JPS63182912A - Signal switching circuit - Google Patents

Abstract

PURPOSE:To make the circuit integration suitable by allowing the 2nd variable impedance means to attenuate an output signal of the 2nd input amplifier in delivering an output of the 1st input amplifier, for example, by a signal switch so as to improve the crosstalk. CONSTITUTION:An input signal Vin2 is fed to a base of a transistor Q2' (TR) being a component of the 2nd input amplifier 1'. A constant current circuit CS1' is a load of an emitter follower TRQ2, to obtain an output signal V0', and since a switch Sa' is turned off in this case, the voltage division by voltage division resistors R1', R2' is far higher than a bias voltage Vref-Vbe, a TR Q3' is turned on and the TR Q2' is turned off. Thus, the impedance of the variable impedance means Q3' is small and the crosstalk component of the input signal Vin2 is bypassed by a power supply Vcc being an AC ground point to reduce the crosstalk.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a signal switching circuit suitable for selecting a negative signal from a plurality of input signals and transmitting it to a subsequent stage. This invention relates to circuit technology that is effective when switching and transmitting high-frequency signals.

[Conventional technology]

In electronic devices, switch circuits are often used to select a desired signal from a plurality of input signals. in this case,
The problem is the crosstalk in the switch circuit, and the crosstalk is large [Television Technology J (1986,
August issue, Publisher Denshi Gijutsu Publishing Co., Ltd., p26, No. 13
An example of the above-mentioned switch circuit is shown in FIG.

The general idea is that a plurality of video input signals and television reception signals are selected and transmitted to the video processing circuit of the receiver, and a signal switching IC with low crosstalk is used.

The present inventors have studied crosstalk reduction in switch circuits. Although the following is not a publicly known technique, it is a technique studied by the present inventors, and the outline thereof is as follows.

In other words, if only crosstalk reduction is considered, a switch with a mechanical structure may be used. However, current electronic devices are becoming increasingly integrated into ICs, and using a switch with a mechanical structure increases the number of external components and increases the number of external connection terminals, which is not desirable. Therefore, a switch circuit using a transistor is used, but the inventors of the present invention have discovered that the following problems occur with the transistor switch.

[Problem that the invention seeks to solve]

A relatively large capacitance component is formed between the base and collector of the transistor. Therefore, in a circuit configuration in which an input signal is supplied to the base of a transistor and an output signal is obtained from the collector, even if the transistor is turned off and the transmission of the input signal is blocked, the input signal is not transmitted through the capacitance component. It is not possible to prevent signal components from leaking to the collector. For this reason, it has been found that the leaked signal component comes to be superimposed on the normal transmission signal, resulting in worsening of crosstalk.

In particular, when the input signal has a high frequency, the leaked signal component becomes large, so it has been desired to improve crosstalk.

An object of the present invention is to provide a signal switching circuit that improves crosstalk and is suitable for IC implementation.

The above and other objects and novel features of the present invention will become apparent from the present specification and the accompanying drawings.

[Failure to solve the problem]

A brief summary of typical inventions disclosed in this application is as follows.

That is, a first input amplifier to which a first input signal is supplied, a second input amplifier to which a second input signal is supplied, and the output of the first input amplifier is supplied to the first input thereof. and a signal switch whose second input is supplied with the output of the second input amplifier, and which selectively transmits one of the outputs of the first and second input amplifiers to its output. wherein the first input amplifier includes a first variable impedance means, the second input amplifier includes a second variable impedance means, and the signal switch transmits the output of the first input amplifier. The second variable impedance means attenuates the output signal of the second input amplifier, and when the signal switch transmits the output of the second input amplifier, the first variable impedance means attenuates the output signal of the second input amplifier. It is characterized in that the output signal of the first input amplifier is attenuated.

[Effect]

According to the above means, for example, when the signal switch transmits the output of the first input amplifier, the l'T of the second input amplifier is
f The impedance of the impedance means decreases and the second
Since the amount of signal transmission regarding the input signal of
The amount of signal transmission related to the input signal is also reduced, and the object of the present invention, which is to improve the crosstalk of the signal switching circuit, can be achieved.

〔Example〕

An embodiment of a signal switching circuit to which the present invention is applied will be described below with reference to FIG. Note that FIG. 1 shows a circuit diagram of a signal switching circuit implemented as an IC.

In this embodiment, from terminal No. 1 to point A constitutes a first signal transmission path I, from terminal No. 2 to point A constitutes a second signal transmission path I, and in the present invention, plural This corresponds to the signal transmission path of

A common signal switch 2 is provided for the plurality of signal transmission paths I, I[, and input signals Vin 1+Vi
It is configured to obtain either one of n2 as the output signal Vout. In addition, switches Sa, Sa.

sb and Sb' are interlocked, and although shown in the drawing as if they were mechanical structures, they are actually composed of transistor switches. In the illustrated switching state, the signal transmission path I is activated and the signal transmission path ■ is inactivated.

Hereinafter, the circuit operations will be explained in order, and parts that perform the same circuit operations will be given the same reference numerals with a ``' to avoid duplication of explanation.

First, select the input signal Vin 1 and output the °C output signal Vout.
The circuit operation in the case where the input signal Vin 2 is obtained and the input signal Vin 2 is cut off will be explained.

The reference voltage Vre f and the transistors Q, ,Q,' are
A bias voltage determined by Vref-Vbe is supplied to the first stage transistors Q, , Q, and ' of the input amplifier 1.1'.

In the input amplifier 1.1', the transistor Qs.

Qs' operates as variable impedance means in the present invention.

The input signal Vin 1 is supplied to the base of the transistor Q2 constituting the first input amplifier 1 according to the present invention.

Constant current circuit C8, emitter follower transistor Q,
becomes a load, and obtains an output signal Vo. At this time, since the switch Sa is in the on state, the voltage divided by the voltage dividing resistors R, , R is lower than the bias voltage Vr e f - Vbe, the transistor Q is turned off, and the impedance seen from its emitter is becomes high impedance.

Therefore, the emitter follower transistor Q.

The output signal Vo is supplied to the base of a transistor Q constituting the signal switch 2.

The input signal Vin 2 is supplied to the bases of transistors Q,' which constitute the second input amplifier 1' in the present invention. Constant current circuit cs,' is emitter follower transistor Q! ', and an attempt is made to obtain the output signal vO', but at this time, the switch Sa' is in the off state, so the divided voltage by the voltage dividing resistors R1' + R4' is smaller than the bias voltage Vr e f - Vbe. much higher, transistor Qs' turns on and transistor Q,' turns off. Therefore, the output impedance seen from the emitter of the transistor Qs' becomes an extremely small value, and the crosstalk component of the input signal Vi port 2 transmitted through the parasitic capacitance between the base and emitter of the transistor Q,' in the off state is 1 which is an alternating current grounding point through a small impedance of Q,′! The signal is shunted to the source VCC and is difficult to be transmitted to the base of the transistor Q,' of the signal switch 2.

The signal switch 2 includes a first differential pair transistor Q.

Q5, the second differential pair transistors Q4'+Q5' constitute a current barrier, and the first . Transistor Q, which becomes a common load for the second differential pair transistor Q, ,Q, ,Q,',Q,' , emitter follower output transistor Q8, switches sb, sb', constant current circuit cs , , cs;

Since the switch sb is on, the first differential pair transistors Q, , Q are in the operating state, and the switch Sb
' is off, the second differential pair transistors Q4r and Q5' are inactive.

Therefore, the first differential pair transistor Q41Q in the operating state
s, current mirror Qs r Q? , emitter follower output transistor Q8 constitute a voltage follower circuit, and the input signal Vin 1 is taken out as the output signal Vout.

A transistor Qs' as a variable impedance means is provided in the second input amplifier 1' (if not, a part of the input signal Vin 2 supplied via the base-emitter junction of the transistor Qt' is in the off state). A current mirror Q, , Q is formed through a large capacitance C' between the base and collector of the transistor Q4'.

The input transistor Q traps the signal and causes undesirable losstalk.

On the other hand, according to the present invention, the impedance of the variable impedance means Qs' is set to a small value, and as described above, the crosstalk component of the input signal Vin2 is bypassed to the power supply Vcc, which is the AC grounding point. It becomes possible to reduce the amount of crosstalk.

Next, the input signal Vinl is cut off, and the input signal δ Vin2 is
The circuit operation when selecting is explained below.

In this case, the switches Sa and Sb are turned off, and the switches Sa' and Sb' are turned on. This switch Sa
As a result, the divided voltage across the voltage dividing resistors R, , R, becomes much higher than the bias voltage Vr e f -Vbe, transistor Q is turned on, and transistor Q is turned off. Therefore, the output impedance seen from the emitter of transistor Q has an extremely small value, and the crosstalk component of input signal Vin 1 transmitted via the parasitic capacitance between the pace emitter of transistor Q in the off state is The signal is bypassed to the power supply Vcc at a point, and is difficult to be transmitted to the transistor Q of the signal switching 52. On the other hand, by turning on the switch Sa, the voltage divided by the voltage dividing resistors R1' and R2' becomes lower than the bias voltage Vref - Vbe, and the transistor Qs turns off, and the impedance seen from its emitter becomes high (
Thus, an output signal Vo' responsive to the input signal Vin 2 is obtained and supplied to the pace of the transistor Q: of the signal switch 2.

Switch sb is turned off, switch Sb' is turned on, and the second differential pair transistors Q4', Qs', current mirror Q, Q, and emitter follower output transistor Q, which are in the operating state, constitute a voltage follower circuit, Input signal Vin 2 is taken out as output signal Vout.

If a transistor Q, as a variable impedance means is not provided in the first input amplifier 1, the part of the input signal Vin 1 supplied via the pace-emitter junction of the transistor Q, is in the off state. ,
The current mirror Q, ,Q through the large capacitance C between the pace and collector of.

is transmitted to the input transistor Q, which causes undesirable losstalk.

On the other hand, according to the present invention, the impedance of the variable impedance means Qs'' is set to a small value, and as described above, the crosstalk component of the input signal Vin 1 is bypassed to the power supply VCC, which is the AC grounding point, and the crosstalk component is It becomes possible to reduce the amount.

The signal switching circuit shown in the above embodiment has the following effects.

(1) A variable impedance means for controlling the amount of signal transmission is provided in a plurality of signal transmission paths, and while signal transmission is being performed by one of the plurality of signal transmission paths, the other signal transmission path is inactive. Since the impedance of the variable impedance means is reduced together with the filter, and signal leakage from the other signal transmission path to one signal transmission path is reduced, the output signals of the plurality of signal transmission paths are connected to a common signal transmission path. Output can be obtained selectively from the circuit.

The effect of improving crosstalk in a signal switching circuit consisting of a plurality of signal transmission paths can be obtained.

(2) The impedance control shown in (1) K above and selective driving of multiple signal transmission paths can be performed by a transistor switch with a simple structure, so the I of the signal switching circuit
The effect that C conversion becomes easy can be obtained.

(3) With the above (11) and (21), the effect of improving signal transmission in the high frequency circuit can be obtained.

Above, the invention made by the present inventors has been specifically explained based on examples, but the present invention is not limited to the above examples (and various changes can be made without departing from the gist of the invention). Needless to say, for example, the number of signal transmission paths can be increased further.

In the above explanation, the invention made by the present inventors was mainly applied to a signal switching circuit, which is the background field of application, but the invention is not limited to this (such as those shown in the known examples). It can be widely used not only for Eirin devices but also for various aCt child devices.

The invention can be used at least in an IC that requires a switch circuit for switching signal transmission.

〔Effect of the invention〕

A brief explanation of the effects obtained by typical inventions disclosed in this application is as follows.

In other words, in addition to transmitting signals through multiple signal transmission paths, when one signal transmission path is interrupted, the impedance is controlled to prevent signal leakage from that signal transmission path to other signal transmission paths. Since it is configured to reduce the crosstalk of the signal switching circuit, it is possible to obtain the effect of improving the crosstalk of the signal switching circuit.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a signal switching circuit to which the present invention is applied. I, l... Signal transmission path, 1, 1'... Input amplifier, 2... Signal switch, Sa, Sa', Sb,
Sb'--Switch, Q, ~Q6...Transistor %Vinl, Min 2--Input signal, Vout
...output signal, c, c'...transistor base-collector capacitance, R,, R,...resistance. β ~ to 2 “- clearing hole

Claims (1)

[Claims] 1. a first input amplifier supplied with a first input signal;
a second input amplifier supplied with a second input signal; a first input thereof supplied with the output of the first input amplifier;
a signal switch whose second input is supplied with the output of the second input amplifier, and which selectively transmits one of the outputs of the first and second input amplifiers to its output; The first input amplifier includes a first variable impedance means, the second input amplifier includes a second variable impedance means, and the signal switch transmits the output of the first input amplifier. The second variable impedance means attenuates the output signal of the second input amplifier, and when the signal switch transmits the output of the second input amplifier, the first variable impedance means attenuates the output signal of the second input amplifier. A signal switching circuit characterized in that the output signal of an input amplifier is attenuated.