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Click here to learn more about balanced amplifier VSWR (new for March 17, 2014!
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 when poorly matched amplifiers are combined. For an explanation on how this works (complete with figures), click here. 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.
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 may 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.
Some things you need to know about balanced amplifiers...
- They usually have excellent input and output return loss, at least over the bandwidth where the couplers provide close to 3 dB power split
- If they are presented with a bad match on the output, the bad match is seen by both amplifiers, but at phase angles differing by 180 degrees.
- 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 (resulting in reduced efficiency.
- The load on the input coupler can be much lower in power handling. It only dissipates the fraction of drive signal that is reflected by the amplifiers.
- 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 four-stage amplifier from Mimix (now part of M/A-COM Tech). The third and fourth stages are balanced, you can see the Lange couplers in the photo. That has to be the world's smallest 50 ohm termination on lower left port of the input coupler, it's dwarfed by the via catch pad! Note that because the input stage is not balanced, the input reflection coefficient is probably not as good as the output reflection coefficient.