An analysis of nonlinear terms in capacitance-voltage characteristic for anti-series-connected varactor-diode pair

Abstract

A variable-capacitance (varactor) diode is widely applied in microwave devices, which utilize the characteristic property that the junction capacitance is varied by an externally applied voltage. In applications of capacitance-controlled devices, such as voltagecontrolled oscillators, the capacitance is varied by the control-bias voltage, whereas the variation in capacitance for microwave RF-signal voltages should be minimized in order to suppress higher-order distortions in the output signal. The capacitance-voltage characteristic of a varactor diode is characterized by a slope exponent ␥. This article discusses the slope exponent that gives a minimum of variation in capacitance for the microwave excitation in the circuit in which diodes are connected in anti-series. The analysis is based upon a time-domainoriented method, in which we first obtain the voltage and current waveforms by solving differential equations with respect to time and then calculate the frequency-domain quantities by Fourier analysis of voltage and current waveforms.

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“…The equivalent circuit of a RF varactor diode with applied reverse voltage is presented in Figure 4(a). By continuous changing the DC bias voltage to the varactor diodes, the effective capacitance can be changed [37]- [39]; the variation of total capacitance with respect to reverse voltage across varactor diode is tabulated in Figure 4(b) [40]. Skywork's varactor diode SMV1281 series, is a surface mount, hyperabrupt varicap diodes which are made for high capacitance tuning ratios and low series resistance for wideband applications [41].…”

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“…As discussed in [19], a capacitor with a bell-shaped C-V curve can be implemented by a pair of varactor diodes connected in antiseries, as shown in Figure 16.…”

“…As discussed in [19], a capacitor with a bell-shaped C-V curve can be implemented by a pair of varactor diodes connected in antiseries, as shown in Figure 16. Neglecting the package parasitic capacitances, the capacitance of each varactor as a function of the applied reversed bias voltage 𝑉 𝐶 is described by:…”

“…By dimensional analysis of (29), the linear and nonlinear capacitances of the DR defined in (19) are found to be:…”