November 21, 2004
Balanced
amplifiers
Click here
to go to our main page on amplifiers. Also, check out our page on
quadrature couplers.
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.
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