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Design >> RF Design >> purposes of the LNA
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Message started by aaron_do on Jan 24^th, 2007, 7:14pm

Title: purposes of the LNA
Post by aaron_do on Jan 24^th, 2007, 7:14pm

Hi everyone,

i'm trying to list the purposes of an LNA and would appreciate any input. I also need to know exactly how necessary it is. Let me know if i'm wrong.

1) Voltage Gain: required to minimize the impact of subsequent circuit blocks noise. Absolutely Necessary in an LNA

2) Matching:
- Prevents signal reflection: Not really necessary
- Ensures matching of previous blocks such as TR switch and Band-select filter. necessary to ensure proper frequency response of the BS filter
- Maximum power transfer: not necessary in CMOS

3) Low Noise: LNA NF sets the minimum value of the overall system NF. depends on system requirements

4) Reverse Isolation: prevents LO leakage to antenna. how necessary is this? In what circumstances?

The reason i'm asking this is because all but the last function can be performed by a passive LC matching network. I'm trying to justify the need for an LNA.

thanks,
Aaron

Title: Re: purposes of the LNA
Post by mg777 on Jan 24^th, 2007, 7:30pm

You must first understand how hard system designers work to get 1 dB of coding gain.

M.G.Rajan
www.eecalc.com

Title: Re: purposes of the LNA
Post by aaron_do on Jan 24^th, 2007, 7:37pm

......you've lost me.

are you saying we definately need and LNA? and if so, why? can you elaborate?

thanks,
Aaron

Title: Re: purposes of the LNA
Post by didac on Jan 24^th, 2007, 11:08pm

Hi,
I think that LNA its a very necessary block in communications systems but there are some scenarios that can be solved without it. For example if your system is limited by interferences and not by noise its not crazy to eliminate it(in fact in UWB systems there are some proposals to erase the LNA on the receiver chain). The main problem it's the isolation with antenna then the problem comes to the mixer(how good is your mixer isolation?).
After saying this I must say that LNA will be with us for many time as a designer.
PD:as a side note the worst thing of using a LNA in CMOS for me its the needing of inductors due to the area they use and their low Q.

Title: Re: purposes of the LNA
Post by mg777 on Jan 24^th, 2007, 11:33pm

You need to weigh the LNA design considerations against how much the SNR means to somebody downstream. Let me put it this way - say you have 4 LNA topologies (including the null one, where you eliminate the LNA). Do you know what the BER waterfall looks like for each case for the typical and worst-case channels?

M.G.Rajan
www.eecalc.com

Title: Re: purposes of the LNA
Post by didac on Jan 25^th, 2007, 12:22am

Obviously when I say that in some scenarios its not crazy to eliminate the LNA I assume that you do a correct system level simulation taking into account the type of channel you will be fighting against(AWGN,Rayleigh,Saleh-Valenzuela and so on...) and see if yout BER requirement can be achieved without the LNA or its a must to use it.

Title: Re: purposes of the LNA
Post by mg777 on Jan 25^th, 2007, 5:40am

didac, you have made excellent points especially with reference to CMOS LNA's. I was replying to aaron_do.

There is a somewhat analogous situation in fiber-optic receivers. While conventional wisdom prefers the transimpedance amplifier (TIA) as front-end, OSNR-limited submarine links use gain blocks that handle the high power with better linearity.

I just posted a poll regarding tool choices for system simulation. See under 'Simulators -> System Simulators'.

M.G.Rajan
www.eecalc.com

Title: Re: purposes of the LNA
Post by aaron_do on Jan 25^th, 2007, 5:29pm

Hi all,

thanks for your replies. Yes I know based on my modulation scheme the required BER at the ADC. Based on what I know of the IF blocks, I also estimated the IRN of the IF blocks at the mixer output, and from there I knew the required gain and NF of the LNA/Mixer stage.

My question was really that if the functions of voltage gain, low noise figure (LC networks can have 3 dB NF even on-chip) and input matching can be performed by an LC network, is there really a need for an LNA?

thanks,
Aaron

Title: Re: purposes of the LNA
Post by didac on Jan 26^th, 2007, 3:33am

Hi to all,
Well if you consider the use of an LC matching network then you use inductors so I think that designing a LNA then its the better choice, you will have the three objectives(voltage gain,low NF and input matching) adding only a few transistors(that add a negligeble area compared with the passives) and gives you more freedom variables to tune your design not only depending on L, C values available. The only point where I doubt its if the linearity of the system its a great issue(systeme limited by interferences like systems operating in free bands where you can find other systems operating near you frquency).

Title: Re: purposes of the LNA
Post by Prabhu on Jan 27^th, 2007, 1:22am

Hi Aaron

In a conventional LNA the voltage gain is partitioned between the input matching network and the actual transistor amplifying stage.
There the Q of the matching network and the resonant load are kept deliberately small ( say around 3 ) so that the LNA gain remains flat over
process shifts.

Now if the entire gain has to be realised using a passive resonant network, the Q of the network will increase to an unacceptable value and a small process shift will result in a large gain variation.

This should be one of the reasons why LNAs are not completely replaced by passive matching networks.

Title: Re: purposes of the LNA
Post by aaron_do on Jan 27^th, 2007, 8:12am

Hi Prabhu,

that sounds like a reasonable answer. I was thinking that using an LC matching network for gain might require a second order network and so the variation would be even more.

Also after going back and running some simulations, I found I could only get about 10 dB gain out of the network. I guess LNAs are very necessary after all.

thanks,
Aaron

Title: Re: purposes of the LNA
Post by carlgrace on Feb 7^th, 2007, 11:04am

aaron_do,

Another reason you need the additional gain of a transistor LNA is to reduce the impact of noise in the back end stages. You can't get enough gain from a passive network to do this. Basically, a well designed receiver will have the majority of its noise impact coming from the LNA. If it is passive, you will have to worry more about the mixer and the baseband circuits.