What is the formula for T-Pad Series Resistor?

The T-Pad Series Resistor formula is R1 = R2 = Z × (K − 1)/(K + 1), where R1—Each series arm resistor (Ω); Z—Characteristic (source = load) impedance (Ω); dB—Desired attenuation; K = 10^(dB/20).

What does R1 mean in the T-Pad Series Resistor formula?

R1 is Each series arm resistor (Ω).

T-Pad Series Resistor

Q: How do you calculate T-Pad Series Resistor?

Use the formula R1 = R2 = Z × (K − 1)/(K + 1). A symmetric T-pad attenuator uses two equal series resistors and one shunt resistor to provide a set attenuation while presenting the characteristic impedance Z to both the source and the load. Enter the values you know in the calculator above and it solves for the remaining variable.

R1 = Z · (K−1)/(K+1)

Calculator

Series Resistor

Characteristic Impedance

Attenuation

Result

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Formula

R1 = R2 = Z × (K − 1)/(K + 1)

Description

A symmetric T-pad attenuator uses two equal series resistors and one shunt resistor to provide a set attenuation while presenting the characteristic impedance Z to both the source and the load. This formula gives the value of each series arm. The T-pad is the standard matched attenuator for equal source and load impedances, used throughout RF and audio test gear.

Variables

R1 — Each series arm resistor (Ω)
Z — Characteristic (source = load) impedance (Ω)
dB — Desired attenuation; K = 10^(dB/20)

Practical Notes

Pair this with the T-pad shunt resistor R3 = 2ZK/(K²−1). Because the pad is matched, it both attenuates and maintains impedance, avoiding reflections on transmission lines.

Related Concepts

All Attenuators & Pads formulas →

Attenuation Factor (K) T-Pad Series Resistor T-Pad Shunt Resistor Pi-Pad Series Resistor Pi-Pad Shunt Resistor Bridged-T Bridge Resistor Bridged-T Shunt Resistor

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