The T-pad Attenuator
A T-pad attenuator is an unbalanced
attenuator network consisting of three non-inductive resistive elements
connected together to form a "T" configuration, (hence its name).
Although not common, this "T" (tee) configuration can also be thought
of as a wye "Y" attenuator configuration as well. Unlike the previous
L-pad Attenuator, which has a
different resistive value looking into the attenuator from either end making it an asymmetrical circuit. The formation of the
resistive elements into a letter "T" shape means that the T-pad attenuator has the same value of resistance looking from
either end. This formation then makes the "T-pad attenuator" a perfectly symmetrical attenuator enabling their input and
output terminals to be transposed as shown.
Basic T-pad Attenuator Circuit
We can see that the T-pad attenuator is symmetrical in its design looking from either end and this type of attenuator design can be used to impedance match either equal or unequal transmission lines. Generally, resistors R1 and R2 are of the same value but when designed to operate between circuits of unequal impedance these two resistor can be of different values. In this instance the T-pad attenuator is often referred to as a "taper pad attenuator".
But before we look at T-pad Attenuators
in more detail we first need to understand the use of the
"K factor" used in calculating attenuator impedances and which can make
the reduction of the maths and our lives a little easier.
The Attenuators "K" Factor
The "K" factor, also known as the "impedance factor" is commonly used with
attenuators to simplify the design process of complex attenuator circuits. This "K" factor or
value is the ratio of the voltage, current or power corresponding to a given value of attenuation. The general equation for
"K" is given as:
So for example, the "K" value for a voltage attenuation of 6dB
will be 10 (6/20) = 1.9953, and an attenuation of 18dB will be
10 (18/20) = 7.9433, and so on. But instead of calculating this "K"
value every time we want to design a new attenuator circuit, we can produce a "K" factor
table for calculating attenuator loss as follows.
Attenuator Loss Table
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and so on, producing an attenuation loss table with as many decibel values as we require for our attenuator design.
T-pad Attenuator with Equal Impedances
We have said previously, that the T-pad attenuator is a symmetrical attenuator design whose input and
output terminals can be transposed with each other. This makes the T-pad attenuator ideal for insertion between two equal
impedances ( ZS = ZL ) to reduce signal levels.
In this case the three resistive elements are chosen to ensure that the input impedance and output impedance match the load
impedance which forms part of the attenuator network. As the T-pad's input and output impedances are designed to perfectly
match the load, this value is called the "characteristic impedance" of the symmetrical T-pad network.
Then the equations given to calculated the resistor values of a T-pad attenuator circuit used for impedance matching
at any desired attenuation are given as:
T-pad Attenuator Equations
where: K is the impedance factor from the table above, and Z
is the source/load impedance.
Example No1
A T-pad attenuator is required to reduce the level of an audio signal by 18dB while matching the impedance
of the 600Ω network. Calculate the values of the three resistors required.
Then resistors R1 and R2 are equal to 466Ω and
resistor R3 is equal to 154Ω, or the nearest preferred values.
Again as before, we can produce standard tables for the values of the series and parallel impedances required to
construct a 50Ω, 75Ω or 600Ω symmetrical T-pad attenuator circuit as these values will always be the same regardless
of application. The calculated values of resistors, R1, R2 and
R3 are given below.
| dB Loss | K factor | 50Ω Impedance | 75Ω Impedance | 600Ω Impedance | |||
| R1, R2 | R3 | R1, R2 | R3 | R1, R2 | R3 | ||
| 1.0 | 1.1220 | 2.9Ω | 433.3Ω | 4.3Ω | 650.0Ω | 34.5Ω | 5K2Ω |
| 2.0 | 1.2589 | 5.7Ω | 215.2Ω | 8.6Ω | 322.9Ω | 68.8Ω | 2K58Ω |
| 3.0 | 1.4125 | 8.5Ω | 141.9Ω | 12.8Ω | 212.9Ω | 102.6Ω | 1K7Ω |
| 6.0 | 1.9953 | 16.6Ω | 66.9Ω | 24.9Ω | 100.4Ω | 199.4Ω | 803.2Ω |
| 10.0 | 3.1623 | 26.0Ω | 35.1Ω | 39.0Ω | 52.7Ω | 311.7Ω | 421.6Ω |
| 18.0 | 7.9433 | 38.8Ω | 12.8Ω | 58.2Ω | 19.2Ω | 465.8Ω | 153.5Ω |
| 24.0 | 15.8489 | 44.1Ω | 6.3Ω | 66.1Ω | 9.5Ω | 528.8Ω | 76.0Ω |
| 32.0 | 39.8107 | 47.5Ω | 2.5Ω | 71.3Ω | 3.8Ω | 570.6Ω | 30.2Ω |
Note, as the amount of attenuation required by the circuit increases the series impedance values for
R1 and R2 also increase while the parallel shunt impedance value of
R3 decreases. This is characteristic of a symmetrical T-pad attenuator circuit used between equal impedances.
T-pad Attenuator with Unequal Impedances
As well as using the T-pad attenuator to reduce signal levels in a circuit with equal impedances, we can also
use it for impedance matching between unequal impedances ( ZS ≠ ZL ).
When used for impedance matching, the T-pad attenuator is called a Taper Pad Attenuator. However, to do so we
need to modify the previous equations a little to take into account the unequal loading of the source and load impedances on
the attenuator circuit. The new equations become.
Taper Pad Attenuator Equations for Unequal Impedances
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where: K is the impedance factor from the table above, and Z1
is the larger of the source/load impedances and Z2 is the smaller of the source/load impedances.
Example No2
A taper pad attenuator connected to a load impedance of 50Ω is required to reduce the level of an audio
signal by 18dB from an impedance source of 75Ω. Calculate the values of the three resistors required.
Then: Z1 = 75Ω (the largest impedance),
Z2 = 50Ω (the smallest impedance) and
K = 18dB = 7.9433 from the table above.
So resistor R1 is equal to 15.67Ω, resistor R2 is equal
to 62Ω and resistor R3 is equal to 36Ω, or the nearest preferred values.
Balanced-T Attenuator
The balanced T-pad attenuator or Balanced-T Attenuator for short, uses two
T-pad attenuator circuits connected together to form a balanced mirror image network as shown below.
Balanced-T Attenuator Circuit
The balanced-T attenuator is also called an H-pad attenuator because the layout of its
resistive elements form the shape of a letter "H" and hence their name, "H-pad attenuators". The resistive values of the
balanced-T circuit are firstly calculated as an unbalanced T-pad configuration the same as before, but this time the values
of the series resistive in each leg are halved (divided by two) to provide a mirror image either side of ground. The total
calculated resistive value of the centre parallel resistor remains at the same value but is divided into two with the centre
connected to ground producing a balanced circuit.
Using the calculated values above for the unbalanced T-pad attenuator gives, series resistor
R1 = R2 = 466Ω ÷ 2 = 233Ω for all four series
resistors and the parallel shunt resistor, R3 = 154Ω the same as before and these
values can be calculated using the following modified equations for a balanced-T attenuator.
Balanced-T Attenuator Equations
T-pad Attenuator Summary
The T-pad attenuator is a symmetrical attenuator network that can be used in a transmission
line circuit that has either equal or unequal impedances. As the T-pad attenuator is symmetrical in its design it can be
connected in either direction making it a bi-directional circuit. One of the main characteristics of the T attenuator, is
that the shunt arm (parallel) impedance becomes smaller as the attenuation increases. T-pad attenuators that are used as
impedance matching circuits are usually called "taper pad attenuators".
T-pad attenuators can be either unbalanced or balanced resistive networks. Fixed value unbalanced T-pad
attenuators are the most common and are generally used in radio frequency and TV coaxial cable transmission lines were one
side of the line is earthed. Balanced-T attenuators are also called H-pad Attenuators due to their design
and construction. H-pad attenuators are mainly used on data transmission lines which use balanced or twisted pair cabling.
In the next tutorial about Attenuators, we will look at another type of T-pad attenuator design
called the Bridged-T Attenuator that uses an additional resistive component in the series line.
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