Varactor C–V Tuning
A varactor (varicap) diode is a voltage-controlled capacitor: reverse bias widens the junction depletion layer and lowers the capacitance. This tool computes the capacitance at the low and high bias limits, the capacitance tuning ratio, and — with a resonator inductance — the resulting frequency tuning range for a VCO or tuned circuit.
Equations & Parameters ▸
\(C(V) = \dfrac{C_0}{\left(1 + V/\varphi\right)^{n}} \qquad \dfrac{C_{\max}}{C_{\min}} = \left(\dfrac{1+V_{\max}/\varphi}{1+V_{\min}/\varphi}\right)^{n}\)
\(f(V) = \dfrac{1}{2\pi\sqrt{L\,C(V)}}, \qquad \dfrac{f_{\max}}{f_{\min}} = \sqrt{\dfrac{C_{\max}}{C_{\min}}}\)
\(f(V) = \dfrac{1}{2\pi\sqrt{L\,C(V)}}, \qquad \dfrac{f_{\max}}{f_{\min}} = \sqrt{\dfrac{C_{\max}}{C_{\min}}}\)
| C0 | Zero-bias junction capacitance (pF). |
| φ | Built-in (junction) potential, typically 0.6–0.8 V for silicon. |
| n | Grading coefficient: 0.5 for an abrupt junction, up to ~1–2 for hyperabrupt. |
| Vmin, Vmax | Reverse-bias control range (V). Low bias → high C; high bias → low C. |
| L | Resonator inductance (nH), optional — gives the frequency tuning range. |
References: B. Razavi, RF Microelectronics, 2nd ed., Prentice Hall, 2012. · S. M. Sze & K. K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley, 2007.
Inputs
pF
At 0 VV
~0.7 for Si0.5–2
V
High C endV
Low C endnH
For f rangeResults
Capacitance
C at Vmin (max)—
C at Vmax (min)—
C tuning ratio—
Frequency tuning
f range—
f tuning ratio—
Diagram