# Bias T Calculator

This tool calculates the Capacitor and Inductor values for a Bias Tee design.

Enter the following:

• Impedance
• Impedance ratio
• Frequency of operation

## Formula

The minimum value of Capacitance is

C = 1/(2*π*f*Xc)

The Inductance value is

L = XL/(2*π*f)

where

• f = Frequency of operation
• Xc = R/n
• XL = R*n
• R = Impedance
• n = Impedance Ratio

## Background

This calculator helps determine the Inductor and Capacitor values for a Bias Tee (also known as Bias T). Note that these are minimum values associated with the circuit.

Bias Tees are used to remotely power low noise amplifiers such that they are located close to the antenna. With either a short cable or no cable between the antenna and amplifier, the antenna cable loss is reduced. This minimizes the noise figure and receiver sensitivity. This in turn maximizes the range of the communication system.

### What is a Bias Tee?

A bias tee is a circuit component used in RF (radio frequency) systems to combine both DC and RF signals onto a single line. It consists of an inductor and a capacitor, usually connected as shown in the picture below.

The inductor blocks RF signals while allowing DC to pass through, while the capacitor blocks DC while allowing RF signals to pass through.

The purpose of a bias tee is to provide power to active components such as amplifiers, which require a DC bias voltage, while also allowing the RF output of the amplifier to be transmitted.

Typically, a bias tee is designed for a 50 ohm impedance, allowing it to match the impedance of the RF system. The capacitance and inductance values are chosen such that the bias tee has negligible effect on the RF signal at the desired frequency.

This ensures that the bias tee does not introduce any signal loss or distortion, allowing for the transmission of wideband RF signals without significant attenuation. The DC input port can be used to provide a bias voltage to an amplifier, and the RF port can be used to pass the amplified signal.

## The design of a Bias Tee

The calculator on this page can be used to design a bias tee at a specific operating frequency, f. It will have a certain bandwidth of operation around this center frequency.

The challenge is when a Bias Tee has to be designed for a remote amplifier that operates over a very large frequency range. In this case, the inductor has to act as a choke and the capacitor has to pass the RF signal, for that entire frequency range.

This requires some tricky modeling and design.

The alternative to designing your own is to use an off-the-shelf Bias Tee like the one below that can be powered with a USB cable or using the DC header. In this case it supports a very wide frequency range of up to 7 GHz.