Updated July 19, 2009
here to go to our main page on amplifiers
here to go to our page on quadrature couplers
here to go to our main page on power combining
here to go to our page on Lange couplers
here to go to our page on coupled-line couplers
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
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...
- They usually have excellent
input and output return loss (as described above)
- 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.
- 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.
- 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!