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Design >> RF Design >> LNA Gain peaking at wrong frequency?
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Message started by tintin on Feb 16^th, 2004, 7:42pm

Title: LNA Gain peaking at wrong frequency?
Post by tintin on Feb 16^th, 2004, 7:42pm

Hi,
The LNA i've designed has an S21 that peaks at 5.1 GHz instead of the intended 5.6GHz. LNA uses standard cascode structure with source degeneration and inductor-capacitor input matching. None of the matching elements are affecting the peak resonance freq. I tried changing the output matching elements too - but though peaking freq. changes, gain drops due to bad matching. What affects the peaking frequency and how can it be modified? Plz help me out.

Title: Re: LNA Gain peaking at wrong frequency?
Post by naren on May 24^th, 2004, 1:54pm

I had same problem how about reducing the "L"or "C" value at the drain of the CG part of your cascode which u must be using to implement the band pass filtering !!!!??? no??

cant u reduce it further?

so that u can maybe shift the tuning frequency up??

without probably affecting input match??

also there is one more effect I am facing.....I am doing an LNA at 900MHz and strangely any change in the input matching network such as a change in Ls or Lg is also changing my center frequency..maybe due to feedback through the substrate network as there seems to be no path thru the cascode itself...which is why we use a cascode in the first place!!!!

Title: Re: LNA Gain peaking at wrong frequency?
Post by mannysingh on Dec 17^th, 2004, 3:44pm

You can increase the Lg to move peak to the right but remember you'll pay for the power gain in terms of NF. It'll go up. If you have little margin for NF, your only choice is to increase the gain. Here's how you increaes the gain:

1) decrease Ls
2) increase W/L ratio
3) Add another Common Source stage with inductive degeneration.

I hope it helps.