The bias tee is a three-ports used for setting the DC bias dot of some electronic components without disturbing other components. The bias tees are alsoa diplexer. The low-frequency port is used to set the bias; the high-frequency port passes the radio-frequency signals but blocks the biasing levels; the combined port connects to the device, which sees both the bias and RF. It is called a tee because the 3 ports are often arranged in the shape of a T.

Conceptually, the bias tee can be viewed as an ideal capacitor that allows AC through but blocks the DC bias and an ideal inductor that blocks AC but allows DC. Although some bias tees can be made with a simple inductor and capacitor, wideband bias tees are considerably more complicated because practical components have parasitic elements.

The bias tees are used to insert DC power into an AC signal to power remote antenna amplifiers or other devices. It is usually positioned at the receiving end of the coaxial cable to pass DC power from an external source to the coaxial cable running to powered device. A bias-T consists of a feed inductor to deliver DC to a connector on the device side and a blocking capacitor to keep DC from passing through to the receiver. The RF signal is connected directly from one connector to the other with only the blocking capacitor in series. The internal blocking diode prevents damage to the bias-T if reverse supply voltage is applied.

Bias tees are used in a variety of applications, but are generally used to provide an RF signal and (DC) power to a remote device where running two separate cables would not be advantageous. Biasing is often used with photodiodes (vacuum and solid state), Microchannel plate detectors, transistors, and triodes, so that high frequencies from the signal do not leak into a common power supply rail. Conversely, noise from the power supply does not appear on the signal line. Other examples include: Power over Ethernet, active antennas, low-noise amplifiers, and down converters.