Balanced amplifiers

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Fellow microwave Dudes, we sure could use some pictures to go along with this text! Send us a picture and you'll receive a cool Microwaves101 pocketknife! Don't make us draw stuff with Microsoft Word, that gets ugly for everyone.

A balanced amplifier has two amplifying devices that are run in quadrature. That is, they are operating 90 degrees apart in phase. A quadrature coupler or splitter on the input phase-shifts the signal 90 degrees at the amplifier inputs, then a second quadrature coupler on the output "un-phase shifts" the signals at the amplifier outputs so they combine in phase. What is the purpose of this? It is an incredible microwave magic trick!

Referring to the figure below, at the input the signals are phase shifted by 90 degrees. That means that signals that reflect from the amplifying devices undergo a 180 degree phase shift and combine out of phase at the RF input. For near-identical devices they subtract from each other when they combine, so they combine to zero volts, and ultimately a great input match. A similar thing happens at the output. The bottom line is this: you can combine stuff with poor reflection coefficients and the amplifier end up matched closely to fifty ohms, so long as the devices are nearly matched in reflection coefficients.

Need a figure!!!

Below is a three-stage amplifier from Mimix. The second two stages are balanced, you can see the Lange couplers in the photo.