# Input and Output matching

1. ## Input and Output matching

Hello all,

I have one quick question..

What is minimum number for input and output matching of LNA? Is it -10dB ? Is there any document to prove this ?

I did a design in which S11=-10dB and S22 =-13dB with a power consumption of only 300uW.

I would really appreciate a quick reply. Its urgent.

Thank you

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2. ## Re: Input and Output matching

LNA is designed for Available Minimum Noise Figure and Input reflection Coefficient simultaneously.In practice, there is a trade-off between these 2 and you find a compromising point to get a Optimum LNA specifications.Therefore saying something doesn't make sense..
If you can obtain Minimum NF and -infinite S11 simultaneously, you LNA will be ideal one but in parctice you should select a compromising point between 2.
And also frequency is important..
If you could this S11=-10dB with a reasonable NF for 5.8GHz, it's quite good..
That's why your question is floating...

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3. ## Re: Input and Output matching

Sorry for this vague question:

My LNA design is at 2.4GHz, Noise figure =2.7dB and gain is 6dB.. P1dB=-21dBm, IIP3=-16dBm

Is this a good design ?

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4. ## Re: Input and Output matching

Originally Posted by Circuits
Sorry for this vague question:

My LNA design is at 2.4GHz, Noise figure =2.7dB and gain is 6dB.. P1dB=-21dBm, IIP3=-16dBm

Is this a good design ?
IIP3 is too low and Gain is also low.Play around the linearity

5. ## Re: Input and Output matching

I take a guess that when you say "-10dB", you mean the reflection coefficient looking into the matched circuit, and your question might be about the quality of the match.

-10dB means 10^(0.1*-10dB) = 0.1. So 10% is reflected. In the case of an LNA, adding an input matching network to transform (say) 50 Ohms input impedance to the ideal impedance for best gain from a device may also mean a poor noise figure, because the impedance for minimum noise is almost never the same as for best gain, and a trade-off compromise is chosen.

-10dB may be the best that can be managed. VSWR is between 1.9 and 2.0. I would hope for better than -14dB for an amplifier if it could be done, but this is not always possible. I have seen near -20dB. Very good designed filters can often claim -25 to -30dB in band. A accurately matched microstrip line on 0.508mm Er=3.5 substrate is shown. It still has radiation leaks and effects across its width but the match is better than -35dB.

-20dB, meaning 1% is reflected, would be considered by most to be a very satisfactory value for an LNA amplifier. VSWR would be about 1.22. Most professional microwave LNA product do not often claim VSWR better than 1.5.

These values of match, while OK for an amplifier, would not be good enough for (say) a waveguide filter carrying 500kW radar pulses with perhaps 2kW average power. 10% of such power not going out where it is supposed to might well cause a little trouble and embarrassment!

IIP3 is too low and Gain is also low.Play around the linearity
BigBoss is right. I would add that the noise figure 2.7dB is not typical of what might be expected from a LNA using devices available over the last decade. One can buy X-Band LNA that claim finished amplifier noise figure of 0.8dB At 2.7GHz, you should find it reasonably easy to manage 2.7dB NF. You probably have room to simply match for ideal gain.

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