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LNA (Low Noise Amplifier) Design Tips? - RF Cafe Forums

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Qacer
Post subject: LNA (low noise amplifier) design tips?
Unread postPosted: Tue Apr 26, 2005 3:23 pm
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Captain

Joined: Sat Apr 23, 2005 2:09 pm
Posts: 17
Location: Tampa, FL
Hello folks,

This is my first time designing a simple LNA (low noise amplifier) project for a class. I was wondering if anyone has any tips especially regarding biasing and feedback. I think I have almost met the design specs, but I'm still have problems getting my output return loss to meet the spec.

I am using a microwave BJT transistor and have incorporated two feedback schemes to get my circuit to be unconditionally stable at a certain frequency range and also get my gain up.


Any good tips from the LNA experts? Thanks!


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Guest
Post subject:
Unread postPosted: Tue Apr 26, 2005 5:28 pm

What are exactly your problems with bias and feedback? Let us know so we might be able to help you out.


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Qacer
Post subject:
Unread postPosted: Tue Apr 26, 2005 9:22 pm
Offline
Captain

Joined: Sat Apr 23, 2005 2:09 pm
Posts: 17
Location: Tampa, FL
One thing that I am curious about pertains to having two types of feedback schemes in the circuit. In reality, would this kind of design be appropriate? Currently, in my design, I have a resistor feedback from collector to base and also a reactive component attached to the emitter.

Aside from using an active feedback scheme, it seems like that was the only option I had to get my circuit stable at a certain range while getting a gain that meets my spec. Is there a resource that can give me some sort of analytical view of the feedback system? I kinda like seeing equations that I can follow around, so it makes more sense to me.

I, somewhat, understand the resistive feedback part using DC analysis, but as for the reactive element on my emitter, I'm a bit clueless with a numerical analysis on how it stabilizes my circuit.

For the bias part, I always thought the Q point would ideally be the mid-point of the supply voltage to allow maximum signal swing, but it seems like for the bias networks I've seen in my book, they don't seem to care about this. They are more interested in maintaining the collector current as temperature varies. Is this always the case for LNAs in microwave frequencies?

I'm sorry if my questions seem off. I'm a bit new to this stuff.

Thanks!


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Itay
Post subject:
Unread postPosted: Wed Apr 27, 2005 12:14 am

Hello,

Your questions seem a bit naitve, yet they are OK.

The bias point should ideally be in the middle of the Ic vs. Vce curve. Yet, you should check if the maximal outpt swing of the LNA per this given operating bias point still gives you a complete swing without clipping. Then if this is the case, then you can work at this bias point (that still will allow a Class A linear operation).

A feedback resistor is a common way to provide a broadband stability. This method is called uniliteralization, since it reduces the effect of the reactive feedback element (the CB junction capacitance, which is a cause for oscillation), making the transistor more unilateral (reducing S12). This method reduces the gain of course, yet doesn't hurt the NF or IP3 as placing a series resitor at the output or at the input of the device.

Attached below is a link to a tutorial that discusses bias schemes (Both active and passive) specific for LNA design:

http://www.odyseus.nildram.co.uk/RFMicr ... rcuits.pdf


Hope this helps.

Should you need any other help, please let me know.

Itay


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Guest
Post subject:
Unread postPosted: Wed Apr 27, 2005 10:18 am

One very important thing people have tendency to forget is that when you will adjust the amplifier on the bench you have to stay very quiet.

Remember any source of noise will add up the end result noise figure....


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Tom
Post subject: LNA Design
Unread postPosted: Tue May 10, 2005 12:15 am

What are the design specs, device and frequency range you require? Techniques vary. Above a few hundred MHz you should use "S" parameters. Your design should start at the input and you must provide the match necessary to get the noise figure you desire. With this known you can claculate the load required to get the desired gain and be stable. Using "S" parameters takes all this into account. By the way an amplifier is never matched. Generally you take a 50 source and add a matching network to present a desired driving impedance to the amp. On the output you take a 50 ohm load and a matching network to present to the amplifier the load necessary to get the desired gain and stablility. This is not matching in the sense of a conjugate match. It is simply a network that provides a desired impedance when attached to 50 ohms. I have a nice little "S" parameter design application on my web page if you need it. It requires that you know something about design so you may want to look at the book by G. Gonzalez, "Microwave Transistor Amplifiers" first.
My web page is at http://members.cox.net/thse-3.14159/download.htm


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Guest
Post subject:
Unread postPosted: Tue May 10, 2005 11:34 am

I think that for LNA the bias point is lower than 50% IDSS.

The best rule for this is to follow the manufacturers recomendations....


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Tom
Post subject: LNA
Unread postPosted: Thu May 12, 2005 12:09 am

The bias point for an LNA is driven by the noise performance you require. Look at the device datasheet and the manufacturer will tell you what current and voltage is required for lowest noise. They will also give you noise parameters for thes conditions. In general less current means lower noise. More current means more noise but more device gain. Front signal handling and intermod are usually important. A good design has to trade off low noise performance against high level signal handling by finding a compromise in bias point that works for most expected conditions.



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