This presentation discusses logarithmic amplifiers, which are electronic devices that convert an exponential input signal into a linear output response. Logarithmic amplifiers are useful for processing signals that vary widely in amplitude, such as in radio frequency communication and audio applications, as they can compress a wide dynamic input range into a more manageable linear output range. The presentation covers the basic circuit diagram of a logarithmic amplifier using a transistor diode configuration, and explains that the output voltage is proportional to the natural logarithm of the input voltage for a fixed resistance value. Applications of logarithmic amplifiers include mathematical operations, decibel measurement, and true RMS conversion.

1. A PRESENTATION ON
LOGARITHMIC AMPLIFIERS
PRESENTEDBY;BIJOYDIPTAGHOSH
ELECTRICALENGINEERING
2ND YEAR(3RD SEM)
11101622011

2. INTRODUCTION
Logarithmic amplifiers, also known as log amps, are electronic
devices designed to convert an input signal's exponential relationship
into a linear output response. They are commonly used in various
applications, including signal processing, communication systems, and
measurement instruments. The primary function of logarithmic
amplifiers is to compress a wide dynamic range of input signal
amplitudes into a more manageable linear range at the output. This is
particularly useful when dealing with signals that span several orders of
magnitude, such as in Radio Frequency communication or audio
applications. By utilizing logarithmic amplifiers, it becomes easier to
accurately process and measure signals that vary widely in amplitude..
The logarithmic amplifiers' ability to convert exponential signals into
linear ones makes them valuable components in various electronic
systems, contributing to improved signal-to-noise ratio and overall
performance.

3. CONTENTS
TOPICS COVERED HERE:
WHAT IS LOGARITHMIC AMPLIFIER
APPLICATIONS OF LOGARITHMIC AMPLIFIERS
CIRCUIT DIAGRAM
TRANS DIODE CONFIGURATION (RELATION BETWEEN OUTPUT
VOLTAGE WITH THE INPUT VOLTAGE AND THE FIXED RESISTENCE)
 Conclusion
 references

4. WHAT IS LOGARITHMIC AMPLIFIER
A log amplifier is an amplifier for which the
output voltage V0 is k times the natural log of the
input voltage Vi, this can be expressed as:
V0=k log ((Vi/ Vr)) ,where Vr is the normalization
constant in volts and K is the scale factor.
The term log amp is generally used in
communication technology, refers to calculate log
of an input signal’s envelope. All logarithmic
amplifiers must therefore specify a signal range
over which they will “log”.

5. APPLICATIONS OF LOGARITHMIC AMPLIFIERS
Logarithmic Amplifiers can be used in many ways,such
as:
To perform mathamatical operations like
multiplication , division , exponentiation.
To calculate the db value of a given quantity
As a true RMS converter

6. CIRCUIT DIAGRAM

7. TRANS DIODE CONFIGURATION
According to the virtual short concept, the voltage at the inverting input terminal of an op-amp will be
equal to the voltage at its non-inverting input terminal. So, the voltage at the inverting input terminal
will be zero volts.
The nodal equation at the inverting input terminal’s node is −
(O-Vi)/R1 + If=O
If=Vi/R1---------->Equation 1
The following is the equation for current flowing through a diode, when it is in forward bias −
If=Is e^(Vf/nVt)-------->Equation 2
where,
 Is is the saturation current of the diode,
 Vf is the voltage drop across diode, when it is in forward bias,
 Vt is the diode’s thermal equivalent voltage.
The KVL equation around the feedback loop of the op amp will be −
O-Vf-Vo=O
 Vf=-V0
Substituting the value of Vf in Equation 2, we get −
If=Is e^(-V0/nVt)
(Vi/R1)=Is e^(-V0/nVt)
ln(Vi/R1Is)=-V0/nRt
V0=-nRtln(Vi/R1Is)
Hence,the output voltage Vo will be proportional to the natural logarithm of the input voltage Vi for a fixed
value of resistance R1. V0 has a negative sign, which indicates that there exists a 1800 phase
difference between the input and the output.

8. CONCLUSION
In summery ,Dc log amp ICs have evoled into small
,easy-to-use, cost effective circuits nicely suited for
certain analog designs. The logarithmic function
convinently compresses wide dynamic range signals
and linearizes sensors with(semi-) exponential
transfer functions.Calibration procedures can
enhance log-amp performance, but are not
necessery in all applications.

9. REFERENCES
WEBSITES VISITED:
www.wikipedia.com
www.powershow.com
BOOKS VISITED:
ELECTRONICS:ANALOG AND DIGITAL, BY NAGRATH, PHI, 2004
ELECTRONIC PRINCIPLES, BY MALVINO,6/e,tmh
Integrated electronics, by millman & halkies,tata mcgrow hill.

10. THANK YOU