Javascripe Electronic Notebook
T-Pad Attenuator
by Martin E. Meserve

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T-Pad Attenuator

The diagram below is a T-Pad Attenuator. It's called a T-Pad because the components of the circuit form the letter T. It consists of two series arm resistors (R1 and R2) and one parallel resistor (R3).

This type of attenuator is considered Unbalanced and is commonly designed with Z1 and Z2 equal. This configuration is then Symmetrical as R1 and R2 are equal values.

Making Z1 not equal to Z2 can cause the calculated circuit values for the series arm resistors to be negative. In these cases you will need to increase the Attenuation, high enough to make the values positive.

Calculations

In the boxes provide below, enter the required Input Impedance (Z1), Output Impedance (Z2), and required Attenuation (A), in db. The resistor values necessary, to realize this configuration, will then be calculated.

Note: After entering your data click anywhere outside of the boxes and the new outputs will be calculated.

Z1 (Ohms)
Z2 (Ohms) A db

The exact resistor values needed for an attenuator, with a Input Impedance (Z1) of x, an Output Impedance (Z2) of x, and an Attenuation (A) of x, are listed below.

x x
x

t_pad.gif

Real World Values

For the design specified above, the nearest standard 5% resistors are listed below. These values, not being exact, will cause the Input/Output Impedances, and the Attenuation, to be slightly higher.

x x
x

Note: Standard resistor values may not be accurate for resistances less than 10 Ohms.

If you require greater accuracy in the resistor values, you can use the closest 1% tolerance resistors, or you can use two standard 5% resistors in parallel to approximate each of the resistor arm values. The resulting resistances will be within 1% of the specified values. The Delta below shows the difference between the original calculated value and the parallel resistor combination.

  RA (Ohms)   RB (Ohms)   RT (Ohms) Delta (%)
R1 x || x = x x
R2 x || x = x x
R3 x || x = x x