JPH05343970A - Signal changeover circuit - Google Patents

Abstract

PURPOSE:To decrease crosstalk between output terminals in the signal changeover circuit in which an input signal is switched and outputted to output terminals of 2 systems. CONSTITUTION:This circuit is the signal changeover circuit having an input terminal 16, a switching terminal 17 and output terminals 18, 19 and emitters of transistors(TRs) 20, 21 whose bases connect to ground in terms of AC are respectively connected to the output terminals 18, 19. Then either of the output terminals 18, 19 is to be worked is selected by a setting voltage of the switching terminal 17 and the current is switched so as to supply a current to the TR 20 or 21 connecting to the output terminal 18 or 19 in the inactive state. By the current switching, since the impedance of the inactive output terminal is low, the leakage of a signal via a junction capacitance of the TR is suppressed and crosstalk between the output terminals is reduced.

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 capable of switching and outputting a high frequency input signal to two output terminals.

[0002]

2. Description of the Related Art A conventional signal switching circuit will be described below. FIG. 2 is a circuit diagram of a conventional signal switching circuit.
2, 3 and 4 are NPN transistors, 5 and 6 are load resistors,
7 is a differential amplifier composed of NPN transistors 1 and 2 and a load resistor 5, 8 is a differential amplifier composed of NPN transistors 3 and 4 and a load resistor 6, 9 and 10 are currents to the differential amplifiers 7 and 8. Is a constant current source, 12 is a bias resistor, 13, 14 and 15 are constant voltage sources, 16 is an input terminal, 17 is a switching terminal, and 18 and 19 are output terminals.

The operation of the signal switching circuit configured as above will be described below. When the set voltage of the switching terminal 17 is higher than the voltage of the constant voltage source 15, the current given by the constant current source 11 flows in the collector of the NPN transistor 9, and the current does not flow in the collector of the NPN transistor 10. That is, the NPN transistor 9 and the constant current source 1
1 and 1 will work as a constant current means. At this time, the differential amplifier 7 is in the operating state, and the signal input from the input terminal 16 is amplified by the differential amplifier 7 and output from the output terminal 18. On the other hand, since no current flows in the differential amplifier 8, it does not operate as an amplifier and no signal is generated at the output terminal 19.

Next, the set voltage at the switching terminal 17 is the constant voltage source 1.
When the voltage is lower than 5, the current given by the constant current source 11 flows in the collector of the NPN transistor 10, and NP
No current flows in the collector of the N-transistor 9. That is, the NPN transistor 10 and the constant current source 11 work as constant current means. At this time, the differential amplifier 8 is in the operating state, and the signal input from the input terminal 16 is amplified by the differential amplifier 8 and output from the output terminal 19. On the other hand, since no current flows in the differential amplifier 7, it does not operate as an amplifier and no signal is generated at the output terminal 18.

As described above, it is possible to switch which of the output terminals 18 and 19 is to be operated by the set voltage of the switching terminal 17.

[0006]

However, in the above-mentioned conventional configuration, when a high frequency signal is used as an input signal, the signal leaks to the output terminal which is not in operation via the junction capacitance of the transistor, so that there are two systems of outputs. There was a problem that crosstalk between terminals deteriorates.

An object of the present invention is to solve the above conventional problems and to provide a signal switching circuit with reduced crosstalk between output terminals.

[0008]

In order to achieve this object, the signal switching circuit of the present invention comprises an emitter of a first transistor whose base is AC grounded to a first output terminal and a first constant current means. Are connected to each other, and the second output terminal is connected to the emitter of the second transistor whose base is grounded in an AC manner and the second constant current means, and the first constant current means has the first output terminal not operating. The current flows only when the second constant current means flows the current only when the second output terminal is not operating.

[0009]

With this configuration, when the output terminal is not in the operating state, the impedance of the output terminal is extremely low, so that the signal leakage can be suppressed and the crosstalk with the operating output terminal can be reduced.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a signal switching circuit according to an embodiment of the present invention. In FIG. 1, 1, 2,
3, 4 are NPN transistors, 5 and 6 are load resistors,
8 is a differential amplifier, 9 and 10 are NPN transistors, 11
Is a constant current source, 12 is a bias resistor, 13, 14 and 15 are constant voltage sources, 16 is an input terminal, 17 is a switching terminal, 18 and 1
Reference numeral 9 is an output terminal, which has the same configuration as the conventional example. Further, 20, 21, 22, and 23 are NPN transistors, 24 is a constant current source, and 25 is a constant voltage source. The bases of the NPN transistors 20 and 21 are connected to the constant voltage source 25, and the emitters are connected to the output terminals 19 and 18, respectively. The base of the NPN transistor 22 is connected to the switching terminal 17 together with the base of the NPN transistor 9,
The base of the NPN transistor 23 is connected to the constant voltage source 15 together with the base of the NPN transistor 10.

The operation of the signal switching circuit of the present embodiment constructed as above will be described below. Switching terminal 1
When the set voltage of 7 is higher than the voltage of the constant voltage source 15, the output terminal 18 is in operation, and conversely, when the set voltage of the switching terminal 17 is lower than the voltage of the constant voltage source 15, the output terminal 19 is in operation. .. The output terminals 18 and 19 do not become active at the same time, and only one of them becomes active. These are the same as the operations of the conventional example.

Further, the set voltage of the switching terminal 17 is the constant voltage source 1.
When the voltage is higher than 5, the current given by the constant current source 24 flows through the collector of the NPN transistor 22,
No current flows in the collector of the transistor 23. That is, the NPN transistor 22 and the constant current source 24 work as constant current means. At this time, since a current flows through the NPN transistor 20 whose base is grounded in an AC manner, the impedance of the output terminal 19 becomes extremely low, and the leakage of signals to the non-operating terminal is significantly reduced. On the other hand, since no current flows through the NPN transistor 21, the impedance of the output terminal 18 in the operating state does not change and the output signal is not affected.

Next, the set voltage at the switching terminal 17 is the constant voltage source 1.
When the voltage is lower than 5, the current given by the constant current source 24 flows through the collector of the NPN transistor 23,
No current flows in the collector of the N-transistor 22. That is, the NPN transistor 23 and the constant current source 24 work as constant current means. At this time, since a current flows through the NPN transistor 21 whose base is grounded in an AC manner, the impedance of the output terminal 18 becomes extremely low, and the signal leakage to the non-operating terminal is significantly reduced. On the other hand, since no current flows through the NPN transistor 20, the impedance of the output terminal 19 in the operating state does not change, and the output signal is not affected.

As described above, according to this embodiment, the impedance of the output terminal which is not in operation can be lowered by switching the current of the grounded base NPN transistor connected to the output terminal, so that the signal leakage is suppressed. Be done. As a result, it is possible to reduce crosstalk with the output terminal in the operating state.

In particular, if the bases of the NPN transistors 9 and 22 are commonly connected to the switching terminal 17 as in this embodiment, the operating state and the impedance of the output terminal can be switched at the same time.

In this embodiment, the NPN transistors 20, 2 are used to switch the impedance of the output terminal.
Although 1, 22, and 23 are used, a PNP transistor may be used instead.

[0017]

As described above, according to the present invention, since the impedance switching transistor and the constant current source are connected to the output terminal, it is possible to suppress the signal leakage to the output terminal which is not in the operating state. It is possible to realize an excellent signal switching circuit capable of reducing crosstalk between them.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a signal switching circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional signal switching circuit.

[Explanation of symbols]

 16 Input Terminal 17 Switching Terminal 18 First Output Terminal 19 Second Output Terminal 20 NPN Transistor (Second Transistor) 21 NPN Transistor (First Transistor) 22 NPN Transistor (Second Constant Current Means) 23 NPN Transistor (First Transistor) Constant current means)

Claims (1)

[Claims] 1. An input terminal, a switching terminal, and a first and a second output terminal, one of which is selectively activated by a voltage applied to the switching terminal. Is connected to the emitter of a first transistor whose base is AC-grounded and the first constant current means, and to the second output terminal, the emitter and second constant of a second transistor whose base is AC-grounded. A current means is connected to the first constant current means, and a current flows only when the first output terminal is not in an operating state, and the second constant current means supplies a current only when the second output terminal is not in an operating state. A signal switching circuit characterized in that a current flows.