JPH1065506A - Signal selection output circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable quick response of an output DC even when switching is performed while the DC of two input signals are different at the time of selectively outputting one of two input signals by providing a time constant circuit inside the feedback loop of a voltage follower circuit. SOLUTION: When a voltage follower circuit 14 is operated, feedback is loaded so as to equalize the base voltages of transistors consisting of a differential amplifier 15. As a result, the voltage equal to an input 1 is generated at the base of a transistor 22, a DC voltage is generated at an input terminal 10 of a time constant circuit 9, and charging to the time constant circuit 9 is performed. In such a state, an AC signal is outputted while being superimposed. When a switch 18 is changed over from such a state to the side of (b), the current of a current source 19 flows to a differential amplifier 17, and the differential amplifier 17 is operated as a voltage follower circuit 16. A current mirror circuit 20 and the current source 19 are also used as the circuits 14 and 16, and dispersion caused by elements 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 selection output circuit for selecting and outputting one of two input signals, and more particularly to switching when two input signals have different DC (direct current levels). The present invention also relates to a signal selection output circuit that can change (response) the output DC immediately.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional signal selection output circuit. A DC signal (input 1) indicated by a dotted line in FIG. 3 is generated from the signal source (1) in FIG. 2 (which also generates a DC voltage at the same time). A DC signal (input 2) indicated by a dashed line in FIG. 3 is generated from the voltage source (2) in FIG. The dashed line DC in FIG.
The level of the signal is lower than that of the DC signal indicated by the dotted line in FIG.

In this state, when the switch (3) in FIG. 2 is switched to the a side as shown in the figure, the capacitor (5) of the time constant circuit (4) is connected to the dotted line in FIG. Is charged to the level of the DC signal. From this state, when the switch (3) in FIG. 2 is switched to the b side, the level of the voltage source (2) is low, so the capacitor (5) of the time constant circuit (4)
Discharges from the level of the dotted line in FIG. 4 to the level of the dashed line.

As a result, an output signal having a changed DC level is obtained at the output terminal (6) by switching the switch (3).

[0005]

However, the circuit of FIG. 2 has a problem that it takes time (T1) to switch from input 1 to input 2 as shown in FIG. In the circuit of FIG. 2, since the discharge time is determined by the time constant of the resistor (7) and the capacitor (5), a problem arises when the time constant is long.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has one end connected to an input terminal, the other end connected to an output terminal, and another end connected to the other end of the resistor. A time constant circuit having a capacitor; a first input signal connected to one input terminal; a second input terminal connected to an input terminal of the time constant circuit; and an output terminal connected to an input terminal of the resistor. A first voltage follower circuit having a dynamic amplifier; a second input signal applied to one input terminal; a voltage applied to the capacitor applied to the other input terminal; and an output terminal connected to an input terminal of the resistor. A second voltage follower circuit having a dynamic amplifier, wherein the first voltage follower circuit or the second voltage follower circuit is operated in response to selection of the first input signal or the second input signal. Characterized by being adapted to derive a signal selected from the output terminal of the time constant circuit.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a signal selection output circuit according to the present invention. (7) is an input terminal to which an input 1 is applied, (8) is an input terminal to which an input 2 is applied, 9) has one end connected to the input terminal (10) and the other end connected to the output terminal (1).
1) A time constant circuit having a resistor (12) connected to the resistor (12) and a capacitor (13) connected to the other end of the resistor (12). A first voltage follower circuit having a differential amplifier (15) having a terminal connected to the input terminal (10) of the time constant circuit (9) and an output side connected to the input terminal (10); A second voltage having a differential amplifier (17) to which the input 2 is applied to one input terminal and the voltage of the capacitor (13) is applied to the other input terminal, and the output side of which is connected to the input terminal (10); It is a follower circuit.

First, the switch (18) in FIG. 1 is in the state on the a side as shown in FIG.
Assume that the current 9) is flowing through the differential amplifier (15).
Then, the differential amplifier (15) operates as a first voltage follower circuit (14). The voltage follower circuit generates a voltage equal to the input voltage.
Therefore, when the first voltage follower circuit (14) operates, feedback is applied so that the base voltages of the transistors constituting the differential amplifier (15) become equal. That is, the output current of the differential amplifier (15) is equal to the current mirror circuit (2
0), and is fed back to the base of the transistor (22) via the base / emitter of the transistor (21).

As a result, a voltage equal to the input 1 is generated at the base of the transistor (22). For example, input 1 now
Is 5V, the input terminal (10) of the time constant circuit (9)
Also generates a DC voltage of 5V. 5 for input terminal (10)
When a DC voltage of V is generated, the time constant circuit (9) is charged, and the voltage of the output terminal (11) also increases to 5V.

While the DC voltage of 5 V is transmitted, an AC signal is also superimposed thereon and output. Next, to switch from this state to the input 2, the switch (18) may be switched to the b side. When the switch (18) is switched to the b side, the current of the operating current source (19) of the differential amplifier flows to the differential amplifier (17). Then, as in the case of the first voltage follower circuit (14), the differential amplifier (17) is connected to the second voltage follower circuit (16).
Works as The current mirror circuit (20) and the operating current source (19) of the differential amplifier are shared between the case of the first voltage follower circuit (14) and the case of the second voltage follower circuit (16). Variation is reduced.

When the input 2 is set to 2 V, the transistor (23) is turned off and the transistor (24) is turned on because the capacitor (13) is charged with 5V. When the transistor (24) is turned on, the base current does not flow through the base of the transistor (21), and the transistor (21) is turned off. At this time, the capacitor (1
The charge of 3) is discharged via the resistor (12) and the constant current source (25). In the circuit configuration of FIG. 1, a loop is formed such that the voltage at the upper end (output terminal (11)) of the capacitor (13) becomes 2 V, so that the input terminal (1
The voltage of 0) is lower. Then, the resistance (1
2) The voltage difference between both ends becomes large, and the value of the discharge current becomes large, so that the discharge time becomes short.

In the above description, the case where the voltage of the capacitor (13) is discharged from a high value to a low value has been described. Conversely, the case where the voltage of the capacitor (13) is charged from a low value to a high value is the same. It is.

[0013]

As described above, according to the present invention, since the resistance of the time constant circuit is incorporated in the feedback loop of the voltage follower circuit, when one of the two input signals is selected and output. In addition, the output DC can be changed (responded) immediately even if switching is performed when the DC of the two input signals is different.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a signal selection output circuit of the present invention.

FIG. 2 is a block diagram showing a conventional signal selection output circuit.

FIG. 3 is a waveform diagram for explaining FIG. 2;

FIG. 4 is a waveform chart for explanation of FIG. 2;

[Explanation of symbols]

 (9) Time constant circuit (14) First voltage follower circuit (15) Differential amplifier (16) Second voltage follower circuit (17) Differential amplifier

Claims (2)

[Claims] A time constant circuit having a resistor having one end connected to the input terminal and the other end connected to the output terminal, and a capacitor connected to the other end of the resistor; and a first input signal connected to one input terminal. Has a differential amplifier whose other input terminal is connected to the input terminal of the time constant circuit and whose output terminal is connected to the input terminal of the resistor.
And a differential amplifier having a second input signal applied to one input terminal, a voltage of the capacitor applied to the other input terminal, and an output terminal connected to an input terminal of the resistor. A voltage follower circuit for operating the first voltage follower circuit or the second voltage follower circuit in response to selection of the first input signal or the second input signal, and the output terminal of the time constant circuit. A signal selection output circuit for deriving a more selected signal. 2. A time constant circuit having one end connected to an input terminal and the other end connected to an output terminal, and a capacitor connected to the other end of the resistor, and a first input signal connected to one input terminal. A first voltage follower circuit having a differential amplifier whose other input terminal is connected to the input terminal of the time constant circuit and whose output terminal is connected to the input terminal of the time constant circuit via a current mirror circuit; A differential amplifier having a second input signal applied to one input terminal, a voltage of the capacitor applied to the other input terminal, and an output terminal connected to the input terminal of the time constant circuit via the current mirror circuit; And a second voltage follower circuit having a second voltage follower circuit, and operates the first voltage follower circuit or the second voltage follower circuit in response to selection of the first input signal or the second input signal. Signal selection output circuit, characterized in that so as to derive a signal selected from the output terminal of the time constant circuit is.