JPS602662Y2 - Narrowband amplifier circuit - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a narrowband amplifier circuit configured using a crystal resonator and a differential amplifier.

First, a conventional amplifier of this type will be explained with reference to FIG.

The circuit that combines a conventional crystal resonator and a differential amplifier is the first
As shown in the figure, the case of the crystal resonator is raised above the ground.

FIG. 2 is an equivalent circuit of the circuit shown in FIG.

In the figure, C and Co are equivalent circuits of a crystal resonator, and C
0"Co't R1=R1'? By setting RL=RL', the output voltage Vout is given by the following equation (1).

The above formula eliminates the influence of C8 and improves the passing characteristics.

It is determined by C9Ro, R□gRt, and shows that transmission characteristics can be obtained with the high sharpness of the crystal resonator.

However, when this circuit is used as part of a high-gain circuit with a total gain exceeding 80 diB, the crystal oscillator case has a kind of antenna effect because the crystal oscillator case is floating from the ground. This has the disadvantage of forming a loop in the high gain circuit and causing oscillation.

In order to solve the above drawbacks, it was necessary to shield the entire narrowband amplifier circuit with a mechanical structure.

Therefore, there was a problem that the overall shape became large.

An object of the present invention is to provide a compact narrowband amplifier that solves the above problems.

In order to achieve the above object, the narrowband amplifier according to the present invention has a narrowband amplifier circuit configured using a crystal oscillator and a differential amplifier. Connect the series circuit of the first resistor and the first resistor,
A capacitor having a capacitance equal to the parallel capacitance of the crystal resonator and a second resistor having a resistance equal to that of the first resistor are connected in series between the signal input terminal and the other input terminal of the differential amplifier. The circuit is connected, the case of the crystal resonator is grounded, and the connection point between the capacitor and the second resistor is grounded via a capacitor having a capacitance of 5 pF or less.

According to the above configuration, the above-mentioned oscillation can be completely prevented and the object of the present invention can be completely achieved.

A more detailed explanation will be given below with reference to the drawings and the like.

FIG. 3 is a diagram showing an embodiment of the circuit according to the present invention.

In the figure, 1 is a signal generator, 2 is a crystal resonator, 3, 4 are capacitors, 5, 6, 7.8 are resistors, 9 is a differential amplifier, and 10 is a level meter.

The resistance values of the resistors 5 and 6, the resistance values of the resistors 8 and 7, the parallel capacitance of the crystal resonator 2, and the capacitance value of the capacitor 3 are made equal to each other.

FIG. 4 is a graph showing the characteristics of the circuit, where the solid line shows the case where a capacitor 4 of an appropriate capacity is connected, and the dotted line shows the case where the capacitor 4 is not connected.

In the conventional circuit shown in FIG. 1, in order to cancel the parallel capacitance of the crystal oscillator 2 over a wide band with the capacitor 3, the differential amplifier 9 needs to have a wide band differential compression characteristic.

For this purpose, it is necessary to lower the input impedance of the differential amplifier, and the resistors 7 and 8 must be set to 100Ω or less.

Next, to determine the passband width of the narrowband pass circuit, (1)
As is clear from the equation, the load impedance due to the bandwidth of the crystal resonator, the signal source impedance, the resistors 5 and 6, and the load impedance is set to a low impedance in order to improve the differential compression characteristics, and the signal source impedance is high impedance. In this case, the signal directly passes through the space to the differential amplifier without passing through the crystal oscillator, and the out-of-band attenuation characteristics are damaged.
It is necessary to do the following.

In the present invention, the signal passing band width is set by changing the value of the resistor 5.6 from several hundred ohms to several ohms.

From this, when the case of the crystal resonator is connected to the ground as shown in Figure 3, the effect of the capacitance between the crystal resonator and the ground is that the crystal resonator 2 with high out-of-band impedance and the resistor 5
The connection point is the largest.

In order to cancel this effect, the capacitance should be 3 as shown in Figure 3.
It can be seen that the deterioration of the out-of-band attenuation can be corrected by connecting a capacitor 4 equal to the ground-to-ground capacitance of the crystal resonator between the connection point of the resistor 6 and the ground.

Note that in order to change the resonance center frequency of the crystal resonator, a capacitor is connected in series with the crystal resonator.

Even if the same thing is done with the circuit according to the present invention, there will be no problem, and in this case, the same result can be obtained by slightly changing the values of capacitor 3 and capacitor 4.

Next, a specific numerical example of the circuit will be shown.

Crystal oscillator 2 is 8.5MH2, resistors 5 and 6 are 500Ω,
When capacitor 3 is set to 5 pF, the out-of-band attenuation is
The capacitance value of capacitor 4 that improves from the dotted line to the solid line in the figure is 1
It is pF.

Although the capacitance value of the capacitor 4 differs depending on the shape of the crystal resonator, it was confirmed in experiments that the capacitance value is between 0 and 5 pF.

As explained above, the narrowband amplifier circuit according to the present invention has the advantage that it does not oscillate even when used in a high gain circuit without mechanical shielding because the crystal oscillator does not act like an antenna. There is.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional narrowband amplifier circuit, FIG. 2 is an equivalent circuit diagram thereof, FIG. 3 is a circuit diagram showing an embodiment of a narrowband amplifier circuit according to the present invention, and FIG. 4 is a narrowband amplifier circuit. 3 is a graph showing measurement results of frequency versus relative loss characteristics. 1... Signal generator, 2... Crystal resonator, 3, 4...
・Capacitor, 5.6.7. Death...Resistor, 9...
Differential amplifier, 10...level meter.

Claims (1)

[Scope of utility model registration request] In a narrowband amplifier circuit configured with a crystal resonator and a differential amplifier, a series circuit of the crystal resonator and a first resistor is connected between a signal input terminal and one input terminal of the differential amplifier, and a series circuit of the crystal resonator and a first resistor is connected between the signal input terminal and one input terminal of the differential amplifier. A series circuit of a capacitor having a capacitance equal to the parallel capacitance of the crystal resonator and a second resistor having a resistance equal to that of the first resistor is connected between the other input terminal of the differential amplifier. . A narrowband amplifier circuit, characterized in that a case of the crystal resonator is grounded, and a connection point between the capacitor and the second resistor is grounded via a capacitor.