Bias Tee - Design

Design

Conceptually, the bias tee can be viewed as an ideal capacitor that allows AC through but blocks the DC bias and an ideal inductor that blocks AC but allows DC. Although some bias tees can be made with a simple inductor and capacitor, wideband bias tees are considerably more complicated because practical components have parasitic elements.

Bias tees are designed for transmission line environments. Typically, the characteristic impedance Z0 will be 50 ohms or 75 ohms. The impedance of the capacitor (XC) is chosen to be much less than Z0, and the impedance of the inductor (XL) is chosen to be much greater than Z0.

X_C = {1 \over j \ \omega \ C} = {1 \over j \ 2\pi \ f \ C} \ ; \ X_L = j \ \omega \ L = j \ 2\pi \ f \ L

Where ω is the frequency in radians per second and f is the frequency in hertz.

Bias tees are designed to operate over a range of signal frequencies. The reactances are chosen to have minimal impact at the lowest frequency.

For wide range bias tees, the inductor must be large at the lowest frequency. A large inductor will have a stray capacitance (which creates its self-resonant frequency). At a high enough frequency, the stray capacitance presents a low impedance shunt path for the signal, and the bias tee becomes ineffective. Practical wide band bias tees must use circuit topologies that avoid the shunt path. For example, a Picosecond Pulse Labs 5580 works from 10 kHz to 15 GHz. (Andrews 2000, p. 3) Consequently, the simple design would need an inductance of at least 800 μH (XL about j500 ohms at 10 kHz), and that inductor must still look like an inductor at 15 GHz. However, a commercial 820 μH inductor has a self-resonant frequency of only 1.8 MHz—four orders of magnitude too low.

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