JPS58215130A - Signal switching circuit - Google Patents

Abstract

PURPOSE:To eliminate level shift and to suppress offset, by using a common load being an output terminal as a constant current source in a signal switching circuit comprising transistors (TRs) mainly, and making its current value to about 1/2 of a current value of a constant current source connected to emitters of a TR group. CONSTITUTION:When a TR19 turns on with a switching signal at a switching signal input terminal 27, a TR20 turns off, and when the TR19 is turned on with a signal applied to an input terminal 25, a TR27 and a diode 21 are made conductive. Further, since the TR20 is turned off, a TR18 and a diode 22 are nonconductive and only the signal at the input terminal 25 is fed to an output terminal 29. When the switching signal turns on the TR20 only, the TR18 and the diode 22 are made conductive. Since the TR is turned off, the TR17 and the diode 21 are made conductive and only the signal applied to an input terminal 26 is fed to the output terminal 26. When a current value I2 of a constant current source 23 to which emitters of the TRs19, 20 are connected is I0, a constant current source 24 is used as the load and the current value I1 is taken as I0/2. The level shift arisen at signal switching is prevented by a simple constitution by setting the condition in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal switching circuit mainly composed of transistors, and aims to simplify the circuit and suppress the influence of the switch on the transmitted signal.

FIG. 1 shows an example of a conventional analog signal switching circuit. N
PN) Transistors 1 and 2 and 3 and 4 constitute first and second differential amplifiers, respectively, the collectors of transistors 1 and 3 are connected to a power supply 15 via a common load 7, and transistors 2° 4 are connected to the power source 15 via a common load 8. Still, the bases of transistors 1 and 4 are commonly connected to the constant voltage source 11, and the bases of transistors 2 and 3 are connected to the switching signal input terminal 12.
It consists of On the other hand, transistors 5 and 6 have equal emitter resistances 9 and 1o, respectively, and input signals to be switched are inputted from input terminals 13 and 14 connected to their bases, respectively. Each collector has an emitter of transistors 1 and 2 forming the first differential amplifier, and a transistor 3 forming the second differential amplifier.
It is connected to the emitter of 4. The two-person power signal input to the input terminals 13 and 14 is switched to the output terminal 16 and outputted by the signal input from the switching signal input terminal.

The operation in the above configuration will be explained.

In FIG. 1, when the voltage of the switching signal at the switching terminal 12 is sufficient to turn on transistors 1 and 4, transistors 2 and 3 are turned off, and the signal input to the signal input terminal 14 turns on transistor 4. From, output terminal 1
6 appears inverted.

On the other hand, the signal at the signal input terminal 13 is negative because transistor 2 is off and transistor 1 is on.
Even if the signal is transmitted to point a, which is the collector of , it is not output to the output terminal 16, and in the end, only the signal at the signal input terminal 14 is output.

Conversely, when the voltage of the switching signal at the switching terminal 12 is lower than that of the constant voltage source 11 and transistors 1 and 4 cannot be turned on, transistors 2 and 3 are turned on, and the signal input to the signal input terminal 13 is turned on. Since transistor 2 is on, it appears at output terminal 16 in an inverted form. - Since the signal at the force signal input terminal 14 is transmitted to point a, since transistor 4 is off and transistor 3 is on, it is not output to the output terminal 16, and in the end, only the signal at the signal input terminal 13 is transmitted. and output.

As described above, the signals applied to the input terminals 13 and 14 are switched by the signal at the switching signal input terminal 12, and the output terminal 16
is output to.

However, in the conventional example, the signal is inverted due to the structure in which the signal is taken out from the collector as described above. Not only signal switching but also level shifting must be performed at the same time, which may seriously impede signal transmission. Furthermore, differences in configuration tend to occur between the set of resistors 7 and 9 and the set of resistors 8 and 10, and as a result, an offset occurs at the time of switching, making it difficult to switch smoothly. Reducing this offset is relatively costly. The present invention is intended to alleviate the above-mentioned drawbacks of the conventional example, and one embodiment of the present invention will be described below with reference to FIG. 2.

In FIG. 2, 17 and 19 and 18 and 20 are a set of cascade-connected NPN transistors, respectively, and the emitters of transistors 19 and 20 are connected to a common first constant current source 23. The diodes 21, 22 are of equal characteristics, each cathode of which is connected to the transistor 1.
7 and 19 and 18 and 20, and each anode is connected to the second constant current source 2 in common.
4 and the output terminal 29.

The input signal is applied to input terminals 25.26 connected to the respective bases of transistors 17.18.

At this time, the bias of transistors 17 and 18 is the same as that of transistor 17 when transistor 19 or 20 is on.
Alternatively, 18 can also be set to be on. The switching signal is applied to input terminals 27 and 28 connected to the bases of transistors 19 and 20, but specifically, the signal applied to input terminal 27 turns on transistor 19 and turns off transistor 20. A signal having a polarity opposite to that of the input terminal 27 is applied to the input terminal 28 so that In other words, the signal is such that any one transistor is turned on.

Next, the operation of the above embodiment will be explained.

The switching signal of the switching signal input terminal 27 causes the transistor 1 to
When transistor 9 is turned on, transistor 20 is off, and when transformer 19 is turned on by the signal applied to input terminal 26, transistor 17 and diode 2 are turned on.
1 is also turned on. On the other hand, since one transistor 20 is off, transistor 18 and diode 22 are off,
Only the signal at input terminal 26 is transmitted to output terminal 29.

Conversely, when the switching signal turns on only transistor 20, transistor 18 and diode 22 turn on. On the other hand, since transistor 19 is off, transistor 17 and diode 21 are off, and only the signal applied to input terminal 26 is transmitted to output terminal 29. Here, the current value 2 of the first constant current source 23 is set to 10, the second constant current source 24 is used as a load, and the current value 1 is set to Al0.

According to the above configuration, the following effects can be obtained.

(-) Since the configuration of the constant current sources 23 and 24 is as described above, there are the following advantages.

The forward voltage drop VD of the diode 21 or 22 that is turned on is as follows.

On the other hand, when the transistor 17 or 18 is turned on, the emitter current Ix is 1-Iz- since I2:IO
It becomes I+ -172In. Therefore, here, Is
-2IS2 If the transistor and diode are selected, VBII=VD. Therefore, if the DC voltage level of the transmission signal is vXN, the DC level at the output is V
oUT is VOIIT =”In+ −VBK +VD
=VII+.

Therefore, by setting the above conditions, it is possible to obtain the effect that level shift is eliminated by signal switching with a simple configuration.

(b) Still, by making the characteristics of the opposing diodes and two transistors, in this case transistors 17 and 18, 19 and 20, and diodes 21 and 22, the same, the offset can be significantly improved while keeping the configuration simple. It has the effect of

This case has the advantage that it can be achieved relatively easily using an integrated circuit.(C) Since the output is basically taken out from the emitter of the input signal transistor, it has the advantage that the signal is not inverted.

(d) In addition to the above, as in the second embodiment shown in FIG. However, by connecting the anode of the diode 32 to the second constant current source 24, the number of input signals that can be switched can be increased without changing the current value of the constant current source.

That is, there is also the advantage that a multi-input selector switch can be easily configured by adding only the configuration of two transistors and a diode as in the above example.

An embodiment using a -PNP) transistor is shown in FIG. PNP) transistor 17'.

When used as transistors 18', 19', and 20', the anodes of the diodes are connected to the emitters of the transistors 17' and 18', and the cathodes of the diodes are connected to the constant current source 24' and the output terminal 29'. However, by making other connections similar to those of the circuit described above, the same effects as described above can be obtained.

As shown above, according to the present invention, level shifts can be removed, offsets can be suppressed, and signals can be prevented from being inverted.
Furthermore, it is possible to configure multiple inputs, making it highly useful as an analog switch, including applications in bipolar ICs.

[Brief explanation of the drawing]

Figure 1 is an electric circuit diagram showing a conventional signal switching circuit;
The figure is an electric circuit diagram showing one embodiment of the signal switching circuit of the present invention, and FIG.
FIG. 4 is an electric circuit diagram when a PNP type transistor is used, which is another embodiment of the present invention. 17.18, 19, 20, 30.31...NP
N transistors, 17', 18', 19', 20
'...PNP transistor, 21, 22,
32, 21', 22'...Diode+
23, 24, 23', 24'...constant current source. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3 Figure 4

Claims (4)

[Claims] (1) A constant current source and a plurality of NPN transistors each having an emitter connected to the constant current source;) A first transistor group consisting of a transistor; and a plurality of NPN transistors each having an emitter connected to each collector of the second group of first transistors. (NPN) comprising a second group of transistors consisting of transistors, a load serving as an output end, and a plurality of diodes whose anodes are connected to the output end of the load in common, and the cathodes of each of the diodes form a pair. It is connected to each emitter of the second transistor group and inputs a switching signal to each base of the first transistor group. A signal switching circuit characterized in that a signal to be switched is input to each base of the second transistor group. (2) A patent claim characterized in that the common load serving as the output end is a constant current source, and the current value thereof is approximately the current value of the constant current source connected to the emitter of the first transistor group. The signal switching circuit according to item 1. (3) a constant current source, a first transistor group consisting of a plurality of PNP transistors whose emitters are each connected to the current source, and a plurality of PNP transistors whose emitters are respectively connected to the collectors of the first transistor group) A second transistor group including a transistor group, a load serving as an output end, and a plurality of diodes whose cathodes are connected to the output end of the load in common, and each of the anodes of the diodes is connected to the second A signal switching circuit connected to each emitter of a transistor group, inputting a switching signal to each base of the first transistor group, and inputting a switching signal to each base of the second transistor group. . (4) Claim 3, characterized in that the common load serving as the output terminal is a constant current source, and its current value is approximately the same as the current value connected to the emitter of the first transistor group. The signal switching circuit described.