This application claims the benefit of priority from prior Japanese Patent Application P2001-303086 filed on Sep. 28, 2001; the contents of which are incorporated by reference herein.
I. Field of the Invention
The present invention relates to a semiconductor device having an oscillating circuit. More particularly, the invention relates to a semiconductor device having a frequency oscillating circuit, such as an LC (which means xe2x80x9cinductive-capacitivexe2x80x9d) oscillating circuit.
II. Background of the Invention
The following will explain an example of the arrangement of a conventional oscillating circuit. FIG. 1 is a circuit diagram illustrating the conventional oscillation circuit arrangement.
In the oscillating circuit of FIG. 1, transistors Q101 and Q102 form a differential pair, and each base of the transistors Q101 and Q102 is connected to a collector of the other transistor. A coil L101 and a capacitor C101, which form an LC resonant tank circuit, are connected between the collectors of these transistors Q101 and Q102.
The collectors of transistors Q101 and Q102 are connected to a power supply voltage VDD via resistors R101 and R102. A constant current source Io is connected to the emitters of transistors Q101 and Q102. Moreover, the constant current source Io is connected to the ground potential GND. The output signal OUT1 from the collector of the transistor Q101 and an output signal OUT2 from the collector of the transistor Q102 are used as a differential pair output, or, alternatively, a single output of one side of the oscillating circuit (e.g., OUT1) is used.
However, the aforementioned oscillating circuit can produce a signal at a fixed frequency, and has a problem in which the frequency cannot be varied.
Accordingly, the present invention is directed to a semiconductor device having a frequency oscillating circuit, such as an LC oscillating circuit.
A semiconductor device according to an embodiment of the present invention includes, for example, a first transistor having a control electrode and having one end and the other end of a current path thereof; a second transistor having a control electrode and having one end and the other end of a current path thereof, the control electrode is coupled to one end of the current path of the first transistor and one end of the current path thereof is coupled to the control electrode of the first transistor; a current mirror circuit that supplies a current to one end of the current path of the first transistor and one end of the current path of the second transistor; an inductor coupled to one end of the current path of the first transistor and one end of the current path of the second transistor; a first capacitor coupled to one end of the current path of the first transistor and one end of the current path of the second transistor; and a second capacitor and a switch element, are coupled to one end of the current path of the first transistor and one end of the current path of the second transistor.
A semiconductor device according to another embodiment of the present invention includes, for example, a first transistor having a control electrode and having one end and the other end of a current path thereof; a second transistor in which a control electrode is coupled to one end of the current path of the first transistor and one end of a current path thereof is coupled to the control electrode of the first transistor; a current mirror circuit that supplies a current to one end of the current path of the first transistor and one end of the current path of the second transistor; an inductor coupled to one end of the current path of the first transistor and one end of the current path of the second transistor; a constant current source coupled to the other end of the current path of the first transistor and the other end of the current path of the second transistor; a first capacitor coupled to one end of the current path of the first transistor and the other end of the constant current source; a second capacitor coupled to one end of the current path of the second transistor and the other end of the constant current source; a third capacitor and a first switch element coupled to one end of the current path of the second transistor and the other end of the constant current source; and a fourth capacitor and a second switch element coupled to one end of the current path of the second transistor and the other end of the constant current source.
Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.