1. Field of the Invention
This invention relates to voltage controlled oscillation circuits.
2. Description of the Related Art
There has been known an emitter-coupled stable multi-vibrator, for example as shown in FIG. 5, which is conventionally used as a voltage-controlled oscillation circuit. The circuit shown in FIG. 5 has a capacitor c1 connected between emitters of the transistor tr1 and tr2 which are further connected to voltage-controlled current sources cs1 and cs2. As further shown in FIG. 5, the base of tr1 is connected to the collector of tr2 and the base of tr2 is connected to the collector of tr1. Thereafter, the collectors of tr1 and tr2 are connected to a power supply terminal VCC (e.g., 3 V) through diode-connected transistors tr3 and tr4 and resistors r1, r2 connected in parallel with the transistors tr3 and tr4. With the above configuration, the transistors tr1 and tr2 are alternately turned on to cause charging to and discharging from the capacitor c1, thereby effecting oscillation operation. This produces voltage waveforms as denoted by a and b in FIG. 6 on terminals a and b. Here, each transistor is equal in size and the current value in each voltage-controlled current source is determined same. The voltage waveforms a and b oscillate with a base-to-emitter voltage VBE of each transistor as an amplitude value. The frequency is determined by the charge/discharge time of the capacitor c1, which is controlled by varying the current values of the voltage-controlled current sources cs1, cs2. For example, the charge/discharge time is reduced by increasing the current value, which means in the voltage waveforms in FIG. 6 the slant of the voltage waveform becomes abrupt to increase the frequency with amplitude value kept constant.
In the configuration of FIG. 5, the transistors tr3 and tr4 are used as diodes to set the amplitude at the voltage VBE (approximately 0.7 V). The resistors r1 and r2 are provided in parallel with these diodes, in order to raise the potential on the terminals a and b to the potential of the power supply terminal VCC when no current flows through these diodes. In order to fix the amplitude value, there is a necessity, for any current value within a variable range of the voltage-controlled current sources cs1, cs2, to give a voltage drop due to the resistors r1, r2 and such current greater than the voltage VBE. If the voltage drop is not set in that manner, a current flows on a resistor r1, r2 side, making it impossible to ensure the amplitude value. Accordingly, there is a necessity of setting at a certain great value the resistance value of the resistor r1, r2 and/or the current value of the voltage-controlled current source cs1, cs2. However, such a voltage-controlled oscillation circuit is usually used for a PLL or the like, and integrated together with other circuit elements in one chip. If the resistors r1, r2 are increased in size, a floating capacitance will increase to decrease the speed in raising the potential. This, in other words, prevents the frequency from being increased. Due to this, if a high frequency oscillation is desired, the current value of the voltage-controlled current sources cs1 and cs2 requires an increase, making it difficult to reduce the power consumption.
Meanwhile, if frequency is increased by decreasing the capacitance of the capacitor cl, there is a limitation in decreasing the capacitance value of the capacitor c2 if the capacitance is considered relative to the floating capacitance.
As stated above, in order to increase the frequency in the FIG. 5 configuration there is no way but to increase the current value of the voltage-controlled current source cs1, cs2, resulting in difficulty in advancing the reduction in power consumption.