JPH0221784Y2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a muting circuit for a differential amplifier, and particularly to a muting circuit that controls the muting of a differential amplifier using a current-driven switching element.

BACKGROUND ART A conventional circuit of this type is shown in FIG. In the figure, PNP transistors 1 and 2 are differential transistors, and the input signal is applied between their bases.
Vi is applied. Both transistors 1,
Load resistors 3 and 4 are connected to each collector of 2, respectively, and a pair of differential outputs are output from each collector.
Vo has been derived. Note that 5 indicates a current source of the differential amplifier.

In order to mute this differential amplifier,
A pair of NPN transistors 6 and 7 are provided, with the emitter of transistor 6 and the emitter of transistor 7
The collectors of the transistors 6 and 7 are commonly connected and connected to one of the differential outputs, and the collectors of the transistors 6 and 7 are commonly connected to the other one of the differential outputs. A muting control signal Vc is supplied to the bases of both transistors 6 and 7 via a resistor 8.

In this configuration, if the muting control signal Vc is set to high level and transistors 6 and 7 are turned on simultaneously to short-circuit the differential outputs, the differential outputs have the same potential and opposite phases, so they cancel each other out. This results in a state of mutating.

However, since the transistors 6 and 7 are current-driven switching elements, when they are turned on, current caused by the base drive current flows out of each emitter and flows into the load resistors 3 and 4, respectively.
As a result, the DC potential of the differential output terminal changes between when the transistor is on and when the transistor is off, resulting in a so-called pop sound being output, which is inconvenient.

FIG. 2 shows another example of a conventional circuit, and parts equivalent to those in FIG. 1 are designated by the same reference numerals. In this example, diodes are used as current-driven switching elements, and the anodes of diodes 9 and 170 are connected in series, and the cathodes are connected to a pair of differential outputs. A muting control signal Vc is applied to the anode connection point via a resistor 8.

In this example as well, when the diodes 9 and 10 are on, the on-currents flow into the load resistors 3 and 4, respectively, causing pop noise.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a muting circuit for a differential amplifier that eliminates pop noises caused by muting on/off.

The muting circuit according to this invention is a differential amplifier muting circuit that performs muting by short-circuiting a pair of differential outputs of a differential amplifier using a current-driven switching element. Another current-driven switching element that has a drive current in the opposite direction to the drive current and is turned on at the same time as the switching element is provided between the differential outputs, and the currents generated by driving both switching elements are set to be equal. It is characterized by becoming.

Embodiments Examples of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram of an embodiment of the present invention.
Parts equivalent to those in the figures are indicated by the same reference numerals. In this example, instead of NPN transistor 7 in Fig. 1,
A PNP transistor 11 is used, the emitter of which is connected to the emitter of transistor 6, and the collector of transistor 11 to the collector of transistor 6. The base of this transistor 11 is supplied with the collector output of an NPN transistor 13 via a resistor 12. The emitter of this transistor 13 is grounded so that it operates as an inverter.

A muting control signal is connected to the base of the inverter transistor 13 via a resistor 15.
Vc is applied, and this signal Vc is also supplied to the base of transistor 6 via variable resistor 14 and resistor 8.

In such a configuration, if the control signal Vc is set to a high level as much as possible for muting, the NPN transistor 6 will be turned on and the differential outputs will be short-circuited to enable muting, but at this time the inverter transistor 13 will be turned on. A drive current is drawn from the base of the PNP transistor 11, and the transistor 11 is turned on.

Therefore, if the transistor 11 is configured to absorb all the emitter current due to the base drive current of the transistor 6, no current due to the drive current when the transistor is on will flow into the load resistor, making it possible to prevent pop noises from occurring. Therefore, there is. For this purpose, it is necessary to equalize the base drive currents of transistors 6 and 11, and this can be done by adjusting variable resistor 14.

In the above case, the current gains of both transistors 6 and 11 are equal, but if they are not equal, it is sufficient to make the emitter current of transistor 6 equal to that of transistor 11, and in this case, the variable resistor is also used. All you have to do is make 14 adjustments.

FIG. 4 is a diagram showing another embodiment of the present invention,
Parts equivalent to those in FIGS. 2 and 3 are designated by the same reference numerals. In addition to diodes 9 and 10, diode 1
6, 17, and diodes 16, 17 are provided.
Transistor 13 is connected by connecting the cathode of 17.
The collector output of is applied to this connection point via a resistor 12. The anodes of diodes 16 and 17 are respectively connected to differential outputs, and the other configurations are the same as the examples shown in FIGS. 2 and 3, and their explanation will be omitted.

In this case as well, if the on-currents of the diodes 9 and 10 and the on-currents of the diodes 16 and 17 are set equal, no drive current will flow through the load resistors 3 and 4 due to the switching on of the switching diodes.

FIG. 5 is a circuit diagram of another embodiment of the present invention,
This is a case where the present invention is applied to the circuit shown in FIG. A pair of PNP transistors 18 and 19 are added, and their emitters and collectors are connected to each of the pair of differential outputs, respectively. The collector output of the NPN transistor 31 is supplied to the bases of both transistors via a resistor 12. This transistor 31, diode 3
A current mirror circuit is formed by the transistor 25, the diode 24, and the resistors 27, 28.

A current mirror circuit is also configured by the transistor 26, the resistor 29, the diode 24, and the resistor 27, and the collector output of the transistor 26 becomes the base drive current of the transistors 6 and 7 via the resistor 8. . An input current to the diode 24 is supplied from the collector of the NPN transistor 21 via the variable resistor 22 and the resistor 23. A muting control signal Vc is applied to the base of this transistor 21 via a resistor 20.

By doing this, when the control signal Vc becomes high level, the base drive currents of transistors 6 and 7 and the base drive currents of transistors 18 and 19 can be equalized by the action of each current mirror circuit. Since this is possible, it is possible to prevent any current caused by the drive current of the muting transistor from flowing through the load resistors 3 and 4 during muting. In this case as well, NPN transistors 6 and 7 and PNP transistors 18 and 19
If the current gains are not equal, variable resistor 2
Of course, it is possible to make the emitter currents of transistors 6 and 7 equal to those of transistors 18 and 19 by adjusting 2.

It is obvious that the method of driving a switching element using this current mirror circuit is also applicable to the circuits shown in FIGS. 3 and 4. Furthermore, the circuit example described above is not limited to the illustrated example, and various modifications can be made.

Effects As described above, according to the present invention, the current flowing into the differential output load due to the drive current of the current-driven switching element for muting is canceled, so that the muting operation is suppressed. No potential fluctuation occurs at the output end during on/off. Then,
It is possible to prevent pop noises from occurring.

[Brief explanation of the drawing]

1 and 2 are diagrams showing a conventional muting circuit of a differential amplifier, and FIGS. 3 to 5 are circuit diagrams of embodiments of the present invention. Explanation of symbols of main parts, 1, 2... Differential transistor, 3, 4... Load resistor, 6, 7... NPN transistor for switching, 9, 10... Diode for switching, 11, 19... For switching PNP transistor, 16, 17... Switching diode.

Claims (1)

[Claims for Utility Model Registration] (1) A muting circuit for a differential amplifier in which muting is performed by short-circuiting a pair of differential outputs of a differential amplifier using a current-driven switching element. Another current-driven switching element, which has a drive current in the opposite direction to the drive current of the switching element and is turned on at the same time as the switching element, is provided between the differential outputs, and the switching element is switched on. A muting circuit characterized in that the currents generated by driving are set equally. (2) The muting circuit according to claim 1, wherein the current-driven switching elements are transistors of opposite conductivity types. (3) The current-driven switching element includes a pair of diodes whose anodes are commonly connected to each other and whose cathodes are connected to the differential output, and a pair of diodes whose cathodes are commonly connected and whose anodes are connected to the differential output. The muting circuit according to claim 1, which is a pair of diodes connected to each other.