1. Field of the Invention
The present invention relates to a voltage controlled oscillating circuit and more particularly, it relates to a voltage controlled oscillating circuit including a variable capacitance element having a bipolar transistor structure.
2. Related Art
A voltage controlled oscillating circuit is an oscillating circuit which can vary an output frequency according to a voltage applied. An example of a conventional voltage controlled circuit includes a transistor as an active element and a resonant circuit connected between the base and emitter of the transistor, and the resonant circuit comprises a variable capacitance element for varying an oscillation frequency.
The variable capacitance element can be implemented using a PN junction in a bipolar transistor, for example. In other words, it can be implemented by using a capacitance variation provided by applying a voltage to the PN junction between the base and the collector of the bipolar transistor. Japanese Patent Laid-Open Publication No. 2002-223127, corresponding to U.S. patent publication 2002/0093377 (see PP. 5, 9-10, FIGS. 6 and 9, etc.) discloses a power amplification circuit using a capacitive component of a PN junction in a bipolar transistor.
When an oscillation frequency is designed to be high in a voltage controlled oscillating circuit in which a variable capacitance element is implemented using such a bipolar transistor, the collector layer must be made thinner in order to increase the maximum oscillation frequency of the bipolar transistor. In order to obtain a sufficient capacitance when the collector layer is made thinner, a forward bias must be applied to the variable capacitance element using the PN junction between the base and the collector.
However, when the forward bias is applied to the PN junction between the base and the collector, a relatively large current flows, and thus current consumption is increased. Furthermore, because the current flows in the variable capacitance element, noise is generated and degrades the noise characteristic of the voltage controlled oscillating circuit.