JPH0526828Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) This invention relates to a signal switching circuit. In particular, it relates to an input signal switching circuit for an oscilloscope.

(Prior art and its problems) Generally, a signal switching device for an oscilloscope has two to four channels of input and one output. The circuit shown in FIG. 5 is a three-channel signal selection circuit, which is an example of a signal switching circuit having three input terminals 1, 2, and 3 and an output terminal 18. In the state shown in the figure, the input terminal 1 is selected and connected to the output stage 17 via the switch circuit 7 which is in the ON state. Switch circuits 7, 8, and 9 turn the signal ON/OFF,
Selects 3 channel signals. in this case
When OFF, the collectors 22 of the input stages 4, 5, and 6 are short-circuited, and a bias current is supplied so that the input stages operate like DC even when OFF. Switching controller 13
(usually TTL level) control signal 10,
Usually one of 11 and 12 is "H",
In other words, a state in which one channel is selected (hereinafter referred to as the first state) is normal, but a state in which all channels are temporarily not selected (hereinafter referred to as the first state), such as in an oscilloscope that can display characters on the CRT surface. A state (hereinafter referred to as the second state) and a state in which two specific channels are selected and signals are added (hereinafter referred to as the third state) are required. On the other hand, since the path through which the bias current flows to the input emitter grounding stages 4, 5, and 6 changes depending on the ON/OFF state of the switch circuits 7, 8, and 9, the currents at the summing point 19, 20 in the second or third state. will change from 0 to 2I. If the variable current sources 15 and 16 that provide bias current to the input stage 4, 5, and 6 that are turned ON and the output stage 17 only flow a constant current, then the changes in the currents 19 and 20 will directly affect the output stage 17. This will appear in the output, and the DC voltage at the output terminal 18 will fluctuate greatly. Conventionally, to compensate for this variation,
In response to the output logic of the switching controller 13, the variable current source 15,
The configuration was to control 16. However, in order to actually logically detect the first to third states mentioned above, the circuit becomes complicated and the number of parts increases. On the other hand, as another method for solving this problem, there is a conventional circuit in which switch sections 7 to 9 are configured as shown in FIG.
When the base voltage 25 of the transistors 46 and 47 is made lower than the base voltage V _B of the transistors 48 and 49, the transistors 48 and 49 are turned on, and the differential input signal is transmitted from the input terminal 22 to the output terminal 23. On the other hand, when the base voltage 25 becomes higher than V _B , the transistors 46 and 47 are turned on, the differential signal is canceled, and only the bias current I flows from the output 23 to the input 22. In other words, the signal is ON/OFF
In either state, the bias current flowing from the output 23 does not change. Therefore, if each of the switch circuits 7 to 9 is implemented using the circuit shown in FIG. 4, the above-mentioned problem of operating point fluctuation can be solved. However, this circuit has a drawback in that the potential difference between the input terminal 22 and the output terminal 23 cannot be reduced. That is, when the potential difference between the input and output terminals 22 and 23 becomes large, the range of the power supply voltage is also limited, and therefore it becomes necessary to perform level shifting in a subsequent circuit to lower the operating point to a lower level. For example, it is difficult to create a level shift circuit with good high frequency characteristics in a multi-stage directly connected amplifier such as an oscilloscope, so the rise in DC voltage at each stage must be kept as small as possible.
Here, the reason why the potential between the input terminal 22 and the output terminal 23 cannot be lowered is that the value of the resistor R cannot be lowered. That is, the resistor R serves as a load resistance for differential signals when the transistors 48 and 49 are ON, and also serves as a drive resistance for the next stage.
The next stage is a parallel feedback circuit, which has a low input resistance, but if the resistor R becomes too small, the current shunted to the resistor R will increase, leading to a gain error. In other words, the value of the resistor R must be large. However, if this is increased, a voltage drop will occur due to the current I flowing through the resistor R when the circuit is OFF, that is, when the transistors 46 and 47 are ON, so 46,
In order to keep V _CB of 47 above 0V, RI + V _BE 46,
A voltage of 47 is required between the input and output (between 22 and 23). In other words, if the value of the resistor R is increased, the potential between the input and output must be increased. Furthermore, even if the value of the resistance R is increased, it will be disadvantageous if the number of channels increases. It is the output terminal 23 of the circuit in Figure 4.
A plurality of channels are connected, and the drive resistance at the next stage becomes 1/the number of channels, leading to a gain error and being disadvantageous.

(Purpose) This invention solves the drawbacks of the conventional example shown in Fig. 5 that the circuit is complicated and the drawback that the potential difference between the inputs of the switch circuit section becomes high as in the conventional example shown in Fig. 4. Add the bias current flowing to the input emitter grounding stage of the channels for the number of channels,
It is equipped with a circuit that detects the number of channels that are OFF based on changes in current, and flows the appropriate current to the input point of the output stage.

(Example) General explanation of the idea In this method of switching signals of multiple channels using a current switch using a transistor or a diode, it is necessary to flow a current through the switch when the switch is turned on. In a circuit where only one channel is always selected, the current flowing through the switch circuit is always one channel's worth, so no matter which channel is selected, it does not affect the bias state of the subsequent circuit. , when attempting to add two channels (third state) or create a state in which all channels are OFF (second state), the current flowing through the switch circuit changes, so it is necessary to correct the change. The present invention has made this correction means as simple as possible and has less influence on the circuit. FIG. 1 shows a signal switching device according to the present invention, and a concrete example thereof will be described in detail below with reference to FIGS. 2 and 3. Note that FIG. 2 shows the case of two-channel input. Signals input from input terminals 1 and 2 are output from output terminal 18. The input emitter grounding stages 4 and 5 are driven with a constant current I as in the conventional example shown in FIG. 5, and the differential signal current appearing at the output is transmitted in a superimposed manner. Reference numerals 30 and 31 indicate an example of a switch circuit, which is a combination of a diode and a transistor, and is turned on and off by controlling the base potentials of the transistors 36, 37, 38, and 39 externally. In this case, if the voltage sources 32, 33 are lower than the potential of the connection points 34, 35, the diodes 40, 41 or 42, 43
is turned on, the bias current I flows, and the differential signal current is superimposed on it and transmitted to the output stage 17. Further, the voltage sources 32 and 33 are connected to the connection points 34 and 3 mentioned above.
When the potential becomes higher than 5, transistors 36 and 37
Alternatively, 38 and 39 are turned on, and the bias current I flows through these transistors. At this time, since the collectors of each transistor are coupled, the differential signal current superimposed on the bias current is canceled at the confluence point 44, and only the bias current I flows toward input stages 1 and 2, and the signal is transmitted to 34 points. Not done. Such input stages 1 and 2 and switch circuit 3
By providing the necessary number of channels with the combination of 0 and 31, signals of the necessary channels can be selected and transmitted to the connection points 34 and 35. If this continues, the current I flowing from the connection point 34 to the input in the second and third states (all channels OFF, two channels added) will be 0 in the second state and 0 in the second state, as described above. In state 3, it changes to 2I, and the operating point of output point 18 changes. Therefore, the fifth
Figure: As in the conventional example, it was connected to a fixed voltage source V ⁺ .
OFF channel bias current supply terminals were connected to each channel and supplied to a voltage source via a resistor R _B as in the embodiment of FIG. As a result, the current flowing through the resistor R _B is the number of OFF channels x 2I, which is 44
The voltage at the point changes depending on the number of OFF channels (or conversely, the number of ON channels). This change is reflected by the emitter follower transistor 45 and the resistor.
R _A causes the current to flow into the connection point 34 as a current change. The operation will be specifically explained in the case of two-channel input as in the embodiment. The number of selected channels is 0 (second state), 1 (first state), 2 (third state).
When the state of
The current flowing through the resistor R _B changes by -2I to 4I, 2I, 0, and the voltage at the junction 44 changes by 2I· _RB . If we set R _A = 2R _B , the change in the current flowing through R _A to 44 points will be a constant voltage at 44 points because the output stage 17 is a parallel feedback circuit, so 2I・R _B /R _A = I and I It changes gradually. In other words, since current is supplied from _RA according to the number of channels selected, the bias current to the output stage does not change, and the operating point of the output point also does not change.

Further, FIG. 3 is a circuit diagram showing another embodiment of the switch circuits 30 and 31, and this circuit has a diode 40.
.about.43 are replaced with transistors 44 and 45.

(Effects) The output DC voltage correction means according to the switch state according to this invention can be realized by adding a simple circuit, and there is no need to set a high potential difference between the input and output of the switch stage. It is possible to realize a circuit configuration that also reduces the DC potential difference between input and output, making it suitable as a signal switch for an oscilloscope.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of the present invention, FIGS. 2 and 3 are circuit diagrams showing an embodiment of the present invention, and FIGS. 4 and 5 are circuit diagrams of a conventional example. 1, 2, 3... Input emitter grounding stage, 18...
Output parallel feedback stage, 7, 8, 9, 30, 33... Current switch circuit, 32, 33... Variable voltage source,
R _B ... Current detection means, 45 ... Emitter follower, R _A ... Voltage-current conversion means.

Claims (1)

[Scope of utility model registration request] In a multi-input single-output signal switch, there are multiple switch circuits that select whether to output the differential signal input as is or to short-circuit the positive and negative sides of the differential signal, and the difference between the output sides of the multiple switch circuits. Connect dynamic signals of the same polarity to create a single differential signal output.
Furthermore, at least two series resistors connecting the positive and negative terminals of the differential signal output of the single output, a connection point of the two series resistors and an emitter are connected, and the base is connected to the line on the short-circuited output side of all of the plurality of switch circuits. A signal switching device comprising a connected emitter follower circuit, and a common power source connected to the base of the emitter follower circuit and the connection point via a resistor.