Bias tee

A bias tee is a kind of multiplexer which has 3 ports arranged in the shape of a T and where frequencies from 1 MHz to 10 GHz pass horizontally through the T and lower frequencies take a 90° turn used to bias transistors and diodes. It is a simple composition of one capacitor and one coil with attention paid to the details.

Construction

The construction of the horizontal bar of the T is based on the rigid coaxial cable with air as dielectric. The radius is chosen to be as large as possible without allowing higher modes.

Capacitor

At one point a small slice is cut out of the center conductor, therefore a capacitor is formed and low frequencies are blocked. This kind of capacitor has the advantage that its is nearly invisible to higher frequencies. To pass frequencies down to 1 MHz the capacitance has to be increased. A dielectric like NPO multiplies the capacitance by a factor of 65. The thickness of the capacitor has to be minimal without leading to electric breakdown in the dielectric, this means to avoid any peaks in the electric field and this means smooth electrodes with rounded edges and a dielectric protruding between the electrodes (doorknob design). A stack of capacitors can be used, but every capacitor needs access to the surface of the inner conductor, because if it's hidden behind another capacitor the high frequencies won't see it, because the electric field needs a lot of time to travel through a dielectric with a high dielectric constant

Coil

A small coil made of fine wire with an air core or MnFeZn-core connects the inner conductor of one of the sides of the capacitor with the a port in the outer conductor leading down the T. Frequencies above 1 GHz hit the coil from the side and apply an equal electric field to the whole coil. Therefore no higher modes are excited within the coil. Because of the inductiveness of the coil almost no current leaks from the center conductor to the port. Frequencies between 1 MHz and 1 GHz do leak into this port, so there is a second coil with a cone shaped core outside of the outer conductor, but inside of a housing to avoid interference with other components. This cone acts like a tapered transmission line transformer. It starts with a high impedance, so a lot of power will be reflected, but the rest will travel down the coil and there is some leakage into the low frequency port.

Oscillations

Any oscillations in the capacitor or the coil or the composed LC circuit are damped by the dielectric and the core. Also the small coil should have about 10 ohm resistance to further damp oscillations and avoid ripple on the transmitted spectrum.

Applications

Biasing for photodiodes (vacuum and solid state), Microchannel plate detectors, transistors, and triodes. High frequencies are not leaking into a common power supply rail and noise from the power supply does not appear on the signal line.

External links

* [http://www.picosecond.com/objects/AN-01e.pdf picosecond]