JP3454642B2 - Signal selection output circuit - Google Patents

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 in particular, switching when two input signals have different DCs (direct current levels). However, the present invention relates to a signal selection output circuit whose output DC can be changed (responsive) 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 (a DC voltage is also generated at the same time). Further, the voltage source (2) in FIG. 2 generates a DC signal (input 2) indicated by the alternate long and short dash line in FIG. DC of the alternate long and short dash line 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) of FIG. 2 is switched to the a side as shown in the figure, the capacitor (5) of the time constant circuit (4) shows the dotted line of FIG. 3 from the signal source (1). 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) is
Discharges from the level of the dotted line in FIG. 4 to the level of the alternate long and short dash line.

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

[0005]

However, the circuit of FIG. 2 has a problem that it takes time (T1) to switch from the input 1 to the 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), there is a problem 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 a resistor having one end connected to an input terminal and the other end connected to an output terminal, and a resistor connected to the other end of the resistor. A time constant circuit having a capacitor, and a first input signal connected to one input terminal, the other input terminal connected to the input terminal of the time constant circuit, and an output terminal connected to the input terminal of the resistor. A first voltage follower circuit having a dynamic amplifier, a second input signal applied to one input terminal, the voltage of the capacitor applied to the other input terminal, and an output terminal connected to the input terminal of the resistor. A second voltage follower circuit having a dynamic amplifier, and operates the first voltage follower circuit or the second voltage follower circuit according 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]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a signal selection output circuit of 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).
A time constant circuit having a resistor (12) connected to 1) and a capacitor (13) connected to the other end of the resistor (12), (14) has an input 1 at one input terminal and an input at the other A first voltage follower circuit (16) having a differential amplifier (15) whose terminal is connected to the input terminal (10) of the time constant circuit (9) and whose output side is connected to the input terminal (10), A second voltage having a differential amplifier (17) having one input terminal to which the input 2 is applied, and the other input terminal to which the voltage of the capacitor (13) is applied, 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 of a side as shown, and the operating current source (1
It is assumed that the current of 9) is flowing in 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 forming the differential amplifier (15) become equal. That is, the output current of the differential amplifier (15) is 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 input 1 develops at the base of transistor (22). For example, input now 1
Is 5V, the input terminal (10) of the time constant circuit (9)
Also, a DC voltage of 5V is generated. 5 to 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 rises to 5V.

In the state where the DC voltage of 5V is transmitted, the AC signal is also superimposed and output. Next, in order 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 through the differential amplifier (17). Then, as in the case of the first voltage follower circuit (14), the differential amplifier (17) has the second voltage follower circuit (16).
To work as. The current mirror circuit (20) and the operating current source (19) of the differential amplifier are commonly used in the case of the first voltage follower circuit (14) and the case of the second voltage follower circuit (16). Variation is reduced.

Now, assuming that the input 2 is 2V, since the capacitor (13) is charged with a voltage of 5V, the transistor (23) is turned off and the transistor (24) is turned on. When the transistor (24) is turned on, no base current flows through the base of the transistor (21), and the transistor (21) is turned off. At this time, the capacitor (1
The electric charge 3) is discharged through the resistor 12 and the constant current source 25. In the circuit configuration of FIG. 1, a loop is configured such that the voltage of the upper end (output terminal (11)) of the capacitor (13) becomes 2V, so that the input terminal (1
The voltage of 0) becomes lower than that. Then, the resistance (1
Since the voltage difference between both ends of 2) becomes large and the value of the discharge current becomes large, 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, but the same applies to the case where the voltage of the capacitor (13) is charged from a low value to a high value. 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. Moreover, the output DC can be changed (responsive) immediately even if the two input signals have different DCs.

[Brief description of 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 chart for explaining FIG.

FIG. 4 is a waveform diagram for explaining FIG.

[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)

(57) [Claims] 1. A time constant circuit having a resistor 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 at one input terminal. A first amplifier 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 resistor
A voltage follower circuit, and a second differential amplifier having a second input signal applied to one input terminal and the voltage of the capacitor applied to the other input terminal, the output terminal of which is connected to the input terminal of the resistor. And a voltage follower circuit for operating the first voltage follower circuit or the second voltage follower circuit according to the selection of the first input signal or the second input signal. A signal selection output circuit, wherein a more selected signal is derived. 2. A time constant circuit having a resistor 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 at one input terminal. And 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 in which a second input signal is applied to one input terminal and the voltage of the capacitor is applied to the other input terminal, and the output terminal is connected to the input terminal of the time constant circuit via the current mirror circuit. And a second voltage follower circuit having: a first voltage follower circuit or a second voltage follower circuit according to the 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.