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LNA amplifier, please Help - RF Cafe Forums

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New2EEWorld

Post subject: LNA amplifier, please Help

Unread post Posted: Thu Feb 03, 2005 6:35 pm

Offline

Lieutenant

Joined: Thu Feb 03, 2005 6:20 pm

Posts: 2

I'm trying to desing and build a simple multistage LNA?

Here are some of the considerations.

frequency desire: 10ghz

Fn= 2db or less if possible.

Which component (discrete,bjt, FET, etc) will be the best one and why will i choose it versus other ones. Cost, etc? (component selection recommendations, the best in the industry)

What other things should i consider for actually builiding one at home with limited equipment?

Temperature estability consideration or drift?

Distortion or liniarity consideration? ( additional circuits to consider)

And what shareware software do you guys consider to be easily accessible and easy to use?

I appreciate any help or comment or feedback, thank you!

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Profile

Bulldog not a Poodle

Post subject:

Unread postPosted: Thu Feb 03, 2005 9:43 pm

ok

what is the application? why is it at 10GHz/2dB NF/etc...and why are you building it at home?

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Joe

Post subject: LNA

Unread postPosted: Thu Feb 03, 2005 9:50 pm

You want either a MESFET, pHEMT, or HJFET. <<2dB NF possible at 10 GHz.

Working at home you will need a packaged device. This won't be a trivial project at home.

Try NEC (CEL), MwT, Eudyna, Filtronics.

Mouser handles NEC, this may be easier.

You should also check the amplifier vendors listed on this site for a MMIC that meets your requirements.

Linearity and distortion will depend on the bias conditions and the input power.

Gain variation over temperature is -.015dB/degree C as a rule of thumb for each stage.

Check the following site for some useful application notes.

http://www.cel.com/eng/eng_notes.asp

I don't know what shareware is available. Perhaps others have suggestions.

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Guest

Post subject: Shareware

Unread postPosted: Fri Feb 04, 2005 9:05 am

Ansoft offers a student version of their software "serenade". It is a full fuctional and has most if not all of the functionality of their full version. You will be limited in the number of components you can use , but if your just designing a LNA, you should not have a problem. You can find a link on RF Cafe under the "software" banner. I would recommend downloading it. It really the only way you will ne able to effectiviely design the amplifier. Of course, the other alternative is to spend $5 , or so, and purchase a MMIC, packaged LNA. Hittite microwave corporation has a packaged LNA that will give <2dB of noise figure from 9 to 17GHz. I believe the model nimber is HMC516LC5.

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New2EEWorld

Post subject: LNA amplifier, please Help

Unread postPosted: Sat Feb 05, 2005 1:58 pm

Offline

Lieutenant

Joined: Thu Feb 03, 2005 6:20 pm

Posts: 2

Thank you for your feedback. I appreciate the useful information.

I also had other question starting with:

Device:

Using a bipolar device (BJT or other), Why isn't this a prefer choice? Do they offer a bad respond at this prefer frequency? Why is MESFET, pHEMT, or HJFET a better choice? Is it the NF value that makes them preferable over others at higher frequencies? Is this the limitations of our technology today?

Design:

I will be testing a combination of different topologies performance for the best LNA. From MNA & MGA, MNA & MNA, and MNA & LNA. I understand the MNA & MNA will be preferred because of the best NF value. Is this also true for practical purposes? Should I consider other configurations?

General:

In your opinion, what would be considered a “good” or “best” performance for a LNA design at 10GHz? (In terms of its specs: gain, NF, input versus output, etc) What topologies would you choose and why? (a simple approach)

Problems:

What problems do you think I will encounter or should consider looking into when designing and simulating or hands on testing?

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Profile

IR

Post subject:

Unread postPosted: Sun Feb 06, 2005 11:09 am

Channel devices are by definition less noisy than BJT's because.

You can have many problems that are related to design or layout of the LNA. Problems like stability, poor linearity etc. When you design the LNA, You should choose a device that gives you the noise circles and the minimum noise figure data together with the optimum reflection coefficient in the source to achieve this noise figure. Most LNA and transistor manufacturers provide this information for low noise transistors/amplifiers.

You can always find design hints in the application notes for the transistors you choose.

Good luck,

IR

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Joe

Post subject: LNA

Unread postPosted: Sun Feb 06, 2005 4:50 pm

At higher frequencies, the MESFET has lower noise figure than BJT's. Below 500 MHz the 1/f noise will dominate and the BJT will have better noise perormance. The HJFET or pHEMT are similar in structure to the MESFET, but the epitaxial layer structure is different. Without getting into a discussion in device physics, the resulting structure has beter gain and noise properties than a regular MESFET.

Best performance for narrow band at 10 GHz <1dB NF

Best noise figure will result from a reactive input match to meet the NFMIN on the Smith chart.

Using 2 devices in a balanced configuration will improve VSWR.

I don't understand what you mean by "MNA & MGA, MNA & MNA, and MNA & LNA"

Stability will be a major consideration. The LNA isn't much good if it's oscillating. Check out of band.

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Bulldog not a Poodle

Post subject:

Unread postPosted: Mon Feb 07, 2005 1:08 pm

try the NEC GaAsFET NE700/710. I think a 2dB NF is very hard to achieve @ 10GHz.

Having viewed the datasheet if you have any problems do let us know

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Joe

Post subject: NE710

Unread postPosted: Mon Feb 07, 2005 3:03 pm

The NE710 was a workhorse MESFET in the amplifier world 15-20 years ago. It was discontinued about 8 years ago. It is probably still available from suppliers that specialize in obsolete parts, but the cost will be $many. This may be good for an existing design, particular where the design has been qualified in a military program and cannot be changed without requalification.

A new device from NEC with better performance is available for $2 to $3.

The NE321 has an NFMIN of .32dB @ 10 GHz. Follow this link to the data sheet.

http://www.cel.com/pdf/datasheets/ne3210s1.pdf

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