Linear RFPA: Push-Pull vs. Single-Ended - RF Cafe Forums
Post subject: Linear RFPA: push-pull vs. single-ended
Unread postPosted: Thu Jun 03, 2004 3:05 am
Does anyone have actual experience in push-pull PA design? What
is the expected improvement in linearity compare to AB class single
postPosted: Thu Jun 03, 2004 3:24 pm
Your question is a
When you design PA, you have 2 requirements which
are contradicted, linearity vs. efficiency. One comes on the account
of the other. Most often there is no PA as a stand alone unit, unless
your linearity requirements are very mild. Your PA will probably
be integrated with some kind of linearizer, of some topology depends
on the bandwidth you have to deliver.
Most push-pull transistors
are intended for high power levels (Mostly above 100W PEP) like
Motorolas' MRF21180. For low and medium power levels most transistors
available are single-ended. The approach for designing a PA is to
get the best efficency by an operating point towards saturation,
and improving the linearity to meet the requirements.
will need to provide more details about your application: Power
levels, frequency of operation etc; please do so and I will be keen
to help you.
Unread postPosted: Fri Jun 04, 2004
Thank you for your post Itay, yet it does not answer
my question, which is very straightforward, is a push-pull amplifier
more linear compare to single-ended one? Provided, that both are
AB class driven with equal conducting angles. Everything you have
mentioned including Gemini packaging and necessity of linearization
around amplifier is of secondary concern, my question is about amplifier
There are tones of publications available about push-pull
amplifiers, but I found only one (http://www.fcsi.fujitsu.com/products/MWpdf/an014.pdf)
scrutinizing the issue with the final conclusion: No, the push-pull
is no more linear than single-ended. This puzzles me.
Mon Jun 07, 2004 9:24 am
If by push/pull you mean a balanced
amplifier, then yes, a balanced amplifier can be more linear. A
balanced amplifier has much, much less 2nd harmonic distortion than
a single ended class AB. So, if your RF band of interest is wider
than an octave, there is a distinct advantage to using a balanced
amplifier. But, if you're only concerned with a narrow band of frequencies,
and the 2nd harmonic will be knocked down by filtering, then there's
really no difference in linearity.
One other nice thing about
a balanced amplifier is that if you lose one transistor, you still
have output. This is good in some high reliability applications
where diminished output is better than no output.
this was what you're looking for?
Unread postPosted: Mon Jun
07, 2004 10:50 am
Thank you for your input,
although I do not understand which topology you are describing.
Could you clarify please what exactly balanced amplifier means within
Unread postPosted: Mon Jun 07, 2004 1:01 pm
Balanced amplifier means 2 different amplifiers
connected in the input and output with hybrid quadrature couplers.
The advantage of this amplifier is that it gets half the input power
level (-3dB) at each input, thus allowing it to work in a greater
backoff and become more linear. At the output, the powers are being
combined and you get twice the power as you would get with a single
amplifier, this gives you a higher P1dB (theoreticaly 3dB higher,
practically less due to the hybrid coupler insertion loss). The
disadvantage of this topology is of course the power dissipation,
which is twice than single ended amplifier.
To your question,
absolutely Push-Pull amplifier is more linear than single ended
one because you utilize the entire cycle of the input signal, namely
the amplifier delivers the entire cycle without cutting it and by
this you produce a linear amplifier with less distortions. You use
class A-B or class B transistor amplifiers and combines them in
Push-pull so at any given time one of them conducts, this can be
done in a balanced amplifier topology or in the push-pull topology.
postPosted: Mon Jun 07, 2004 5:28 pm
of topology for balanced amplifier coincides with yours, but I am
afraid not with Darrin’s. Yet since you are on a position to comment
Darrin’s posting, would you explain please the issue of second harmonic
suppression in balanced amplifier? I am accustomed to idea that
this feature does not belong to balanced amplifier inherently.
Furthermore, I quote your statement “You use class A-B or class
B transistor amplifiers and combines them in Push-pull so at any
given time one of them conducts, this can be done in a balanced
amplifier topology or in the push-pull topology.” How this can possibly
be done in a BALANCED topology?
I’d vote with both my hands for
the idea of enhanced linearity of RF push-pull amplifier due to
conducting angle expanded to 360deg. This is well-known fact at
audio field. The problem is that for some reason this seems not
to work at RF, presumed that linearity concerns spectrum re-growth
close to carrier rather than second harmonic suppression (http://www.fcsi.fujitsu.com/products/MWpdf/an014.pdf).
Not even a single source I found mentions that RF AB class push-pull
amplifier is more linear than single ended counterpart. Can anyone
explain why, or prove opposite?
Unread postPosted: Thu Jul 08, 2004 5:41 pm
The balanced amplifier is different from differential
amplifer, the later can be constructed by 180 degree balun or hybrid.
In most cases the 2fo can not be eliminated as Darin said since
they are out of band. However 2fo can used rather than cancelled,and
it is still very interesting to have push-pull for high power delivery
and ease to match(higher impedance). Another insteresting developement
is for Differential LNA design above 2 GHz, since this structure
absorbs some noise sources which exist in common mode and not need
GOOD rf ground.
If talking same power level, it is clear
there is 3 dB power back off, good? If not, at least the linearity
is not worse then single-ended, OK?
The differential amplifier
is not limited by the concept of push-pull anymore. It can be wokring
at calss A, AB, C, E...
Old Tele man
Post subject: re: RFPA linearity
Unread postPosted: Sun Oct 17, 2004 1:28 am
Joined: Sun Oct 17, 2004 1:07 am
a very good book on this subject is:
Steve C. Cripps,
RF Power Amplifiers for Wireless Communications, 1999;
Artech House (microwave library), Inc.; Norwood, MA;
Devil sez: "...Yes, but it's a DRY heat!"