**Voltage Variable Capacitors:**

**VVC Operation –** Voltage Variable Capacitors diodes (VVCs) are also known as **varicaps**, **varactors**, and as **tuning diodes**. Basically, a Voltage Variable Capacitors is a reverse biased diode, and its capacitance is the junction capacitance. Recall that the width of the depletion region at a pn-junction depends upon the reverse bias voltage, (Fig. 21-1). A large reverse bias produces a wide depletion region, and a small reverse bias gives a narrow depletion region. The depletion region acts as a dielectric between two conducting plates, so the junction behaves as a capacitor.

The depletion layer capacitance (C_{pn}) is proportional to the junction area and inversely proportional to the width of the depletion region. Because the depletion region width is proportional to the reverse bias voltage, C_{pn} is inversely proportional to the reverse bias voltage. This is not a direct proportionality; instead C_{pn} is proportional to 1/V^{n}, where V is the reverse bias voltage, and n depends upon doping density.

Figure 21-2 shows the doping profiles for two types of Voltage Variable Capacitors classified as **abrupt junction** and **hyperabrupt junction** devices. In the abrupt junction WC. the semiconductor material is uniformly doped, and it changes abruptly from p-type to n-type at the junction. The hyperabrupt junction device has the doping density increased close to the junction. This increasing density produces a narrower depletion region, and so it results in a larger junction capacitance. It also causes the depletion region width to be more sensitive to bias voltage variations, thus it produces the largest capacitance change for a given voltage variation. VVCs are packaged just like ordinary low-current diodes.

**Equivalent Circuit:**

The complete equivalent circuit for a Voltage Variable Capacitors is shown in Fig. 21-3(a), and a simplified version is given in Fig. 21-3(b). In the complete circuit, the junction capacitance (C_{J}) is shunted by the junction reverse leakage resistance (R_{J}). The resistance of the semiconductor material is represented by R_{S}, the terminal inductance is L_{S}, and the capacitance of the terminals (or the device package) is C_{C}. Because L_{S} is normally very small and R_{J} is very large, the equivalent circuit can be simplified [Fig 21-3(b)] to R_{S} in series with C_{T}, where C_{T} is the sum of the junction and terminal capacitances, (C_{T} = C_{J} + C_{C}). The Q-factor for a VVC can be as high as 600 at a 50 MHz frequency. However, Q-factor varies with bias voltage and frequency, so it is used only as a figure of merit for comparing the performance of different VVCs.

**Specification and Characteristics:**

A wide selection of nominal WC capacitances is available, ranging approximately from 6 pF to 700 pF. The capacitance tuning ratio (TR) is the ratio of C_{T} at a small reverse voltage to C_{T} at a large reverse voltage. In the partial specification for a VVC shown in Fig. 21-4, the tuning ratio is listed as C_{1}/C_{10}. This is the ratio of the device capacitance at 1 V reverse bias to that at a 10 V reverse bias. Using the 400 pF minimum capacitance (C_{T}) listed for a 1 V bias, the capacitance is changed to 400 pF/ 14 when the bias is 10 V. The specification also lists the Q-factor, as well as maximum reverse voltage, reverse leakage current, and the maximum forward current that can be passed when the device is forward biased.

A typical graph of capacitance (C_{T}) versus reverse bias voltage (V_{R}) for a hyperabrupt junction VVC is reproduced in Fig. 21-5 together with the VVC circuit symbol. It is seen that C_{T} varies (approximately) from 500 pF to 25 pF when V_{R} is changed from 1 V to 10 V. It should be noted from the specification in Fig. 21-4 that the nominal capacitance has a large tolerance (400 pF to 600 pF), and this must be taken into account when using the C_{T}/ V_{R} graphs.

**Applications:**

The major application of Voltage Variable Capacitors is as tuning capacitors to adjust the frequency of resonance circuits. An example of this is the circuit shown in Fig. 21-6, which is an amplifier with a tuned circuit load. The amplifier produces an output at the resonance frequency of the tuned circuit. The VVC provides the capacitance (C_{T}) of the resonant circuit, and this can be altered by adjusting the diode (reverse) bias voltage (V_{D}). So, the resonance frequency of the circuit can be varied. C_{1} is a coupling capacitor with a capacitance much larger than that of the VVC, and R_{2} limits the VVC forward current in the event that it becomes forward biased.