JPH08186481A - Crosstalk elimination circuit for analog switch - Google Patents

Abstract

PURPOSE: To suppress the occurrence of a crosstalk in an analog switch by using a grounded-collector transistor(TR) in each input of the analog switch and connecting its emitter resistance to the stitching output of the analog switch. CONSTITUTION: When an analog switch 1 selects the signal of an input TR 2, an emitter resistance 4 is connected to the TR 2 through the switch 1, and the signal of the TR 2 appears in the output. At this time, an input TR 3 is cut off because the emitter resistance is not connected. However, a finite impedance exists also on the input side which is not selected by the switch 1, and some signal appears there by this impedance; but it is attenuated by a capacitor 6. When the analog switch 1 selects the signal of a TR 3, the capacitor 6 is set to such value that its influence is insignificant because the impedance of the resistance 4 is low. At this time, the TR 2 which is not selected by the switch 1 is cut off, and the signal is attenuated by a capacitor 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal switching circuit, and more particularly to a video signal switching circuit including a high frequency component.

[0002]

2. Description of the Related Art In recent years, many video signal sources have been introduced into the world. For example, not only video tape recorders, but also laser disks, home game consoles, satellite receivers, and the like. The signal switching circuit that selects one's own required signal from among them,
It is indispensable for any video display terminal.

As described above, a custom switching IC or the like may be used as a circuit for switching a complicated video source, but switching may be performed by combining an analog switch for cost reduction. However, when trying to switch the video signal with the analog switch, crosstalk inside the switch becomes a problem. To eliminate this, circuits such as transistors for blocking unselected signals are often used.

FIG. 3 shows an example thereof, 1 is an analog switch, 2 and 3 are input transistors connected to their input sides, and a defeat is provided between the emitter output circuits of the input transistors 2 and 3 and the ground. Transistors 7 and 8 are connected, and this defeat transistor 7 and 8
A selection switching signal is given via.

[0005]

However, by using such a defeat transistor,
Although good crosstalk characteristics can be obtained, there is a problem that the cost increases as a result.

The present invention solves the above-mentioned problems. A good crosstalk characteristic can be obtained without using a new defeet transistor or the like in order to eliminate the crosstalk in the analog switch, and the cost is low. The purpose is to realize a crosstalk elimination circuit for various analog switches.

[0007]

In order to achieve the above object, a crosstalk removing circuit for an analog switch according to the present invention comprises an analog switch, an input transistor connected to each input terminal thereof, and an emitter of the input transistor. The capacitor is connected between the output and ground, and the output resistance is connected between the analog switch output terminal and ground.

[0008]

According to the present invention, with the above configuration, a collector-grounded transistor (emitter follower) is formed at each input of the analog switch, and the emitter resistance of the transistor is connected to the switching output of the analog switch. The emitter resistance is connected to the input transistor, and the input transistor not selected by the analog switch is cut off because the emitter resistance is not connected. As a result, the signal on the non-selected side is not input to the analog switch, so that crosstalk does not occur inside the analog switch.

[0009]

Embodiments of the present invention will be described below. (Embodiment 1) As shown in FIG. 1, a crosstalk removing circuit for an analog switch comprises an analog switch 1, an input transistor 2, an input transistor 3, an output resistor (emitter resistor) 4, a capacitor 5 and a capacitor 6, Capacitors 5 and 6 are connected between the emitter outputs of the transistors 2 and 3 and the ground, and an output resistor 4 is connected between the output terminal of the analog switch 1 and the ground.

In this circuit, two video signals a and b
Are input transistor 2 and input transistor 3, respectively.
Through the analog switch 1. Here, when the analog switch 1 is selecting the signal of the input transistor 2, the analog switch 1 is connected to the input transistor 2.
Since the emitter resistor 4 is connected through the signal, the signal of the input transistor 2 appears at the output.

On the other hand, at this time, the input transistor 3 is cut off because the emitter resistance is not connected.
However, the input on the side not selected by the analog switch 1 also has a finite impedance, which causes some signal to appear, but the capacitor 6 attenuates this.

On the contrary, when the signal of the input transistor 3 is selected, the influence of the capacitor 6 is set to such a value that there is no problem because the impedance of the emitter resistor 4 is low. At this time, as before, the input transistor 2 not selected by the analog switch 1 is cut off, and the signal is further attenuated by the capacitor 5. (Embodiment 2) In this embodiment, as shown in FIG. 2, instead of the capacitors 5 and 6 described in the embodiment 1, filters 5a and 6a composed of inductors and capacitors are used, and the filters 5a and 6a are selectively used. Attenuate specific frequencies. Since the operation is the same as that of the first embodiment,
The details are omitted.

[0013]

As is apparent from the above description of the embodiments, according to the present invention, the crosstalk inside the analog switch can be effectively performed by the circuit having a simple structure without using the defeat transistor in addition to the input transistor. Can be removed, and it has excellent cost and performance.
Very informative.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a crosstalk removing circuit of an analog switch according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a crosstalk removing circuit of an analog switch according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a crosstalk removing circuit of a conventional analog switch.

[Explanation of symbols]

 1 analog switch 2 transistor (collector ground) 3 transistor (collector ground) 4 output resistance 5 capacitor 5a filter 6 capacitor 6a filter

Claims (2)

[Claims] 1. A switching circuit using an analog switch as means for switching a plurality of signals,
A crosstalk elimination circuit of an analog switch that uses a collector-grounded amplifier circuit (emitter follower) for its input and connects a capacitor that attenuates the non-selection signal between the emitter and ground. 2. A switching circuit using an analog switch as means for switching a plurality of signals,
A crosstalk elimination circuit of an analog switch that uses a collector grounded amplifier circuit (emitter follower) for its input and connects a filter consisting of an inductor and a capacitor that attenuates a non-selection signal between the emitter and ground.