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Passive Attenuators

Passive Attenuators

A Passive Attenuator is a special type of electrical or electronic bidirectional circuit made up of entirely resistive elements.

Passive attenuators are basically two port resistive networks 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.

A passive 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 Passive 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.

Attenuator Connection

passive attenuators connection

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

simple passive attenuators

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 (B). 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:

Decibel Attenuation

decibel attenuation

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.

Passive Attenuators Example No1

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.

passive attenuators calculation

The antilog (log-1) of -1.6 is given as:

attenuator antilog

Then if the output voltage produced with 32 decibels of attenuation, an input voltage of 2.0 volts is required.

Attenuator Loss Table

Vout/Vin 1 0.7071 0.5 0.25 0.125 0.0625 0.03125 0.01563 0.00781
Log
Value
20log
(1)
20log
(0.7071)
20log
(0.5)
20log
(0.25)
20log
(0.125)
20log
(0.0625)
20log
(0.03125)
20log
(0.01563)
20log
(0.00781)
in dB’s 0 -3dB -6dB -12dB -18dB -24dB -30dB -36dB -42dB

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 what a passive attenuator is how it can be used to reduce or “attenuate” 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 Attenuators Design

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.

Attenuator Types

passive attenuators type

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 center 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.

Switched Passive Attenuators

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. Multi-pole 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.

Switched Attenuator

switched attenuator

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.

Therefore, the total amount of attenuation provided by the circuit would be the sum of all four attenuators networks that are 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.

Passive Attenuators Summary

  • 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.
  • 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.
  • Attenuators can be fixed, fully variable or variable in known steps of attenuation, -0.5dB, -1dB, -10dB, etc.
  • An attenuator can be symmetrical or asymmetrical in form and either balanced or unbalanced.
  • Fixed attenuators also known as a “pad” are used to “match” unequal impedances.
  • An attenuator is effectively the opposite of an amplifier. An amplifier provides gain while an attenuator provides loss, or gain less than 1 (unity).
  • Attenuators are usually passive devices made to from simple voltage divider networks. The switching between different resistances produces adjustable stepped attenuators and continuously adjustable ones using potentiometers.

To simplify the design of the attenuator, a “K” (for constant) value can be used. This “K” value is the ratio of the voltage, current or power corresponding to a given value of dB attenuation and is given as:

K factor equation

We can produce a set of constant values called “K” values for different amounts of attenuation as given in the following table.

Attenuator Loss Table

dB 0.5 1.0 2.0 3.0 4.0 5.0 6.0 10.0 20.0
“K” value 1.0593 1.1220 1.2589 1.4125 1.5849 1.7783 1.9953 3.1623 10.000

and so on, producing a table with as many “K” values as we require.

Fixed value passive attenuators, called “attenuator pads” are used mainly in radio frequency (Rf) transmission lines to lower voltage, dissipate power, or to improve the impedance matching between various mismatched circuits.

Line-level attenuators in pre-amplifier or audio power amplifiers can be as simple as a 0.5 watt potentiometer, or voltage divider L-pad designed to reduce the amplitude of an audio signal before it 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 otherwise 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.

39 Comments

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  • Three-way speaker

    Thanks again. Attenuators is another term for antenna. They go opposite effect to amplifiers. I was just curious, i know you don’t need to answer this because this is for engineers only. But technicians need to know this. The constant yellow wire in Car audio is from the battery and antenna. This antenna/12 volts: How much power added does it supply to the after market car amplifier? In terms of providing the necessary voltage to have a signal conduction? Added boost. They connect the yellow constant power wires to the amplifier to give it added power. If in case the radio or pre-amplifier, doesn’t work. No sounds. Thanks…

    • Wayne Storr

      Ref. ” Attenuators is another term for antenna. ” – No it is not.

      Another term for an Antenna is “Aerial” – Both convert electromagnetic fields into ac signals of the same frequency. (transmission is the reverse).

      An Attenuator is a network of resistors, capacitors, or both which reduces a signals amplitude in precise, predetermined steps.

      Then they are not the same.

  • Musse Abdulle

    send me this note please.
    thanks

  • Royal Rajputana

    Yes

  • Peter

    “attenuation is 1mV/1V which is equal to 0.001 or a reduction of 1,000th.”

    no it’s not a reduction of 1000th, it is a reduction of 1000 or 1000th of the original signal. (A reduction of 1000th would give 0.999 of the original signal.

  • Eng mohamed Abdiraxmaan

    Waa cashar aad uwanaagsan

  • nani

    how can we download the pdf version

  • Kelly

    These are really good tutorials. Hats off! I have two questions:
    1) To attenuate a guitar amplifier, say a Marshall 1959 Superlead (~100W), which type of attenuator would be ideal, L, T or PI?
    2) While the equations and examples provide good information on the component resistances required to provide a given level of signal reduction, there appears to be no information on the power rating required of the restive elements. How is the power rating of the different resistive networks determined?

  • jama abdirizak yasiin

    hi evryone

  • John Killam

    I want to create a commercial getter to defuse t v commercials that are to broadcast at higher decibels.Any thoughts welcomed!

  • Kostas

    Great articles, I’m glad I stumbled on your website .

    Supposing we want a fixed attenuation and not a variable one (no potentiometers/voltage dividers) , what is the benefit of using resistors in parallel between the + line and the common line? Can we just use a single resistor (R1) in series in the + line?

    Also what is the benefit of the T and Pi configurations?

    Lastly, I think it could be relevant to mention in this page that the resistors used in attenuators should be picked so that they can handle high power.

  • Conny

    If i want build an attenuator for 16 ohm -3db I get these values from a site:
    R1 4.67 ohm
    R2 38.78 ohm

    My question are, how close to the values do I need to be? is 5 ohm close enough? or is it spot on to the values?

    Conny

    • Wayne Storr

      You can use “preferred values” of 4.64 ohms (E96) or 4.64 ohms (E192) if you want a more accurate attenuation. Using a 5.1 ohm resistor would not give an exact -3dB attenuation.

  • Ahmed El-Qady

    that’s great thanks a lot.

  • david

    what is a correct attenuator types to build a cable to match a trs line out to trs mic in (external recorder like a Zoom to a dslr)

  • StarBridge

    How to solve a star network. 4 identical resistors RA on the +X, -X,+Y, -Y coordinates. Then, 2 identical resistors RB in series connect the -X & + Y on Q2. Lets call the mid point of this intersection point A, which is 45 deg from axis center. Two more identical resistors RB in series connect the +X & -Y on Q4. Lets call the midpoint of this intersection point B, which is 45 deg from axis center. (reflection). A resistor RC connects -X & +X. A potentiometer connects points A &B. Power applied at +Y & -Y and output comes from -X & +X.

  • Ahmed

    that’s great thanks a lot, I really appreciate that. this is amazing

  • Manjunatha GN

    good for beginner.

  • Kuldeep Verma

    Ckt to dedect fat of milk

  • khiem T

    Do they require for Attenuator yearly calibration for example: attenuator 25w-3db? How is calibration with attenuator is solid state shield 100%?

  • Brijesh kumar B Sharma

    Quit informative, useful.

  • s. kumar

    very useful informations for the beginner.

    thank you