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Mis-matching & matching (was Dumb question...)

B

billcalley

Jan 1, 1970
0
Hi Guys,

From what I have been able to absorb now from all of your really
terrific responses is that it is totally and completely impossible to
LC match, even theoretically, a purposefully mis-matched active
device*, and then look back into that mis-matched network and see a
perfect 50 ohms. However, the purposefully mis-matched active device
WILL get to see the impedance it wants to see. And all this is due to
the reciprocal nature of LC matching networks. Thus, it would be
wise, as you have all mentioned, to design any non-50 ohm LNA or PA to
be as close to 50 ohms as possible, or to design the next connecting
stage so that it properly works with something other than 50 ohms --
'cause there is no way to "fix" this mis-match issue with a LC
matching network. Is that correct, or have I misunderstood
something? (I can't believe I didn't know -- or didn't understand --
this stuff from the get-go! A major glitch in my knowledge-base,
that's for sure.)

Many Thanks,

-Bill

*The mis-match created so as to optimize an LNA transistor's input for
NF, or a PA transistor's output for P1dB, to name two common reasons.

(HAD TO OPEN-UP A NEW THREAD BECAUSE GOOGLE GROUPS APPARENTLY REFUSED
TO POST THIS, EVEN AFTER TWO TRIES -- OR AT LEAST IT DID NOT SHOW UP)
 
J

Joerg

Jan 1, 1970
0
billcalley said:
Hi Guys,

From what I have been able to absorb now from all of your really
terrific responses is that it is totally and completely impossible to
LC match, even theoretically, a purposefully mis-matched active
device*, and then look back into that mis-matched network and see a
perfect 50 ohms. However, the purposefully mis-matched active device
WILL get to see the impedance it wants to see. And all this is due to
the reciprocal nature of LC matching networks. Thus, it would be
wise, as you have all mentioned, to design any non-50 ohm LNA or PA to
be as close to 50 ohms as possible, or to design the next connecting
stage so that it properly works with something other than 50 ohms --
'cause there is no way to "fix" this mis-match issue with a LC
matching network. Is that correct, or have I misunderstood
something? (I can't believe I didn't know -- or didn't understand --
this stuff from the get-go! A major glitch in my knowledge-base,
that's for sure.)

Many Thanks,

-Bill

*The mis-match created so as to optimize an LNA transistor's input for
NF, or a PA transistor's output for P1dB, to name two common reasons.

(HAD TO OPEN-UP A NEW THREAD BECAUSE GOOGLE GROUPS APPARENTLY REFUSED
TO POST THIS, EVEN AFTER TWO TRIES -- OR AT LEAST IT DID NOT SHOW UP)

Bill, your original post did show up, twice. I don't use Google Groups
but sometimes I search groups via Google. The results are a bit mixed
lately, used to be 100% accurate but no more after they "fancified"
their GUI.
 
M

Mark

Jan 1, 1970
0
Hi Guys,

From what I have been able to absorb now from all of your really
terrific responses is that it is totally and completely impossible to
LC match, even theoretically, a purposefully mis-matched active
device*, and then look back into that mis-matched network and see a
perfect 50 ohms. However, the purposefully mis-matched active device
WILL get to see the impedance it wants to see. And all this is due to
the reciprocal nature of LC matching networks. Thus, it would be
wise, as you have all mentioned, to design any non-50 ohm LNA or PA to
be as close to 50 ohms as possible, or to design the next connecting
stage so that it properly works with something other than 50 ohms --
'cause there is no way to "fix" this mis-match issue with a LC
matching network. Is that correct, or have I misunderstood
something? (I can't believe I didn't know -- or didn't understand --
this stuff from the get-go! A major glitch in my knowledge-base,
that's for sure.)

Many Thanks,
yes I think that is correct for reciprocal matching networks i.e
standard L C or transformer..

you can do it with pads but it makes no sense to use pads in this
application..

I did think of another way, CIRCULATORS are non-reciprocal devices, so
in theory you CAN do it using a circulator... but a circulator is
probably not practical for your application. The theory is
interesting, you may want to read up on them... but for your
application you probably just need to compromise the match between
noise figure and filter flatness.

Mark
 
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