JPH118841A - Protection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate an abnormal surge voltage without deteriorating the electrical characteristic of a high frequency amplifier circuit covering up to a specific high frequency band by connecting in series at least two pairs of diodes which are connected in parallel to each other with their polarities set opposite to each other and connecting these pairs of diodes between the input or output side of the high frequency amplifier circuit and the ground. SOLUTION: Three pairs of diodes 4a which are connected in parallel to each other with their polarities set opposite to each other are connected in series between an output line 2 of a high frequency amplifier circuit 1 used to a CATV(cable television), etc., and the ground. In such a constitution, a protection circuit 4 is obtained. Meanwhile, a protection circuit 5 is also obtained by connecting in series two pairs of three diodes 5a in the forward direction and connecting these pairs of diodes 5a in parallel to each other with their polarities set opposite to each other. As a result, the circuit 1 can be protected from the abnormal serge voltage without deteriorating its distortion and impedance characteristics covering up to a high frequency band of 1000 MHz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection circuit for a surge voltage of a high-frequency amplifier circuit used in a cable television (CATV) or the like.

[0002]

2. Description of the Related Art Since a high-frequency amplifier circuit used in a CATV transmission line or the like is provided in a repeater provided in the air, a branch amplifier, or a main amplifier, it is connected via a coaxial cable or a power line connected thereto. An abnormal high voltage is likely to enter due to lightning or induced lightning, and a protection circuit for protecting the amplifier circuit element from such an abnormal surge voltage is provided immediately before or immediately after the protection circuit. As such a protection circuit, there are generally a method using an arrester or a coil, a method using a diode, and the like, but a circuit combining a diode is often used because of its excellent blocking characteristics. . As such a circuit, as shown in FIG. 7A, diodes 25 are connected in parallel between the output 2 (or input 6) of the high-frequency amplifier circuit 1 and the ground so that the polarities are opposite to each other. FIG.
As shown in (b), in addition to the configuration of (a), a circuit is provided in which both diodes 25, 25 are reverse-biased by power supplies 26, 27. Reference numeral 3 is an output terminal.

By providing such a circuit, a positive high voltage among the surge voltages causes energy to be released from the signal line to the ground, and a negative high voltage causes the energy to flow from the ground to the signal line. It protects the high-frequency amplifier circuit from abnormal high voltage generated by vibration. Although the protection circuit shown in (a) has a sufficient effect of removing an abnormal surge voltage, it is difficult to maintain good distortion characteristics of the line, and this may cause beat disturbance and deterioration of cross modulation. ,
As shown in (b), by providing a reverse bias to each of the diodes 25, 25, these distortion characteristics are improved.

[0004]

[0005] Among the CATV lines, a line that enables bidirectional communication has a transmission band of, for example, 5 MHz.
In this case, the band of the telephone signal and the data signal from the terminal is 5 MHz to 50 MHz, and the signal from the center station such as the television signal is 50 MHz.
１０００1000 MHz. Therefore, the high frequency amplifier circuit amplifies the frequency in the band of 50 MHz to 1000 MHz.
It is necessary to prevent the electrical characteristics from deteriorating over a wide band up to the high frequency range of 00 MHz. However, it is difficult for the above-described conventional protection circuit to maintain good characteristics in this band, and in particular, impedance characteristics are reduced in a high frequency range close to 1000 MHz. )). Therefore, improvement of the characteristics of the protection circuit has been demanded. Therefore, an object of the present invention is to provide a protection circuit for removing an abnormal surge voltage without deteriorating the electrical characteristics of a high-frequency amplifier circuit up to a high frequency band of 1000 MHz.

[0005]

In order to solve the above-mentioned problems, a protection circuit according to the first aspect of the present invention comprises two diodes connected between an input or output of a high-frequency amplifier circuit and a ground, the polarities being opposite to each other. At least two sets connected in parallel are connected in series.

According to a second aspect of the present invention, at least two protection circuits are provided between the input or output of the high-frequency amplifier circuit and the ground.
Two sets of diodes connected in series in the forward direction are connected in parallel so that the polarities are opposite to each other.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a configuration diagram of a protection circuit. The protection circuit is provided between an output line 2 of a high-frequency amplifier circuit 1 and a ground. FIG. Are connected in series so as to be in opposite directions. In addition, as shown in FIG.
A similar characteristic can be obtained even if three sets of a are connected in series in the forward direction and two sets are connected in parallel so that the polarities are opposite. In addition, (a) the number of diode sets and (b)
The number of diodes connected in series is not limited to three or three, but may be two or two or more.

FIG. 2 is a block diagram showing a case where this protection circuit is applied to an amplifier installed at a service entrance drawn from a trunk line to a subscriber's home in a CATV line capable of bidirectional communication. FIG. This amplifier 10
Has an internal high-frequency amplifier circuit 14, and a protection circuit of the present invention is provided on the output side of the high-frequency amplifier circuit 14 in order to protect the high-frequency amplifier circuit 14 from abnormal surge voltage due to induced lightning that enters through a terminal. I have. In the figure, 11 is an input terminal, 12 is a low-pass filter, 13 and 16 are high-pass filters, 17 is a power supply separation filter, 18 is a power supply circuit, 20 is an output terminal, 21 is a center station, 22 is an antenna, and the protection circuit 4 Is provided between the output line of the high-frequency amplifier circuit 14 and the ground.

The low-pass filter 12 is provided in parallel with the high-frequency amplifier circuit 14 for transmitting telephone signals, data signals, and the like from terminals to the center station 21, and transmits high-frequency signals such as television signals from the center station 21. The signal is amplified by the high frequency amplifier circuit 14. still,
In order to protect the surge voltage from the direction of the center station 21, a known protection circuit composed of a coil or the like is provided on the input side of the high-frequency amplifier circuit 14 (not shown), and the protection circuit also includes the diode. It is good also as composition.

FIGS. 3 to 6 show various characteristics of the amplifier 10 shown in FIG. 2, and show changes in characteristics of the protection circuit due to a difference in the number of diodes. First, FIG. 3 shows distortion characteristics, where A is 110 MHz, B is 220 MHz, C
Is 520 MHz, D is 620 MHz, E is 720 MHz
At each frequency. The horizontal axis indicates the number of series-connected diodes of the protection circuit, where 1 indicates the conventional protection circuit shown in FIG. 7A, 2 and 3 indicate the case of two or three sets, and the protection circuit of the present invention. ing. "Nil" is the characteristic without the protection circuit. As shown in the figure, the distortion characteristics when a protection circuit configured by connecting two or more sets of diodes in series are closer to the characteristics without the protection circuit than one set of diodes, and the distortion characteristics are not deteriorated. Therefore, there is no need to provide a reverse bias to the diode as shown in FIG.

FIG. 4 shows the frequency characteristic of the gain of the amplifier, FIG. 5 shows the return loss characteristic at the input terminal 11, and FIG. 6 shows the return loss characteristic at the output terminal 20. FIG. 7A shows a case where the number of the diodes is 1 in FIG.
Shows the characteristics of the conventional protection circuit shown in FIG.
3 shows a set of protection circuits of the invention. As shown in the figure, the gain of F with one set of diodes deteriorates as the frequency increases, whereas the gain of G and H with two or more sets of diodes connected in series is greatly improved as compared with the gain of F. ing. Also, the return loss characteristic is 300 MHz on the input side.
From the above, it can be seen that the output side has two or more sets of diodes connected in series at 500 MHz or higher and has a smaller value than that of one set, which is improved. This indicates that the VSWR has also been improved, indicating that the impedance characteristics have been improved up to a high frequency of 1000 MHz. 3 to 6
The characteristics shown in FIG. 1 show the same characteristics even when the circuit of FIG. 1B is used for the protection circuit.

As described above, a circuit in which at least two sets of two diodes connected in parallel in the reverse direction to the protection circuit are connected in series, or a set of two diodes connected in series in the forward direction is two If circuits connected in parallel so as to be in opposite directions are used, both the distortion characteristics and the impedance characteristics of the high-frequency amplifier circuit are not degraded up to the high frequency band of 1000 MHz.

[0013]

As described above in detail, according to the protection circuit of the present invention, it is possible to remove an abnormal surge voltage without deteriorating the distortion characteristics and impedance characteristics of the high-frequency amplifier circuit up to the high frequency band of 1000 MHz. Can be.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an example of an embodiment of a protection circuit according to the present invention.

FIG. 2 is a circuit block diagram showing a specific example in which a protection circuit of the present invention is provided at an output of a high-frequency amplifier circuit.

FIG. 3 is a diagram illustrating distortion characteristics of the amplifier of FIG. 2;

FIG. 4 is a diagram illustrating gain characteristics of the amplifier of FIG. 2;

FIG. 5 is a diagram illustrating return loss characteristics on the input side of the amplifier of FIG. 2;

FIG. 6 is a diagram illustrating return loss characteristics on the output side of the amplifier in FIG. 2;

FIG. 7 is a diagram showing a conventional protection circuit.

[Explanation of symbols]

1. high frequency amplifier circuit, 2. output line, 3. output terminal, 4. protection circuit, 4a diode, 5. protection circuit, 5a diode, 6, input line

Claims (2)

[Claims] 1. A protection circuit comprising at least two sets of two diodes connected in parallel so that polarities are opposite to each other in series between an input or output of a high-frequency amplifier circuit and ground. 2. A set in which at least two diodes are connected in series in a forward direction between an input or output of a high-frequency amplifier circuit and a ground, and two sets are connected in parallel so that the polarities are opposite to each other. Protection circuit.