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Design of LNA (Read 3985 times)
Iamalearner
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Design of LNA
Mar 21st, 2004, 4:55pm
 
Hi all,
I am a humble learner in the field of RFIC. I am trying to design a Low Noise Amplifier. Its a very simple design. I have yet not used any type of feedback in it.

I am also a starter in Spectre RF tool. When i try to do PSS analysis and find the voltage gain of LNA, I get a negative gain in dB.

I am trying to figure out what i can do rectify this and get a gain of 15dB atleast. Can anyone help me how to do this ??

thanks in advance, probably as i learn the subject more, i would able to ask questions that are substantial, not trivial Smiley

thankyou
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August West
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Re: Design of LNA
Reply #1 - Mar 21st, 2004, 11:14pm
 
You do not normally need to use the PSS analysis when determining the gain. A simple DC analysis followed by either an AC or SP analysis is sufficient. The only time a PSS analysis would be needed is if you wanted to apply a large signal and determine the gain while the LNA was in compression, or if you wanted to determine the gain with a large blocker tone present.

-August
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learner
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Re: Design of LNA
Reply #2 - Mar 21st, 2004, 11:48pm
 
Hi August west
Thankyou very much for replying
I was going through a Spectre RF manual wherein they had found voltage gain of LNA using PSS analysis. I simply followed those steps. Can you please tell me more about DC analysis, SP analysis and how they differ from PSS analysis. I am asking this so that next time I am able to decide on my own which analysis to use for what. If you can refer me to some documentation, that will be helpful too. But i really need to learn all this.
So please help me!
thank you very much
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August West
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Re: Design of LNA
Reply #3 - Mar 22nd, 2004, 10:22pm
 
The AC, SP, and noise analyses linearize the circuit about the DC operating point before calculating gain and noise. This mimics the situation where you are just applying a small input signal to compute the gain, or you are computing the noise with no input at all.

The PSS analysis allows you to apply a large signal; large meaning large enough to cause the LNA to distort. You can use this while sweeping the input power to compute the compression point.  Or you can simply apply a large signal that represents an out-of-band blocker, and then perform a PAC, PSP, or PNoise analysis to compute the small-signal gain and noise in the presence of the large blocker.

The first set of analyses, AC, Noise, and SP are analyses that can be performed using Spectre or Spice. The second set, PSS and its small signal partners, PAC, PSP, and PNoise, require SpectreRF.

August
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kokabanga
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Re: Design of LNA
Reply #4 - May 20th, 2004, 3:45am
 
I am going to answer your main problem of having low gain

there can be 2 reasons

1) the inductor u are using to tune at the LC tank at the out put(assuming u are using a cascode) maybe very lossy

i.e at the resonant frequency u are designing for

gm*Rl<<<1

=> 10*log10*(Av)<0!!!!

so u must use a inductor with a high Q!!!!

which is actually my PhD!!!

2) or maybe your mos/BJT is plain not even in the active region in which case gm<<<<1 => gm*Rl<<<<1

=> 20*log10(Av)<<<0

thus u have two reasons

so first check if your mosfet/BJT is in the region where it will have high gm and good linearity which is obviously the active region for MOS .

do mail me back if u need any help

will TRY to HELP!!!

Grin
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naren
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