Looking at the schematic for the Shure PA770 Antenna Combiner (found in their FCC listing) we can see the design is fairly straightforward, some amplification before combining via a typical Wilkinson divider.

What isn't immediately obvious to me is why the designer chose to use baluns/transformers to first convert the single-ended input to differential, amplify both signals independently, before converting the differential signal back to a single-ended output:

Front-end schematic of the Shure PA770

As can be seen in some internal photos on an FCC listing of their more modern combiners, these transformers aren't small (nor inexpensive), not to mention the duplication of the RF transistor and biasing circuitry:

Internal image of the front-end of the Shure PA821SWB

What are the benefits to this design vs a more typical single-ended power amplifier architecture?


2 Answers 2


One reason is that balanced amplifiers tend to suppress even harmonics, and reducing the 2nd harmonic means you don't need as much filtering.

In a broad band amplifier (an octave or more), filtering out the 2nd harmonics might be difficult since the second harmonic of the lowest frequency can fall within the upper end of the bandwidth you're trying to amplify, so having it suppressed by means of a balanced amplifier could be a big advantage.


I can only speculate, but

  • two amplifiers giving you the sum of their powers might be easier to build than one amplifier with twice the power
  • if there's some calibration / feedback, you can very nicely achieve great balance that way
  • "single-ended" means you don't need a negative rail (you wouldn't need one in AC coupling anyways, but still) for a higher Vpp

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