lab reportondesign and testing of logarithmic and
anti-logarithmic amplifier using op-amp
Bona fide record of work done byKARTHIKEYAN (13L223)KRISHNA
KUMAR (13L224)KUMAARAN.S.G (13L225)Dissertation submitted in
partial fulfillment of the requirements for the degree ofBACHELOR
OF ENGINEERINGBranch: ELECTRONICS AND COMMUNICATION engineering
FEBRUARY_2015department of ELECTRONICS AND COMMUNICATION
engineeringPSG COLLEGE OF TECHNOLOGY(Autonomous Institution)PSG
COLLEGE OF TECHNOLOGY(Autonomous Institution)COIMBATORE 641 004
design and testing of logarithmic and anti-logarithmic amplifier
using op-amp
Bona fide record of work done byKARTHIKEYAN (13L223)KRISHNA
KUMAR (13L224)KUMAARAN.S.G (13L225)
Dissertation submitted in partial fulfillment of the
requirements for the degree of
bachelor OF ENGINEERING
Branch: ELECTRONIC AND COMMUNICATION ENGINEERING
FEBRUARY_2015
. Dr. S. Subha Rani Faculty guide Head of the
DepartmentACKNOWLEDGEMENT
I would like to express my sincere thanks to Dr. R.
RUDRAMOORTHY, the principal, PSG College of Technology, for
providing us with various infrastructure and facilities in the
college.
I am indebted to Dr. S. SUBHA RANI, Professor and Head of the
Department of Electronics and Communication Engineering, PSG
College of Technology, for her continued support and
motivation.
I would like to express my sincere gratitude to our faculties in
charge Mrs. L.THULASIMANI and Dr. K.R.RADHAKRISHNAN for their
constant motivation, direction and guidance in the laboratory.
I also extend my heartfelt thanks to our class advisor Dr. P.
VISALAKSHI for her continuous support.
I would like to thank the student friends who contributed in
their own special way for the betterment of this report. Last but
not the least I thank my family members the Almighty and who have
been a guiding light in all my endeavors.
ABSTRACT
Selecting the right operational amplifier for a specific
application requires you to have your design goals clearly in mind
along with a firm understanding of what the published
specifications mean .This paper addresses the issue of
understanding data sheetspecifications.
This paper begins with background information. First,
introductory topics on the basic principles of amplifiers are
presented, including the ideal op amp model. As an example, two
simple amplifier circuits are analyzed using the ideal model.
Second, a simplifiedcircuit of an operational amplifier is
discussed to show how parameters arise that limit the ideal
functioning of the operational amplifier.
The paper then focuses on op amp specifications. Texas
Instruments data book, Amplifiers, Comparators, and Special
Functions, is the basis for the discussion on op amp
specifications. Information is presented about how Texas
Instruments defines and tests operational amplifier parameters.
Table of ContentsACKNOWLEDGEMENT3ABSTRACT4A. INTRODUCTION:6Fig
A.1 Basic Op- amp (LOGARITHMIC)6ANTI LOG AMPLIFIER6Fig A.2 Basic
Op- amp (ANTI LOGARITHMIC)72. AIM:73. COMPONENTS REQUIRED:73.
DESIGN STEPS:83.1 LOGARITHMIC AMPLIFIER:83.2 ANTI-LOGARITHMIC
AMPLIFIER:84. THEORY:94.1 LOGARITHMIC AMPLIFIER9Fig 4.1.1 Basic Op-
amp (LOGARITHMIC)9Logarithmic amplifier operation94.2
ANTILOGARITHMIC AMPLIFIER10Fig 4.2.1 Basic Op- amp
(LOGARITHMIC)10Anti log amplifier operation115. CIRCUIT:125.1
LOGARITHMIC AMPLIFIER:125.1.1 MODEL GRAPH:125.1.2 TABULATION:125.2
ANTI-LOGARITHMIC AMPLIFIER:135.2.1 MODEL GRAPH:135.2.2
TABULATION:136. PROCEDURE:146.1 LOGARITHMIC AMPLIFIER :146.2
ANTI-LOGARITHMIC AMPLIFIER:147. PRE LAB QUESTIONS:148. POST LAB
QUESTIONS:159. SIMULATION:1610. RESULT:1711 . CONCLUSION:1712 .
REFERENCES:1713.APPENDIX18
INTRODUCTION:Alog amplifieris anamplifierfor which the output
voltageVoutisKtimes thenatural logof the input voltageVin. This can
be expressed as,
whereVrefis the normalization constant in volts andKis the scale
factor.
Fig 1.1 Basic Op- amp (LOGARITHMIC)The relationship between the
input voltageand the output voltageis given by:
whereandare thesaturation currentand thethermal voltageof the
diode respectively.ANTI LOG AMPLIFIER
Anti log amplifier is one which provides output proportional to
the anti log i.e. exponential to the input voltage.If Viis the
input signal applied to a Anti log amplifierthen the output is
Vo=K*exp(a*Vi) where K is proportionality constant, a is
constant.
Fig 1.2 Basic Op- amp (ANTI LOGARITHMIC)
1. AIM:
To design and test the logarithmic and anti-logarithmic
amplifier using op-amp to obtain its voltage transfer
characteristics.
2. COMPONENTS REQUIRED:
COMPONENTSRANGEQUANTITY
IC7411
Diode D0A791
Resistor 10K1
MSO1
Breadboard-1
Connecting wires ,probes-As required
3. DESIGN STEPS:3.1 LOGARITHMIC AMPLIFIER:
Taking natural log on both sides,
3.2 ANTI-LOGARITHMIC AMPLIFIER:If =IO [ [I=If = ] =
= VO= - (RIO)The value of =1(taken)VT Thermal voltage Io
Saturation current.VT and Io are taken for the diode D0A79 at a
particular temperature (room temp - 250 C)R=10KVT =26mVIo =0.3A for
diode D0A79
4. THEORY:
Antilog computation may require function such as lnx, logx or
sinhx. This can be performed continuously with op-amps. Log
amplifiers can easily perform the function of digital voltmeter or
spectrum analysis. It is also used to compress the dynamic range of
a signal. 4.1 LOGARITHMIC AMPLIFIERIn the log-amp circuit the diode
is connected in the circuit. Input given to inverting terminal
through resistor. Diode current I =IO [ where VT Thermal voltage
and Io is reverse saturation current. Output voltage V0=VT where ,
VT and Io R are constant. Hence Vo ln(Vin), so it is used to find
the logarithmic value of input. Fig 4.1.1 Basic Op- amp
(LOGARITHMIC)
Logarithmic amplifier operation
It is obvious from the circuit shown above that negative
feedback is provided from output to inverting terminal.Using the
concept of virtual short between the input terminals of an opamp
the voltage at inverting terminal will be zero volts.(Since the non
inverting terminal of opamp is at ground potential). The
logarithmic circuit can be redrawn as follows
The current equation of diode is given as Id= Ido*(exp (V/Vt)-1)
where Idois reverse saturation current,V is voltage applied across
diode; Vtis the voltage equivalent of temperature. Hence applying
KCL at inverting terminal of opamp, we get (0-Vin)/R1+ Id= 0
implies Id= Vin/R1Substituting the equation for current in the
above equation we get Ido*(exp (V/Vt)-1) = Vin/R1. Assuming exp
(V/Vt) >> 1 i.e. V>>Vtand V = Vo,we get Ido*exp (-Vo/
Vt) = Vin/R1. Applying Antilog on both sides we get Vo= Vt* ln
(Vin/(R1*Ido)).
Gain of logarithmic amplifierGain of amplifierK = -Vt4.2
ANTILOGARITHMIC AMPLIFIER
In the anti-logarithmic amplifier, feedback resistor is
connected and input is given to inverting terminal through diode.
Diode current I =IO [ the output voltage of antilog amplifier is
Vo=Io R where Io R, ,VT are constant. Hence Vo anti-log(Vin)
Fig 4.2.1 Basic Op- amp (LOGARITHMIC)
Anti log amplifier is one which provides output proportional to
the anti log i.e. exponential to the input voltage.If Viis the
input signal applied to a Anti log amplifierthen the output is
Vo=K*exp(a*Vi) where K is proportionality constant, a is
constant.
Anti log amplifier operation
It is obvious from the circuit shown above that negative
feedback is provided from output to inverting terminal. Using the
concept of virtual short between the input terminals of an op amp
the voltage at inverting terminal will be zero volts.(Since the non
inverting terminal of op amp is at ground potential).
The current equation of diode is given as Id= Ido*(exp (V/Vt)-1)
where Idois reverse saturation current, V is voltage applied across
diode; Vtis the voltage equivalent of temperatureApplying KCL at
inverting node of op amp we get Id= (0-Vo)/R = Io*(exp (Vin/ Vt))
(assumed Vin/ Vt>> 1)Hence Vo= -Io*R*(exp (Vin/Vt)).
Gain of Anti log amplifierGain of Anti log amplifierK= -Io*R
5. CIRCUIT:5.1 LOGARITHMIC AMPLIFIER:
5.1.1 MODEL GRAPH:
5.1.2 TABULATION:Vin(v)Vo(v)
0.10.20.30.40.50.60.70.80.91.0TheoreticalPractical
-91.09-109.19119.73-127.21-133.01-137.76-141.76-145.23-148.3-151.04
-67.9-72.3-89.8-102.5-111.2-117.3-123.3-130.8-135.9-141.6S
5.2 ANTI-LOGARITHMIC AMPLIFIER:
5.2.1 MODEL GRAPH:
5.2.2 TABULATION:Vin(v)Vo(v)
-0.01-0.02-0.03-0.04-0.05-0.06-0.07-0.08-0.09-0.1-0.2-0.3-0.4-0.5TheoreticalPractical
0.0090.0280.0560.0940.1510.2270.4840.7960.9421.123.686.488.2110.57
6. PROCEDURE:6.1 LOGARITHMIC AMPLIFIER : Connections are made as
per the circuit diagram The input voltage is applied to the circuit
between (0-1)V of very small voltage. The output voltage is noted
and it is plotted.
6.2 ANTI-LOGARITHMIC AMPLIFIER: Connections are given as per
circuit diagram The input of very small voltage is applied to the
circuit between (0-1)V The output voltage is noted and the graph is
plotted.7. PRE LAB QUESTIONS:
1. What are the limitations of basic log amplifier?The emitter
saturation current Is varies from transistor to transistor and with
temperature . Thus a stable reference voltage cannot be obtained.2.
How log amplifier using diode is thermally compensated?
Temperature compensation by op amp A2 which provides a non
inverting gain
8. POST LAB QUESTIONS:
1. Applications of log and anti log amplifier. Multiplication
and division, powers and roots Compression and decompression True
RMS detection Process control
2. How log amplifier using diode is thermally compensated?
9. SIMULATION:
LOGARITHMIC AMPLIFIER:
ANTI LOGARITHMIC AMPLIFIER:
10. RESULT:
Thus, the logarithmic and anti-logarithmic amplifier using
op-amp was designed and tested and its voltage transfer
characteristics were plotted.
11 . CONCLUSION:
Early op amps were made from vacuum tubes consuming lots of
space and energy. Later op amps were made smaller by implementing
them with discrete transistors. Today, op amps are monolithic
integrated circuits highly efficient and cost effective
12 . REFERENCES: www.electronicshub.org
www.electronicstutorial.ws www.wikepedia.com www.daenotes.com
www.allaboutcircuits.com
13.APPENDIX
13
