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LNA Stability...Need help

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kmmaran

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Hi Everyone,

I am having problem with my LNA, and like to know, if I reduce the gain of the LNA, can I improve the stability (K).
Basically, the K come with the device, and biasing the LNA will help the K, but in my case I am using Module, and biasing does not promise good K, can the matching help on this. (Input and Output matching.

I am using MAX2373 LNA from MAXIM.

Thanks All.
 

Definitely the input/output matching can improve the stability of the LNA.

For matching follow the instructions given by Maxim:

"The input- and output-matching networks are sensitive to layout-related parasitic inductions. To minimize parasitic inductance, keep traces short and place components as close as possible to the chip. To minimize parasitic capacitance, minimize the area of the plane."
 

One typical method to improve the K is reducing the gain by means of a shunted resistor at the output. If you have spare gain, you can try this.

In ADS2006 in the palette probe componets, there are a couple of elements called SProbe and Sprobe2 which check the loop gain between the active device and the matchin network, they can be used when K<1.
 

You should read an app note by Agilent titled "Practical Amplifier Design by Ken Payne" (search for it, it's freely available). There is a comprehensive treatment of stability analysis with EM and practical layout for amplifier in pages 20-40. I was burned by parasitic inductance of my ground plane causing instability so take heed when manufacturers warn you of parasitic inductance.
 

Hi Guys,

I have a designed a LNA and downconvertion circuit at 2.4 but the LNA is highly unstable. As soon as I supply the voltage to the LNA (RF3866 from RFMD) I have a high level output at 3.05 GHz. Actually I have a oscillator at this frequency. Can anybody help me?

Regards
Hadi
 

It is very easily to make a 32dB of gain high-frequency LNA to oscillate. Actually is harder to don’t make it oscillate.
Cannot say too much without visualizing the design, but I recommend to take care about layout, grounding, input matching, and specifically position and the distance between the drain inductors L1 and L2.
 

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