Bridged-Tee Attenuator Calculator

Attenuation (dB)

Impedance (Z₀)

R₁

R₂

* Formulas assume equal source and load impedance (Z₀).

Let A_dB be the attenuation in dB.

Voltage loss ratio: L = 10^{A_dB / 20}

Resistor: R₁ = Z₀ × (L − 1)

Resistor: R₂ = Z₀ × L / (L − 1)

Bridged-T attenuator in level control and audio/RF chains

A bridged-T attenuator uses four resistors arranged as a T-network with a “bridge” resistor across the series arms. Properly designed, it can provide adjustable or fixed attenuation while keeping the input and output impedance nearly constant.

Compared with simple series pads, the bridged-T topology is often used in:

Precision stepped attenuators for test equipment
Audio line-level controls with constant load impedance
RF level pads where impedance matching must be preserved
Multi-stage attenuator ladders in measurement front-ends

Topology and resistor roles

A bridged-T network can be visualized as:

Two series resistors in the main signal path (R_s1 and R_s2)
One shunt resistor to ground at the junction of the series arms (R_sh)
One bridge resistor connecting input and output nodes (R_b)

When the values are chosen according to the desired attenuation and system impedance Z₀, the input and output both “see” approximately Z₀ over the operating band.

Attenuation and impedance targets

For a symmetrical bridged-T attenuator in a system with characteristic impedance Z₀ and target attenuation A_dB, the key design goals are:

Input impedance ≈ Z₀
Output impedance ≈ Z₀
Voltage attenuation = A_dB at the design frequency

Let the voltage attenuation ratio be:

K = 10^{(A_dB / 20)}

The calculator uses standard bridged-T design equations to solve for the four resistor values so that both impedance and attenuation criteria are satisfied for the specified Z₀ and K.

Advantages of bridged-T over simple pads

While simpler π or T pads are often sufficient, the bridged-T network offers useful benefits in some applications:

Better constant-impedance behavior across a range of switched settings when used in stepped attenuators.
Reduced insertion loss variation when stages are cascaded, maintaining more predictable gain structure.
Convenient implementation of high-attenuation steps without excessively large resistor ratios.

In many audio and measurement systems, multiple bridged-T sections are combined on rotary switches to form accurate, low-distortion level controls.

Using the Bridged-T Attenuator Calculator

Enter the system impedance: Specify Z₀ according to your line or load (e.g. 50 Ω RF, 75 Ω video, 600 Ω legacy audio).
Set the desired attenuation: Enter A_dB in decibels for the attenuation you require at that stage.
Read the resistor values: The calculator outputs values for R_s1, R_s2, R_sh and R_b for a constant-impedance bridged-T network.
Map to real components: Choose the nearest standard resistor values and check the resulting attenuation and impedance with a simulator or measurement if high accuracy is required.

Worked example – 20 dB bridged-T in a 50 Ω system

Z₀ = 50 Ω
A_dB = 20 dB → K = 10^(20/20) = 10

With these inputs, the calculator determines a set of resistor values such that:

The input and output both measure close to 50 Ω
The through signal is attenuated to 1/10 of its original voltage (1/100 in power)
Power is shared among the resistors according to their position in the network

You can then round the results to preferred values (E24/E96 series), recalculate the exact attenuation, and choose resistor power ratings appropriate to the maximum signal level.

Design tips for bridged-T attenuators

Start with realistic attenuation steps: Common step sizes are 1 dB, 2 dB, 3 dB, 6 dB, 10 dB and 20 dB. Extremely high or very small steps may require tight-tolerance parts.
Consider total chain loss: In a multi-stage ladder, overall insertion loss adds up. Plan the gain structure so upstream and downstream stages remain in their linear region.
Check power dissipation: For RF or line-level applications, calculate the power in each resistor at maximum level, and specify adequate wattage and thermal management.
Layout matters at high frequency: Keep leads short, minimize stray capacitance and inductance, and respect the characteristic impedance of the PCB or coaxial environment.

With the bridged-T attenuator calculator, you can move directly from target attenuation and line impedance to practical resistor values, making it easier to construct accurate step pads, gain trims and level controls for RF, audio and test systems.

FAQ about Bridged-Tee Attenuator Calculator

Why would I choose a bridged-T attenuator instead of a simple π or T pad?

Bridged-T networks are especially useful when you need:

Better constant-impedance behavior over multiple switch positions
High attenuation steps without extreme resistor ratios
Precision stepped attenuators for test equipment or audio control

Simple π or T pads are easier for single fixed values, but bridged-T is often preferred in multi-stage, precision applications.

Does a bridged-T attenuator work for both RF and audio frequencies?

Yes. The underlying resistive network is frequency-independent in theory.

At RF, layout parasitics and connector geometry become important; at audio, noise, distortion and absolute resistor values matter more.

As long as you design and lay out the network appropriately, the same bridged-T topology works well across a wide range of frequencies.

How accurate is the attenuation after I round to standard resistor values?

Rounding to E12/E24/E96 values introduces small deviations from the ideal design.

For many practical uses, an error of ±0.2–0.5 dB is acceptable.

If you need very tight accuracy:

Use 1% (or better) resistors
Simulate the exact rounded network
Adjust values iteratively to minimize the attenuation error and keep the impedance close to Z<sub>0</sub>.