open-circuit voltage (with ) and the short-

circuit current (with ).

As the voltage gain of the emitter follower is close to unity, the open-circuit output voltage is

approximately the same as the source voltage . The short-circuit current can be found as

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Therefore the output resistance is

The overall output resistance is

Conclusion:

The emitter follower is a circuit with deep negative feedback, i.e., all of its output vout = ve is fed back to become part of its input vbc. The fact that this is a negative feedback can be seen by:

Due to this deep negative feedback, the voltage gain of the emitter follower is smaller than unity. However, the circuit is drastically improved in terms of its input and output resistances. In fact the

emitter follower acts as an impedance transformer with a ratio of , i.e., the input resistance is

times greater than and the output resistance is times smaller than

.

Comparing this with the input resistance and output resistances of the common-emitter transistor circuit, we see that the emitter follower circuit has very favorable input/output resistances.

Although the emitter follower does not amplify voltage, due to its high input resistance drawing little current from the source, and its low output resistance capable of driving heavy load, it is widely used as both the input and output stages for a multi-stage voltage amplification circuit.

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EDC Lab Manual