What is the formula for Bridged-T Bridge Resistor?

The Bridged-T Bridge Resistor formula is Rb = Z × (K − 1), where Rb—Bridge resistor across the series arms (Ω); Z—Characteristic impedance = series-arm value (Ω); dB—Desired attenuation; K = 10^(dB/20).

What does Rb mean in the Bridged-T Bridge Resistor formula?

Rb is Bridge resistor across the series arms (Ω).

Bridged-T Bridge Resistor

Q: How do you calculate Bridged-T Bridge Resistor?

Use the formula Rb = Z × (K − 1). A bridged-T attenuator keeps the two series arms equal to the characteristic impedance Z and varies only two resistors—the bridge resistor across the top and the shunt resistor to ground—to set the attenuation while staying matched. Enter the values you know in the calculator above and it solves for the remaining variable.

Rb = Z · (K − 1)

Calculator

Bridge Resistor

Characteristic Impedance

Attenuation

Result

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Formula

Rb = Z × (K − 1)

Description

A bridged-T attenuator keeps the two series arms equal to the characteristic impedance Z and varies only two resistors—the bridge resistor across the top and the shunt resistor to ground—to set the attenuation while staying matched. This formula gives the bridge resistor. Because only two resistors change with attenuation, the bridged-T is the basis of constant-impedance variable (stepped) attenuators.

Variables

Rb — Bridge resistor across the series arms (Ω)
Z — Characteristic impedance = series-arm value (Ω)
dB — Desired attenuation; K = 10^(dB/20)

Practical Notes

The matching shunt resistor is Rs = Z/(K−1), so Rb × Rs = Z². Switching just these two resistors gives calibrated step attenuators with constant input/output impedance.

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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