JP2956985B2 - Drive circuit for crystal oscillator - Google Patents

Description

The present invention relates to a driving circuit for a crystal oscillator for driving a Colpitts oscillation circuit including a crystal resonator.

[Conventional technology]

Generally, in a communication device, a crystal oscillator is used as an oscillation source. Oscillators are required to have a stable oscillation frequency even when the power supply voltage changes due to temperature changes.In particular, they are used as standard oscillation sources for portable cordless phones and cellular phones. The following characteristics are required for such a crystal oscillator.

In particular, the temperature compensation type crystal oscillator must have good temperature characteristics.

The oscillation frequency does not change with changes in load. That is, the load fluctuation characteristics are good.

Good voltage fluctuation characteristics.

Low power consumption.

Small and lightweight.

Output voltage must be 1V _PP or more.

Inexpensive.

As a driving circuit for a crystal oscillator or a temperature-compensated crystal oscillator that satisfies these requirements, the one shown in FIG. 2 is used.

In FIG. 2, a crystal unit X and a trimmer capacitor
A Colpitts type oscillation circuit is constituted by Cp, capacitors C1, C2, C4, C5, C6, resistors R1, R2, R3, R4 and transistor T1. The transistor T1 and the transistor T2 constituting the Colpitts oscillation circuit are cascode-connected. The collector of the transistor T2 is connected to the power supply via the resistor R5. The emitter of the transistor T1 is connected to the base of the transistor T2 via the capacitor C3.
The output is taken out via. This output is a load 3 consisting of a capacitor component _CL and a resistance component _RL.
Is supplied to Note that the capacitor C8 is provided for removing noise.

As shown in FIG. 2, by cascode-connecting the two transistors T1 and T2, the above-mentioned required characteristics, low power consumption, output voltage of 1 V _PP or more, low cost, small size and light weight are realized. .

[Problems to be solved by the invention]

FIG. 2 shows a general circuit configuration of a conventional crystal oscillator drive circuit, but has the following drawbacks in load variation characteristics.

That is, the capacitor from the collector of transistor T2
Since the output is performed via C7, the output impedance is high (corresponding to the resistance value R5 in the example of FIG. 2). Therefore, the capacitor component C _L, which is the load 3 of the oscillator, and the resistance component R _L
Changes, the operating state of the transistor T2 changes, and as a result, the oscillation frequency of the transistor T1 forming the oscillation circuit is affected. As described above, the fluctuation of the load appears as the fluctuation of the oscillation frequency, and good frequency stability cannot be obtained.

In order to suppress the frequency fluctuation due to the load fluctuation as described above, it is necessary to shut off the transistor T1 and the transistor T2 in an alternating manner. Therefore, it is necessary to increase the capacity of the capacitor C6. However, when the capacitance of the capacitor C6 is increased, the gain may be reduced as a result, and a desired output voltage may not be obtained.

In general, a crystal oscillator used for communication equipment is often connected to an input gate of a PLL (phase control loop) IC. PLLIC is usually composed of a C-MOS, and mainly has a capacitor component rather than a resistance component. When this capacitor component (corresponding to the C _L in FIG. 2) varies, the oscillation frequency fluctuates as described above.

An object of the present invention is to minimize the output impedance and to have a large output capacity, thereby absorbing fluctuations in load capacitance, and suppressing the influence on the oscillation circuit section even if a load fluctuation occurs. It is an object of the present invention to provide a driving circuit for a crystal oscillator that can obtain good frequency stability.

[Means for Solving the Problems] A drive circuit for a crystal oscillator according to the present invention is for driving a Colpitts oscillation circuit including a crystal oscillator,
A second transistor connected to a first transistor constituting a Colpitts oscillation circuit, wherein a second transistor is connected to a collector of the first transistor and an emitter of the second transistor, and a base of the second transistor;
It is configured by connecting a capacitor between the emitters. The oscillation output is derived from the collector of the first transistor.

[Operation] In the present invention, the first stage in the amplification stage of the oscillation circuit is used.
And the emitter of the second transistor are connected. Therefore, power consumption is reduced. Further, the Colpitts oscillation circuit including the first transistor becomes an emitter load of the second transistor,
Transistors constitute an emitter follower circuit. This reduces the output impedance. Further, a capacitor is connected between the base and the emitter of the second transistor connected to the first transistor of the oscillation circuit. Therefore, the capacitance of the capacitor viewed from the emitter side can be regarded as a capacitance larger than the actual capacitance as a product of the capacitance of the capacitor and the amplification factor of the transistor.

As described above, the current loss can be reduced by the low output impedance and the large capacitance, and the influence on the oscillation circuit can be reduced by absorbing the fluctuation of the load capacitance.

〔Example〕

FIG. 1 shows a crystal oscillation circuit and a driving circuit according to an embodiment of the present invention.

In the figure, a drive circuit 2 is connected between a Colpitts oscillation circuit 1 and a power supply, and a load 3 is connected to an output of the Colpitts oscillation circuit 1.

Colpitts oscillation circuit 1 includes a crystal oscillator X, and trimmer capacitor C _P, and capacitor C11, C12, C13 and C14, and resistors R11, R12 and R13, is constituted by the first transistor T1. The base of the first transistor T1 is connected crystal resonator X via a capacitor C12, the quartz crystal resonator X is trimmer capacitor C _P and the capacitor C1
It is connected to ground via one parallel circuit. Also,
A capacitor C13 is connected between the base and the emitter of the first transistor T1, and the emitter is connected to the ground via a parallel circuit of the capacitor C14 and the resistor R13.
Further, the power supply voltage divided by the resistors R11 and R12 can be applied to the base of the first transistor T1.

The drive circuit 2 has a second transistor T2 whose emitter is connected to the collector of the first transistor T1 of the Colpitts oscillation circuit, a capacitor C15, and a resistor R14. The collector of the second transistor T2 is directly connected to the power supply, and the base is connected to the power supply via the resistor R14. The oscillation circuit 1 including the first transistor T1 is connected to the emitter of the second transistor T2, and the second transistor T2 forms an emitter follower circuit. Further, the base of the second transistor T2,
A capacitor C15 is connected between the emitters.

The collector output via a capacitor C16 from the connection point of the emitter of the second transistor T2 is derived, wherein the resistance component R _L and the capacitor component of the first transistor T1
A load 3 composed of C _L is connected.

 Next, the operation and effect of the embodiment will be described.

In the Colpitts type oscillation circuit as described above, a signal of a frequency determined by the crystal oscillator is amplified by the first transistor T1, and a signal of a predetermined frequency can be supplied to the load 3.

At this time, for example, when each resistance value is set as shown in FIG. 1 and the power supply is 5 V, the output voltage obtained at the collector of the first transistor T1 is approximately 2.5 V, and the output current is reduced to about 2.5 V.
It can be 1 mA.

Therefore, the operating impedance during oscillation is 2.5V / 1mA = 2.5KΩ. This load of about 2.5 KΩ becomes the emitter load of the second transistor T2. Since the second transistor T2 forms an emitter follower circuit, the output impedance viewed from the output capacitor C16 is 1 / h _FE times (h _FE : second
Current amplification factor of transistor T2).

That is, when about 100 h _FE, output impedance is 2.5 k / 100 = 25 [Omega], and becomes a very low value.

The capacitor C15 connected between the base and the emitter of the second transistor T2 appears as a capacitor having a capacitance value of C15 × h _FE ≒ C5 × 100 due to the Miller effect.

As described above, according to the driving circuit of the present embodiment, the output impedance and the output capacitance are as shown in Table 1 below, and a driving circuit having the intended low output impedance and large capacitance can be realized. .

Therefore, the capacitor component C _L of the load 3 and the resistance component R _L
, The rate of change of the frequency with respect to the variation of the frequency is reduced, and good frequency stability can be obtained.

The frequency stability (Δf / f) of the conventional example and the present embodiment will be compared below.

As is clear from Tables 2-1 and 2-2, the present embodiment has a variation rate of 1/10 or less with respect to the variation of the load as compared with the conventional example.

〔The invention's effect〕

As described above, according to the present invention, in the second transistor in which the emitter is connected to the collector of the first transistor constituting the oscillation circuit, the second transistor 2 is
Since the oscillation circuit is an emitter follower circuit having an emitter load, and a capacitor is connected between the base and the emitter of the second transistor, a driving circuit having low output impedance and large capacitance can be realized. An oscillation output with good frequency stability against fluctuations can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a crystal oscillator and its driving circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional crystal oscillator and its driving circuit. 1 Colpitts oscillation circuit, 2 Driving circuit, 3 Load, T1, T2, first and second transistors, X Crystal resonator, C11 to C17, Capacitor, R11 to R14 resistance.

Claims (1)

(57) [Claims] A second transistor connected to a first transistor constituting a Colpitts oscillation circuit, wherein a second transistor is connected to a collector of the first transistor and an emitter of the second transistor; A driving circuit for driving a crystal oscillator, wherein a capacitor is connected between the base and the emitter of the transistor, and an oscillation output is derived from the collector of the first transistor.