Recently the mobile communications, which are represented by the cellular phones, have been developed rapidly. A VCXO (Voltage Controlled Crystal Oscillator) is necessary to compose of a PLL and so on. The oscillation frequency of the VCXO is usually controlled by variation of capacitance of a varactor diode. In order to get wide variation of the frequency, it is necessary for the variable capacitance circuit of the VCXO to have wide variation, But, it is difficult to realize the varactor diode on an IC chip.
First, we propose to apply a Miller capacitor to the variable capacitance circuit. As a result, it is verified that the variable capacitance circuit, which is operated with small amplitude, is so effective for the variable capacitance circuit of the wide variable range oscillator for IC.
Next, we propose to apply this variable capacitance circuit to a crystal oscillator using a bipolar junction transistor. As a result, when the external capacitance of the MOSFET is about 10 [pF] and the capacitance ratio of the crystal resonator is 220, the VCXO using this variable capacitance circuit is shown to have wide variation of the frequency over 400 [ppm].