JP5514036B2 - Audio amplifier and electronic device using the same - Google Patents

Description

  The present invention relates to an amplifier that amplifies an audio signal.

  An electronic device having a function of reproducing an audio signal such as a CD player, an audio amplifier, a car stereo or a portable radio or a portable audio player has an amplifier that amplifies the audio signal (hereinafter referred to as an audio amplifier). It is common to provide. Examples of the amplifier for audio include an AGC circuit, an electronic volume circuit, a power amplifier provided in front of a speaker, or a voltage follower type input buffer and output buffer.

  The audio signal is amplified by an amplifier and is finally output as sound from a speaker or headphones. The amplitude (level) of the audio signal is adjusted by controlling the gain of the amplifier or by controlling the attenuation rate of the attenuator.

Japanese Patent Laid-Open No. 2005-117489 JP 2005-217710 A JP 2004-222077 A Japanese Patent Laid-Open No. 2005-21796 Japanese Patent Laid-Open No. 11-75287

  The audio amplifier amplifies the audio signal with reference to a certain bias voltage. When this bias voltage is not the ground voltage (0 V), for example, when it is set to ½ of the power supply voltage Vdd, noise is generated when the power is turned on. FIG. 1 is an operation waveform diagram of the audio amplifier when the power is turned on. In the following waveform diagrams and the like, the vertical axis and the horizontal axis are appropriately enlarged or reduced for easy understanding, and each waveform shown is also appropriately simplified for easy understanding. ing.

  As the power is turned on, the power supply voltage Vdd increases from 0V. Ideally, the bias voltage Vbias that is ½ times the power supply voltage Vdd also rises continuously from 0 V together with the power supply voltage Vdd. However, the audio amplifier is composed of an inverting amplifier or a non-inverting amplifier using an operational amplifier, and the operational amplifier does not operate normally in a low voltage region where the power supply voltage Vdd is around 0V. Therefore, the output voltage Vout of the audio amplifier changes discontinuously from an ideal waveform as indicated by a broken line or a one-dot chain line. This discontinuity stimulates the user's hearing as a shock noise (pop noise).

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and one of exemplary purposes of an aspect thereof is to provide an audio amplifier that suppresses generation of noise at the time of activation.

  One embodiment of the present invention relates to an amplifier for audio. The amplifier for audio includes a bias circuit that divides a power supply voltage to generate a bias voltage, an on / off switchable amplifier circuit that amplifies an input audio signal based on the bias voltage, and a bias circuit A first switch provided between the output terminal and the output terminal of the amplifier circuit; and a controller that controls the on / off state of the first switch and the on / off state of the amplifier circuit.

“Amplifier circuit is on” means that an output signal corresponding to the input signal is generated. “Amplifier circuit is off” means that the input signal and the output signal are irrelevant, and the output is A state that has become high impedance.
According to this aspect, by turning on the first switch at the time of activation, the potential of the output terminal of the audio amplifier rises according to the bias voltage regardless of the operation of the amplifier circuit, so that noise can be suppressed.

  Another embodiment of the present invention is also an audio amplifier. This audio amplifier includes a bias circuit that divides a power supply voltage to generate a bias voltage, a variable resistor that attenuates an input audio signal with reference to the bias voltage, and an output signal of the variable resistor at its non-inverting input terminal. Input operational amplifier, first switch provided between the output terminal of the bias circuit and the output terminal of the audio amplifier, and provided between the output terminal of the operational amplifier and the output terminal of the audio amplifier The second switch and its first input terminal are connected to the output terminal of the operational amplifier, its second input terminal is connected to the output terminal of this audio amplifier, and its output terminal is connected to the inverting input terminal of the operational amplifier. And a control unit for controlling the first and second switches and the selector.

  Also according to this aspect, by turning on the first switch at the time of startup, the potential of the output terminal of the audio amplifier rises according to the bias voltage regardless of the operation of the operational amplifier, so that noise can be suppressed.

Yet another embodiment of the present invention is an electronic device. This electronic apparatus includes the audio amplifier according to any one of the above-described aspects that amplifies an audio signal, and an audio output unit that outputs an output signal of the audio amplifier.
According to this aspect, since the noise output from the audio output unit when the power is turned on is reduced, the listener can listen to the audio comfortably.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  The audio amplifier according to the present invention can suppress noise when the power is turned on.

It is an operation waveform diagram at the time of power-on of a general audio amplifier. It is a block diagram which shows the whole structure of the electronic device using the audio amplifier which concerns on 1st Embodiment. 3 is a time chart showing the operation of the audio amplifier of FIG. 2. It is a circuit diagram which shows the structure of the amplifier for audios concerning 2nd Embodiment. FIG. 3 is a circuit diagram illustrating a configuration example of an operational amplifier in FIG. 2.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are electrically connected in addition to the case where the member A and the member B are physically directly connected. It includes the case of being indirectly connected through another member that does not affect the connection state.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

(First embodiment)
FIG. 2 is a block diagram showing the overall configuration of the electronic apparatus 2 using the audio amplifier 100 according to the first embodiment. The electronic device 2 is a device including means for outputting sound, such as an audio amplifier, a car stereo, a portable audio player, and a mobile phone terminal. The audio amplifier 100 according to the present embodiment is mounted on the electronic device 2. For example, the audio amplifier 100 may be an electroacoustic transducer such as a speaker or an earphone, or a volume circuit that adjusts the volume of an audio signal output from the line-out.

  The electronic device 2 includes an audio amplifier 100 and an audio output unit 3. An audio signal generation unit (not shown) converts audio data stored in a disk medium, a memory, or a hard disk, or input from outside by wired or wireless, into an analog audio signal S1.

  The audio amplifier 100 amplifies the audio signal S1 with a preset gain. In this specification, “amplification” includes not only the case where the gain is higher than 1, but also the case where the gain is lower than 1, that is, attenuation. Therefore, “gain (amplification factor)” in this specification is a concept including an attenuation factor, and “amplifier” is a concept including an attenuator.

  The audio output unit 3 includes a resistor R5, a mute transistor 8, a power amplifier 4, and an electroacoustic conversion element 6. The electroacoustic transducer 6 may be a headphone or a speaker, or may be used as a line-out instead. The resistor R <b> 5 is provided between the output terminal 104 of the audio amplifier 100 and the input terminal of the power amplifier 4. The power amplifier 4 amplifies the audio signal S <b> 2 whose level is adjusted from the audio amplifier 100. The mute transistor 8 fixes the potential of the input terminal of the power amplifier 4 to the ground voltage (0 V) in the on state, and prevents the noise from the electroacoustic transducer 6 from being output. The resistor R5 and the mute transistor 8 can be omitted.

  Next, the configuration of the audio amplifier 100 will be described in detail. The audio signal S1 is input to the input terminal 102 of the audio amplifier 100 via a coupling capacitor (not shown) and an input resistor (not shown) provided in series with the signal path. The audio amplifier 100 amplifies the audio signal S1 input to the input terminal 102 with a set gain, and outputs the amplified audio signal S2 from the output terminal 104.

  The audio amplifier 100 includes a bias circuit 10, a variable gain amplifier 20, a first switch 30, a charge circuit 40, and a control unit 50.

  The bias circuit 10 divides the power supply voltage Vdd to generate a bias voltage Vbias. The bias circuit 10 includes a first resistor R1, a second resistor R2, and a capacitor C1. The resistance values of the first resistor R1 and the second resistor R2 are equal, and the bias voltage Vbias is the midpoint voltage Vdd / 2 of the power supply voltage Vdd and the ground voltage. The capacitor C1 is provided to stabilize the bias voltage Vbias and remove noise from the power supply voltage Vdd.

The variable gain amplifier 20 amplifies the input audio signal S1 with reference to the bias voltage Vbias, and changes its amplitude. The variable gain amplifier 20 includes ladder resistors Rd _{1 to} Rd _N , switches SW _{0 to} SW _N , an operational amplifier 22 and a buffer 24. Ladder resistors Rd _{1 to} Rd _N and switches SW _{0 to} SW _N form a variable resistor 26. The ladder resistors Rd _{1 to} Rd _N are connected in series to form a resistor string. The audio signal S1 is applied to one end of the resistor string, and the bias voltage Vbias is applied to the other end via the buffer 24. A switch SW _i is provided between the connection point between the adjacent resistors Rd _i and Rd _{i + 1} and the output terminal of the variable resistor 26. The variable resistor 26 attenuates the audio signal S1 with reference to the bias voltage Vbias.

The operational amplifier 22 is a voltage follower (buffer) whose output terminal and inverting input terminal are connected. The output terminal of the variable resistor 26 is connected to the non-inverting input terminal of the operational amplifier 22.
The gain of the variable gain amplifier 20 is maximized when the switch SW ₀ is turned on and minimized when the switch SW _N is turned on.

  The variable gain amplifier 20 is configured to be switched on and off according to the control signal CNT2. The OFF state of the variable gain amplifier 20 can be realized by cutting off the bias current of the operational amplifier 22. Alternatively, the operational amplifier 22 can be turned off by providing a switch in series with its output terminal and turning the switch off.

  The first switch 30 is provided between the output terminal 12 of the bias circuit 10 and the output terminal of the variable gain amplifier 20. The on / off state of the first switch 30 is switched according to the control signal CNT1.

  The control unit 50 generates control signals CNT1 and CNT2 to control the states of the operational amplifier 22 and the first switch 30.

The charge circuit 40 is connected to the output terminal 12 of the bias circuit 10 and is provided for charging the output terminal 12 and the capacitor C1. For example, the charge circuit 40 includes a transistor Q1, a third resistor R3, and a fourth resistor R4. The third resistor R3 and the fourth resistor R4 are connected in series between the power supply terminal and the ground terminal, and divide the power supply voltage Vdd. When R3 = R4, Vdd / 2 is generated. The transistor Q1 is an NPN bipolar transistor, the emitter thereof is connected to the output terminal 12 of the bias circuit 10, and the midpoint voltage Vdd / 2 is inputted to the base thereof. The charge circuit 40 causes the output terminal of the bias circuit 10 to be Vdd / 2−Vbe.
The battery is rapidly charged with the target value. Vbe is the base-emitter voltage of the bipolar transistor, and is about 0.7V.

  The above is the configuration of the audio amplifier 100. Next, the operation will be described. FIG. 3 is a time chart showing the operation of the audio amplifier 100 of FIG. The high level of the control signal CNT1 corresponds to the on state of the first switch 30, and the low level corresponds to the off state. The high level of the control signal CNT2 corresponds to the on state of the variable gain amplifier 20, and the low level corresponds to the off state.

At time t0, the power is turned on by the user, and activation starts. Almost simultaneously with the start of activation, the control signal CNT1 becomes high level and the control signal CNT2 becomes low level, the first switch 30 is turned on and the variable gain amplifier 20 is turned off. The switch SW _N of the variable resistor 26 is turned on, the other switches _SW 0 _{~SW N-1} is turned off.

  After time t0, the power supply voltage Vdd increases with time. The bias voltage Vbias obtained by dividing the power supply voltage Vdd also increases following the power supply voltage Vdd. At this time, since the output terminal 104 of the audio amplifier 100 is connected to the output terminal of the bias circuit 10 via the first switch 30, the output signal S2 of the audio amplifier 100 rises in accordance with the bias voltage Vbias. To go.

  After a predetermined second period τ2 has elapsed from the start of activation (time t1), the control signal CNT2 is switched to the high level, and the variable gain amplifier 20 is turned on. After a predetermined first period τ1 has elapsed from the start of activation (time t2), the control signal CNT1 is switched to the low level, and the first switch 30 is turned off. The second period τ2 is set longer than the time required for the power supply voltage Vdd to reach the minimum operating voltage of the operational amplifier 22. That is, after time t1, the operational amplifier 22 operates normally as a voltage follower.

After the time t1, the switch SW _N is turned on, the bias voltage Vbias is input to a non-inverting input terminal of the operational amplifier 22. When the operational amplifier 22 operates as a voltage follower, the potential of the output terminal 104 of the audio amplifier 100 is stabilized so as to coincide with the bias voltage Vbias.

  If the output of the variable gain amplifier 20 is in a high impedance state and the first switch 30 is turned off at the same time, the output terminal 104 is in a high impedance state and the potential becomes indefinite, which may cause noise. Therefore, the first period τ1 is set to a length equal to or longer than the second period τ2.

  The above is the operation of the audio amplifier 100.

  As described above, according to the audio amplifier 100 of FIG. 2, in the region where the power supply voltage Vdd is low and the variable gain amplifier 20 does not operate normally, the first switch 30 causes the potential S2 of the output terminal 104 to follow the bias voltage Vbias. be able to. After the power supply voltage Vdd rises to a region where the variable gain amplifier 20 operates normally, the first switch 30 is turned off and the variable gain amplifier 20 is turned on to cause the potential S2 of the output terminal 104 to follow the bias voltage Vbias. be able to.

  As shown in FIG. 3, since the output signal S2 rises gently and continuously, the noise generated from the electroacoustic transducer 6 can be made almost zero.

  Further, the charging circuit 40 provides the following effects. If the mute transistor 8 is on during time t0 to t1, the charge of the capacitor C1 is discharged via the first switch 30, the resistor R5, and the mute transistor 8. When the resistance value of the resistor R5 is smaller than the resistance value of the first resistor R1, the discharge current of the capacitor C1 due to the resistor R5 and the resistor R2 exceeds the charging current due to the resistor R1, so that the bias voltage Vbias may not sufficiently increase. There is. In this case, by providing the charge circuit 40, the capacitor C1 can be reliably and rapidly charged.

(Second Embodiment)
FIG. 4 is a circuit diagram showing a configuration of an audio amplifier 100a according to the second embodiment. The audio amplifier 100a in FIG. 4 is different from the audio amplifier 100 in FIG. 2 in the configuration of the variable gain amplifier 20a, and the other configurations are the same.

  The variable gain amplifier 20a in FIG. 4 includes a second switch 28 and a selector 29 in addition to the variable gain amplifier 20 in FIG. The second switch 28 and the selector 29 are provided to switch the variable gain amplifier 20a between on and off states.

  The second switch 28 is provided between the output terminal of the operational amplifier 22 and the output terminal 104 of the audio amplifier 100a. The first input terminal P1 of the selector 29 is connected to the output terminal of the operational amplifier 22, its second input terminal P2 is connected to the output terminal 104 of the audio amplifier 100a, and its output terminal P3 is the inverting input of the operational amplifier 22. Connected to terminal (-).

  The control unit 50a controls the first switch 30, the second switch 28, the buffer 24, and the variable resistor 26. Specifically, the control unit 50a turns on the first switch 30 for a predetermined first period τ1 from the start of activation, and then turns off the first switch 30. The second switch 28 is turned off for a predetermined second period τ2 from the start of activation, and then turned on. The selector 29 is turned on to the first input terminal P1 side for a predetermined third period τ3 from the start of activation, and then turned on to the second input terminal P2 side. The second switch 28 and the selector 29 are controlled according to the control signals CNT2a and CNT2b, respectively.

  The second period τ2 and the third period τ3 may be equal. The first period τ1 is preferably longer than the second period τ2 and the third period τ3, as in the first embodiment.

  According to the audio amplifier 100a of FIG. 4, the variable gain amplifier 20a can be turned on and off by controlling the second switch 28 and the selector 29. As a result, as with the audio amplifier 100 of FIG. 2, noise at startup can be suppressed.

  FIG. 5 is a circuit diagram showing a configuration example of the operational amplifier 22 of FIG. The operational amplifier 22 includes transistors M1 to M8 and a bias current source 23. The transistors M1 to M5 constitute an input stage, and the transistors M7 and M8 constitute an output stage. The transistors M1 and M2 are N-channel MOSFETs and constitute a differential pair. The transistors M3 and M4 are P-channel MOSFET current mirror circuits and operate as a load for the differential pair. The bias current source 23 generates a bias current Ibias. The transistor M6 is provided on the path of the bias current Ibias. The transistor M5 forms a current mirror circuit together with the transistor M6, and generates a tail current It corresponding to the bias current Ibias.

  The transistors M8 and M7 are stacked between the power supply line Vdd and the ground line GND. The transistor M7 forms a current mirror circuit together with the transistor M6, and generates a current corresponding to the bias current Ibias.

The first gate switch SW21 is provided between the gate and source of the transistor M7. The second gate switch SW22 is provided between the gate and source of the transistor M8. The first gate switch SW21 and the second gate switch SW22 are controlled to be turned on / off according to the control signal CNT2. When the first gate switch SW21 and the second gate switch SW22 are turned on, the output stages M7 and M8 are turned off, and the output of the variable gain amplifier 20 is in a high impedance state. As a result, the variable gain amplifier 20 can be turned off.
Alternatively, the operational amplifier 22 may be turned off and the variable gain amplifier 20 may be turned off by turning off the bias current source 23 and stopping the bias current Ibias.

  The configuration of the operational amplifier 22 is not limited to that shown in FIG. 5 and may be another configuration.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 2 ... Electronic device, 3 ... Audio | voice output part, 4 ... Power amplifier, 6 ... Electroacoustic transduction element, 8 ... Mute transistor, 10 ... Bias circuit, 20 ... Variable gain amplifier, 22 ... Operational amplifier, 24 ... Buffer, 26 ... Variable resistor 28 ... second switch 29 ... selector 30 ... first switch 40 ... charge circuit 50 ... control unit 100 ... audio amplifier 102 ... input terminal 104 ... output terminal R1 ... first resistor , R2 ... second resistor, R3 ... third resistor, R4 ... fourth resistor.

Claims (10)

A bias circuit that generates a bias voltage obtained by dividing the power supply voltage;
An amplifier circuit having a buffer circuit for receiving the bias voltage , configured to be switched on and off, and amplifying an input audio signal with reference to an output voltage of the buffer circuit ;
A first switch provided between a connection point between the output terminal of the bias circuit and the input terminal of the buffer circuit, and the output terminal of the amplifier circuit;
A control unit for controlling the on / off state of the first switch and the on / off state of the amplifier circuit;
An audio amplifier comprising: The controller is
The first switch is turned on for a predetermined first period from the start of activation, and then turned off.
2. The audio amplifier according to claim 1, wherein the amplifier circuit is turned off for a predetermined second period from the start of activation and then turned on.   The amplifier circuit is
  An operational amplifier connected to the output terminal and the inverting input terminal;
  One end thereof is connected to an input terminal to which the audio signal is input, the other end is connected to an output terminal of the buffer circuit, and a variable resistor that attenuates the audio signal with reference to the output voltage of the buffer circuit;
  The audio amplifier according to claim 1 or 2, characterized by comprising: Wherein, the second period supplied from the activation start of a predetermined bias current of the amplifying circuit turns off the amplifier circuit by stopping, then according to claim 1 or 2, characterized in that on Audio amplifier. The amplifier circuit includes an operational amplifier, and the operational amplifier includes a transistor constituting an output stage thereof, and a gate switch connected to a gate of the transistor,
The audio amplifier according to any one of claims 1 to 3 , wherein the amplifier circuit is configured to be switched between an on state and an off state according to an on state and an off state of the gate switch . The amplifier circuit is
An operational amplifier;
Including an output switch provided in series with the output terminal of the operational amplifier;
The audio amplifier according to any one of claims 1 to 3 , wherein the amplifier circuit is configured to be switched between an on state and an off state in accordance with an on / off state of the output switch . A bias circuit that divides the power supply voltage and generates a bias voltage;
A variable resistor for attenuating the input audio signal with respect to the bias voltage;
An operational amplifier in which the output signal of the variable resistor is input to the non-inverting input terminal;
A first switch provided between the output terminal of the bias circuit and the output terminal of the audio amplifier;
A second switch provided between the output terminal of the operational amplifier and the output terminal of the audio amplifier;
A selector having a first input terminal connected to the output terminal of the operational amplifier, a second input terminal connected to the output terminal of the audio amplifier, and an output terminal connected to the inverting input terminal of the operational amplifier; ,
A control unit for controlling the first and second switches and the selector;
An audio amplifier comprising: The controller is
The first switch is turned on for a predetermined first period from the start of activation, and then turned off.
The second switch is turned off for a predetermined second period from the start of activation, and then turned on,
8. The audio amplifier according to claim 7 , wherein the selector is turned on to the first input terminal side for a predetermined third period from the start of activation, and then turned on to the second input terminal side. Audio amplifier according to any one of claims 1 to 8, characterized by further comprising a charge circuit for charging the output terminal of said bias circuit. An audio amplifier according to any one of claims 1 to 9 for amplifying an audio signal;
An audio output unit for outputting an output signal of the audio amplifier;
An electronic device comprising:

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