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Small Signal Amp./PA Matching - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


nickc
Post subject: small signal amp./PA matching Posted: Tue Jul 18, 2006 5:12 pm

Captain

Joined: Tue Jul 18, 2006 9:52 am
Posts: 6
hey,

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.
thanx
nick


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IR
Post subject: Posted: Wed Jul 19, 2006 10:51 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

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.

_________________
Best regards,

- IR


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nickc
Post subject: Posted: Wed Jul 19, 2006 11:35 am

Captain

Joined: Tue Jul 18, 2006 9:52 am
Posts: 6
hey

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 match??

nick


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IR
Post subject: Posted: Wed Jul 19, 2006 2:29 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
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.

_________________
Best regards,

- IR


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nickc
Post subject: Posted: Wed Jul 19, 2006 3:01 pm

Captain

Joined: Tue Jul 18, 2006 9:52 am
Posts: 6
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.

nick


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IR
Post subject: Posted: Thu Jul 27, 2006 3:10 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

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:

http://www.odyseus.nildram.co.uk/RFMicr ... torial.pdf

Good luck!

_________________
Best regards,

- IR


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Jeanalmira
Post subject: Posted: Tue Jan 30, 2007 9:43 pm

General


Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi :

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 impedance).
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.

Regards,
Jean









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