Overview

The bias tee is an essential component for applying DC voltage to any component that must also pass RF/microwave signals, most commonly an RF amplifier that requires a DC supply. For narrowband applications, bias tee design and construction are relatively straightforward, provided attention is paid to component self-resonant frequencies (SRFs). For broadband applications, however, bias tee design and construction are nontrivial, and attention to component characteristics is paramount to a successful, high-performance design. In this article, we examine narrowband bias tee design, component SRFs, and how they impact the design, then extend those ideas to broadband bias tees.  We will also compare the electrical and physical performance attributes of different types of broadband bias tee designs including discrete circuits with conical inductors as well as MMICs. 

Basic Functions of RF Bias Tees

Bias tees are utilized anytime the designer is required to apply DC bias to an RF line. This is common practice in remote RF electronics where it is generally far more efficient, in terms of both size and cost, to run the DC current up the center conductor of a coaxial cable to power the remote equipment than it is to run separate wires for power supply alone. Bias tees in remote systems of this nature are commonly high-performance, connectorized modules.

Bias tees are also found in “local” applications on high-density, RF/microwave/millimeter wave circuit boards that have dozens of these devices, each occupying only a handful of square millimeters. 

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