Microwave Integrated Circuits
Professor Jayanta Mukherjee
Department of Electrical Engineering
Indian Intitute of Technology Bombay
Module 7
Lecture No 29
Noise
Hello, welcome to another module of this course, in NPTEL mocks program, microwave
integrated circuits. In the previous modules in this week we had covered the design design aspect
using gain circle. In those modules, we covered how to achieve a particular gain whether it is a
transducer power gain or available power gain or operating power gain. So in this module we
will be covering another important aspect of amplifier design which is the noise aspect. In the 1
st
part, we will be covering what is noise, how to characterise noise and inthe 2
nd
part, we will
be
discussing about noise circle and how to achieve a particular noise figure.
And then we will also see what is in noise figure. What does the term, noise figure mean and
how to achieve if you have given noise figure specifications and how we can achieve that using
noise figure circuits.
So, 1
st
question is, what is noise?
(Refer Slide Time:
1:27)
Noise if you have any idea about statistical statistics, noise is known as a noise is a random
process. What do we mean by random process? 1
st

say a voltage. It has, at a particular instant it will have a definite value. But say, noise voltage if
you say, it does not have any particular value. Instead, what we can characterise it is with some
average or moments or what we call probability density function. Now, an ensemble of such
random variables over trying if time is also now available, then such a process is called a random
process. And noise is one such random process.
Now how to as I said, you cannot have a deterministic value. For example I say that the voltage,
if you measure the voltage of a battery you might find it to be 2 volts then you say that the
voltage of the battery is 2 volts. But
on the other hand let’s
say noise voltage, at any instant it
may not be the same value. It keeps on varying. The value of the noise voltage keeps on varying.
But then, the average value if you find the average of the noise voltage, then that remains
constant.
So that is how we characterise noise. Even though we cannot exactly specify what the real
voltage will be at a particular instant of time, we can say what the average will be over time. So
in that sense, there are 2 types of average actually.
2
2
lim
1
( )
( )dt
( )
( )
T
T
n
n t
n t
T
T
n t
n t P
n dn
One is what is known as the time average. That is represented by this symbol and it is defined as
like this. So it is like the average of noise voltage or any other random process with time.