Joel Kolstad wrote:
On a reciever, I'm using a MMIC LNA at VHF

Andy writes:
Joel, all of the advice I've read on this post is good advice, but
I didn't see any reference to the Linville or Stern stability factors.

Devices, ESPECIALLY at VHF, which are matched into source
and load often oscillate. Hell, they USUALLY oscillate.
Hence , S12 is a major factor in these designs....

Most of the time, devices in this range must be mismatched
to some degree to provide stability.

Motorola used to have an excellent App Note on amplifier
matching, and the degree of mis-matching to ensure stability
It was called " Effects of Y parameters" or " Calculating Stability
in... " or some such, and the number was in the 100s or 200s.

If you know the Sparamters ( or Y parameters, for the app note)
you can easily do a Linville on the numbers to see if the LNA
is unconditionally stable for all source and loads.....

It probly won't be.....

Then, you can do a Stern to see what degree of mismatching is
needed to just reach the stability threshold.... If you program
the equations into a computer ( I use BASIC but an EXCEL
spreadsheet will work well also) you can quickly decide what you
want to do. Note that the amplifier configuration is the source
of the parameters and not just the device. For instance, common
base is often unconditionally stable while common emitter more
often than not is never unconditionally stable. The S or Y
parameters for the amp in these configurations show it
when a Linville is applied.

It may be that optimum noise figure can't be reached with
unconditional stability...... so you just do the best you can.......
Personally, I've always found the best noise figure while
diddling the circuit, rather than calculating it..... but that's
just my own experience...


Good luck on your design. And don't get so hung up on this
"maximum power transfer Thevin" stuff, since any amplifier that
is Thevenin matched on it's output could only achieve 50% efficiency,
and most amplifiers do a lot better than that...... for the simple
reason that max power transfer isn't the main goal of an amplifier.....

Andy W4OAH in Eureka, Texas