L-type Attenuator / L-pad

In its basic form, the L-pad Debilitating is a very simple voltage dividing network used in many electrical and electronic circuits to produce a lower voltage.The difference this time is that this type of debilitator is used to create a weakening of a transmission line in frequency-dependent circuits or to match impedances of unstable welding and load networks.

The L-pad debilitator consists of two resistant elements in series connected by the ratio between these two resistors, which form a network of voltage dividers, along a voltage source, as shown below.

We can see that the debilitating design of the L-pad is the same as the voltage dividing circuit used to slightly reduce the input voltage. While the two resistors are serially connected throughout the entire input voltage, the output signal or voltage is taken only through one resistance, the two resistant elements form the shape of an inverted letter "L" and therefore their name "L". For this type of circuitry, slimming is given as Vout/Vin.

Input resistance is serial with R1 output, and resistance R2 is parallel to output and therefore load. Then the output voltage provided by this "L" arrangement is divided into a factor equal to the ratio of these two resistance values, as shown.

Since the L-pad is made of debilitating fully resistant components, there is no phase shift in the debilitator. Placing the debilitator between the source and the load should not change the welding voltage and therefore the resistance the source sees should always remain the same. Since the two resistance elements have constant values, if the impedance of the load is not infinite, weakening occurs and therefore its impedance changes. As a result, the L-pad debilitating can provide only one impedance match in one direction.

L-pad debilitators are widely used in audio applications to reduce a larger or stronger signal to ensure maximum power transfer when matching impedance between source and load. However, if the impedance of the weld is different from the impedance of the load, the L-pad debilitating can be made to fit both impedances together, not separately.

This is due to the fact that the arrangement of resistant elements does not produce the same impedance when viewed from both sides of the network. In other words, the L-pad is a debilitating asymmetric debilitating, and therefore, if a weakening network is required to match two unequal impedances in both directions, other types of debilitating, such as the symmetrical "T-pad" or "Pi-pad", should be used.

L-pad Debilitating with Equal Impedance

As we mentioned earlier, a passive debilitator is a resistant network designed to reduce the power or signal level of an audio or radio frequency signal without creating any distortion of the signal. Sometimes the output of a sound amplifier can be very high, and it is necessary to weaken this signal to feed it to a speaker. For example, suppose we want to reduce the power transmitted from an amplifier with output source impedance (ZS) 8Ω, which feeds a speaker load (ZL) 8Ω, by 6dB. The values of R1 and R2 resistances are as follows.

Looking at the welding impedance direction, this will be the equation for the L-pad debilitating circuit, which connects between two equal impedances (ZS = ZL):

A "K" value can be used in the debilitating equation above to simplify the design and mathematics of the debilitating. This "K" value is the ratio of voltage, current or power corresponding to a certain weakening value. The general equation for "K" is given as follows:

In other words, the value "K" for a voltage weakening of 6dB in our example will be 10(6/20) = 1.9953. For slimming, let's put this value into two equations:

Then, between the two equal impedances facing the welding impedance ZS direction, the value of the serial resistance is R1 4Ω and the value of the parallel resistance is R2 8Ω.

The problem with this type of L-pad debilitating configuration is that the impedance pairing is in the direction of serial resistance R1, and the impedance "mismatch" is towards parallel resistance R2. The problem with this is that as the level of weight loss increases, this dissonance becomes larger and the value of parallel resistance at high slimming values becomes the fractions of an Ohm.

For example, at a weakening of -32dB, the R1 and R2 values are 7.8Ω and 0.2Ω, which effectively short-circuits the speaker at 200mΩ, which can have a serious impact on the amplifier's output circuit.

One way to increase weight loss without overloading the source is to pair impedance, load impedance with circuit in the direction of ZL. But now that we are looking at the L-pad debilitating circuit by parallel resistance, the equations are somewhat different. Then, with impedance matching between equal impedances and from the load, the values or resistors of R1 and R2 are calculated as follows:

If we know that it removes weakening to -32dB, the value of the resistors is R1 = 310Ω and R2 = 8.2Ω, respectively, and these values are safe enough for the welding circuit to which it is connected.

Unequal Impedance L-pad Debilitating

So far, we've looked at connecting the L-pad Debilitator between equal impedances to weaken a signal. But we can also use the "L-pad debilitator" to match the impedances of the two unequal circuits. This impedance pairing may be in the direction of larger or smaller impedance, but not in both. The debilitating configuration will be the same as before, but the equations used to match the two unequal impedances are different as shown.

Between two unequal impedances, the impedance match is towards the smaller of the two impedances from the source.

Correct Impedance Match to Small Impedance

Between two unequal impedances, the impedance pairing is towards the larger of the two impedances from the load.

Correct Impedance Match to Larger Impedance

L-pad Debilitating Sample

A signal transmission line with a 75Ω welding impedance will be connected to a signal strength meter with a maximum of -12dB representation of 50Ω impedance. Calculate the values of the resistors required in an L-pad debilitating circuit to operate the meter at maximum power.

With impedance matching towards a smaller value of 50Ω, the resistances R1 and R2 are calculated as follows.

Resistance R1 equals 59.6Ω and R2 equals 22.2Ω or the nearest preferred values.

The L-pad can be used to perfectly match one impedance to another by providing a debilitating, constant amount of slimming, but the resulting circuit becomes "lossy". However, if a constant amount of attenuation is not significant and only minimal insertion loss is required between welding and load, the L-pad debilitator can be used to match two unequal impedances using the following equations to calculate resistors.

Minimum InsertIon Loss

Resistance R1 is on the side of the larger impedance, and resistance R2 is on the side of the smaller impedance, and in our example above this will be 75Ω and 50Ω respectively. Therefore, the minimum insertion loss in decibels of an L-pad debilitator that binds between a source and a load is given as follows:


In this tutorial, we found that an L-pad debilitating circuit is a passive and fully resistant network that can be used to reduce the power of a signal when matching impedances of welding and load. L-pad debilitators are widely used in audio electronics to reduce the sound signal generated by an amplifier transmitted to a speaker or headphones.

However, one of the main drawbacks of the "L-pad debilitator" is that the L-pad debilitator is a fixed impedance device, so at low power settings the debilitator converts all energy that is not sent to the load into heat. In addition, at much higher frequencies or when a debilitating circuit is required, it perfectly matches the input and output, and other improved attenuating designs are used.

In the next lesson on Passive Attenuators, we will look at another debilitating design called T-pad Debilitating, which uses three resistant components to produce a balanced debilitator.