Power combining

Updated September 3, 2011

Click here to go to our page on Doherty combiners

Click here to go to our page on balanced amplifiers

Click here to go to our main page on couplers and splitters

New for May 2011! Click here to learn about a special consideration for solid state power amplifiers!

Click here to learn about the load-pull effects on quadrature and in-phase combiners

Click here to check out a presentation that the Unknown Editor gave at a IEEE chapter meeting in December 2008 on the topic of power combiners (and much more!)

We've posted a wide variety of couplers and splitters. It's time we explored one of the main applications of splitters, which is combining amplifiers. This will take some time, and some class participation. For December 2008, there are some new examples using the Lim Eom network which are linked from the bottom of this page.

Combining can be done with reactive splitters, in phase matched splitters (Wilkinsons), 90 degree hybrids and 180 degree hybrids. It's important to understand the trade space of why you'd pick one over another.

The two signals must be coherent, that is, they must be at the exact same frequency, and close in phase and amplitude. Being at the same frequency is not an issue when you are combining power amplifiers, because the signals started out from the same source before you split it. Stop here if you don't get that point, and consider your brother-in-law's offer of going 50% on a Quiznos franchise... at least you'll always have toasted snacks coming your way!

Why power combine?

All power amplifier technologies have limitations. Power combining allows you to overcome them. By far the most active area of power combining is in solid state power amplifiers, often abbreviated SSPAs.

Over the years the holy grail of power combining is to replace a microwave tube with a solid state power amp. At first, tube engineers laughed at the puny efforts of solid state engineers, it takes a ridiculous amount of one watt amps to replace a hundred or even kilowatt amp. But over the years, as GaAs became pHEMT, and silicon carbide and GaN technology developed, suddenly they (we) don't look so stupid. Once you get to a magic number of eight devices needed to form the tube replacement, it becomes doable. How did we pick eight as the magic number? It's a pidooma.

In a binary combiner, when the loss per combined section is -1 dB (not a very great splitter), the max power output would occur when eight amplifiers are combined. Sixteen amplifiers combined would have less power than eight in this case. Also, consider that the yield of an assembly with more than eight power amplifiers can be very low, and rework sometimes creates more problems than it solves.

Solid state power amps are not the only reason for power combining. High-power transmitters that need to be highly reliable (like television transmitters) can employ redundant tube amplifiers that are power-combined to make up to 50,000 watts of power. Because the power combiner allows the amps to be isolated from each other, it is possible to turn on one amplifier, and leave the other one (or two!) as spares for when the first one burns out. These are very special systems, the isolation loads take a pounding when an amplifier is turned off. Often the loads are liquid cooled, that's the best way to remove 10 kilowatts of unwanted dissipation! Perhaps one of our readers can send us more information on this interesting topic...

Combiner considerations

Bandwidth

The bandwidth of many power combining schemes is very narrow, but in some cases that won't matter. For ultra-wideband applications, stay away from spatial combiners and coupled designs. A corporate structure using wideband Wilkinson power splitters can provide as much bandwidth as anyone really needs.

Update August 2011! Wait, we have to eat our words about bandwidth, thanks to Mark! Here's a spatial combiner that offers 2-20 GHz:

http://capwireless.com/pdf/pdf_pa/spatium/990004%20Rev.%205A%20_CHPA0220-2_.pdf

If you see those CAP guys, tell them they owe Microwaves101 for that free plug...

Efficiency

Maintaining low loss in a combiner is very important, especially in a corporate combiner. If you lose 1 dB in each stage, by the time you get to a three stage combiner you're throwing away half of your output power! Often overlooked are the routing losses associated with distributing the splits out to all those amplifiers... who cares if your Wilkinson has 0.2 dB loss if you lose 0.5 dB routing to the next split? For "N-way" efficiency, nothing beats a radial combiner.

There is a famous paper about binary versus spatial combining by Robert York, we found an on-line copy here. This paper is referred to simply as "York's Paper" around the industry.

Size

The size of will be a function of the technique, as well as the media. For example, a radial combiner using stripline will be much smaller than a radial combiner using waveguide.

Isolation

Isolation is one of the most important qualities of a combiner network. Ideally you don't want any of the amplifiers to "see" each other. In practice it is often difficult to achieve 20 dB isolation between all branches of the network, but that might be enough. The most common problem of poor isolation in a combiner is that spurious oscillations can occur.

Graceful degradation

When you combine power sources (power amplifiers driven at the same phase angle), if you have good isolation, an amplifier can fail in the network and output power of the network will degrade gracefully.

However, it isn't as simple as calculating the fraction of power amplifiers that are left operating (thanks to Shane for the correction). Some of the "leftover" power gets dissipated in the isolation resistor network.

Time for a Microwaves101 rule of thumb!

In an N-way combiner, if one or more amplifiers fail, the output power will (ideally) be reduced to the square of the fraction of working amplifiers. For example, one failure out of eight results in 76.6 % power [equal to(7/8)^2]. One failure out of four results in 56.3% power. If one amp fails in a two-way combiner, you only have 25% of the original combined power (in decibels, -6 dB). Yikes!

Phase errors

Your power-combined amplifier will have phase errors, and these will cause you to lose power. It's your job to minimize this problem! Phase errors can occur within the power splitter, the individual amplifiers, or the power combiner

Below is a plot that we generated using Excel, to compute loss in power versus RMS phase errors, in this case for a four-way combiner. In computing degradation due to phase errors, be sure to convert power to voltage first. If anyone wants, we can supply the spreadsheet that did this calculation.

Time for another Microwaves101 rule of thumb!

If you want to have less than 0.5 dB combiner loss due to phase errors, make sure that your RMS phase error is less than 20 degrees.

Amplitude errors

Variations in amplitude also cause a loss in power in an SSPA. But it is not as epic as phase errors. Typically you will have all of your amplifiers in gain compression and they are likely to be within 1 dB of each other. The loss in efficiency at this point is just a percent or two (much less than 0.1 dB).

Yes, we need to follow up that statement with some math some day...

Harmonic effects

What style you pick has a big effect on second and third harmonics. This topic is covered in Practical RF System Design, by William F. Egan, this is a good book to buy if you want to move up the RF food chain. We'll summarize the outcomes according to Egan but skip the math. We also had some great help on this topic from Jack K. of Matrix Test Equipment!

90 degree combiners reduce the amplitudes of second order distortion products by 3 dB compared to the individual amplifiers because the level at each amplifier is 3 dB below a single stage amplifier and the slope of the second order distortion versus signal level is 2:1 so reducing the signal by 3 dB reduces the distortion by 6 dB and the signal to distortion changes by 3 dB.

Third order distortions products which include 3A, 2A+B, A+B+C are canceled while 2A-B and A+B-C do not cancel. The level of the 2A-B and A+BC are lower by 6 dB because the level at each amplifier is 3 dB below a single stage amplifier and the slope of the third order distortion versus signal level is 3:1 so reducing the signal by 3 dB reduces the distortion by 9 dB and the signal to distortion changes by 6 dB.

Unfortunately it is the 2A-B and the A+BC products that fall in band and cause problems.

180 degree combiners ideally eliminate even order harmonics and intermods. They have no effect on odd-order intermods and harmonics.

Where do the "missing" signal products go? into the isolation load!

Coupled, corporate, radial or spatial?

If you need to combine more than two amplifiers, you need to make this decision. The following figures were taken from U.S. Patent 4,933,651, Multichannel Combiner Divider, from a discussion under "prior art", to illustrate the first three cases. Thanks to Myron for correcting the patent number! Google now has the best patent search available on the web, in case you were wondering...

Coupled combiners

The coupled combiner seems simple enough, but each coupler needs a different coupling coefficient to be successful. The bandwidth of the approach is limited, and often the loss is high, even when waveguide is used. If you opt to use this, we wish you luck! The coupling coefficients (ignoring accumulating "real" losses) need to follow the sequence:

1/k, 1/(k-1), 1/(k-2), 1/(k-3)...

for an eight-way combiner, the coupling coefficients in dB are:

-9.03
-8.45
-7.78
-6.98
-6.02
-4.77
-3.01

Corporate combiners

A corporate combiner is shown below. This is a "third order" binary combiner, which combined eight sources (coherent amplifiers). This is a very straightforward combiner to develop, however, the loss can pile up, with each additional split the final combiner needs to span an ever-increasing distance. Also, the isolation between each amplifier is not equal.

Radial combiners

Below is a radial combiner. This type of design is also not for the faint of heart, however, it offers the most bandwidth. Wilkinson's original concept was a radial combiner.

Radial combiners often suffer from poor isolation, and they can have tricky line impedance requirements. But they provide the least loss, and the highest bandwidth.

Spatial combiners

This image came from US patent 5,214,394, High-efficiency, bi-directional spatial power combiner amplifier. In this combiner, a two-dimensional array of amplifiers is illuminated from a feed horn. Each amplifier has two antennas at opposite polarization, vertical for input and horizontal for output for example. An ortho-mode transducer or circulator can be used to separate the input and output signals. By locating the input and output an the same side of the array, the inventors have solve the problem of heat sinking.

For the spatial combiner, it is difficult to achieve uniform power split to each amplifier, and there is "spillover loss" associated with the illumination going outside the array. Gain is low, in the case of the reflecting array above, you can't exceed the isolation of the two polarizations; you might need a second, smaller array to drive the first one. The bandwidth will be narrow, and it is hard to imagine that you could make such a combiner cheaply. Other than that, Mrs. Lincoln, how was the play?

Reactive, quadrature, 180 degree or in-phase?

More to come..

Reactive combiners

The term "reactive" in this case means that no resistors are used to terminate the errors of an out-of-phase condition (like they would in a Wilkinson structure). Reactive combiners can sometimes suffer from oscillations which are the result of the "odd mode" where adjacent amplifiers become 180 degrees out of phase. Also, if one amplifier fails, you are in a world of hurt as it will surely pull down the other amplifiers.

Quadrature (90 degree) combiners

This is by far the most popular amp combining method, when you only have two amplifiers to combine. Examples include the Lange, branchline, and overlay couplers.

The input and output return losses of amplifiers are vastly improved when 90 degree combiner is used. The reflected power from the amps is dissipated in the load on the "isolated" port. For an explanation of this, go to our page on quadrature couplers.

Using quadrature combiners can provide an appreciable degree of immunity to load-pull effects, for example, when a solid-state power amplifier is connected to an antenna that has less than perfect impedance match. Learn about this phenomenon here!

180 degree combiners

180 degree splitters include the rat-race, and the waveguide magic tee.

The VSWR of the amplifiers is NOT reduced in a 180 combiner.

In-phase isolated combiners

These provide isolation between the amplifiers, the classic example is the Wilkinson. The Wilkinson splitter also has a fairly wideband response and by adding more sections the bandwidth can be increased.

The Gysel is also a popular in-phase combiner. It offers a distinct advantage over the Wilkinson, in that the isolation loads are one-ports and can be pulled away from the splitter, such that much more power can be dissipated. If you broke into the transmitter floors inside the Empire State Building you might find an entire "herd" of Gysels combining redundant tubes that power up a score of commercial broadcasting systems! By the way, you'd think that having a megawatt of RF power from one site might cause a little concern about health issues, compared to all the hype about handsets.

Now it's time for some examples...

Jack's three-way splitter used as a three-way combiner

Moved to this page.

Lim-Eom splitter used as a combiner

Three-way combiner on this page

Two-way combiner ion this page