A variable capacitance circuit has a plurality of circuits each having capacitor and switch, and, by controlling the number of the switches to be ON, variably controls the capacitance value. Such a variable capacitance circuit is used, when there is a need for variably setting a capacitance value of a certain node according to various conditions.
For example, at an output terminal of a power amplifier used for a mobile phone or the like, an impedance-matching circuit is equipped. An impedance-matching circuit has an inductor and a capacitor. For enabling a fine adjustment of impedance of an impedance-matching circuit, possible options are to enable a variable control of the inductance value of the inductor, or to enable a variable control of the capacitance value of the capacitor. In general, a variable capacitance circuit for variably controlling the capacitance value is used. A variable capacitance circuit is also used when there is a need for variably setting the capacitance value for various situations other than a power amplifier.
Such a variable capacitance circuit has circuits in parallel, each of which has capacitor and MOS transistor between an output terminal of a power amplifier and the ground, and, by turning the MOS transistors ON, increases the capacitance of the output terminal.
Descriptions relating to a variable capacitance circuit are in Japanese patent laid-open publications No. 2008-166877 and No. 2000-150779.
However, in the power amplifier in the above examples, high-power signals are delivered to the output terminals. For example, when impedance of an output of a mobile phone is temporarily mismatched, there will be a case that the output power momentarily exceeds 30 dBm. This power means, if converted into a voltage amplitude of an AC (alternate current) signal with 50Ω matching, an amplitude of plus/minus 10V with 0V as the center voltage.
Hence, there is a concern for the variable capacitance circuit connected to the output terminal of the power amplifier such that an exceptionally high-power voltage applied to the capacitor element and thus the capacitor is broken. Therefore, a circuit configuration with considering withstanding voltage of the capacitor element is desired.
The above AC signal occurring at the output terminal with a large amplitude with 0V as the center voltage has a voltage exceeding the power-supply voltage, and has, on the other hand, a large minus voltage. Hence, PN junction of a drain of MOS transistor within the variable capacitance circuit becomes conductive, which causes distortion of the waveform of the output signal.