Last edited: 2024-10-27 18:28:33

**What is a varactor? The varactor, which stands for variable capacitor, is a diode with a well defined capacitance that can be controlled with the reverse voltage applied to it.**

The varactor's depletion region width $d$, seen in the illustration of the varactor diode in the figure above, increases when the reverse voltage over the diode increases and can be likened to the distance between the plates of a plate capacitor being increased as seen in

$C = \frac{A \varepsilon}{d},$where $C$ is the capacitance, $A$ is the cross section area of the plate capacitor, $\varepsilon$ the dielectric constant and $d$ is the distance between the two plates. The increase of $d$ due to the increase of reverse voltage causes the capacitance of the varactor to decrease. These characteristics enable the varactor's capacitance to be controlled and set to a specific value. All varactor diodes do however have a maximum allowed reverse voltage known as the breakdown voltage. Exceeding the breakdown voltage could break the varactor which is why the reverse voltage must be kept below this limit.

The varactor's circuit can be seen in the figure above. As can be seen, it is a combination of the symbol of a diode and a capacitor.

The figure below shows the SPICE (Simulation Program with Integrated Circuit Emphasis) model used for modeling the varactor as an equivalent circuit. It is derived from *Skyworks Solutions Inc*'s datasheets. The SPICE model contains parasitic components $C_p$ and $R_s$ and the package inductance $L_s$. The difference compared to *Skyworks Solutions Inc*'s datasheet is the change from the diode model to a simple variable capacitance $C_\text{var}$.

While the SPICE model shown in the figure above works for simulating S-parameters, a nonlinear model that accounts for the voltage swing across the varactor is required if more accurate results are wished for. The circuit simulation software *Advanced Design System* provides a solution for this using a built-in component called Symbolically Defined Devices (SDD). The SDD enables the creation of equation-based nonlinear components that are a function of the port voltages, currents, and their derivates. In the figure below an alternative schematic using the SDD can be seen.

To model the nonlinear varactor using the SDD, an equation describing the capacitance as a function of reverse voltage $C(v)$ is required. This can either be obtained by fitting a curve to measured data or, if available, be found in the datasheet of the varactor. Integrating $C(v)$ with respect to $v$ gives the charge $Q(v)$ as a function of reverse voltage which can be used as input for the SDD.

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