on 5/16/01 9:51 AM, Billy Ward at billydeanward@hotmail.com wrote:
>> Conjugate matching is not required for unconditional stability. That's
>> incorrect. Many low noise amplifiers are purposely mismatched because
>> simultaneous conjugate match generally does not equal minimum noise figure.
>
> I was not taking minimum noise figure into the context of what I said. I am
> willing to learn. I purposely mismatch for best noise figure. in low-level
> stages, also. However, I am curious how that makes my statement incorrect
> that Conjugate Matching is not required for unconditional stability.
A simultaneous conjugate match is unconditionally stable. However, any
other match may be unconditionally stable as well.
Unconditional stability is not just so simple as defining a match at one
frequency. After all, the conjugate match is technically going to exist
over a relatively narrow band for the most part (most of what we do is
narrow band). Certainly, if I match an amplifier for a conjugate match at
14 MHz, it will NOT be conjugately matched at 28 MHz. Is the amp unstable
then? Perhaps, perhaps not.
Many of us when testing linears for stability, test them with open circuits
on the input and output when keying down. We rotate the tank circuit
through various positions to test for stability. Certainly, this is not a
conjugate matching system either, yet if an amplifier is properly designed,
it will be stable.
Simultaneous conjugate match insures stability at the frequency of the
match, but not anywhere else. It is NOT a condition for stability.
I'll comment on the rest of your post at a later time, if someone else
doesn't beat me to it. I hope W8JI comments on it.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|