Balanced Attenuator Pad

- an overview of the balanced attenuator pad used for balanced RF and other systems including audio voice and television applications.

Balanced attenuator pads are can be seen in a variety of circuits. While the unbalanced formats for the Pi and T section attenuator pads are probably the most widely used, balanced attenuator pads need to be used for balanced systems.

Balanced attenuators are used for balanced RF systems, but they are probably more widely used for balanced audio systems where the characteristic impedance is 600 ohms and for some television systems as well.


Balanced attenuator basics

There are a number of formats that can be adopted for balanced attenuators. The most commonly used are the balanced Pi attenuator and balanced T attenuator - these are basically balanced versions of the familiar Pi and T attenuator pads.

Balanced Pi attenuator pad:

The balanced Pi attenuator is shown in the diagram below. It can be seen from this that the series resistor in the top of the Pi section of the attenuator is shared between the two lines, rather than being completely contained within the non-earth line in the case of the unbalanced version. As a result the value of the series resistor is half that of the value of the resistor in the equivalent position on the unbalanced Pi attenuator.

The balanced version of the Pi attenuator pad showing how the series resistance is shared between both lines.
Balanced Pi attenuator pad

The resistor numbers relate to those used in other pages of this tutorial. Resistor values can be taken from those used in the "Attenuator Resistor Values" table provided on another page of this tutorial.

Balanced T attenuator pad:

The balanced T attenuator has a total of five resistors. As may be imagined, the resistors in the top of the T section are half the value of the equivalent resistors in the unbalanced version of the attenuator pad.

As there are two resistors that are effectively split between the two lines, the balanced T attenuator pad has one more resistor than the balanced Pi attenuator.

The balanced version of the T attenuator pad showing how the series resistance is shared between both lines.
Balanced T attenuator pad

As above, the resistor numbers relate to those used in other pages of this tutorial. Resistor values can be taken from those used in the "Attenuator Resistor Values" table provided on another page of this tutorial.

By Ian Poole


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