JP2007067548A - Mute circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of a conventional mute circuit for controlling a mute operation of a sound output apparatus that takes much time for restoration from mute-on on the way of an ordinary operation. <P>SOLUTION: The mute-on operation in an ordinary operation disclosed herein is carried out by turning on a switch SW to discharge electric charges in a capacitor Cm so as to lower a base voltage Vin1 of a transistor Q8 more than a base voltage Vref2 of a transistor Q9. A diode D1 is connected to the base of the transistor Q8 so as to clamp the base voltage Vin1 so that the base voltage Vin1 is not lower than a voltage Vref1 supplied from a clamp reference power supply. Thus, the time until the Vin1 exceeds the Vref2 is reduced by re-charging after the SW is turned off so as to quicken the restoration from the mute-on state. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a mute circuit that suppresses generation of unnecessary sound such as abnormal noise when power is turned on in an electronic circuit such as a low-frequency amplifier, and more particularly to a mute operation during steady operation of the electronic circuit.

  A low-frequency amplifier used in an audio output device or the like has a mute function in order to suppress abnormal noise generated when the power is turned on. On the other hand, even during normal operation when the power source of the audio output device or the like is in a steady state, the mute function is used for the purpose of blocking unnecessary input signals to the low frequency amplifier. For example, the mute function in normal operation is used to switch preset radio stations on the radio by button operation, and the mute function is activated in conjunction with the operation of the preset button to switch the audio at the time of tuning. Becomes smooth.

  FIG. 4 is a circuit diagram showing a configuration of a conventional mute circuit. This mute circuit is supplied with Vcc as a drive power source common to a target circuit such as a low-frequency amplifier to be muted. The charging circuit 2 receives the supply of Vcc and generates and outputs a current for charging the capacitor Cm. When the supply of Vcc is started, the capacitor Cm that has been in a discharged state until then is charged by the charging circuit 2, and the base voltage Vin1 of the transistor Q8 rises.

  The transistor Q8 forms a differential pair with the transistor Q9, and the emitters of both transistors are connected to a common constant current source I. A constant voltage source is connected to the base of the transistor Q9, and the base voltage Vin2 is kept at the reference voltage Vref. In the differential pair consisting of npn transistors Q8 and Q9, when Vin1 is lower than Vin2, Q8 is turned off and Q9 is turned on, and current I flows between the emitter and collector of Q9. On the other hand, when Vin1 is higher than Vin2, Q8 is on and Q9 is off, and basically no current flows between the emitter and collector of Q9.

  A current mirror circuit comprising transistors Q7 and Q5 is connected between Q9 and Vcc, and a mirror current corresponding to the emitter-collector current of Q9 flows through Q7 through Q7. The current flowing through Q5 is output from the terminal OUT as a mute control signal for controlling the mute operation of the target circuit.

FIGS. 5 and 6 are timing diagrams for explaining the operation of the mute circuit. FIG. 5 is a mute operation when Vcc is started up, and FIG. 6 is a diagram for explaining a mute operation during a normal operation. 5 (a) and 6 (a) show the time change of the base voltage Vin1 of the transistor Q8, and the voltage Vin1 changes according to the charging voltage of the capacitor Cm. FIGS. 5B and 6B show temporal changes in the mute control signal output from the output terminal OUT. 5, when the Vcc is raised at time t _0, the charging of the capacitor Cm is started, until time t ₂ when the capacitor Cm is charged voltage is fully charged reaches the saturation level, Vin1 is gradually rises . The operation of the differential pair described above, in the lower period than Vin1 is Vref from time t _0, the current I flows in Q9, the current I is output from the terminal OUT as the mute control signal. In response to the mute control signal, the target circuit such as a low frequency amplifier turns on the mute operation. On the other hand, when the charging of the capacitor Cm proceeds and Vin1 becomes a value in the vicinity of Vref, the current balance between the transistors Q8 and Q9 constituting the differential pair is rapidly reversed (time t ₁ ). After time t _1, in the normal operation state where Vin1 exceeds the Vref, current output from the terminal OUT as the mute control signal is essentially zero. In response to the mute control signal, the target circuit turns off the mute operation.

The mute operation in the normal operation state is realized by the switch SW that can discharge the capacitor Cm. The switch SW is kept off in the mute off state, and the capacitor Cm is maintained at a charging voltage higher than Vref. On the other hand, when the mute is turned on, the switch SW is turned on to discharge the capacitor Cm, and then the switch SW is returned to the off state. As a result, the same operation as the above-described Vcc supply start is performed. FIG. 6 shows the mute operation in this normal operation state. Vcc is the period from the time t ₃ when in the normal operating state is maintained in a steady state until the time t _4, the switch SW is turned on. When turning on the switch SW at time t _3, the capacitor Cm is discharged rapidly, and the voltage drops to approximately ground potential. When returning the switch SW to OFF state at time _{t 4,} Vin1 as with from time _{t 0} at Vcc startup until _{t 2} is increased (time _t 4 _~t 6). Current output from the terminal OUT as the mute control signal, from the time t _3, until time t ₅ vicinity Vin1 exceeds Vref, next current value I, during this time, it is possible to mute the desired behavior the circuit is turned on.

  In the conventional mute circuit, when the switch SW is turned on to turn on the mute operation in the normal operation state, the capacitor Cm is almost discharged. As a result, the voltage of the capacitor Cm at the time when the switch SW is turned off to turn off the mute operation is close to the initial discharge voltage at the start of the supply of the drive power supply Vcc. Charging starts. That is, since the difference between the voltage at the start of charging and the reference voltage Vref at which the differential pair is inverted is relatively large, it takes a relatively long time to charge the capacitor Cm to Vref. Here, the required amount and allowable amount of delay time (off-time recovery time) from when the switch SW is turned off to when the mute operation is switched to the off state may vary depending on the purpose of use of the mute operation in the normal operation state. Therefore, depending on the purpose of use, the conventional mute circuit has a problem that the off-time recovery time may become unnecessarily long.

  The mute circuit according to the present invention is discharged by each of the supply stop of the drive power to the target circuit and the discharge operation of the mute switch, and is charged by the start of supply of the drive power or the end of the discharge operation of the mute switch. A reference voltage for supplying a predetermined reference voltage, using a capacitor to be started, and generating a mute control signal for controlling on / off of the mute operation of the target circuit based on a charging voltage of the capacitor A differential pair including a source and a pair of transistors, a differential voltage input terminal having a reference voltage terminal connected to the reference voltage source and a control voltage terminal connected to the capacitor, the control voltage terminal A differential circuit that changes an on / off state of the two transistors according to the applied charging voltage and changes an operation state; To generate a current to charge the capacitor, and to change the charge voltage from an initial discharge voltage before starting the supply of the driving power supply or a switch operation discharge voltage during a discharge operation of the mute switch to a full charge voltage. A charging circuit for changing the operation state of the differential circuit from a state corresponding to mute on to a state corresponding to mute off, and a control signal generation for generating the mute control signal based on the operation state of the differential circuit A clamp circuit for clamping the switch operation discharge voltage to a predetermined voltage between the initial discharge voltage and the reference voltage.

  In another mute circuit according to the present invention, the clamp circuit includes a bias circuit that generates a bias current according to a current flowing through a differential transistor that is turned on when the mute is on, of the pair of transistors, A circuit generates the switch operation discharge voltage at the control voltage terminal based on the bias current.

  In another mute circuit according to the present invention, the bias circuit includes a current mirror circuit that generates a mirror current of a current flowing through the differential transistor and uses the current as a bias current.

  In another mute circuit according to the present invention, the bias circuit is inserted in the path of the bias current, and an offset voltage is generated that generates an offset voltage corresponding to a target value of the switch operation discharge voltage based on the bias current. And a diode having one terminal connected to the control voltage terminal and the other terminal to which the offset voltage is applied, the diode having the initial discharge voltage applied to the one terminal, When the offset voltage is applied, the forward bias is applied.

  In the mute circuit according to another aspect of the present invention, the offset voltage generation circuit supplies a voltage according to the target value of the switch operation discharge voltage, and an emitter is connected to the other terminal of the diode. And a clamp transistor that is turned on by the clamp reference power source that is supplied to the base between the emitter and the collector.

  In another mute circuit according to the present invention, the clamp transistor has an emitter-base diode junction direction that is the same as the diode direction with respect to the emitter.

  According to the present invention, when the mute operation in the normal operation state is turned on, the mute switch is operated to discharge the capacitor. This discharge changes the charging voltage of the capacitor from the fully charged voltage to the initial discharging voltage, but the switch circuit is set to a value between the reference voltage and the initial discharging voltage between the terminals of the capacitor. Since the discharge voltage is applied, the capacitor stops discharging in a state where the charge voltage becomes the switch operation discharge voltage. When the mute operation is turned off, charging starts from this state, so that the charge voltage quickly exceeds the reference voltage, the operation state of the differential circuit is inverted, and the mute control signal corresponds to mute off. Transition to the state. Therefore, it is possible to shorten the off-time recovery time.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

  FIG. 1 is a circuit diagram showing a schematic configuration of a mute circuit according to the present embodiment. The mute circuit is configured as a semiconductor integrated circuit (IC), for example, and is used as a part of a main unit such as various audio output devices using the low frequency amplifier, for example, for the purpose of controlling the mute operation of the low frequency amplifier. It is incorporated in the circuit of the main device.

  A voltage Vcc is supplied to a Vcc terminal, which is an external connection terminal provided in an IC package, as a drive power source common to a low frequency amplifier or the like that is a control target (target circuit) of a mute operation. One terminal of each of the capacitor Cm and the resistor R4 is connected to the Vm terminal which is an external connection terminal. The other terminal of the capacitor Cm is grounded. The other terminal of the resistor R4 is configured to be grounded via the switch SW.

  Capacitor Cm is charged by a charging circuit including transistor Q3. On the other hand, when the supply of the drive power supply Vcc is stopped by turning off the main power supply of the main unit, the charging circuit is also stopped, the capacitor Cm is discharged due to a leak current or the like, and the voltage at the terminal Vm decreases toward the initial discharge voltage. . For example, the initial discharge voltage becomes a ground potential or a value close thereto. The capacitor Cm is also discharged by turning on the switch SW. Specifically, when the switch SW is turned on, the terminal Vm side of the capacitor Cm is grounded via the resistor R4.

  In addition, the voltage at the terminal Vm to which one terminal of the capacitor Cm is connected is clamped by a clamp circuit including the diode D1 so as not to fall below a predetermined voltage.

  A differential input signal is applied between the bases of Q8 and Q9 in the differential pair composed of transistors Q8 and Q9. Here, since the base voltage of Q9 which is the reference voltage terminal is fixed to a predetermined reference voltage applied from the reference voltage source Vref2, the operation of the differential circuit including the differential pair is the control voltage terminal. It switches according to the change of the voltage applied to the base of Q8. As described above, the voltage applied to the base of Q8 is determined according to the charge state of the capacitor Cm and the control by the clamp circuit. The operation of the differential circuit is basically based on the charge voltage of the capacitor Cm being the reference voltage. Switches according to whether it is higher or lower than Vref2. The mute circuit generates a mute control signal based on the switching of the operation state of the differential circuit according to the charging state of the capacitor Cm, and outputs the mute control signal from the output terminal OUT to the target circuit. Based on this, the mute operation is turned on / off.

  Hereinafter, the configuration and operation of the mute circuit will be described in detail. First, the charging circuit will be described. The charging circuit has resistors R1 and R2 connected in series between Vcc and the ground potential, and one terminal connected to the connection point between these R1 and R2, and smoothes the voltage divided by R1 and R2 to ripple And a capacitor Cr for removing. The voltage smoothed by the capacitor Cr is applied to the base of the npn transistor Q1 through the resistor R3. The emitter of the transistor Q1 is applied with a predetermined positive voltage by the constant voltage source Vr, and the collector is connected to the power source Vcc via the diode-connected transistor Q2. Q1 is turned on by applying a positive voltage to the base, and a current flows between the emitter and the collector. Then, the collector current of Q1 generates a mirror current I6 in Q3 which forms a current mirror circuit together with Q2, and this is supplied to the terminal Vm as a charging current of the capacitor Cm.

  The differential circuit includes a differential pair composed of npn transistors Q8 and Q9 and a constant current source I commonly connected to the emitters of the transistors Q8 and Q9. As described above, the base voltage Vin1 of Q8 changes according to the charging voltage of the capacitor Cm, while the base voltage Vin2 of Q9 is set to Vref2. In the differential pair, when Vin1 is lower than Vin2, Q8 is off and Q9 is on, and current I flows between the emitter and collector of Q9. On the other hand, when Vin1 is higher than Vin2, Q8 is turned on and Q9 is turned off, and the current I from the constant current source flows between the emitter and collector of Q8, and basically a current flows between the emitter and collector of Q9. Absent. A diode-connected transistor Q7 is inserted between Q9 and Vcc, and a current basically flows through Q7 through the collector of Q9.

  The clamp circuit includes a current mirror circuit composed of Q7 and Q6. This current mirror circuit generates a bias current I2 at Q6, which is a mirror current corresponding to the current flowing through Q9. An offset voltage generating circuit and a diode D1 are connected to the collector of Q6. The offset voltage generation circuit includes a transistor Q4 and a clamp reference power supply Vref1 that is a constant voltage source. The collector of Q6 is connected to the emitter of Q4, the collector of Q4 is grounded, and the base is applied with a predetermined positive voltage from Vref1, which is a constant voltage source. Q4 is turned on by applying a voltage Vref1 to the base, and a current I4 flows. The emitter voltage in the ON state is set to a voltage corresponding to Vref1. The diode D1 has an anode connected to the connector of Q6 (emitter of Q4) and a cathode connected to the base of Q8 (or terminal Vm).

  When the cathode voltage decreases, the diode D1 is forward-biased and passes a current I3 that is a part of the collector current of Q6. In this forward bias state, the cathode voltage is set according to the anode voltage. That is, in the normal operation state where Vcc is rising, the voltage at the terminal Vm to which the cathode of the diode D1 is connected is clamped so as not to fall below the voltage corresponding to the offset reference voltage Vref1.

  In the present mute circuit, in order to set a lower limit for the voltage at the terminal Vm, the diode D1 is arranged such that its cathode is connected to the terminal Vm. Correspondingly, Q4 is a pnp-type transistor in which the direction of diode connection between the emitter and the base viewed from the collector of Q6 is the same as that of the diode D1. Here, when the junction potential difference between the emitter and base of the transistor is expressed as Vbe, the voltage of the emitter of Q4 in the ON state is Vref1 + Vbe. The junction potential difference of the diode D1 can also be basically handled as Vbe, and considering its direction, the cathode voltage of the diode D1 becomes Vref1. In other words, in this configuration, the clamp voltage at the terminal Vm can be basically set to the offset reference voltage Vref1 without the influence of Vbe, so that the clamp voltage is hardly affected by the temperature change and the accuracy of the mute control is improved. Is possible.

  Transistor Q5 forms a current mirror circuit together with Q7. The current mirror circuit composed of Q5 and Q7 constitutes a control signal generation circuit that generates a mute control signal based on the operating state of the differential circuit, and a mirror current I5 corresponding to the current flowing through Q9 is used as a mute control signal. Output from the output terminal OUT to the target circuit.

  FIG. 2 is a timing chart for explaining the operation of the present mute circuit when the drive power supply Vcc is raised. FIG. 2A shows the time change of the base voltage Vin1 of the transistor Q8. The voltage Vin1 changes according to the charging voltage of the capacitor Cm. FIG. 2B shows a time change of the mute control signal output from the output terminal OUT.

The mute operation when the drive power supply Vcc is raised will be described with reference to FIG. At time t _0, when the Vcc is raised, the charging of the capacitor Cm is started. The voltage of the capacitor Cm at the start of charging is an initial discharge voltage that is basically 0 V. The charging voltage of the capacitor Cm is a saturation level (full charge voltage) at which the capacitor Cm is fully charged from the initial discharge voltage. until the time _{t 2} to reach, Vin1 is gradually rises. The operation of the differential pair described above, in the lower period than Vin1 is Vref2 from time t _0, the current I flows in Q9, a current I5 output from the terminal OUT as the mute control signal and a value corresponding to the current I Become. In response to the mute control signal, the target circuit such as a low frequency amplifier turns on the mute operation. On the other hand, when the charging of the capacitor Cm progresses and Vin1 becomes a value near Vref2, the current balance between the transistors Q8 and Q9 constituting the differential pair is rapidly reversed (time t ₁ ). After time t _1, in the normal operation state where Vin1 exceeds the Vref2, current I5 output from the terminal OUT as the mute control signal is essentially zero. In response to the mute control signal, the target circuit turns off the mute operation.

  Next, the mute operation in the normal operation state will be described. The mute operation during the normal operation state is realized by the switch SW. The switch SW is kept off in the mute off state, and keeps Vin1 at a voltage higher than Vref2. In this state, Q9 is in an off state, and the current I5 output from Q5 to the output terminal OUT is basically 0. To switch from this state to the mute-on state, the switch SW is turned on to discharge the capacitor Cm, and Vin1 is set to a voltage lower than Vref2. As a result, Q9 is turned on, and the current I5 output from Q5 to the output terminal OUT becomes I. On the other hand, to switch from the mute on state to the mute off state, the switch SW is returned to the off state. As a result, the capacitor Cm is charged with the current I6 from the charging circuit, Vin1 recovers to a voltage higher than Vref2, and the output current at the terminal OUT becomes zero.

FIG. 3 is a timing chart for explaining the operation of the mute circuit in the normal operation state. 3A and 3B, FIG. 3A shows the time change of the base voltage Vin1 of the transistor Q8, and FIG. 3B shows the time change of the mute control signal output from the output terminal OUT. . Vcc is the period from the time t ₃ when in the normal operating state is maintained in a steady state until the time t _4, the switch SW is turned on. When the switch SW is turned on, the charging voltage of the capacitor Cm decreases. When this charging voltage falls below Vref2, the state of the differential pair Q8, Q9 is inverted, Q8 is turned off, and Q9 is turned on. When Q9 is turned on, the clamp circuit described above operates. That is, a current flows through Q4, the anode voltage of the diode D1 is set to Vref1 + Vbe, and the forward-biased diode D1 functions so that its cathode voltage does not fall below Vref1. As a result, even if the switch SW is turned on, the charging voltage of the capacitor Cm can be reduced only to Vref1. Note that Vref1 is set to a voltage lower than Vref2 so that when the switch SW is turned on, the state of the differential pair is inverted and mute-on is realized. When returning the switch SW to OFF state at time t _4, Vin1 by recharging by current I6 starts to rise from Vref1 (time t _4). Unlike the conventional mute circuit described with reference to FIG. 6, the charging voltage at the start of recharging is Vref1 higher than the initial discharge voltage (0 V), so that the time until it exceeds Vref2 is shortened compared to the conventional case. . Incidentally, in FIG. 3A, the change of the charging voltage shown in FIG. 6A is shown by a dotted line for comparison. In this muting circuit, the current output from the terminal OUT as the mute control signal, as shown in FIG. 3 (b), from time t ₃ to time t _{5 'near} the Vin1 exceeds Vref2, the current value I. That is, the off-time recovery time is a period t _{4 to} t ₅ ′, which is shorter than the conventional period t _{4 to} t ₅ . In addition, in the conventional mute circuit, the off-time recovery time is constant, but in this mute circuit, the off-time recovery time according to the target circuit is set by changing the clamp voltage Vref1 when the switch SW is turned on. Is possible.

1 is a circuit diagram showing a schematic configuration of a mute circuit according to an embodiment of the present invention. FIG. 5 is a timing diagram for explaining a mute operation when a drive power supply Vcc of the mute circuit according to the embodiment of the present invention is started up. FIG. 6 is a timing diagram illustrating a mute operation in a normal operation state of the mute circuit according to the embodiment of the present invention. It is a circuit diagram which shows the structure of the conventional mute circuit. FIG. 10 is a timing diagram for explaining a mute operation when a driving power supply Vcc of a conventional mute circuit is raised. It is a timing diagram explaining the mute operation in the normal operation state of the conventional mute circuit.

Explanation of symbols

  Cm, Cr capacitors, SW switches, Q1-Q9 transistors, R1-R4 resistors, Vref1 offset reference power supply, Vref2 reference voltage source.

Claims (6)

Using a capacitor that is discharged by each of the supply stop of drive power to the target circuit and the discharge operation of the mute switch, and charging is started by the start of supply of the drive power or the end of the discharge operation of the mute switch, In a mute circuit for generating a mute control signal for controlling on / off of a mute operation of the target circuit based on a charging voltage of the capacitor,
A reference voltage source for supplying a predetermined reference voltage;
A differential pair comprising a pair of transistors and having a reference voltage terminal connected to the reference voltage source and a control voltage terminal connected to the capacitor as a differential signal input terminal, is applied to the control voltage terminal A differential circuit that changes an on / off state of the two transistors according to the charging voltage and changes an operation state;
A current is generated based on the drive power supply to charge the capacitor, and the charge voltage is changed from an initial discharge voltage before starting the supply of the drive power supply or a switch operation discharge voltage during a discharge operation of the mute switch to a full charge voltage. A charging circuit that changes the operating state of the differential circuit from a state corresponding to mute on to a state corresponding to mute off;
A control signal generation circuit that generates the mute control signal based on an operating state of the differential circuit;
A clamp circuit for clamping the switch operation discharge voltage to a predetermined voltage between the initial discharge voltage and the reference voltage;
A mute circuit comprising: The mute circuit according to claim 1, wherein
The clamp circuit includes a bias circuit that generates a bias current corresponding to a current flowing through a differential transistor that is turned on when the mute is turned on among the pair of transistors.
The bias circuit generates the switch operation discharge voltage at the control voltage terminal based on the bias current;
A mute circuit characterized by The mute circuit according to claim 2,
The mute circuit, wherein the bias circuit includes a current mirror circuit that generates a mirror current of a current flowing through the differential transistor and uses the current as a bias current. In the mute circuit according to claim 2 or 3,
The bias circuit includes:
An offset voltage generation circuit that is inserted into the path of the bias current and generates an offset voltage corresponding to a target value of the switch operation discharge voltage based on the bias current;
A diode having one terminal connected to the control voltage terminal and the offset voltage applied to the other terminal;
Have
The diode is forward biased with the initial discharge voltage applied to the one terminal and the offset voltage applied to the other terminal;
A mute circuit comprising: The mute circuit according to claim 4, wherein
The offset voltage generation circuit includes:
A clamp reference power supply for supplying a voltage corresponding to the target value of the switch operation discharge voltage;
A clamp transistor having an emitter connected to the other terminal of the diode, a path of the bias current between the emitter and the collector, and being turned on by the clamp reference power source supplied to a base;
A mute circuit comprising: The mute circuit according to claim 5,
The clamp transistor has an emitter-base diode junction orientation that is the same as the diode orientation with respect to the emitter,
A mute circuit characterized by

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