biastee

A bias tee, or bias-T, is a passive RF and microwave component that combines or separates a DC bias from an RF signal on a single transmission line. It enables a DC-powered device, such as an amplifier, mixer, laser diode, or transistor, to be biased while an RF signal is simultaneously applied or extracted.

Construction relies on two paths: a DC path that presents a high impedance to RF (typically an inductor or RF choke) to feed the device with DC, and an RF path that blocks DC (typically a capacitor) to carry the RF signal to or from the device. The RF port sees the capacitor that passes RF but blocks DC, while the DC port sees the inductor that passes DC but presents a high impedance to RF. Some designs use a transformer or microstrip layout, with careful impedance matching and shielding.

Applications include RF and microwave systems where a device must be biased without disturbing the RF signal, such as biasing transistors, diodes, or laser diodes in communication systems, biasing stages in amplifiers, LO injection in mixers, or powering active components in test equipment. In fiber optics, an optical bias tee combines a DC current for a laser diode with a high-frequency modulation signal to produce modulated light.

Performance characteristics vary with design but commonly include bandwidth (from kilohertz to many gigahertz), DC current capability, RF isolation between the DC and RF ports, and insertion loss on the RF path. Designers must consider return loss, leakage of DC into the RF source, and the physical layout to minimize parasitics.