T type attenuator design equations derived from first principles

The t type attenuator is a very useful circuit and is used quite extensively to not only attenuate signals but also to match inputs and outputs. This post presents the derivation of the design equations for this type of attenuator. The equations have been derived for a symmetric t type attenuator with equal I/O impedances. Please visit the SPG website and select the relevant post from the complementary menu items.

Calculate t type attenuator resistors for equal input and output impedances.

A javascript script is available to calculate the resistors needed for the t type attenuator in the case of equal input/output impedances. Given the required attenuation and “characteristic impedance” the script will calculate the resistors required. Please visit the SPG website and select the “complementary” menu item and then select the javascript from the list of items.

The T type attenuator and its characteristics

The T type attenuator is a very useful circuit block that has the capability to simultaneously match two transmission lines and attenuate the signals traveling on them. It is a symmetric circuit and when its impedances are chosen correctly the I/O can be exchanged without affecting the signals. Unequal impedances can also be used with appropriate modifications. These and other characteristics are explained in the “T pad” post on the Signal Processing Group Inc, website. Please visit for more info.