Eliminating Bias Tees from Push-Pull Amplifier Outputs Using TCM3-452X+ 3:1 Unbalanced-to-Balanced Transformer

Push-pull amplifiers are used in many multi-octave systems to enhance amplifier output power and efficiency, suppress unwanted harmonics and improve system dynamic range.  In a push-pull amplifier design, two matched transistors or amplifiers are supplied by the source signal in parallel and at a 180° phase relationship.  When re-combined through a second 180° phase shifting combiner, the fundamental signals are in-phase and combined at twice the power of each half, while the even order harmonics are out-of-phase, creating ideal cancellation of these unwanted signals.  In reality, the phase and amplitude unbalance of the splitting-combining circuits result in degradation of the ideal cancellation.  However, the net result is still significant suppression of the even harmonics, typically by 20 to 40 dB. 

Techniques for Improving Impedance Mismatch

Impedance matching is a complex subject cloaked in a certain degree of mystery. Whenever a circuit fails or systemic problems are encountered, impedance matching is most often attributed as the cause. As a result, there are many situations in which it becomes necessary to match the impedance of a load to that of the source in order to maximize power transfer. There are many different techniques for impedance matching, and the best application of each will depend on the situation. This article will review the basics of impedance matching and describe some of the effective techniques commonly used to overcome impedance mismatch in a circuit.