JPH08186443A - Crystal oscillator - Google Patents

Abstract

PURPOSE: To make the device suitable for an operation using the power source of a low voltage, to reduce power consumption, to fix and comparatively enlarge an output amplitude and to stably operate the device with the fixed amplitude even against a temperature change. CONSTITUTION: This device is provided with a Colpitts type crystal oscillation circuit using a first transistor 11 and a second transistor 20 for buffering and amplifying out the oscillated output of the first transistor 11 while being cascade-connected to the first transistor 11, and equipped with a third transistor 29 for applying the voltage of a connecting point to the base of the second transistor 20 while being connected to the output of a constant current circuit 28 and a constant voltage source 31 for applying a fixed voltage to the base of the third transistor 29.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention operates on a low power supply voltage,
The present invention relates to a crystal oscillator that consumes low power and that can obtain an oscillation output with a constant amplitude regardless of changes in temperature.

[0002]

2. Description of the Related Art Recently, a crystal oscillator has been widely used as a reference for frequency, time, etc. of various electronic devices. However, in such an electronic device, it is desired to reduce the power consumption in order to effectively use a power source such as a battery having a limited power capacity, in addition to a reduction in size and weight, particularly in a portable device or the like. ing. Therefore, for example, a conventional portable device has been used with a power supply voltage of 5 V, but recent devices have been operated with a low voltage of 3 V or lower, and have low power consumption. Is desired.

FIG. 3 is a circuit diagram showing an example of a conventional crystal oscillator. In this crystal oscillator, a buffer amplifier 3 is connected to a cascode at the output of a Colpitts-type crystal oscillation circuit 2 using a crystal oscillator 1. When such a crystal oscillator is operated by a low-voltage power supply 4 having a power supply voltage of about 3 V, for example, two transistors are connected in series to the single power supply 4, and the power supply voltage is effectively used. This is advantageous in terms of power consumption because it can be done, but the power supply voltage is divided into two. Then, with respect to the divided power supply voltage, the operating point of each transistor is V due to temperature change.
It is necessary to set a certain allowance in consideration of changes in be (base-emitter voltage) and the like. For this reason, the range in which the power supply voltage can be effectively used is further reduced. For example, for a power supply voltage of 3 V, the amplitude of the oscillation output is 0.5.
It becomes Vp-p or less.

In such a crystal oscillation circuit having a small oscillation output amplitude, for example, a C-MOS type logic circuit cannot be stably driven. Therefore, for example, as shown in FIG. 4, a crystal oscillator in which a Colpitts-type crystal oscillation circuit 2 using a crystal oscillator 1 and a buffer amplifier 3 are connected in parallel to a power supply 4 can be considered. This has the advantage that a relatively large output amplitude can be obtained. However, in such a configuration, the oscillation circuit 2 and the buffer amplifier 3 are connected in parallel to the power supply 4, which naturally consumes a large amount of power and is not suitable for portable equipment.

Further, as a common problem of the oscillation circuits shown in FIGS. 3 and 4, there is a fluctuation of the output amplitude when the temperature changes. This tendency is caused by the consumption of a low voltage power source. The less power the oscillator has, the more prominent it appears. That is, in the case of a normal transistor, Vbe is about -2 mV.
It is known to change with / ° C. Here, for example, in the oscillator shown in FIG. 3, the power supply voltage is 3V, the emitter voltage of the oscillation transistor is 0.5V, and the collector current is 300V.
It shall be designed to operate at room temperature at μA.

Here, in consideration of the actual use environment of this kind of oscillator, when a temperature range of room temperature ± 50 ° C. is required as a standard, the above-mentioned Vb with respect to temperature change of room temperature ± 50 ° C.
e changes about ± 0.1V. Therefore, the emitter potential of the oscillation transistor changes to 0.5V ± 0.1V, that is, 0.4V to 0.6V, and the rate of change reaches 1.5 times. Therefore, the collector current of the oscillation transistor is 2
The amplitude of the oscillation output changes from 40 μA to 360 μA.
Since the output amplitude fluctuates 1.5 times because it is roughly proportional to the collector current, the temperature drops extremely in electronic equipment that operates with almost no margin using a low-voltage power supply. However, if it rises or rises, it may cause inconvenience such as a circuit in the subsequent stage not operating.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is suitable for operation using a low-voltage power supply, has low power consumption, and has a constant and relatively large output amplitude. Moreover, it is an object of the present invention to provide a crystal oscillator that operates stably with a constant output amplitude even when the temperature changes.

[0007]

According to the present invention, a Colpitts type crystal oscillating circuit using a first transistor and a cascode connected to the first transistor are provided. The oscillation output of the first transistor is buffer-amplified and output. A second transistor, a third transistor connected to the output of the constant current circuit and applying a voltage at the connection point to the base of the second transistor, and a constant voltage source for applying a constant voltage to the base of the third transistor. And is provided.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the circuit diagram shown in FIG. Reference numeral 11 in the figure denotes an NPN type transistor of a crystal oscillation circuit, the base of which is a crystal oscillator 12
Also, the varicap diode 13 is grounded via the series. A control voltage Vc is applied to the varicap diode 13 from a control terminal 15 via an AC cut resistor 14 so that the resonance frequency of the crystal resonator 12 can be slightly changed. The base of the first transistor 11 is grounded via the bias resistor 16, the capacitor 17 is interposed between the base and the emitter, and the emitter is grounded via the capacitor 18. And capacitor 1
An emitter resistor 19 is connected to 8 in parallel.

A NPN type second transistor 20 of a buffer amplifier is connected to the first transistor 11 in a cascode. That is, the collector of the first transistor 11 is connected to the emitter of the second transistor 20, and the collector of the second transistor 20 is connected to the positive terminal of the power source 22 via the LC tuning circuit 21 tuned to the oscillation frequency, for example. Connected. The negative terminal of the power source 22 is grounded. Further, the collector of the second transistor 20 is connected to the output terminal 24 via the capacitor 23, and the oscillation output is output from the output terminal 24.

The output of a current mirror type constant current circuit 28 consisting of two transistors 25 and 26 and a constant current source 27 is grounded via the emitter / collector of a PNP type third transistor 29. There is. The connection point between the output of the constant current circuit 28 and the emitter of the third transistor 29 is connected to the base of the second transistor 20 via the resistor 30. Further, a constant voltage Vb of about 1.2V is applied to the base of the third transistor 29 from the constant voltage source 31 to place the third transistor 29 in the active region. And the second transistor 2
A bias resistor 31 is inserted between the base of 0 and the base of the first transistor 11.

With such a configuration, the oscillation output of the first transistor 11 can be buffer-amplified by the second transistor 20 and output from the output terminal 24. Then, due to the change in temperature, the first transistor 1
When Vbe1 of 1 (base-emitter voltage) fluctuates, the same Vbe3 is generated in the third transistor 29 as well.
Cause changes. This change in Vbe3 changes the voltage at the connection point between the constant current circuit 28 and the third transistor 29, cancels the change in the emitter potential of the first transistor 11 and maintains a constant emitter potential.

Therefore, regardless of changes in temperature, the first
Since the emitter potential of the transistor 11 can be kept constant and the collector current can be kept constant, the output amplitude can be kept constant. Since the first and second transistors 11 and 20 are inserted in series with the power supply 22 to divide the power supply voltage into two and efficiently use the power supply voltage, for example, a power supply voltage of 3V is used.
It can be operated with a small current consumption of about 0 μA. Further, in this case, for example, as shown in the graph of the output waveform in FIG.
1.2V for 10KΩ load as shown by curve A
It can be driven with a large amplitude of (p-p) or more, and the logic circuit or the like can be stably driven as it is.
On the other hand, in the case of the conventional oscillation circuit as shown in FIG. 3 and FIG. 4, the amplitude of the oscillation output is small as shown by the curve B in FIG. , The circuit in the latter stage cannot be driven stably.

The present invention is not limited to the above embodiment. For example, in the above embodiment, the load of the buffer amplifier is LC.
Although the tank circuit is used, a fixed resistor having an appropriate value may be used. In this case, the amplitude of the oscillation output is smaller than that using the LC tank circuit,
Advantages of smaller parts volume

[0014]

As described above in detail, according to the present invention, it is possible to provide a crystal oscillator which is suitable for low voltage operation, has low power consumption, and operates stably with a constant output amplitude even when the temperature changes. .

[Brief description of drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a diagram showing the output amplitude of one embodiment of the present invention in comparison with the output amplitude of a conventional crystal oscillator.

FIG. 3 is a circuit diagram showing an example of a conventional crystal oscillator.

FIG. 4 is a circuit diagram showing another example of a conventional crystal oscillator.

[Explanation of symbols]

 11 First Transistor 12 Crystal Oscillator 20 Second Transistor 22 Power Supply 28 Constant Current Circuit 29 Third Transistor

Claims (5)

[Claims] 1. A Colpitts type crystal oscillating circuit using a crystal resonator and a first transistor, and a second cascode connected to the first transistor for buffer-amplifying and outputting the oscillation output of the first transistor. Connected to the output of the transistor and the constant current circuit, the voltage at the connection point is given to the base of the second transistor, and the change in the emitter potential of the first transistor due to the change in the base-emitter voltage of the transistor is canceled to be constant And a constant voltage source that supplies a constant voltage to the base of the third transistor, the crystal oscillator. 2. A crystal oscillator according to claim 1, wherein the third transistor is provided with an output current of a current mirror type constant current circuit between the emitter and the collector. 3. A crystal oscillator according to claim 1, wherein the load of the second transistor is a tank circuit tuned to the oscillation frequency. 4. A crystal oscillator according to claim 1, which operates at a power supply voltage of 3 V or less. 5. A crystal oscillator according to claim 1, wherein the oscillation output frequency is 10 MHz or higher.