JPH0797732B2 - Audio output amplifier circuit - Google Patents

Description

Description: [Object of the Invention] (Field of Industrial Application) [0001] The present invention relates to a multistage audio output transistor amplifier circuit, and more particularly to a circuit for preventing the generation of shock noise when power is turned on. .

(Prior Art) A multi-stage audio output amplifier circuit used in, for example, an audio device is a semiconductor integrated circuit.
Shown in the figure. That is, 1 is an audio signal input terminal, 2 is a preamplifier circuit, 3 is an output stage amplifier circuit, 4 is an audio signal output terminal, 5 is a current mirror circuit, and a PNP type input transistor Q ₁ and a PNP input transistor Q ₁ respectively. Form the first and second output transistors Q ₂ and Q _3, and the first output transistor Q ₂ supplies the operating power supply current of the preamplifier circuit 2 from the V _cc power supply end, The output transistor Q ₃ is connected so that the operating power supply current of the output stage amplifier circuit 3 is supplied from the V _cc power supply end. On the other hand, 6 is a ripple filter circuit composed of resistance elements R ₁ and R ₂ connected in series between the V _cc power supply terminal and the ground terminal, and a capacitor C connected between the interconnection node and the ground terminal. is there. The output voltage (the terminal voltage of the capacitor C) of the ripple filter circuit 6 is input to 7 and when this input voltage is a predetermined value or more, the output current is turned on and the output current is changed to the input transistor Q ₁ of the current mirror circuit 5. Is a bias circuit for supplying to. The bias circuit 7 has a collector connected to V _cc supply terminal, base the ripple first transistor Q ₄ of connected NPN type to the output node of the filter circuit 6, the emitter and the ground terminal of the transistor Q ₄ A resistor R ₃ and two diodes D ₁ and D ₂ connected in series between the resistor and the base, and a collector connected to the resistor R ₃ and the diode D _1. The second NPN type transistor Q ₅ connected to the collector-base interconnection point of the input transistor Q ₁ and the resistance element connected between the emitter of the transistor Q ₅ and the ground terminal
It consists of R ₄ .

Next, the operation of the audio output amplifier circuit when the power is turned on will be described. When the V _cc power is turned on, the capacitor C is charged according to the time constant of the lip filter circuit 6,
Bias circuit 7 when the terminal voltage rises to 2V _BE (V _BE is the base-emitter voltage of the NPN transistor)
The transistors Q ₄ and Q _{5 of} are turned on and a bias current starts to flow, and the bias current output causes the current mirror circuit 5 to operate to supply the operating current to the amplifier circuits 2 and 3 of the respective stages. To start the operation of. That is, in the audio output circuit, one bias circuit 7 controls so that the amplifier circuits at the respective stages start operating simultaneously.

By the way, a differential amplifier circuit is often used as the preamplifier circuit 2, and at the time when the operation of the differential amplifier circuit starts, the negative feedback capacitor connected to the inverting side input terminal is in an uncharged state. Therefore, the input on the inverting side has almost dropped to the ground level. On the other hand, in the case of a circuit that does not require an input coupling capacitor, the non-inverting side input terminal rises almost at the same time as the operation of the amplifier circuit starts, so that the non-inverting side input and the inverting side input are in an unbalanced state. There is a period. Even when an input coupling capacitor is connected to the non-inverting side input terminal, it is necessary to make the charging time constant for this coupling capacitor and the charging time constant for the negative feedback capacitor at the inverting side input terminal uniform. Since it is almost impossible in terms of the configuration, as described above, there is a state where the potential between the pair of input terminals is unbalanced at the time of starting the operation of the differential amplifier circuit. Therefore, when the amplifier circuits of the respective stages of the audio output amplifier circuit simultaneously start operating as described above, the above-mentioned potential difference between the input terminals of the preamplifier circuit 2 appears up to the output terminal 4 of the output stage amplifier circuit 3, and this output A shock sound (shock sound when the power is turned on) is generated from a speaker, headphones, etc. connected to the terminal 4. In order to prevent the generation of this shock noise, a muting circuit is added to the output stage amplifier circuit 3 to perform a muting operation when the power is turned on. However, even if the muting circuit is not used, the shock noise is generated. Circuits to prevent may be desired.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned circumstances, and an audio output amplifier circuit capable of preventing the generation of a shock noise at power-on even without using a muting circuit. The purpose is to provide.

[Configuration of the Invention] (Means for Solving Problems) The present invention has at least a preamplifier circuit and an output stage amplifier circuit, and the preamplifier circuit has an inverting input terminal and a non-inverting input terminal. A voice output amplifier circuit configured by a differential amplifier circuit in which a negative feedback capacitor is connected to the inverting input terminal, wherein a time constant circuit in which an output voltage rises according to a set time constant when power is turned on. Connected to the time constant circuit and the preamplifier circuit, the output voltage of the time constant circuit being the first
Is connected to the first bias circuit for supplying a bias current to the preamplification circuit, the time constant circuit and the output stage amplification circuit, and the output voltage of the time constant circuit is the first voltage. A second voltage that is higher than the second voltage and supplies a bias current to the output stage amplifier circuit.
And a bias circuit of.

(Operation) Even if a potential unbalanced state temporarily exists at a pair of input terminals of the audio signal input stage amplifier circuit when the power is turned on, the operation of the output stage amplifier circuit does not occur after this unbalanced state does not exist. By setting the operation start time of the bias circuit so as to start, it is possible to prevent the occurrence of a shock sound at the time of turning on the power due to the unbalanced state.

Embodiment An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an audio output amplifier circuit integrated into a circuit, which is different from the audio amplifier circuit described above with reference to FIG.
(1) In addition to the first bias circuit 7 having the same configuration as the bias circuit 7 in FIG. 3, a second bias circuit 8 having an operation start input voltage level higher than that is added,
(2) The first current mirror circuit 9 is driven by the bias current output of the first bias circuit 7 to supply the operating power supply current to the preamplifier circuit 2, and the bias current output of the second bias circuit 8 is supplied. The second current mirror circuit by
It is different in that it is connected so as to drive 10 to supply an operating power supply current to the output stage amplifier circuit 3 and is otherwise the same, so the same reference numerals as those in FIG. 3 are used and the description thereof is omitted.

Like the first bias circuit 7, the second bias circuit 8 has an input node connected to the output node of the ripple filter circuit 6, and the second bias circuit 8 has a configuration similar to that of the first bias circuit 7 with respect to the transistor Q. The first bias circuit is the same except that one diode D ₃ is added in series to the resistance element R ₃ between the emitter of _{4 and} the base of the transistor Q ₅ , and the others are the same. The same symbols as each element in 7 are attached. Then, the first current mirror circuit 8, becomes an input transistor Q ₆ of the PNP from the output transistor Q ₇ Prefecture, the input transistor
Q ₆ is connected to the bias current output node of the first bias circuit 7. Similarly, the second current mirror circuit 10 is also composed of a PNP type input transistor Q ₈ and an output transistor Q _9, and this input transistor Q ₈ serves as a bias current output node of the second bias circuit 8. It is connected.

Next, the operation of the audio output amplifier circuit when the power is turned on will be described. When the V _cc power supply is turned on, the capacitor C is charged according to the time constant of the ripple filter circuit 6 and its terminal voltage rises. In this case, the terminal voltage is 2V
_{When BE} (V _BE is the base-emitter voltage of the NPN transistor), first the transistors Q ₄ and Q _{5 of} the first bias circuit 7 are turned on, and the bias current begins to flow.
By this bias current output, the first current mirror circuit 8 operates to supply an operating current to the preamplifier circuit 2 to start its operation. After this, the terminal voltage is further V _F (V _F is the forward voltage of the diode, and V _F =
When V _BE is a) becomes increased to 3V _BE, the second bias circuit 8 also begin bias current flows through the transistor Q _4, Q ₅ is turned on, by the bias current output second current mirror circuit 10 Operates to supply an operating current to the output stage amplifier circuit 3 to start the operation.

According to the audio output amplifier circuit of the above-mentioned embodiment, the operation start time of the preamplifier circuit 2 and the output stage amplifier circuit 3 is determined by the terminal voltage of the capacitor C in the ripple filter circuit 6 being V _F
It is possible to shift by a time corresponding to the charging time required to rise by a minute. In this case, even if the above-described unbalanced state of the potential between the input terminals temporarily occurs in the preamplifier circuit 2 at the time of power-on, the operation of the output stage amplifier circuit 3 after the unbalanced state disappears. By setting the circuit constants so that the power supply starts, it is possible to prevent the generation of shock noise at power-on due to the above-mentioned unbalanced state. That is, according to the above-mentioned embodiment, it is possible to prevent the generation of shock noise when the power is turned on without using the muting circuit, the second bias circuit 8 is added as compared with the conventional example, and the current mirror circuit (first This can be realized by a relatively simple configuration in which two current mirror circuits 9 and 10 are added while omitting (FIG. 3).

FIG. 2 shows an example in which the present invention is applied to an audio output amplifier circuit having a muting circuit, which is different from the audio output amplifier circuit shown in FIG. 1 in that it has a muting circuit 11 and this muting. Two NPN-type power transistors Q ₁₁ and Q ₁₂ in the output stage which are controlled by the circuit 11 and which are push-pull connected are different from each other in FIG. Attached.
The muting circuit 11 has a collector connected to the _Vcc power supply terminal and a base connected to the output node of the ripple filter circuit 6 between an NPN transistor Q ₁₃ and the emitter of the transistor Q ₁₃ and the ground terminal. to the resistance element R _4, R ₅ connected in series, the pace to the interconnection point of the resistance element R _4, R ₅ are connected, and an NPN transistor Q ₁₄ whose emitter is grounded, the transistor Q ₁₄ Collector and V
_The base is connected to the resistor element R ₆ connected between the _cc power supply terminal and the collector of the transistor Q ₁₄ , and
Muting N connected between the _Vcc power supply terminal and the base of the power transistor Q ₁₂ on the low potential side between the emitters.
It consists of a PN transistor Q ₁₅ .

In the muting circuit 11, the transistors Q ₁₃ and Q ₁₄ are in the off state immediately after the power is turned on, and the muting transistor Q ₁₅ is in the on state because the base current is supplied via the resistance element R _6. As a result, the power transistor Q ₁₂ on the low potential side is driven on, so that the audio signal output terminal 4 is at the ground potential. After the power is turned on, the first bias circuit 7 and the second bias circuit 8 start to operate sequentially in the same manner as in the above embodiment. After that, when the transistors Q ₁₃ and Q _{14 of the} muting circuit 11 are turned on by the terminal voltage of the capacitor C of the ripple filter circuit 6, the muting transistor Q ₁₅ is turned off due to insufficient base current, Muting operation is turned off and output stage power transistors Q ₁₁ and Q
_Twelve normal push-pull operations are possible. In this way, the muting operation is turned off because the voltage obtained by dividing the voltage lower than the terminal voltage of the capacitor C by the base-emitter voltage V _{BE of the} transistor Q ₁₃ by the resistance elements R ₄ and R ₅ is the transistor Q. This is when the base-emitter voltage of ₁₄ (about 0.7 V) is reached, and this timing can be determined by the resistance value ratio of the resistance elements R ₄ and R ₅ . In this case, if the timing is set such that the operation of the output stage amplifier circuit 3 starts at the same time as the muting operation is turned off, it is possible to make the sound output rise smoothly. When the present invention is applied to the audio output amplifier circuit having the muting circuit 11 as described above, the DC potential difference between the two channels used by connecting the audio output amplifier circuit by BTL (balanced transformerless) becomes a problem. In this case, it is possible to align the rising timings of the audio outputs of the audio signal output terminals of both channels, which is particularly effective.

Incidentally, the present invention is not limited to the above embodiment, in the multistage audio output amplifier circuit, the multistage amplifier circuit is divided into two or more systems so as to have a difference in the operation start time at power-on,
A bias current determination circuit (corresponding to the bias circuit of the above-mentioned embodiment) and a bias current supply circuit (corresponding to the current mirror circuit of the above-mentioned embodiment) are provided for each of the above-mentioned systems, and the output stage amplifier circuit corresponds to the input stage amplifier circuit. The operation may be started after the operation is started. In this case, the bias current determination circuit and the bias current supply circuit are not limited to those in the above embodiment, and various modifications can be made.

[Effects of the Invention] As described above, according to the audio output amplifier circuit of the present invention, it is possible to prevent the generation of a shock sound when the power is turned on without using the muting circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an audio output amplifier circuit of the present invention, FIG. 2 is a circuit diagram showing another embodiment of the same, and FIG. 3 is a circuit diagram showing a conventional audio output amplifier circuit. . 1 ... Audio signal input terminal, 2 ... Preamplifier circuit, 3 ...
Output stage amplifier circuit, 4 ... Audio signal output terminal, 6 ... Ripple filter circuit, 7,8 ... Bias circuit, 9,10 ... Current mirror circuit, 11 ... Muting circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-55-91215 (JP, A) JP-A-55-91214 (JP, A) JP-A-55-8169 (JP, A) Actual development Sho-56- 176513 (JP, U)

Claims (3)

[Claims] 1. A differential amplifier having at least a preamplifier circuit and an output stage amplifier circuit, wherein the preamplifier circuit has an inverting input terminal and a non-inverting input terminal, and a negative feedback capacitor is connected to the inverting input terminal. A voice output amplifier circuit configured by a dynamic amplifier circuit, which is connected to the time constant circuit and the preamplifier circuit, wherein the output voltage rises according to a set time constant when the power is turned on, A first bias circuit for supplying a bias current to the preamplifier circuit when the output voltage of the time constant circuit reaches a first voltage; and the time constant circuit and the output stage amplifier circuit, A second constant voltage output from the time constant circuit higher than the first voltage;
A second bias circuit that supplies a bias current to the output stage amplifier circuit when the voltage becomes equal to the voltage. 2. A muting circuit for turning off the audio output when the power is turned on is further provided, and the operation of the output stage amplifier circuit is started at the same time when the muting operation of the muting circuit is turned off. The audio output amplifier circuit according to claim 1, characterized in that 3. The muting circuit is supplied with the output voltage of the time constant circuit, and when the input voltage of the muting circuit reaches a predetermined value, the muting operation is turned off. Claim 2 to
The audio output amplifier circuit according to the item.