Lumped-element branchline coupler

New for February 2023.  This content was split from our main branchline coupler page and is currently being expanded.

Lumped elements can be used to approximate transmission lines in a branchline coupler. The structure for a quarter wave transmission line can be realized with a pair of shunt capacitors of equal value, separated by a series inductor (a "pi" network). By optimizing the inductor and capacitor values, different line impedances can be "faked". Why would you want to use shunt elements? So you make branchline couplers at lower microwave frequencies (UHF through S-band) and not have to deal with huge transmission line lengths.

Here's a nice lumped element branchline effort using LTCC.  The capaictors are created as parallel plates with just one layer of tape between them.

Here's a two-section branchline lumped-element branchline coupler in MMIC representation, with the quarter wavelength arms changed into lumped inductive and capacitive elements. It's a double-box structure on four-mil GaAs. It worked great at S-band, with less than 1 dB resistive loss in spite of all those spiral inductors! Such a structure can make an efficient power combiner for the IF output of a higher-frequency image-rejection mixer.

That MMIC dores not have ground-signal-ground (GSG) interfaces for RF probing, which seems old-school.  But it's purely passive, so teh yileld of visually-god die is probably close to 100%.

Sorry, we don't have a design model of that MMIC! But we will offer this: if the input port is on the upper left, the lower left  port is the isolated port, the upper right is the reference phase line an dthe lower right is 90 degrees delayed phase line.