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Design >> RF Design >> why 50 ohms impedance matching for low noise amplifier
https://designers-guide.org/forum/YaBB.pl?num=1284194476

Message started by suryakumari on Sep 11^th, 2010, 1:41am

Title: why 50 ohms impedance matching for low noise amplifier
Post by suryakumari on Sep 11^th, 2010, 1:41am

why 50 ohms impedance matching in designing LNA,,why not 75 ohms..is there any specific reason for choosing that number???
can anyone tel me

waiting for reply

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by pancho_hideboo on Sep 11^th, 2010, 3:43am

See the followings.
http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/5#5
http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/3#3
http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/1#1
http://www.designers-guide.org/Forum/YaBB.pl?num=1065493598

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by suryakumari on Sep 12^th, 2010, 1:10am

Thank u.
I read that discussions but i am not clear .

I am new to rf :( please can anyone clarify my basic Q'ns so that i can move in rf world

(1) matching impedance of LNA depends on the type of antenna we are using..is my statement is correct??? and 50ohms for which antenna???
(2)If there is no filter between antenna and LNA then what about the impedance matching for LNA ???
(2) If there is a filter between antenna and LNA then what about the impedance matching for LNA and filter ???
(3)what is the effect of transmission line or co-axial cable on impedance matching???

Thanks
Suryakumari

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by pancho_hideboo on Sep 12^th, 2010, 1:28am

saam wrote on Sep 12^th, 2010, 1:10am:

please can anyone clarify my basic Q'ns so that i can move in rf world

I don't think so.
With understanding only these easy questions, you can not still even touch a door of RF World.

saam wrote on Sep 12^th, 2010, 1:10am:

(1) matching impedance of LNA depends on the type of antenna we are using..
is my statement is correct???

Correct.

saam wrote on Sep 12^th, 2010, 1:10am:

and 50ohms for which antenna???

Impedance of conventional commercial product's antenna is 50ohm.
http://www.designers-guide.org/Forum/YaBB.pl?num=1245527790

saam wrote on Sep 12^th, 2010, 1:10am:

(2) If there is no filter between antenna and LNA
then what about the impedance matching for LNA ???

You have to match input impedance of LNA to output impedance of antenna.

saam wrote on Sep 12^th, 2010, 1:10am:

~~(2)~~(3) If there is a filter between antenna and LNA
then as matching for antenna-fliter and filter-LNA.

You have to care about termination's condition of filter.
So you have to apply it as matching for antenna-filter and filter-LNA.

saam wrote on Sep 12^th, 2010, 1:10am:

~~(3)~~(4) what is the effect of transmission line or co-axial cable on impedance matching???

Consider a behavior of Γ in smith chart.
If line is perfectly lossless, Γ keeps a constant magnitude(that is a constant VSWR) and changes its phase.

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by suryakumari on Sep 12^th, 2010, 2:07am

Thank u

loss of rf coaxial cabel is minimum around 50ohm characteristics impedance....can anyone tel me the reason??

suppose if v use 50 ohms co-axial cable for minimum loss then do v have to use the antenna having radiation resistance of 50 ohms also

do v have to match the input impedance of LNA as 50 ohms???

If there is no feeder line between antenna and LNA then as you said we have to match i/p impedance of LNA to the o/p impedance of antenna.
If If there is no feeder line between antenna and LNA then what about the matching of LNa???

Thank u
suryakumari

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by pancho_hideboo on Sep 12^th, 2010, 2:12am

saam wrote on Sep 12^th, 2010, 2:07am:

loss of rf coaxial cabel is minimum around 50ohm characteristics impedance....
can anyone tel me the reason??

Simply total loss of wall is minimum around Rc=50ohm in TEM-mode of coaxial-cable.

Rc value for true minimum loss is 76.7ohm not 50ohm.
But losses of them are almost same.

See any beginner text book on microwave engineering.

http://en.wikipedia.org/wiki/Coaxial_cable

saam wrote on Sep 12^th, 2010, 2:07am:

suppose if v use 50 ohms co-axial cable for minimum loss
then do v have to use the antenna having radiation resistance of 50 ohms
also do v have to match the input impedance of LNA as 50 ohms???

Ideally yes.

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by suryakumari on Sep 12^th, 2010, 2:47am

Rc value for true minimal loss is 76.7ohm not 50ohm.
But losses of them are almost same.

then why don't they use 76.7 ohms ???
as you said conventional commercial antenna impedance is 50 ohms ..is it answer for my Q'n???

Thank u so much :)

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by pancho_hideboo on Sep 12^th, 2010, 2:51am

saam wrote on Sep 12^th, 2010, 2:47am:

then why don't they use 76.7 ohms ???
as you said conventional commercial antenna impedance is 50 ohms ..
is it answer for my Q'n???

Read http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245/5#5 and http://en.wikipedia.org/wiki/Coaxial_cable surely.

Again from practical point of view, losses of Rc=50ohm and Rc=76.7ohm are almost same.

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by suryakumari on Sep 12^th, 2010, 5:03am

Thank u :)

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by loose-electron on Sep 14^th, 2010, 6:14pm

50 (or for the fussy types, 52 ohms) is compromise between 77 ohms and 32 ohms

Best power handling is 30-32 ohms, and has been used for high power broadcast antenna systems in the past.

Best cable loss characteristic is at around 74 - 80 ohms and is commonly used in 75 ohm CATV systems to minimize system losses --

See:
http://www.microwaves101.com/encyclopedia/why50ohms.cfm

for a pretty good treatment of the subject.

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by pancho_hideboo on Sep 15^th, 2010, 8:31am

loose-electron wrote on Sep 14^th, 2010, 6:14pm:

and is commonly used in 75 ohm CATV systems to minimize system losses --

I don't think so.

Input impedance of folded-dipole antenna is 300ohm.
This is a reason why Rc of parallel feeder is 300ohm.

If we feed this folded-dipole by single-ended, it is easy to use 1/4-balun.
So Rc of coaxial cable is chosen as 75ohm historically.

Title: Re: why 50 ohms impedance matching for low noise amplifier
Post by loose-electron on Sep 15^th, 2010, 1:48pm

Or use a 6:1 to go from 300 ohms of a folded dipole to
50 ohms.

Parallel wire (aka twin lead) comes in 300 and 200 ohm variations.

Go research the history of it.

pancho_hideboo wrote on Sep 15^th, 2010, 8:31am:

loose-electron wrote on Sep 14^th, 2010, 6:14pm:

and is commonly used in 75 ohm CATV systems to minimize system losses --