Eom power combiner example #2
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New for December 2008!
The Lim Eom six-port network has some unique properties. Recently
it was used by the original author Jong-Sik Lim and his team to
power-combine a pair of amplifiers as reported at the IEEE 2001
MTT Symposium. The paper was titled A New Balanced Amplifier
using Six-Port Power Divider. On first glance, this looks like
the best thing since sliced bread: A network than can combine two
amplifiers without the need for isolation resistors. Resistors are
always needed in "conventional" combining approaches such
as Lange couplers, branchlines
or Wilkinsons. Resistors drive
up the cost of a power combiner, and complicate its construction.
The Lim-Eom network can be used
as a three-way splitter, and the three split ports can provide arbitrary
amplitudes by adjusting leg impedances. It also provides a two-way
split function, and it actually can provide two two-way splitters
from the six-port network. This is the property that was exploited
in the paper referenced above.
Lim at al refer to the
amplifier as a "balanced amp".
We usually reserve the word "balanced" to mean combined
in quadrature. Maybe we could put this definition to a vote some
We rearranged the port numbers
so that port one is at the middle of the top, and port 2 is at the
middle of the bottom because these are going to be ports 1 and 2
of our power amp. Note that when used as an equal two-way splitter,
there is a great deal more symmetry then when it is used as an equal
three-way splitter. All of the lines around the perimeter have impedance
SQRT(2)xZ0. The line though the center of the network can be any
impedance and affects the bandwidth of some circuit parameters.
In the network below we left it at Z0 (fifty ohms).
All of the ports are matched.
If this was a three-port network, this would not be possible from
basic network theory.
When port 1 is the input, the
power is (ideally) equally split between ports 3 and 4 at the center
frequency. Similarly, if port 2 was the input, ports 5 and 6 would
each receive half the incident power. Here's the 3 dB power split:
No need to plot the split between
port 2 two and ports 5 and 6, it is the same by symmetry. Going
back to basic network theory, you can't make a three-port circuit
that is lossless and impedance matched. The Lim Eom splitter provides
two three-port functions which are both matched yet the circuit
is ideally lossless. How did they pull off this magic trick? It's
simple: the network is a six-port, not a three-port, even though
it can be made to operate like two three-ports.
Now let's throw in some ideal
amplifiers between ports 3 and 5, and between ports 4 and six (which
is what Lim at al did). Now ports 1, 3 and 4 form the divider,
and ports 2, 5 and 6 form the combiner.
Here is the response with the
ideal, perfectly matched, 10 dB gain amplifier.
What would happen if the input
and output ports of the amplifier were mismatched? The mismatch
would mess show up at the input and output, just line when Wilkinson's
are used to power combine.
Here's a thought... why not add
some quarterwave sections to the network so that the reflection
coefficients cancel? This is sometimes done with Wilkinsons to improve
Here's the response. Say, what's
that, at 800 MHz the combined amplifier has more gain than the ideal
amp we created it from? Isn't that perpetual motion?
Actually, its due to feedback,
and it spells trouble in the form of instability (oscillations are
likely). Looking back at the six-port network, there is a DC
connection between both amplifiers' inputs and outputs. Yikes!
Maybe we should check the frequency response of the isolation of
the ports that are connected to each amplifier. Ideally the input
should be fully isolated from the output of an amplifier, but this
is not the case. Below are the magnitude and phase of the unwanted
Because the feedback cannot be
reduced to zero, the use of the Lim-Eom six-port splitter as a two-way
combiner will probably be limited to narrrow-band applications,
like the example the authors cited in their paper. We recommend
that you only attempt this combiner on well-matched amplifiers that
have DC blocks on both ports. One unfortunate side effect of all
of this is that you will have to build bias networks into each of
the amplifiers, in a balanced amplifier using Lange couplers for
example you can bias the circuits right though the divider and combiner
It is a cool idea, and it eliminates
the need for resistors in a combining network, but just be aware
of the potential for instability when you try it. By the way, the
feedback issue was NOT described in the IEEE paper, even though
IEEE peer-reviews all of their papers. Maybe some of the peers were
asleep on the job!
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