Author Topic: LNA Question  (Read 2551 times)

0 Members and 1 Guest are viewing this topic.

Offline jmsiglerTopic starter

  • Regular Contributor
  • *
  • Posts: 59
  • Country: us
LNA Question
« on: July 30, 2016, 06:47:50 pm »
Hello,

What performance trade off does a LNA make to achieve its low noise factor compared to a regular amplifier? Why would it be a bad idea to use a LNA other places in the signal chain besides right after or close to the input? Is it mainly a cost thing?
 

Offline German_EE

  • Super Contributor
  • ***
  • Posts: 2399
  • Country: de
Re: LNA Question
« Reply #1 on: July 30, 2016, 07:01:29 pm »
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

Warren Buffett
 

Offline jmsiglerTopic starter

  • Regular Contributor
  • *
  • Posts: 59
  • Country: us
Re: LNA Question
« Reply #2 on: July 30, 2016, 07:17:26 pm »
Good point, I probably should have used google first. Thanks for the response!
 

Offline whollender

  • Regular Contributor
  • *
  • Posts: 58
  • Country: us
Re: LNA Question
« Reply #3 on: July 30, 2016, 08:00:39 pm »
The big tradeoff for LNAs is usually performance at higher power levels.  So LNAs will usually start distorting at lower power levels than other amplifiers.  They also tend to be more expensive than simple gain block type amplifiers, so it usually only makes sense to put the LNA at the beginning of the receiver chain where the low noise figure really makes a difference.
 

Offline radar_macgyver

  • Frequent Contributor
  • **
  • Posts: 760
  • Country: us
Re: LNA Question
« Reply #4 on: July 30, 2016, 10:03:37 pm »
In order to push the noise figure very low, LNA designs tend to be narrow band. They might still have gain outside their specified bandwidth, but other properties like saturation power (P1dB) and linearity (IP3) suffer outside the specified bandwidth.

Amplifier design: you have noise figure, power output and bandwidth. Pick two.
 

Offline rfbroadband

  • Supporter
  • ****
  • Posts: 186
  • Country: us
Re: LNA Question
« Reply #5 on: July 30, 2016, 11:17:47 pm »
trade offs are:
1) input impedance match
2) bandwidth
3) large signal performance (P1dB, IP3, IP2)

1) the optimum NF is achieved at impedances other than 50 or 75 Ohm...-> google for noise matching
2) matching  for lowest possible noise will (in many cases) be narrow band
3) in order to operate the device for best noise performance requires a bias point that is non optimal for large signal behavior of your amp.

I provided a very basic, simplified explanation. The trade offs depend on the type of device (MOSFET, BJT) and the type of circuits you choose (common gate, common source etc).

You could study a few text books that derive the equations for each case to get a deeper understanding.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf