Practical Bias Tee Calculator
Practical Bias Tee Calculator
Taylor Microwave, Inc. · Circuit sizing and RF performance
Your tested baseline: C1 = 1 µF and L1 = 3.3 mH at 5 kHz, with 50 Ω RF ports. C2 = 10 µF is an example; enter your actual bypass capacitance.
Values remain fixed while you change frequency. The sizing button adjusts only C1 and L1. The DC-port RF termination defaults to 50 Ω for instrument-style isolation measurements; use your supply’s estimated RF resistance to model operation with that supply.
Equations, definitions, and practical limits
All RF results are evaluated at the entered frequency. Both RF ports use the same reference impedance. C2 remains connected across the DC port.
Zseries = ESR1 + 1/(jωC1). Zbypass = ESR2 + 1/(jωC2). Znode = Zbypass ∥ RDC-port. Zbranch = RL + jωL1 + Znode.
ABCD matrix: A = 1 + Zseries/Zbranch; B = Zseries; C = 1/Zbranch; D = 1. Denominator = A + B/Z0 + CZ0 + D. S21 = 2/denominator; S11 = (A + B/Z0 − CZ0 − D)/denominator; S22 = (−A + B/Z0 − CZ0 + D)/denominator.
For DC-port isolation, H = Znode/Zbranch. S31 = S21 × H × √(Z0/RDC-port); S32 = (1 + S22) × H × √(Z0/RDC-port). Isolation = −20 log₁₀|S31| or −20 log₁₀|S32|. The undriven RF port is matched. DC-port reference impedance equals its entered RF termination. Use 50 Ω for all ports for conventional 50 Ω S-parameter measurements. A 0 Ω DC termination has no power-wave isolation definition.
Higher return loss and isolation indicate less reflection and leakage, respectively. Lower insertion loss means better RF transmission. At Port 1, incident power divides among transmission to Port 2, reflection, leakage into the DC termination, and internal component loss.
Baseline scaling: C1 (µF) = (5/fkHz)(50/Z0); L1 (mH) = (16.5/fkHz)(Z0/50). This preserves your tested reactance ratios; it does not guarantee a universal minimum or bandwidth.
Required DC supply = DUT DC voltage + I × L1 DCR. DC-port RF resistance is not used as a DC series resistor. Bypass-capacitor leakage and other wiring resistance are excluded.
RF voltage at the matched DUT = √(delivered power × Z0); RF peak = √2 × RMS. RF voltage is not added to the required DC supply. The bypass capacitor must be rated for the DC supply voltage and RF/transient stresses.
This lumped model excludes component self-resonance, stray capacitance, layout, saturation and frequency-dependent supply reactance. Enter ESR and winding resistance at the operating frequency. The default RF winding resistance equals the example DCR and excludes additional AC/core losses. L1 and C2 can resonate and strongly load the RF line. Results are estimates, not measured specifications.