The values in the above two equations are reactances of the inductor (L) and capacitor (C) at the circuit’s operating frequency. Therefore, we have two important definitions we can use to calculate the impedance ratio: The equivalent impedance drop across the bias tee (measured from RFOUT to the output side of the capacitor) needs to match the transmission line LO_OUT The impedance looking into the DC port should be much larger than the impedance looking into the RF output This impedance ratio needs to be included in the design of the bias tee under the following design targets: The impedance ratio in a bias tee refers to the ratio between the impedance along the DC path and the impedance seen by the signal as it travels along the output path. Impedance Ratio in a Simple Bias Teeīias tees are designed such that we attempt to hit a particular impedance ratio. The DC signal is simply the power being provided to the VDD pin, which will power the amplifier, and the internal circuitry of the amplifier will route the input power in the required manner. The concept is simple the inductive impedance of the inductor blocks the AC signal, while the capacitor blocks the DC signal from passing to the load in this system. The circuit that was used previously is shown below. In the earlier example with our RF power amplifier, a bias tee was used to provide power to the amplifier, while also passing RF output power from the same pin. The simplest type of bias tee is a 2-input device an inductor is used to pass DC power from one input port, and a capacitor is used to pass AC power to the output. What is a Bias Tee?Ī bias tee is a simple circuit that uses an inductor and a capacitor to direct the flow of AC and DC power. These devices are available both as high-power modules for long-range broadcasting, but they can be built as small circuits on a PCB for lower-power RF systems. In this article, I’ll outline how to design this circuit and what the important parameters are for ensuring high-power propagation into your load component. In a previous project with an RF power amplifier layout, there is a bias tee in the design that is used to provide DC power to the amplifier while also allowing RF power to propagate towards the output SMA connector. It is a 2-input, 1-output device that uses reactive elements to provide a power flow along different directions. RF bias tees are used to separate DC and AC power along an interconnect.

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