T-Pad Attenuator
Pi-Pad Attenuator (π)
L-Pad Attenuator
Resistor Attenuator Calculator
A resistor attenuator is a passive network that reduces a signal’s amplitude by a precise number of decibels while maintaining impedance matching between source and load. Attenuators appear in RF, audio, video, and telecom systems wherever you need to drop signal level without distortion or mismatch. The calculator above takes your desired attenuation in dB, the source and load impedances, and the topology (T-pad, Pi-pad, or L-pad) and returns the exact resistor values along with the voltage ratio and power ratio.
Attenuation Basics
dB = 10 × log10(Pout / Pin) — power ratio in decibels
Voltage ratio = 10(−dB/20)
Power ratio = 10(−dB/10)
6 dB halves the voltage (ratio = 0.5). 20 dB reduces voltage to 10% (ratio = 0.1). 3 dB halves the power. These ratios are independent of impedance — decibels describe the ratio, not the absolute level. For more on signal levels, see our Decibel Calculator.
T-Pad Attenuator
The T-pad uses three resistors: R1 in series at the input, R3 in series at the output, and R2 shunting to ground between them. It is the most common topology for RF and general-purpose attenuators because it provides impedance matching in both directions.
Symmetric T-Pad (Zs = Zl = Z0)
When source and load impedance are equal, R1 = R3 and the formulas simplify. Let K = 10(dB/20):
R2 = Z0 × 2K / (K² − 1)
Worked Example — 50 Ω / 6 dB T-Pad
R1 = R3 = 50 × (1.9953 − 1) / (1.9953 + 1) = 50 × 0.9953 / 2.9953 = 16.6 Ω
R2 = 50 × 2 × 1.9953 / (1.9953² − 1) = 50 × 3.9906 / 2.9812 = 66.9 Ω
Use standard E24 resistors: R1 = R3 = 16 Ω, R2 = 68 Ω. This gives a 50 Ω matched attenuator that reduces the signal by approximately 6 dB — a common pad for reducing signal generator output to protect sensitive receiver inputs in RF test setups.
Asymmetric T-Pad (Zs ≠ Zl)
When source and load impedances differ, R1 ≠ R3. The general equations use both Zs and Zl with the attenuation factor K. The calculator handles this automatically — enter the two impedances and the desired dB, and it returns all three resistor values. This is common when interfacing 50 Ω RF equipment with 75 Ω video systems.
Pi-Pad Attenuator
The Pi-pad (π-pad) uses two shunt resistors (R1 at the input, R3 at the output) and one series resistor (R2) between them. It is the electrical dual of the T-pad — same attenuation and impedance matching, different resistor arrangement. The Pi-pad is preferred when shunt elements are easier to implement in the circuit layout or when the physical construction favours parallel connections (e.g. stripline or microstrip RF boards).
Symmetric Pi-Pad (Zs = Zl = Z0)
R2 = Z0 × (K² − 1) / 2K
Worked Example — 75 Ω / 10 dB Pi-Pad
R1 = R3 = 75 × (3.1623 + 1) / (3.1623 − 1) = 75 × 4.1623 / 2.1623 = 144.4 Ω
R2 = 75 × (3.1623² − 1) / (2 × 3.1623) = 75 × 9.0 / 6.3246 = 106.7 Ω
Standard values: R1 = R3 = 150 Ω, R2 = 110 Ω. This builds a 75 Ω matched 10 dB pad suitable for video and cable TV signal paths where 75 Ω impedance is the system standard.
L-Pad Attenuator
The L-pad uses only two resistors: R1 in series and R2 shunting to ground. It is simpler and cheaper but only provides impedance matching looking into one direction — the source sees the correct impedance, but the load does not (or vice versa). L-pads are common in audio speaker applications where the amplifier output impedance is low and the goal is to reduce volume to a specific speaker without affecting the amplifier’s load.
L-Pad Formulas
R2 = Zs × K / (K − 1) — shunt resistor
Because the L-pad does not match impedance in both directions, it is not suitable for systems where reflections matter (RF, transmission lines). For audio and low-frequency applications where impedance matching is less critical than simplicity and cost, L-pads work well.
Choosing the Right Topology
Pi-pad — same performance as T-pad, different layout. Preferred when shunt components are easier to place (stripline, microstrip, connectorised attenuators).
L-pad — two resistors, unidirectional match. Best for audio speaker volume control and situations where cost and simplicity outweigh bidirectional matching.
For symmetric impedances (Zs = Zl), T-pad and Pi-pad give identical attenuation with the same number of components — choose based on layout convenience. For asymmetric impedances (e.g. 50 Ω to 75 Ω), both T and Pi topologies handle the mismatch; the L-pad cannot unless attenuation and impedance ratio align to specific combinations.
Common Impedance Standards
50 Ω — RF test equipment, antennas, coaxial systems (SMA, BNC, N-type). Most laboratory and military RF work uses 50 Ω.
75 Ω — video, cable TV, satellite, and broadcast (F-type, BNC-75). Chosen for minimum signal attenuation in coaxial cable.
600 Ω — legacy audio and telecom balanced lines. Still used in broadcast audio, studio wiring, and telephone trunk circuits.
Worked Example — 600 Ω / 20 dB T-Pad
R1 = R3 = 600 × (10 − 1) / (10 + 1) = 600 × 9/11 = 490.9 Ω
R2 = 600 × 20 / (100 − 1) = 12000 / 99 = 121.2 Ω
Standard values: R1 = R3 = 490 Ω (or 470 Ω + 22 Ω in series), R2 = 120 Ω. This attenuator drops a 600 Ω audio line by 20 dB — reducing voltage to 10% of the input — while keeping the impedance matched at both ports.
Power Handling and Resistor Selection
The series resistors in a T-pad (or the series resistor in a Pi-pad) carry the full signal current and dissipate the most power. For a 1 W input signal through a 6 dB attenuator, about 750 mW is absorbed by the resistor network — split unevenly across the three resistors. The calculator shows the power dissipated in each resistor so you can select the correct wattage rating.
For RF attenuators, use non-inductive resistors (metal film or thin film) to maintain performance at high frequencies. Carbon composition and wirewound resistors have parasitic inductance and capacitance that degrade attenuation accuracy above a few MHz. Surface-mount thin-film resistors in 0402 or 0603 packages work up to several GHz.
Cascading Attenuators
Attenuators in series add in dB. A 6 dB pad followed by a 10 dB pad gives 16 dB total attenuation. This is useful when you need an attenuation value that does not yield convenient resistor values as a single stage — build it from two standard stages instead. As long as each stage is impedance-matched, cascading introduces no additional error.
Frequently Asked Questions
Last updated: March 2026