JPH062347Y2 - Signal switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an improvement of a signal switching circuit, and more particularly to a signal switching circuit that prevents shock noise during switching.

(B) Conventional technology By selectively switching a plurality of input signals by a control signal,
An IC (integrated circuit) version of the output signal switching circuit is described on page 976 of "'85 Sanyo Semiconductor Handbook Monolithic Bipolar Integrated Circuit Edition" issued by CQ Publishing Co., Ltd. on March 20, 1960. Has been done. The signal switching circuit has a circuit configuration as shown in FIG. In FIG. 2, (1) is a first input terminal, (2) is a second input terminal, (3) is an output terminal, and (4) is a control terminal.

Now, if the "H" level control signal is applied to the base of the transistor (6) forming the differential amplifier circuit ( 5 ), the transistor (6) is on and the transistor (7) is off. , The transistor (8) is on and the transistor (9) is off. The first input signal from the first input terminal (1) is a capacitor (10), transistors (11), (8), and (1
It is led to the output terminal (3) via 2).

Further, in this state, if the "L" level control signal is applied to the base of the transistor (6), the transistor (6) is turned off and the transistor (7) is turned on. The transistor (8) turns off and the transistor (9) turns on. Then, the second input signal from the second input terminal (2) receives the capacitor (13), the transistors (14), (9), and (12).
Through the output terminal (3).

Therefore, according to the circuit of FIG. 2, the first and second input terminals are
Control terminal (4) for the first and second input signals from (1) and (2)
Selectable according to the control signal from the output terminal
It can be derived in (3).

(C) Problems to be solved by the device By the way, in the transistors (15) and (16) shown in FIG. 2, the power source (17) is connected to the base, and the emitters are respectively resistors (18).
And (19) to the bases of the transistors (14) and (11) to bias the bases of the transistors (11) and (14). The resistors (18) and (19) are resistors for a buffer, and the first and second input terminals (1) and
A relatively large resistance value is required to efficiently apply the first and second input signals from (2) to the bases of the transistors (11) and (14).

Now, if the control signal from the control terminal (4) is at "L" level, the transistor (7) is turned on, so the transistors (9) and (14) are turned on. At this time, the transistor (14)
The emitter voltage of its base current is a transistor (15)
Since it is supplied from the emitter of V _B −2V _BE −ΔV
(However, V _B is the voltage of the power supply (17), V _BE is the base-emitter voltage of the transistor, ΔV is the transistor (14)
It is the product of the base current and the resistance (18). Therefore, a DC voltage equal to the emitter voltage is also generated at the output terminal (3). On the other hand, since at this time the transistor (6) is off, the transistor (11) is also turned off, the DC voltage V _B -V _BE from the emitter of the transistor (16) to the base is applied.

When the control signal is switched to the “H” level, the transistor (6) of the differential amplifier circuit ( 5 ) is turned on and the transistor (7) is turned on.
Turns off. On the other hand, since the collector voltage of the transistor (6) decreases, the transistor (11) starts to turn on, but at this time, the base current of the transistor (11) is supplied by the discharge of the charge accumulated in the capacitor (10), emitter voltage of the transistor (11) keeps the state of _V B _{-2 V bE,} then lowered to _V B _{-2V bE -ΔV} depending on the time constant. That is, in the circuit of FIG. 2, the voltage drop Δ
Due to V, there is a problem that the DC voltage fluctuates when switching the input signal and shock noise occurs.

(D) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and one of the first and second input signals is generated according to the first and second control signals having opposite polarities. In the signal switching circuit for switching and selecting, the first input signal is applied to the base, the first transistor operates according to the first control signal, and the second input signal is applied to the base, and the first control signal is applied according to the second control signal. A second transistor that operates, a first current supply circuit that supplies a base current of the first transistor in response to the first control signal, and the second transistor
A second current supply circuit for supplying a base current of the second transistor according to a control signal.

(E) Function According to the present invention, the base current flowing when the first transistor is turned on is supplied to the base of the first transistor according to the control signal. It can be kept constant regardless of whether it is on or off.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention. (20A) is a first input terminal to which a first input signal is applied, and (20B) is a second input terminal.
Second input terminal to which an input signal is applied, (21A) and (21B) are capacitors for blocking direct current, and (22A) and (22B) are first and second control terminals to which a control signal of opposite polarity is always applied. , (23) is an output terminal, (24A) is a first transistor to which the first input signal is applied, and (25A) is the first transistor.
A transistor that applies a bias voltage to the base of (24A),
(26A) is a resistor for buffer, (27A) is the first control terminal (22A)
A second transistor ( 28A ) which is turned on / off according to a control signal from the second transistor (27A) and a diode (2
9A) and a first current mirror circuit consisting of a transistor (30A), ( 31A ) is a diode (32A) and a transistor (33A)
A second current mirror circuit, (34) is a first switch circuit, (35) is a second switch circuit having the same configuration as the first switch circuit (34), and (36) is the first switch circuit. A transistor to which the first input signal from (34) or the second input signal from the second switch circuit (35) is applied to the base.

Next, the operation will be described. If the "L" level control signal is applied to the second control terminal (22B) of the second switch circuit (35), the third current mirror circuit ( 28B ) will not operate in response to the control signal. , The second transistor (27B) and the transistor (30B) are off, therefore the first transistor
The power supply voltage (+ V) is applied to the emitter of (24B) through the resistor (37B).
_CC ) is added, so the first transistor (24B) is off. Therefore, the second input signal from the second input terminal (20B) is not transmitted, and the operation of the second switch circuit (35) is stopped.

When a "H" level control signal is applied to the first control terminal (22A) of the first switch circuit (34), the first current mirror circuit ( 2
8A ) and an equal current I is applied to the second transistor (27A).
A), flows into the transistor (30A). Then, the first transistor (24A) and the transistor (39A) start to turn on. At this time, a current equal to the current I flowing through the collector-emitter path of the second transistor (27A) flows through the transistor (38A), so that the diode (32A) forming the second current mirror circuit ( 31A ) has almost no current. A current of / β (where β is the current amplification factor) flows. Therefore, a current equal to the current (I / β) is supplied from the collector of the transistor 33A to the base of the first transistor 24A. When the first transistor (24A) is turned on, the current flowing through its collector-emitter path is approximately I. Therefore, the base current of the I / β can sufficiently cover the base current of the first transistor (24A). Therefore, no current flows from the emitter of the transistor (25A) to the base of the first transistor (24A), and the base voltage of the first transistor (24A) is V _B −V _BE (where V _B is the power source (40)). Voltage) and keep that state. Therefore, the first transistor (24A)
When is turned on, its emitter voltage becomes V _B -2V _BE , and a DC voltage equal to it is generated at the output terminal (23). Therefore, the first input signal from the first input terminal (20A) is obtained at the output terminal (23) via the capacitor (21A), the first transistor (24A), the transistors (39A) and (36).

Next, the case where the control signal is switched to the opposite state will be described. "L" level control signal is the first control terminal (22A)
, The operation of the first current mirror circuit ( 28A ) is stopped, the collector of the second transistor (27A) rises to almost the power supply voltage, and the first transistor (24A) is turned off. The operation of the circuit (34) is stopped.

On the other hand, when the "H" level control signal is applied to the second control terminal (22B), the third current mirror circuit ( 28B ) starts to operate and the collector voltage of the second transistor (27B) starts to decrease. Then, the first transistor that was off until now
The base current is supplied from the collector of the transistor (33B) to the base of (24B) by the above-described operation, and the first transistor (24B) is turned on. At this time, since the base current is not supplied from the emitter of the transistor 25B, the base voltage of the first transistor 24B remains V _B -V _BE . Therefore, when the first transistor (24B) is turned on, its emitter voltage is also V _B -2 V _BE, and the same equal DC voltage is generated at the output terminal (23). That is, the DC voltage equal to that in the case where the first input signal is derived is generated at the output terminal (23), and the DC voltage does not fluctuate at all.

Therefore, avoid the effect of shock noise when switching the second input signal from the second input terminal (20B), and
B), the first transistor (24B), and the transistor (39B),
It can be led to the output terminal (23) via (36).

As a result, according to the circuit of FIG. 1, the first and second control terminals are
According to the control signals from (22A) and (22B), the first and second
It is possible to selectively switch the input signal without including shock noise at the time of switching and lead it to the output terminal (23).

Although the first and second control terminals (22A) and (22B) are always applied with control signals of opposite polarities, they may be configured with a differential amplifier circuit as in the prior art. Alternatively, an inverter may be inserted.

(G) Effect of the Invention As described above, according to the present invention, it is possible to prevent the shock noise generated when the signal switching circuit is switched. or,
If the base current is supplied according to the current supply means as in the embodiment, it is suitable for an IC.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional signal switching circuit. (20A) ... first input terminal, (22A) ... first control terminal, (23) ... output terminal, (24A) ... first transistor, (40) ... power supply, (26A)
... resistor, (27A) ... second transistor, ( 28A ) ... first current mirror circuit, ( 31A ) ... second current mirror circuit, (33A)
... transistor.

Claims (1)

[Scope of utility model registration request] 1. A signal switching circuit for switching and selecting one of a first input signal and a second input signal in response to first and second control signals having opposite polarities, wherein a first input signal is applied to a base. 1 transistor, a first current mirror circuit that operates according to the first control signal to operate the first transistor, and an output current of the first current mirror circuit is 1 / β (where β is the current amplification of the transistor). Rate) current, a second current mirror circuit that supplies the output current of the first current conversion means to the base of the first transistor, and the second input signal is applied to the base. Second
A transistor, and a third current mirror circuit that operates according to the second control signal to operate the second transistor,
Second current conversion means for converting the output current of the third current mirror circuit into a current of 1 / β; and a fourth current mirror circuit for supplying the output current of the second current conversion means to the base of the second transistor. A signal switching circuit comprising: