Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: microwave osc
Unread postPosted: Wed May
11, 2005 3:55 pm
i need the schematics for microwave osc with
2000 - 3000 Ghz aprox
if somebody can helpme please
send a mail for me
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.
Tue Apr 26, 2005 9:22 pm
Joined: Sat Apr
23, 2005 2:09 pm
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.
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.
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
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
Your questions seem a bit naitve, yet they are
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
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
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.
any source of noise will add up the end result noise figure....
Post subject: LNA Design
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
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