Small Signal Amp./PA Matching - RF Cafe Forums
Post subject: small signal amp./PA matching Posted: Tue
Jul 18, 2006 5:12 pm
Joined: Tue Jul 18,
2006 9:52 am
when we match input/output
for power gain in small signal amp we match the input/output impedances
conjugately to 50 Ohm and the S11 and S22 are -15 dB or below or
something like that which indicates good matches. i thought I understood
matching networks farily well until i started reading on PA's. in
a PA the output is matched to an impedance that is found using loadpull
analysis so the matching network is not a conjugate match. however,
in books and in many PA publications the S22 is shown and it is
usually pretty good -15 dB or something like that. What does S22
mean in PA's? I thought since output is not conjugate match the
S22 shouldn't be that good.
if we assume that the S22 that
is usually shown when talking about PA's is small signal S22 then
how come the match could be so good when the designer actually didn't
match for it---it was matched for loadpull! any comments/suggestions.
Post subject: Posted:
Wed Jul 19, 2006 10:51 am
Mon Jun 27, 2005 2:02 pm
PA matching is different than matching a small signal
amplifier for maximal output power. In PA many times you don't necessarily
want to get the maximal output power, but a good linearity or efficiency.
This results in different impedance contours. The best way to measure
it is with Loadpull. So with the Loadpull you can sweep the output
impedance to different locations on the Smith chart for different
conditions (Like best linearity or efficiency, which come on the
expense of each other).
The best S22 (The lowest) doesn't
necessarily means that your PA will give you what you wanted to
achieve from your design, because you match for other conditions
and not for lower S22.
Post subject: Posted:
Wed Jul 19, 2006 11:35 am
Joined: Tue Jul
18, 2006 9:52 am
thanks for the reply
IR. but let's say i did loadpull for a class A (linear) PA and found
out that I need to match 50 Ohm to A+jX impedance, which is different
than the actual output impedance of the transistor (or set of transistor
cells). Hence, the match that I do will not be a conjugate match,
right? So, my question is that then why in books and in technical
published papers people talk about S22 and it is usually quite good
and they highlight it also? Even Cripps, on page 19 or 20 does so
for a simple linear Class A PA design.
as i said in the earlier
post, it could be that the S22 is shown for small signal but then
why is it good because the match wasn't done for small signal S22/conjugate
Posted: Wed Jul 19, 2006 2:29 pm
Mon Jun 27, 2005 2:02 pm
This is because in Class A design you get a good linearity and bad
efficiency, so you need to match for conjugate in order to get maximal
output power to improve your efficiency. Besides Class A there are
other classes like A-B or B, in which you will get a degraded linearity
but good efficiency. Therefore, in those matching for conjugate
is not required and you need to match for good linearity, which
will not necessarily yield a good S22. I was referring in my previous
post to those type of amplifiers -especially Class A-B, which is
most common in Base-Stations PA's.
Posted: Wed Jul 19, 2006 3:01 pm
Tue Jul 18, 2006 9:52 am
hey IR, i don't know if
i fully agree---i am not sure. even in class A you don't do conjugate
match of you transistor out impedance; rather you transform 50 Ohms
to some impedance that you want your transistor to see that you
get from loadpull. let's say your transistor output impedance is
10-j3; in PA (eve for class A) you don't match for 10+j3 rather
some other value that you find from loadpull (let's say it is 15+j3)
and will provide optimal output power but not necessarily maximum
power gain. so you transform your 50 Ohm to this 15+j3. so your
small signal S22 shouldn't be quite good because instead of having
a conjugate match of 10+j3 you have something else. but look at
some PA papers and if they give out S22, in many cases, it is as
if it is conjugate matched to the transistor's actual output impedance.
may be i don't have a clear concept of S11/S22 or something but
it doesn't make sense.
Post subject: Posted: Thu Jul 27, 2006 3:10 pm
Joined: Mon Jun 27, 2005 2:02 pm
The transistor can be matched to any condition.
The conditions can be e.g.: NFmin, Specific Gain, Maximal Output
Power etc. For some of these conditions (NFmin, G) there are families
of countours that are plotted on a Smith chart, which specify the
impedance the transistor needs to be matched to for a given Gain
and/or NF. You can match to fulfill 2 conditions simultansously
(An intersection of 2 countours).
These impedances will not necessarily
will give you the best S11 or S22 but they will provide you a maximal
gain or minimal NF.
Attached is a link for an application
note with some examples:
subject: Posted: Tue Jan 30, 2007 9:43 pm
Joined: Tue Mar 15, 2005 11:43 pm
Just want to add notes that the PA matching
that has been mentioned about is large- signal matching. As we understand
that the output impedance at small-signal conditions is different
than large-signal conditions.
Referring to Cripps method that
large-signal output impedance is lower than small-signal output
impedance. (The large-signal impedance is 1/2 to 1/3 of small signal
I think it's one of the reason that large-signal
matching is not conjugate matching to the small-signal S-para (S22).
I hope it makes sense.