By AWR with contributions from Dr. Dominic FitzPatrick of PoweRFul Microwave
The use of printed microstrip couplers in power amplifiers operating at microwave frequencies above 1GHz has been well established and numerous papers have been written about the design of such structures. It is surprising, however, that little has been written about the subject of printed baluns (BALanced to UNbalanced)— electrical devices that convert between a balanced signal and an unbalanced signal. Even more surprising is that the dominant approach to power combining at VHF and UHF is via coaxial cables, which comes with inherent assembly, cost, and thermal issues.
A balanced amplifier is particularly desirable in the design of high-power, highefficiency amplifiers and is based upon driving two transistors 180° out of phase. This has thermal and 2nd harmonic rejection benefits. The alternative to coaxial baluns, which can overcome the cost and assembly issues using printed planar transmission lines, can be more difficult to design, which may be one reason why it has not been more widely adopted. A key issue has been the lack of adequate linear transmission line models.