Hi gwhite,

I would have to agree with you on most everything you have said through this
thread. I once saw my boss (with his "PHD") try to model and match a power
amp based on the small signal parameters off the datasheet. He insisted that
the stated input and output impedances were characteristic parasitics of
that amp and wouldn't change between a small or large signal. It was kind of
pathetic to watch him struggle for over a month on the matching network, and
I think he had resorted to guessing in the end.

I've often questioned why manufactures put small signal parameters on their
datasheets? Makes no sense to me. Even if they do publish some large signal
parameters it is unlikely to be the exact same mode of operation that you
need for your project.

Playing with triple stub tuners on PA's has shown me that there are many
combinations of input and output impedances that appear to give similar
results at any one frequency, but give different results at others
frequencies. So it's a matter of finding the input and output impedance that
give you adequate performance over the entire scope of your project.

Thomas

"gwhite" wrote in message
...
Richard Clark wrote:

On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
wrote:

Wow this is a long thread. Don't really know where I should put my two
bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about
50W.
Pretty easy stuff if you have the right tools and know how to use them.
The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) - stub tuner - FET - stub tuner - 50
ohm
dummy pad - spectrum analyzer. Then just tune the stubs for the
performance
you desire, these include: efficiency (thermal issues), harmonic
content,
spurious emissions, load VSWR considerations, cold start, ect. Then
remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can
handle
the voltage/power, send the design off to the enviro test lab, and head
home
early for the day.

Cheers,
Thomas

Hi Thomas,

Thanx, your two bits were worth more than the academic plug nickel.
This is something that our original poster should hearken to as his
needs were obviously production oriented.

I doubt you understand what he wrote. I can't fathom why you would be
concerned
with the OP when your own difficulties are so acute.

Bench experience will trump
cut-and-paste theory in a heart-beat.

How would you know?

However, triple stub is pretty aggressive. How long did it take you
to flatten response?

How long will it take you to figure out that he wrote not a wisp of a word
on
what the "output-Z" of the amplifier is? He did write that he determines
how
the amp was loaded to acheive power, something I've been saying is the
prime
concern.

Thomas Magma wrote:


I've often questioned why manufactures put small signal parameters on their
datasheets? Makes no sense to me.

They might be of some use for specific cases. For example, if the PA is class
A, is used well backed off because of high PEP-to-avg ratios of the signal, and
you've managed to get the output load dialed in, s-params can be useful for a
first cut at the amplifier *input* match. I've always still had to do some
tweeking though. Also, with some work and considering the load-line match, they
can give you an idea of what gain can be accomplished. This might already be in
the data sheet though, as you mention.

Even if they do publish some large signal
parameters it is unlikely to be the exact same mode of operation that you
need for your project.

One of the large signal parameters I like best is how much power the device can
dissipate. ;-) Voltage breakdowns and Imax are nice too. ;-) ;-)