An Attenuator is a special type of electrical or electronic bidirectional circuit made
up of entirely resistive elements. An attenuator is a two port resistive network designed to weaken or "attenuate" (hence
their name) the power being supplied by a source to a level that is suitable for the connected load. The attenuator
reduces the amount of power being delivered to the connected load by either a single fixed amount, a variable amount or in a
series of known switchable steps. Attenuators are generally used in radio, communication and transmission line applications
to weaken a stronger signal.
The Attenuator is a purely passive resistive network (hence no supply) which is used in
a wide variety of electronic equipment for extending the dynamic range of measuring equipment by adjusting signal levels, to
provide impedance matching of oscillators or amplifiers to reduce the effects of improper input/output terminations, or to
simply provide isolation between different circuit stages depending upon their application as shown.
Simple attenuator networks (also known as "pads") can be designed to produce a fixed degree of
"attenuation" or to give a variable amount of attenuation in pre-determined steps. Standard fixed attenuator networks
generally known as an "attenuator pad" are available in specific values from 0 dB to more than 100 dB. Variable and switched
attenuators are basically adjustable resistor networks that show a calibrated increase in attenuation for each switched step,
for example steps of -2dB or -6dB per switch position.
Then an Attenuator is a four terminal (two port) passive resistive network (active types
are also available which use transistors and integrated circuits) designed to produce "distortionless" attenuation of the
output electrical signal at all frequencies by an equal amount with no phase shift unlike a passive type RC filter network,
and therefore to achieve this attenuators should be made up of pure non-inductive and not wirewound resistances, since
reactive elements will give frequency discrimination.
Simple Passive Attenuator
Attenuators are the reverse of amplifiers in that they reduce gain with the resistive voltage divider circuit
being a typical attenuator. The amount of attenuation in a given network is determined by the ratio of: Output/Input.
For example, if the input voltage to a circuit is 1 volt (1V) and the output voltage is 1 milli-volt (1mV) then the amount of
attenuation is 1mV/1V which is equal to 0.001 or a reduction of 1,000th.
However, using voltage, current or even power ratios to determine or express the amount of attenuation that
a resistive attenuator network may have, called the attenuation factor, can be confusing, so for the passive
attenuator its degree of attenuation is normally expressed using a logarithmic scale which is given in decibels
(dB) making it easier to deal with such small numbers.
Degrees of Attenuation
An attenuators performance is expressed by the number of decibels the input signal has decreased per frequency
decade (or octave). The decibel, abbreviated to "dB", is generally defined as the logarithm or "log" measure of the voltage,
current or power ratio and represents one tenth 1/10th of a Bel. In other words it takes 10 decibels to make one bel. Then by definition,
the ratio between an input signal (Vin) and an output signal (Vout) is given in decibels as:
Note that the decibel (dB) is a logarithmic ratio and therefore has no units. So a value of -140dB represents
an attenuation of 1:10,000,000 units or a ratio of 10 million to 1!.
In passive attenuator circuits, it is often convenient to assign the input value as the 0 dB reference point. This
means that no matter what is the actual value of the input signal or voltage, is used as a reference with which to compare the output
values of attenuation and is therefore assigned a 0 dB value. This means that any value of output signal voltage below this reference
point will be expressed as a negative dB value, ( -dB ). So for example an attenuation of -6dB indicates that the value is 6 dB
below the 0 dB input reference. Likewise if the ratio of output/input is less than one (unity), for example 0.707, then this corresponds
to 20 log(0.707) = -3dB. If the ratio of output/input = 0.5, then this corresponds to 20 log(0.5) = -6 dB, and so on, with standard
electrical tables of attenuation available to save on the calculation.
A passive attenuator circuit has an insertion loss of -32dB and an output voltage of 50mV. What will be
the value of the input voltage.
The antilog (log-1) of -1.6 is given as:
Then if the output voltage produced with 32 decibels of attenuation, an input voltage of 2.0 volts is required.
Attenuator Loss Table
and so on, producing a table with as many decibel values as we require for our attenuator design.
This decrease in voltage, current or power expressed in decibels by the insertion of the attenuator into
an electrical circuit is known as insertion loss and minimum loss attenuator designs match circuits of unequal impedances
with a minimum loss in the matching network. Now that we know that the passive attenuator reduces or "attenuates"
the power or voltage level of a signal, while introducing little or no distortion and insertion loss, by an amount expressed in
decibels, we can begin to look at the different attenuator circuit designs available.
Passive Attenuator Designs
There are many ways in which resistors can be arranged in attenuator circuits with the
Potential Divider Circuit
being the simplest type of passive attenuator circuit. The potential or voltage divider circuit is generally known as an
"L-pad" attenuator because its circuit diagram resembles that of an inverted "L". But there are other common types of
attenuator network as well such as the "T-pad" attenuator and the "Pi-pad" (π) attenuator depending upon how you
connect together the resistive components. These three common attenuator types are shown below.
The above attenuator circuit designs can be arranged in either "balanced" or "unbalanced" form with the
action of both types being identical. The balanced version of the "T-pad" attenuator is called the "H-pad" attenuator while the
balanced version of the "π-pad" attenuator is called the "O-pad" attenuator. Bridged T-type attenuators are also available.
In an unbalanced attenuator, the resistive elements are connected to one side of the transmission line
only while the other side is grounded to prevent leakage at higher frequencies. Generally the grounded side of the attenuator
network has no resistive elements and is therefore called the "common line".
In a balanced attenuator configuration, the same number of resistive elements are connected equally to
each side of the transmission line with the ground located at a centre point created by the balanced parallel resistances.
Generally, balanced and unbalanced attenuator networks can not be connected together as this results in half of the balanced
network being shorted to ground through the unbalanced configuration.
Instead of having just one attenuator to achieve the required degree of attenuation, individual
attenuator pads can be connected or cascaded together to increase the amount of attenuation in given steps
of attenuation. Multipole rotary switches, rocker switches or ganged push-button switches can also be used to connect or bypass
individual fixed attenuator networks in any desired sequence from 1dB to 100dB or more, making it easy to design and construct
switched attenuator networks, also known as a step attenuator. By switching in the appropriate attenuators, the attenuation
can be increased or decreased in fixed steps as shown below.
Here, there are four independent resistive attenuator networks cascaded together in a series ladder
network with each attenuator having a value twice that of its predecessor, (1-2-4-8). Each attenuator network may be
switched "in" or "out" of the signal path as required by the associated switch producing a step adjustment attenuator
circuit that can be switched from 0dB to -15dB in 1dB steps and the total circuit attenuation is the sum of all four
attenuators switched "in". So for example an attenuation of -5dB would require switches SW1
and SW3 to be connected, and an attenuation of -12dB would require switches
SW3 and SW4 to be connected, and so on.
- 1. An attenuator is a four terminal device that reduces the amplitude or power
of a signal without distorting the signal waveform, an attenuator introduces a certain amount of loss.
- 2. The attenuator network is inserted between a source and
a load circuit to reduce the source signal's magnitude by a known amount suitable for the load.
- 3. Attenuators can be fixed, fully variable or variable in
known steps of attenuation, -0.5dB, -1dB, -10dB, etc.
- 4. An attenuator can be symmetrical or asymmetrical in form
and either balanced or unbalanced.
- 5. Fixed attenuators also known as a "pad" are used to "match"
- 6. An attenuator is effectively the opposite of an amplifier.
An amplifier provides gain while an attenuator provides loss, or gain less than 1 (unity).
- 7. Attenuators are usually passive devices made from simple
voltage divider networks. Switching between different resistances forms adjustable stepped attenuators and continuously adjustable
ones using potentiometers.
Fixed value attenuators are also called "attenuator pads" and are used in radio frequency (Rf) transmission
lines to lower voltage, dissipate power, and to improve the impedance matching between circuits. Line-level attenuators in
pre-amplifier or power amplifier can be as simple as a 0.5 watt potentiometer, or voltage divider L-pad designed to reduce the
amplitude of the audio signal that reaches the speaker, reducing the volume of the output. In measuring signals, high power
attenuator pads are used to lower the amplitude of the signal a known amount to enable measurements, or to protect the measuring
device from high signal levels that might damage it.
In the next tutorial about Attenuators, we will look at the most basic type of resistive
attenuator network commonly called a "L-type" or "L-pad" attenuator which can be made using just two resistive components. The
"L-pad" attenuator circuit can also be used as a voltage or potential divider circuit.