measure

Characterization of an RF amplifier - 1db Compression Point - part 2



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today we have part two where we're gonna

measure

the 1db compression point of the

amplifier

let's go so guys to measure the 1db

compression point of this amplifier

we're gonna use the same setup

we used it in the last video we have

here the spectrum analyzer a signal

generator

here we have the power supply that's

supplying 12 volts to the amplifier but

first

let's try to understand what is the 1db

compression point in an amplifier when

we are dealing with an amplifier and we

plot the gain

characteristic of the amplifier in a db

scale here so we have db input power

to db output power the transfer

characteristics

will have a slope of one and we can see

it here

gain of five so with negative 10 db of

input power

we have negative 5 db of output power

with negative 9 and db of input power

we have negative 4 db of output power

so as this scale here is logarithm

we're gonna have a slope of one one

thing we need to have in mind is that

the radio frequency amplifier

is modeled here by a small signal

behavior

so this behavior here of the ideal

amplifier

is from the linearized point of view of

a small signal amplifier

the 1db compression point parameter of

an amplifier is related

with the transition from small signal to

large signal behavior we need to have in

mind that a practical amplifier have

limited output power so in some points

the amplifier starts to saturate

and to compress the output the 1db

compression point is the point

where the amplifier gain is reduced by

1 db because of compression so in this

example here we can see

that the amplifier behaves in small

signal region

until we reach something like negative

7 db negative 8 db of input power

and at this point the output power of

the amplifier starts to compress

when the ideal curve and the practical

curve becomes 1db apart

we have the 1db compression point of the

amplifier in a practical point of view

the output power became compressed

because now the power is going to

harmonics of the signal as the amplifier

is behaving in large signal and this

mouse signal model cannot be applied

anymore we're gonna measure the 1db

compression point

using the spectral analyzer and after

you're gonna see the non-linearities and

the large signal behavior

using a real-time oscilloscope i

configure the signal analyzer to 100 mag

because this frequency is close to the

frequency i'm going to use the amplifier

here on the spectrum analyzer i center

the output

here on the center of the screen and it

we're gonna sweep the input power

and we're gonna see when the amplifier

gain drops by one db i will manually

sweep the input power

and i'm gonna annotate here on the

notebook the output power so you can

calculate

the gain at each step so let's go

negative 25 dbe

negative 24 negative 23 negative 22

negative 21 and let's go now we have a

table of input power

output power and the outer power that is

the gain of the amplifier

9db is exactly what we get on the last

video and as the input power is sweeper

to a higher power we can see

that again starts to be compressed and

you can see that

here at input power of negative 14

negative 15 we have 1 db of difference

from the starting point here as we

started with 9.4 db

of gain and here the compression point

is about

8.6 8.2 i made the mean

of these two output powers here 1db

compression point

look at from the output of the amplifier

is

negative 6.1 db and we're gonna see in

the next video that is very close to the

third order interception point

plus 10 db and this is common behavior

for linear amplifiers in the next video

we're going to learn what is the third

order intercept

it's very clear here that if i try to

put more power at the input of the

amplifier the output

will not grow anymore in a real

application it's really important to use

the amplifier

well below the 1db compression point so

we have

a linear and small signal operation of

the amplifier we can see what happens

with the signal when you start to

approach

the 1db compression point using an

oscilloscope

so we are here at

negative 20 db of input and we can see

that the signal still looks like a sign

and

and the second and the third harmonic

are almost 50 db

below the fundamental i will start to

increase the input level

and we're going to start approach the

1db compression point from the input

perspective the 1db compression point

is negative 12 db that we measured here

but we can see that when we start to

approach it

here we we are at negative 18 db

the harmonic content of the signal

starts to increase very happily and

as we approach here we are approaching

the 1db

the theoretical 1db compression point

and we can see

that now this signal is heavily

distorted and we are entering

large signal behavior look at this

now the signal is totally compressed

here

on top and we are entering

large signal behavior at signals close

or above

the 1db compression point or linear

small signal model

cannot be applied anymore so guys thank

you for watching the video

i hope you enjoy it if so please

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see you

in the next video