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New to RF, trouble understanding transformers and baluns - RF Cafe
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
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Post subject: New to RF, trouble understanding transformers
and baluns Posted: Fri Mar 20, 2009 8:30 pm
Joined: Fri Mar 20, 2009 8:22 pm
Hi..I'm new to
the RF world and found this extremely educative forum just now. I'm
a new grad and learning about high power rf amps.
I see that
in high frequency range, broadband, hybrid couplers are used to split
and combine two amps in parallel to get the required power output. While
in low RF frequencies like 10 MHz to 500 MHz range, transformers and
baluns are used to achieve the same result. I'm having a hard time understanding
Like what is 9:1 or 4:1 and how do we determine
what coax to use, whether 25 ohms or 50 ohms, and what value of ferrite
cores to use ? I know this is a lot of questions, but I'm sure all of
them lead to the same answer. Your help much appreciated.
Post subject: Re: New to RF, trouble
understanding transformers and balunsPosted: Thu Apr 09, 2009 10:05
Joined: Wed Apr 08, 2009 7:53 am
Regarding to the use of combiners or
baluns to double balance power amplifiers I think it´s more a matter
of "type of application" than "frequency rage". Of course, the type
of application is correlated to frequencies ranges ..
you get a good matching performance characteristic, and with baluns
you get a reduction in second harmonic power. So, depending on your
application and system characteristics you decide whether to use one
or the other. For example, for modern cellular signals, because of many
reasons, Class AB power amplifiers is the best choice, these class of
amplifiers already has a low 2nd harmonic power performance characteristic,
so, if needed, splitters/combiners is the preferred choice to make a
double balanced amplifier.
Both baluns and combiners are capable
to be designed with a 50ohm combined port, so it is not necessary to
use a different coax than 50ohm.
You do not always will need
to bias the power amplifiers with a ferrite core inductor.
drain bias circuit, if you have room on the pcb, using a 1/4 wave length
is a good practice.
In the gate bias circuit, you can also use a
1/4 wave length, or because you have low current, you will find standard
commercial coils.. in both cases (1/4 wave length or coil) for the gate
stability, it is a good practice to use ferrite beads (with this is
like all the stability problems are gone)...
Well, perhaps this
is a too short answer, I avoided an academic explanation because these
are very general questions, and if I had to justify every affirmation
I wrote it will be a very long answer.
Hope I could help you.
Post subject: Re: New to RF, trouble understanding
transformers and balunsPosted: Mon Apr 13, 2009 2:05 am
Joined: Thu Sep 25, 2003 1:19 am
RDO-RF said it all, but I'll try add to his comments.
When discussing the process of splitting and combining amplifiers,(often
called cascode) the type of splitters used have very specific advantages
First I will list splitters, not transformers.
When the incoming power is split, half
the power is applied to the amplifers, the power out of the ampliers
is re-combined by the combiner.
Therefore, the power is 3dB higher
out of the combiner than out of each amplifier. For example, if each
amplifer is outputing 27dBm, the output of the combiner is 30dBm. So
the linearity of the amplifer is 3dB better, then one amplifier by itself.
90 degree split/combine (quadrature coupler)
This method is called
a balanced amplier.
Because the power is split and finally recombined
in phase, it the has the same advantages as the 0 degrees version.
But this method has benefit the 0 edgree doesn't. If each amplifier
has poor S11, the S11 at the input of the splitter will still look good.
(as long as each amplifiers are matched for S11.
The S11 is so
good because the path from one amp reflect with 360 deg phase, while
the other refect back with 180 deg phase shift, causing the power to
Another benefit of the balanced amplifer is odd mode
cancelation. Odd mode is related to IP3, so IP3 will be an addition
3dB better than a single amp.
180 deg split/combine
is referred to as a push-pull amplifier.
Either uses BALUNS or 180deg
The method splits and combine is phase as well, but does
not have the improve the S11 the way that the 90 deg does.
180deg splitters cause common mode rejection which is related to IP2.
So IP2 will be better than compared to an indivisual amplifier. The
push-pull method is often used for power ampliers because the full voltage
swing of the signal can be utilied.
You don't typically see
the 0deg method because it doesn't offer addition improve such as S11,
IP3 or IP2.
90deg versions tend to half the lowest bandwidths.
The above applies to low and high frequencies. The only difference
is lower frequency version tend to use "core and wire", while higher
frequency uses microstrip lines.
Impedance transformers using
"core and wire" is reserved for lower frequencies. If impedance transformation
is needed for higher frequencies, typically caps and inductors are used,
I hope this is clear, let me know,
Post subject: Re: New to RF, trouble
understanding transformers and balunsPosted: Sun May 03, 2009 3:09 am
Joined: Fri Mar 20, 2009 8:22 pm
Thanks a lot for the explanations RDO-RF and Rod. Now it makes
sense to me why we use push-pull amplifiers or 180 deg splitters and
impedance transformers. Also I now understand why I see a low second
harmonic when there are Class AB Power amps.
Thanks once again..These
were very informative explanations