Noise Figure for Low-Noise Amplifier

[@YY]

This might sounds stupid but can you find any reading or prove regarding the noise figure?

I understand that for LNA, the lower the noise figure the better the quality of the LNA. However, how low can we consider the amplifier actually qualified as a LNA? Noise figure lower than 3 dB?

Can anyone advise on this? some reading suggestion will be great.

 

[kpc]

Hi,

Yes, NF is an important parameter for LNAs. But when you look at an LNA datasheet it differentiates from other amplifiers with also other parameters like P1dB and IP3 points. Since LNAs are used in the "RF Front End" stage of an RF receiver, they are designed for being able to process low noise signals. This also means that their saturation points at the output will also be lower with respect to the standard power amplifiers. NF values determine the quality of LNA, so in simple terms lower is better.

You can look at RF books, especially chapters about "receiver design" for referance. Sayre's Complete Wireless Design and Pozar's Microwave Engineering books may help.

 

[jiripolivka]

This might sounds stupid but can you find any reading or prove regarding the noise figure?

I understand that for LNA, the lower the noise figure the better the quality of the LNA. However, how low can we consider the amplifier actually qualified as a LNA? Noise figure lower than 3 dB?

Can anyone advise on this? some reading suggestion will be great.

Definition of low-noise LNA depends on the type of a system in which a LNA is to be used.

LNAs are typically low-noise amplifiers to be used in VHF and higher-frequency communication systems where sky noise contributes only lightly to receiver+ antenna thermal noise (below say 30 MHz, galactic and sky noise is higher than thermal noise, so using a low-noise amplifier in a communication system is not giving better results).
An exception may be a LNA used in medical NMR systems which are enclosed, do not receive external noise but a good signal/noise ratio is needed belo 30 MHz.
Satellite communications utilize microwaves and the sky noise is close to several Kelvins, so using a LNA with 0.5 dB noise figure is a real advantage. Radar systems detecting aiplanes also "look" in the cold sky, and a good LNA offers a longer detection range.

 

[mtwieg]

I can confirm that in NMR/MRI, a NF of 0.5dB is considered standard for HF up to UHF.

 

[D.A.(Tony)Stewart]

Consider that if you are on the fringe of receiving a signal and you just need 1% more range, this can be as little as 0.1dB improvement in NF of the LNA. Materials such as GaAs are better suited than Silicon for LNA's

https://cache.freescale.com/files/rf_if/doc/white_paper/RFLNAWP.pdf

 

[jiripolivka]

Consider that if you are on the fringe of receiving a signal and you just need 1% more range, this can be as little as 0.1dB improvement in NF of the LNA. Materials such as GaAs are better suited than Silicon for LNA's

https://cache.freescale.com/files/rf_if/doc/white_paper/RFLNAWP.pdf

Concerning "fringe" area, I think an improvement can rather be achieved by using a higher-gain antenna. In microwave terrestrial communication systems, using better LNAs does not improve signal-to-noise ratio in receivers as the receiving antenna "looks" to the ground that emits close to 300 K noise temperature. Then a better LNA contributes not to a better system noise figure but rather offers a way to interference.

For low-noise applications, always analyze the complete system for good results.

 

[D.A.(Tony)Stewart]

Path loss is always biggest issue and antenna gain is easiest to implement, but when optimal already 0.1dB makes a big difference in conductive path. THis loss can be found in mis-wired baluns, RF connectors and coax quality. A former co-worker of mine invented/designed the Bell microwave telcom repeater network in Canada using only Andrews very high gain Antennae.


... and his only personal reward was getting 3.4 dB to 3.3 dB loss in splitters normally only possible to get 3.5dB loss in thousands of cascaded repeaters which translated into wider spacing of repeaters.

A microwave engineer I knew from Motorola in Arizona, designed the Iridium Satcom radios which I design the test set for my former company C-MAC who planned to build them in Florida. The LNA Tx/Rx chips had exceptionally low loss so I had to choose a VNA with 0.01 dB resolution and calibrate as such for 0.1dB losses. (Anritsu VNA) 6GHz with a flip chip socket.

 

[Alper özel]

1- The most common way to establish whether if an amplifier can be considered as LNA or not is biasing. While LNA's are mostly biased in linear region other amplifiers are in saturation region. But this is not enough alone of course.

2- Behavior of the LNA must be linear as possible as it can be. So unlike other transistors, LNA's do not have non-linear outputs. At least not as much as other amplifiers. This is about both first 1st entry and input matching.

3- If an amplifier has noise figure below 3 dB in high frequency range it may be considered as LNA, but for fundamental band, NOT.

 

[D.A.(Tony)Stewart]

note , previous reply#3 is due simply to that typical reduced noise BW in the receiver filters occurs at baseband frequencies.

It does not matter if the RF system operates on 100 MHz or at 2450 MHz, the noise power / Hz will be the same, if the radio channel bandwidth is the same.
Thermal noise at room temp (290K) is the reference level of -174 dBm/Hz and you simply multiply it by the actual bandwidth of the radio channel.

If the amplifier is known to have a 3dB NF, the internal noise source adds an equal noise to the input noise before amplification.