New to RF, trouble understanding transformers and baluns - RF Cafe Forums
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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 the concept.
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 am
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 ..
With combiners/splitters 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.
For the 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 and disadvantages.
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 cancel.
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
The method is referred to as a push-pull amplifier.
Either uses BALUNS or 180deg splitters.
The method splits and combine is phase as well, but does not have the improve the S11 the way that the 90 deg does.
The 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, microstrip stub.
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