Bridged-T Attenuator

Bridged-T Attenuator

The Bridged-T Attenuator ( T ) is another purely resistive design that is a variation on the symmetrical T-pad Attenuator we looked at previously. As its name implies, the bridged-T attenuator has an additional resistive element forming a bridged network across the two series resistors of the standard T-pad. This additional resistor enables the circuit to reduce the level of a signal by the required attenuation without changing the characteristic impedance of the circuit as the signal appears to "bridge" across the T-pad network. Also the two series resistances of the original T-pad are always equal to the input source and output load impedances. The circuit for a "bridged-T attenuator" is given below.

Bridged-T Attenuator Circuit

Resistor, R3 forms the bridge network across a standard T-pad attenuator. The two series resistors, R1 are chosen to equal the source/load line impedance. One major advantage of the bridged-T attenuator over its T-pad cousin, is that the bridged-T pad has a tendency to match itself to the transmissions lines characteristic impedance. However, one disadvantage of the bridged-T attenuator circuit is that the attenuator requires that its input or source impedance, ( Z_S ) equals its output or load impedance, ( Z_L ) and therefore cannot be used for impedance matching.

The design of a bridged-T attenuator is as simple as for the standard T-pad attenuator. The two series resistors are equal in value to the lines characteristic impedance and therefore require no calculation. Then the equations given to calculated the parallel shunt resistor and the additional bridging resistor of a bridged-T attenuator circuit used for impedance matching at any desired attenuation are given as:

Bridged-T Attenuator Equations

where: K is the impedance factor, and Z is the source/load impedance.

Example No1

A bridged-T attenuator is required to reduce the level of an 8Ω audio signal line by 4dB. Calculate the values of the resistors required.

Then resistors R1 are equal to the line impedance of 8Ω, resistor R2 is equal to 13.7Ω and the bridging resistor R3 is equal to 4.7Ω, or the nearest preferred values.

As with the standard T-pad attenuator, as the amount of attenuation required by the circuit increases, the series bridge impedance value of resistor R3 also increase while the parallel shunt impedance value of resistor R2 decreases. This is characteristic of a symmetrical bridged-T attenuator circuit used between equal impedances.

Variable Bridged-T Attenuator

We have seen that a symmetrical bridged-T attenuator can be designed to attenuate a signal by a fixed amount while matching the characteristic impedance of the signal line. Hopefully by now we know that the bridged-T attenuator circuit consists of four resistive elements, two which match the characteristic impedance of the signal line and two which we calculate for a given amount of attenuation. But by replacing two of the attenuators resistive elements with either a potentiometer or a resistive switch, we can convert a fixed attenuator pad into a variable attenuator over a predetermined range of attenuation as shown.

Variable Bridged-T Attenuator

So for example above, if we wanted a variable bridged-T attenuator to operate on an 8Ω audio line with attenuation adjustable from -2dB to -20dB, we would need resistive values of:

Resistor values at -2dB

Resistor values at -20dB

Then we can see that the maximum resistance required for an attenuation of 2dB is 31Ω and at 20dB is 72Ω. So we can replace the fixed value resistors with two potentiometers of 100Ω each. But instead of adjusting two potentiometers one at a time to find the required amount of attenuation, both potentiometers could be replaced by a single 100Ω dual-gang potentiometer which is electrically connected so that each resistance varies inversely in value with respect to the other as the potentiometer is adjusted from 2dB to 20dB as shown.

Fully Adjustable Bridged-T Attenuator

By careful calibration of the potentiometer, we can easily produce in our simple example, a fully adjustable bridged-T attenuator in the range of 2dB to 20dB. By changing the values of the potentiometers to suit the characteristic impedance of the signal line, in theory any amount of variable attenuation is possible by using the full range of resistance from zero to infinity for both VR1a and VR1b, but in reality 30dB is about the limit for a single variable bridged-T attenuator as the resistive values become to small. Noise distortion is also a problem.

Taking this idea one step further, we could also produce a stepable bridged-T attenuator circuit by replacing the potentiometers with fixed value resistances and a ganged rotary switch, rocker switches or push-button switches and by switching in the appropriate resistance, the attenuation can be increased or decreased in steps. For example, using our 8Ω transmission line impedance example above.

We can calculate the individual bridge resistances and parallel shunt resistances for an attenuation of between 2dB and 20dB. But as before, to save on the maths we can produce tables for the values of the series bridge and parallel shunt impedances required to construct either an 8Ω, 50Ω or 75Ω switchable bridged-T attenuator circuit. The calculated values of the bridging resistor R2 and parallel shunt resistor R3 are given below.
 

dB Loss	K factor	8Ω Line Impedance		50Ω Line Impedance		75Ω Line Impedance
		R2	R3	R2	R3	R2	R3
2.0	1.2589	30.9Ω	2.1Ω	193.1Ω	12.9Ω	289.7Ω	19.4Ω
4.0	1.5849	13.7Ω	4.7Ω	85.5Ω	29.2Ω	128.2Ω	43.9Ω
6.0	1.9953	8.0Ω	8.0Ω	50.2Ω	49.8Ω	75.4Ω	74.6Ω
8.0	2.5119	5.3Ω	12.1Ω	33.1Ω	75.6Ω	49.6Ω	113.4Ω
10.0	3.1623	3.7Ω	17.3Ω	23.1Ω	108.1Ω	34.7Ω	162.2Ω
12.0	3.9811	2.7Ω	23.8Ω	16.8Ω	149.1Ω	25.2Ω	223.6Ω
16.0	6.3096	1.5Ω	42.5Ω	9.4Ω	265.5Ω	14.1Ω	398.2Ω
20.0	10.00	0.9Ω	72.0Ω	5.6Ω	450.0Ω	8.3Ω	675.0Ω

Note that the two fixed series resistors R1 of the circuit will always be equal to the transmission lines characteristic impedance.

Then using our 8Ω transmission line as our example, we can construct a switchable bridged-T attenuator circuit as follows using the resistive values calculated in the table.

Switchable Bridged-T Attenuator

So for the bridging resistance set by VR1a at the -10dB point, the total resistance is equal to the sum of the individual resistances as is given as:

5.2 + 4.1 + 3.3 + 2.6 + 2.1 = 17.3Ω

Likewise, for the parallel shunt resistance set by VR1b, the total resistance at the -10dB point will be equal to:

1.0 + 1.2 + 0.6 + 0.9 = 3.7Ω

Note that both of these resistive values of VR1a = 17.3Ω and VR1b = 3.7Ω correspond to the -10dB attenuation we calculated in the above table.

We have seen that the Bridged-T attenuator is a purely resistive fixed type symmetrical attenuator which can be used to introduce a given amount of attenuator loss when inserted between equal impedances with the bridged-T design being an improved version of the more common T-pad attenuator. In some ways we can also think of the bridged-T attenuator as a modified Pi-pad attenuator we will look at in the next tutorial. One of the main disadvantage of this type of circuit is that due to the bridging resistor, this type of attenuator circuit can not be used for the matching of unequal impedances.

The bridged-T attenuator design makes it easy to calculate the resistances required for the network because the values of the two series resistances are always equal to the characteristic impedance of the transmission line making the attenuator symmetrical. Once the desired amount of attenuation is determined the maths involved in calculating the remaining resistance values is fairly simple. Also this type of attenuator design allows for the bridged-T pad to be adjustable by changing only two of the resistive elements for potentiometers or switched resistors were as the standard T-pad attenuator would need three.

In the next tutorial about Attenuators, we will look at a different types of attenuator design called the Pi-pad Attenuator which uses only three resistive component, one in the series line and two in the parallel shunt line.