Understand baluns for highly integrated RF modules

March 29, 2011 //By Mark Forbes and Mark Gorbett
Understand baluns for highly integrated RF modules
Mark Forbes, Mentor Graphics Corp. and Mark Gorbett, Microwave Assurance, LLC, show why it is essential to understand balun technology to achieve highly integrated RF modules

Today, RF/MMIC engineers designing multi-chip modules require “circuit-level” EM simulation and modeling to meet the increased demands for higher levels of system integration. Modules providing full system-level functionality require passive support circuitry in addition to typical active components.   These passive components typically include circuits such as 90-degree couplers, 180-degree couplers, in-phase couplers, filters, diplexers, and transmission line structures.  

Baluns are very important support circuitry used in high-frequency circuit design.   The 180-degree balun is a major component in heterojunction bipolar transistor (HBT) as well as pseudo high-electron mobility transistor (pHEMT) push-pull amplifiers, balanced mixers, balanced frequency multipliers, phase shifters, balanced modulators, dipole feeds, unbalanced to differential converters for differential signaling, and numerous other applications.    Additionally, analog circuits requiring balanced inputs and outputs to reduce noise and minimize high order harmonics, and improve the dynamic range of the circuits [1] are also good candidates to benefit from this type of balun structure.

Derivation of a Marchand Balun
A balun, by definition, is a transformer used to connect balanced or differential transmission-line circuits to unbalanced or single-ended transmission-line circuits.   Several different kinds of balun structures have been developed over the years; however, new interest in transmission-line type baluns emphasizes the need for them to be planar, compact, and more suitable for mixers and push-pull power amplifiers.[2]   In addition, several types of baluns have been used for microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs).  

The most popular is the planar version of the Marchand balun because it is easy to implement and provides wide bandwidth.[1] The Marchand balun has a documented wider bandwidth compared to other balun designs due to improved phase and amplitude balance.   Looking at the development of the Marchand balun from the typical balun design illustrates why it has a superior physical layout and measurement results.   Figure 1a shows the typical balun layout and Figure 1b shows the

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