The use of a traditional coupler as a Balun has many advantageous. A single design can be adapted for both Class A applications and Push Pull applications. The method is best suited for narrow to moderate bandwidth circuits. The traditional coupler relies on a differential 90 degree signal split while the Balun is designed for a 180 degree split. The conversion of the coupler to the Balun requires an additional delay in the direct coupled leg. It is the practical application of this method that is the subject of this note.

The ideal low pass filter topology offers a reasonable 90 degree phase shift and requires much less re estate than the traditional delay line. When the Low Pass transfer function pole is properly placed the insertion loss is minimum while the phase goes through 90 degrees. The second step in the process is to convert the ideal low pass topology to a distributed semi-lumped structure.

Full application note