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MSG and matching - RF Cafe Forums
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guest Post subject: MSG and matching Posted: Tue Jan 24, 2006 5:10
pm hello guys
when you are designing a standard small signal
amplifier you match your input/output to get as much of the MSG as possible.
but what about power amplifiers?
i checked MSG of a transistor
and it came out to be 13 dB at center frequency but the transistor is
being used for a PA. true or false: -since my output is not conjugate
matched but is matched for optimal load i will see much less than 13
dB of gain because of this matching (optimal laod matching).
i think the above sentence is true then my question is how can i get
more gain out of the transistor (upto MSG of 13 dB) without using another
stage. my guess is it is not possible considering i can't change my
bias because my bias is restricted by the class of PA i am using and
the amount of swing i need at the load. thanx for help
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IR Post subject: Posted: Wed Jan 25, 2006 3:31 pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm Posts:
373 Location: Germany Hello guest,
Yes you are right in
your description. MSG is ususally being considered for low-power amplifiers.
For PA, the matching is done per various criteria as Efficiency,
Linearity or maximal output power and not necessarily for gain. The
matching can be obtained by performing Load-Pull measurement. The outcome
of these measurements is a set of countours on the Smith Chart.
_________________ Best regards,
- IR
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guest Post subject: Posted: Mon Jan 30, 2006 6:16 am
hello guys
need to clarify about MSG and PA again. when we say
that PA has gain of, let's say, 5 dB then it means Pgain = Pout - Pin
= 5 dB. as i said in my earlier post the MSG of the transistor was 13
dB. my question is does MSG directly relates to Pgain---higher MSG will
give me higher Pgain? in other words Pgain is directly proportional
to MSG?
the thing is that my design required 15 dBm of output
power with no spec for input power level so i designed it for 15 dBm
of output power but i needed 10 dBm of input power to get that much
output power. i am wondering if this is a poor design because i am requiring
so much input power but i wasn't sure the exact relationship b/w MSG
and Pgain. thanks
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IR Post subject: Posted:
Mon Jan 30, 2006 4:01 pm
Site Admin
Joined: Mon Jun
27, 2005 2:02 pm Posts: 373 Location: Germany Hello,
MSG is proportional to the Power Gain.
MSG is the maximal
gain available from an amplifier without causing it to oscillate - This
is the gain at which the Rollet factor K=1.
If your gain is 5dB
and the MSG is 13dB, then I would say that you have some problem with
your design: Mismatching in the input and/or output, wrong bias or just
a failed device. The Power Gain shouldn´t be that far from the MSG.
_________________ Best regards,
- IR
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guest Post subject: Posted: Mon Jan 30, 2006 11:49 pm
thanks IR; yes there is a mismatch at both the input and output
because the match is for output power and not for maximum power swing--this
transistor is being used for a PA. the matches were determined using
source and loadpull...also the bias is not ideal for MSG either because
i need certain bias for a specific class operation....you think both
these factors will reduce the gain this much, from 13 dB to 5 dB? thanks
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rk Post subject: Posted: Tue Jan 31, 2006 1:40
am Matching for MSG vs. Pmax shouldn't result in 8dB difference.
There's something wrong with your design.
Which doesn't neccessarily
make you a bad person.
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IR Post subject: Posted:
Tue Jan 31, 2006 1:42 am
Site Admin
Joined: Mon Jun
27, 2005 2:02 pm Posts: 373 Location: Germany Yes that could
be.
I will need more information in order to help you. Please
feel free to send me an email if this is more convenient . My email
is: itre27@012.net.il
Thanks.
_________________ Best
regards,
- IR
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Guest Post subject:
Posted: Wed Feb 01, 2006 2:10 am i think your design might be right
and you can get much lower than your MSG. Remember, MSG is calculated
using small signal and the transistor is being used for PA and most
probably you are running it at compression (not small signal) so gain
will be less. On top of that, you are not cojugately matched (most probably
optimal load match for max. power/efficiency, etc) so that reduces your
gain too. so, i would say yeah 8 dB down from you MSG is not that odd.
-jk
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guest Post subject: Posted: Wed Feb
01, 2006 7:33 am i might have figured out the problem. why do we
switch from MSG=mag(S21)/mag*S12) to MAG=mag(S21)/mag*S12)* (k-sqrt(k^2
-1)) when k>1? my k was greater than one but i was looking at MSG
and not MAG. now MAG is closer to what i'm getting but i would still
like to know why this switch. is just that the amplifier is now stabl
(k>1) and you can get only MAG gain out of it. if that is true then
i think MSG wording is confusing. i thought it meant that the circuit
is stable and you can get MSG gain from it but i guess what it means
is that the amplifier is conditionally stable (with the condition that
you don't want to extract more than MSG).
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IR
Post subject: Posted: Wed Feb 01, 2006 6:00 pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm Posts: 373 Location:
Germany Hello,
MSG is obtained from the MAG, simply by narrowing
the equation to S21/S12 when K=1 (You can try and see it for yourself).
It is a good design practice not to come close to MSG and keep
some margin from this value. Now the actual gain you will get from your
amplifier is dependent on the matching you provide at the input and
output. As I mentioned in one of the previous posts, you can do matching
according to different criteria, and for each matching criteria you
will get a different gain value.
_________________ Best regards,
- IR
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guest Post subject: MSG and matchingPosted:
Tue Feb 07, 2006 12:23 pm Quote: On top of that, you are not
cojugately matched (most probably optimal load match for max. power/efficiency,
etc) so that reduces your gain too. so, i would say yeah 8 dB down from
you MSG is not that odd.
What is the difference between being
conjugately matched (max power flow) and optimal load match, if any?
Im familar with the terms of Power gain, Transducer gain and Available
gain, also the MSGs/MAGs. Currently I've only been tuning so that the
trade-off is between gain, noise and matching (vswrs at input, output
or either one).Is Efficiency, Linearity or maximal output power mostly
conciderations to take into account when dealing with large signal amplifiers?
Sincerly, RF-student
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IR Post subject:
Posted: Tue Feb 07, 2006 5:10 pm
Site Admin
Joined:
Mon Jun 27, 2005 2:02 pm Posts: 373 Location: Germany Hello,
There is no difference between the two. If you have maximal power
flow that means that you are matched to the optimum.
Yes linearity,
efficiency and maximum output power are related to large-signal amplifiers
- Power Amplifiers.
_________________ Best regards,
- IR
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lifan Post subject: Posted: Fri
Feb 10, 2006 2:17 am
Captain
Joined: Fri Feb 10, 2006
1:20 am Posts: 10 Hello,
what is MSG?
thank
you.
_________________ Friends are all over the world.
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IR Post subject: Posted: Sun Feb 12, 2006 4:38
am
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373 Location: Germany Hello lifan
MSG - Maximal
Stable Gain.
It is the maximal gain that an amplifier can produce
without becoming unstable (oscillate). It is given by:
MSG=S21/S12
At this Gain the Rollet factor K equals 1.
_________________
Best regards,
- IR
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Jeanalmira Post
subject: Posted: Tue Feb 28, 2006 5:03 am
General
Joined: Tue Mar 15, 2005 11:43 pm Posts: 65 Location: Singapore
Hi :
I agree with IR, that the large signal gain should be
close with MSG. It can be varied between 1 dB range. However, if your
gain is so much off by 8 dB. I think it's probably caused by mismatch
impedance of input and output. I encountered this problem before,
have you checked the return loss? I suspect maybe your output impedance
too low. If there's mismatch, maybe you should check the matching for
power matching.
Or you may want to check your device, whether
it is in saturation region, so it affects the efficiency and linearity?
If you provice me more details, I may be able to help.
Thanks and Regards, Jean
Posted 11/12/2012
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