JPH0129852Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of an oscillation circuit, and particularly to an oscillation circuit that operates stably and is implemented as an integrated circuit. An integrated circuit oscillation circuit as shown in FIG. 1 has been proposed by the same applicant as the present applicant. In FIG. 1, reference numeral 1 indicates differentially connected first and second transistors 2 and 3;
a positive feedback amplifier comprising a constant current transistor 4 connected to a common emitter of transistors 2 and 3, and a feedback capacitor 5 connected between the collector of the second transistor 3 and the base of the first transistor 2; 6 is an oscillation element such as a crystal resonator whose one end is connected to the collector of the second transistor 3; and 7 is a differentially connected third and fourth transistor 8 and 9; and a fifth transistor 10 connected to the common emitter of the third transistor 8 and whose collector current changes according to a control signal applied to the base; a capacitor 11 connected between the base and collector of the third transistor 8; A resistor 1 connected between the bases of the third and fourth transistors 8 and 9
This is a variable capacitance reactance section consisting of 2. Therefore, in the circuit shown in FIG. 1, the variable capacitance reactance section 7 has a variable capacitance reactance as a whole.
Cx, and has a configuration in which the variable capacitance reactance Cx is connected in parallel to the oscillation element 6. Therefore, by changing the value of the variable capacitance reactance Cx, the oscillation circuit can generate an output oscillation signal in a predetermined frequency range. By the way, the variable capacitance reactance Cx is as follows: Cx=R・g _n・C (1) (where, R is the resistance value of the resistor 12, g _n is the mutual conductance, and C is the capacitance reactance of the capacitor 11.) , transconductance
Since g _n is proportional to the current I flowing through the fifth transistor 10, if a control signal is applied to the base of the fifth transistor 10 to change the current I, the variable capacitance reactance Cx changes according to the control signal. It becomes something. By the way, the output oscillation signal obtained at the output terminal 13 of the oscillation circuit has an amplitude determined by the power supply voltage (+V _CC ) and the ground potential, as shown by the solid line A in FIG. On the other hand, since a bias voltage of +V _B is applied to the bases of the third and fourth transistors 8 and 9 of the variable capacitance reactance section 7 by the bias power supply 14, the base voltage of the third transistor 8 is as shown in FIG. It will look like the broken line B. As is clear from comparing Figure 2 A and B, the output oscillation signal is +
When the voltage of the output oscillation signal decreases below V _B , the voltage of the output oscillation signal
The voltage becomes lower than the base voltage of transistor 8. Therefore, since the output terminal 13 is also connected to the collector of the third transistor 8 of the variable capacitance reactance section 7 , and the voltage of the output oscillation signal is also the collector voltage of the third transistor 8, the voltage of the output oscillation signal is is the third transistor 8
The fact that the voltage is lower than the base voltage of
This means that the collector-base voltage of the transistor 8 becomes reverse biased, and the third transistor 8 becomes saturated. When the third transistor 8 is saturated, the variable capacitance reactance section 7 no longer functions normally, and the characteristics become unstable or oscillation stops. The present invention has been developed in view of the above points, and will be described below based on embodiments with reference to the drawings. FIG. 3 shows an embodiment of the present invention, which is characterized in that the emitter of the constant current transistor 4 of the positive feedback amplifier section 1 is connected to a predetermined potential point instead of to ground. That is, in FIG. 3, first, second, and third bias resistors 15, 16, and 17 connected in series are provided;
and a bias transistor 18 whose base is connected to the connection point of the third bias resistors 16 and 17.
is provided. Since the emitter of the bias transistor 18 is connected to the emitter of the constant current transistor 4, the emitter voltage V _E of the constant current transistor 4 is
It is determined by the values of the third bias resistors 15 to 17 and the base-emitter voltage of the bias transistor 18. In addition, in Figure 3,
Circuit elements that are the same as those in FIG. 1 are given the same figure numbers and their explanations are omitted. By the way, when the third transistor 8 of the variable capacitance reactance section 7 is saturated, the collector voltage V _C of the third transistor 8 is as follows, where V _B is the base voltage of the fourth transistor 9, V _C = V _B - V _BE +Vsat...(2) ( _VBE is the voltage between the base and emitter of the transistor, and Vsat is the saturation voltage between the collector and emitter of the transistor.) On the other hand, when the emitter voltage V _E of the constant current transistor 4 of the positive feedback amplifier section 1 is determined, the minimum collector voltage Vmin of the second transistor 3 of the positive feedback amplifier section 1 becomes Vmin=V _E +2Vsat...(3) , by the collector voltage of the second transistor 3, the third
In order to prevent saturation in the transistor 8, the condition V _C <Vmin (4) is required. Therefore, from the second to fourth equations, the V _E must be determined so that V _E >V _B −V _BE −Vsat (5). Figure 4 shows the output oscillation signal when V _E = V _B , and the output oscillation signal (solid line A) is composed of the power supply voltage (+V _CC ) and the emitter voltage of constant current transistor 4 (+V _E ). It has a swing width determined by
This shows that the base voltage of the third transistor 8 does not drop below the base voltage (broken line 2). As the emitter voltage V _E of the constant current transistor 4, for example,
2V, the base voltage of the fourth transistor 9, for example
If the voltage is 3V, stable operation can be obtained. As described above, according to the present invention, since the second transistor 8 of the variable capacitance reactance section 7 does not become saturated, it is practical in that a reliable oscillation signal can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an oscillation circuit that is an object of improvement of the present invention, FIG. 2 is a characteristic diagram showing its oscillation output signal, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. FIG. 4 is a characteristic diagram showing the oscillation output signal. Explanation of main drawing numbers, 1 ...Positive feedback amplifier section, 4...
constant current transistor, 7 ... variable capacitance reactance section, 8... third transistor, 9... fourth transistor.

Claims (1)

[Scope of utility model registration request] A pair of differentially connected transistors, a constant current transistor connected to a common emitter of the pair of transistors, and a positive feedback capacitor connected between the base of one of the transistors and the collector of the other of the pair of transistors. , a positive feedback amplification section having the collector of the other transistor as an output terminal from which an oscillation output signal is obtained, an oscillation element connected to the output terminal of the positive feedback amplification section for determining an oscillation frequency, and a differential It includes a pair of connected transistors, a capacitor connected between the base and collector of one of the pair of transistors, and a bias power supply that applies a reference voltage to the bases of the pair of transistors, and the collector of the one transistor is connected to the base of the transistor. a variable capacitance reactance section connected to the output terminal; and a voltage supply circuit that applies a predetermined emitter voltage to the emitter of a constant current transistor of the positive feedback amplification section, so that one transistor of the variable capacitance reactance section is not saturated. An oscillation circuit characterized in that the emitter voltage is set to a value substantially equal to or larger than the output voltage of the bias power supply.