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Post subject: LNA (low noise amplifier) design tips?
postPosted: Tue Apr 26, 2005 3:23 pm
Sat Apr 23, 2005 2:09 pm
Location: Tampa, FL
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!
Unread postPosted: Tue Apr 26, 2005
What are exactly your problems with bias and feedback?
Let us know so we might be able to help you out.
Unread postPosted: Tue Apr 26, 2005
Joined: Sat Apr 23, 2005 2:09
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.
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.
Unread postPosted: Wed Apr 27, 2005 12:14 am
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:
Hope this helps.
Should you need any
other help, please let me know.
Unread postPosted: Wed Apr 27, 2005 10:18
One very important thing people have tendency to forget is
that when you will adjust the amplifier on the bench you have to stay
Remember any source of noise will add up the end
result noise figure....
Unread postPosted: Tue May 10, 2005 12:15 am
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.
web page is at http://members.cox.net/thse-3.14159/download.htm
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
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.Posted