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Balanced amplifiers

Updated July 19, 2009

A balanced amplifier has two amplifying devices that are run in quadrature. That is, they are operating 90 degrees apart in transmission phase. A quadrature coupler or splitter on the input phase-shifts the two signals 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 main purpose of a balanced amplifier? It pulls off an incredible microwave magic trick! See our page on quadrature couplers for a better explanation of the disappearing reflection coefficient.

The reflection coefficient external to a balanced amplifier is passed on to the individual amplifiers in all its glory, albeit at 180 degrees out of phase. Our Sniffer circuit will help you predict this phenomenon. Balanced amplifiers are more immune to load pull effects than in-phase power combining schemes, because the two reflection coefficients are seen 180 degrees out of phase. Click here to learn more.

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.

Some things you need to know about balanced amplifiers...

1. They usually have excellent input and output return loss (as described above)
2. If they are presented with a bad match on the output, the bad match is seen by both amplifiers, but at phase angles differening by 180 degrees.
3. The load that terminates the isolated port on the output can see sizable heat dissipation if the phases of the two amplifiers (or their amplitudes) are not exactly the same.
4. The use of balanced amplifiers at millimeterwave is quite common. The loss of a Lange coupler at W-band may actually be less than at X-band, because metal loss per wavelength actually decreases with frequency.

Below is a three-stage amplifier from Mimix. The second two stages are balanced, you can see the Lange couplers in the photo. That has to be tyhe world's smalledt 50 ohm termination on lower left port of the input coupler, it's dwarfed by the via catch pad!

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