How to maintain NF? reducing the signal power...

[brmadhukar]

How to maintain NF?

Hi,
Can I use a small value capacitor in series to increase impedance to reduce the signal power to my LNA. Since capacitor is lossless does it mean my NF will not increase.
B R

 

[Humungus]

Your approach is not correct because you'll change input impedance of your system. Then, source and input impedances will be unmatched, giving rise to increased noise figure

 

[unkarc]

re: How to maintain NF?

Hi,

Whatever you do or apply for reducing the input signal to the LNA, the signal-to-noise power ratio suffers, so your NF also suffers.

The idea with using a (more or less lossless) capacitor can be made an advantage if you use it for negative feedback inside a relatively wideband amplifier (much like a wideband inductive feedback like made by Norton ) to maintain the lowest NF possible.

I wonder if you have one big signal that causes trouble and you want it to attenuate but near to that big signal there is one or some more weak signals you wish to amplify? In this case the only remedy is to use a higher dynamic range LNA (it costs more unfortunately and consumes more DC input power). Band-pass filters/selective amplifiers are also possible but they depend on details you can consider.

rgds, unkarc

 

[brmadhukar]

Hi,
I have a RFIC with sufficient gain but higher NF. I want to reduce the NF but should maintain the total system gain constant. This is possible with only low noise amp. but the gain is the problem. If I have more gain than necessary the thermal noise itself will be more.
Can I somehow improve the NF without LNA gain.
B R

 

[cswang]

Re: re: How to maintain NF?

Hi,
I wonder if you have one big signal that causes trouble and you want it to attenuate but near to that big signal there is one or some more weak signals you wish to amplify? In this case the only remedy is to use a higher dynamic range LNA (it costs more unfortunately and consumes more DC input power). Band-pass filters/selective amplifiers are also possible but they depend on details you can consider.

rgds, unkarc

You could design high gain and low gain two mode to reduce your design!

"A CMOS Cellular Receiver Front-Ends from specification to relization " wroted by Michiel Steyaert is a good book for this topic!

 

[unkarc]

brmadhukar,

The very first stage of you system establishes the NF so you cannot escape using an LNA with having better NF than the one you have now.

There is a simple shareware software for studying system design with NF etc issues: RfWorkbench from Kirt Blattenberger, see
**broken link removed**
You can enter your system in block diagram forms with the appropiate data for each block (gain/loss, NF, IP, etc) and you may play around with the numbers to find a solution for your problem.

to cswang:

Thanks for the interesting info, I have not read that.

rgds, unkarc

 

[egemini]

LNA

Hello,
you can easily design a descrete LNA using NEC parts like NE68139.
Use a constant current biasing, so that you can stabilise the operating point.
thanks

 

[toonafishy]

How about using drain/emitter series inductance? This changes the input match without adding any additional noise to the system.

 

[zgx]

using parallel gate-drain negative-feedback also reduce the gain and improve the NF.

 

[flatulent]

no need for NF retention

If you have so much signal power that it overloads your system, you do not need to keep the NF low. The signal to noise ratio is high to start with. You should consider removing the amplifier from the signal chain or using one with less gain.

 

[Rayengine]

Hi,
A correct biasing current for LNA is most important in maintaining a good Noise Figure. Try Simulation and also on-board tuning is necessary for optimize the NF.

Rgs

 

[Humungus]

Has somebody experience at layouting CMOS LNAs?

 

[dd21]

CMOS LNA　does not look like a good solution at all. Usually, LNA use the GaAs or SiGe. BJT is not very good at noise figure, but is ok for lower frequency such as lower than 800MHz.

 

[brmadhukar]

Hi,
Thnx all for the info. I have tried using the capacitor. I used 47pF but, I could not get the SRF of the Cap, it increased the NF by 1.0dB and when shorted the NF did not change much.

brmadhukar

 

[jamez]

You can use series cap and shunt ind make a match circuit with low noise increase.