Quantifying Accuracy of Balanced Circuits

Balanced circuits have existed for a very long time, having first been developed to reduce noise and crosstalk in telephone circuits. Implementation in push-pull amplifiers was another early use, simplifying output coupling to open-wire balanced feeders used in many antenna systems. As unbalanced coaxial lines replaced balanced lines, the ability of balanced circuits to reduce even-order harmonics became the primary factor for their use. In addition to classic push-pull designs, other circuits take advantage of topologies that use the antiphase properties of balanced circuits to provide cancellation of unwanted signals. Applications include hybrid couplers that provide isolation between ports, and the well-known double-balanced mixer that offers isolation at RF and LO frequencies, easing post-mixer filtering requirements. Recently, balanced circuits have become popular for dealing with the difficulty of maintaining a low impedance ground plane in microwave MMICs. With very low ground (or common mode) currents, the performance of the ground plane in a balanced circuit has much less effect on performance than in an unbalanced design.