JPS643363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amplifier circuit that suppresses a transient output that occurs during a power-on transition.

FIG. 1 shows a conventional amplifier circuit used in a power amplification IC. The power amplifier 2 is an IC amplifier that includes a plurality of elements therein, and a power supply 6 is connected to a power supply terminal 4 via a switch 8 . An input signal is applied to the input terminal 12 from a preamplifier (not shown), and its amplified output is taken out through the output terminal 14. A speaker 18 is connected to the output terminal 14 via a capacitor 16 for output coupling.

The power amplifier 2 is provided with a bootstrap terminal 20 for configuring a bootstrap circuit and a feedback terminal 22 for an external connection element to a feedback circuit that determines the AC gain of the power amplifier 2. Therefore, bootstrap terminal 2
A bootstrap capacitor 24 is connected between the output terminal 14 and the output terminal 14, and the driving power source 6 is connected to the bootstrap terminal 20 via a resistor 26. Further, a feedback resistor 29 is connected between the input and output terminals of the power amplifier 2, and the feedback terminal 2
A series circuit consisting of a feedback resistor 28 and a capacitor 30 for removing the DC component of the output signal of the power amplifier 2 is connected to the power amplifier 2. Note that 32 is a grounding terminal (GND).

In this amplifier circuit, when switch 8 is turned on, power supply voltage V _CC rises rapidly as shown at A in FIG. 2 and is applied to power amplifier 2 .
At this time, the voltage V _CC of the power supply 6 is applied to the series circuit of the resistor 26, capacitors 16 and 24, and the speaker 18, regardless of the power amplifier 2, and
A spike appears at the output of power amplifier 2, as shown in _B1 . In addition, when the power amplifier 2 shifts to a steady state after turning on the power supply 6, as shown in FIG. 2B,
A transient output occurs as shown in _B2 . All of these transient outputs cause the generation of transient sounds such as pumping noise, and especially in ultra-small radios and tape recorders that perform stereo reproduction only with headphones, these transient sounds cause discomfort.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an amplifier circuit that suppresses the transient output that occurs when power is turned on.

That is, in the amplifier circuit of the present invention, a bootstrap circuit is installed, and a first capacitor (capacitor 30) for removing the DC component included in the output signal is installed in a feedback circuit installed between the input and output terminals. a second capacitor (capacitor 38) that is connected to a drive power source for driving this amplifier via a resistor and removes a ripple component from the drive power source; and a second capacitor (capacitor 38) that is charged through the resistor when the drive power source is supplied. a potential detector that detects a potential change that occurs in the resistor as the capacitor is charged; a switch circuit that prevents charging of the bootstrap capacitor during a period from the start of supply of the drive power until the potential of the resistor shifts to a specific potential according to the detection output of the potential detector; and a charging circuit that charges the second capacitor during a period from the start of supply of the driving power until the potential of the resistor shifts to a specific potential.

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

FIG. 3 shows an embodiment of the amplifier circuit of the present invention, and the same parts as those of the amplifier circuit shown in FIG. 1 are given the same reference numerals.

In the figure, a drive power source 6 is connected to a power source terminal 34 via a switch 8, and a ripple filter circuit 40 consisting of a resistor 36 and a capacitor 38 is also connected. That is, in order to remove the ripple component of the drive power source 6, a capacitor 38 is connected to the drive power source 6 via a resistor 36. The terminal 42 is a terminal for external connection of the IC.
Therefore, when the power supply 6 is turned on, the capacitor 38 is charged through the resistor 36, but since a potential difference occurs in the capacitor 38 during charging, the capacitor 38
A potential detector 44 is connected between the terminals of the capacitor 8 in order to detect potential changes during charging of the capacitor 38.

Further, a switch circuit 46 is installed between the power supply terminal 34 and the bootstrap terminal 20 on the side of the power amplifier 2 to control the supply of the driving power supply 6 via the resistor 26. This switch circuit 46 can be composed of switching elements such as transistors.
P represents an equivalent contact. That is, the switch circuit 46 is controlled based on the detection output of the potential detector 44,
The equivalent contact 46P is kept in the off state until the potential of the resistor 36 shifts to a specific potential. Therefore, along with the bootstrap capacitor 24,
Application of the power supply voltage V _CC to the capacitor 16 on the output side of the power amplifier 2 is maintained in a cutoff state by the off state of the equivalent contact 46P.

A feedback circuit installed between the input and output terminals of the power amplifier 2 includes a capacitor 3 that removes the DC component contained in the AC signal of the output signal of the power amplifier 2.
0 is installed, and a charging circuit 48 is installed to charge this capacitor 30 when the power is turned on. This charging circuit 48 is connected to the potential detector 44
The charging current flows from the feedback terminal 22 to the capacitor 30 through the resistor 28 until the potential of the resistor 36 shifts to a specific potential, and the capacitor 30 is precharged when the power is turned on. .

In the above configuration, it is assumed that an input signal is applied to the input terminal 12 from a signal source such as a preamplifier (not shown) at the same time as the switch 8 turns on. When the switch 8 is turned on, the potential at point A of the power supply terminal 34 rises rapidly as shown at A in FIG.

Furthermore, based on the turning on of the power supply 6, a charging current flows through the capacitor 38 of the ripple filter circuit 40 via the resistor 36, and the potential at point B on the positive side of the capacitor 38 is as shown by the curve shown in B in FIG. Then, after a certain transient time, the voltage shifts to the power supply voltage V _CC which is a steady voltage value. In this case, the time constant T is given by the product of the resistance value R ₃₆ of the resistor 36 and the capacitance C ₃₈ of the capacitor 38 (T=R ₃₆ ·C ₃₈ ). Such a change in the potential of the capacitor 38 appears as a voltage drop across the resistor 36 due to the charging current.

The potential detector 44 detects the potential change appearing across the resistor 36 and outputs an output that rises with a certain time constant, and detects the turning on of the power supply 6 shown in A in FIG. 4, and simultaneously turns on the power. An output (power supply voltage V _CC ) having no time constant at the rising point P is generated.

The switch circuit 46 receives the output of the former which rises with the time constant of the potential detector 44, and is in the off state for a certain period of time _Td from when the switch 8 is turned on, as shown in FIG. 4C. After, delay time
Hold T _s to transition to on state. Therefore, after the bootstrap terminal 20 is cut off from the power supply 6 for a certain period of time _Td while the switch circuit 46 is in the OFF state, it shifts to the power supply voltage V _CC as the switch circuit 46 shifts to the ON state.

Further, the charging circuit 48 becomes operational in response to the supply of the power supply voltage V _CC which is the latter output of the potential detection circuit 44 . Due to the operation of this charging circuit 48,
As shown at D in FIG. 4, the potential at point D of the feedback terminal 22 shifts to a voltage V _C corresponding to the power supply voltage V _CC that instantly rises at the same time as the power is turned on, and as a result,
The capacitor 30 is in a precharged state, and its charging voltage V _C30 becomes the voltage V _C. In other words, the feedback terminal 22 is connected to the
The voltage V _C is maintained until T _C ).

By the way, the power amplifier 2 has a switch circuit 46.
At time _T is in normal operating condition. For this reason, the time T _x
Now, while the power amplifier 2 is transitioning to the operating state,
Since the output section is cut off by the switch circuit 46, a muting state occurs in which no signal is output, and T _M (= _TC + T _X ) in D in FIG. 4 becomes the muting time.

Then, as shown in E of FIG. 4, the potential of the output terminal 14 of the power amplifier 2 becomes 1/2 _of the power supply voltage V _CC (V _CC /
2) and stabilize. Therefore, during the time T _d during which the switch circuit 46 is maintained in the off state after the power is turned on, the series circuit of the resistor 26, capacitors 24, 16, and speaker 18 connected between the power supply terminal 34 and the ground via the switch circuit 46 are cut off from the power supply 6, there is no steep potential rise due to the capacitors 24 and 16, and no spikes B ₁ and B ₂ , which are transient outputs, are generated as shown by broken lines.

Therefore, the output point F of the power amplifier 2 is maintained in a non-signal state during the muting time _TM , and the audio output shown at F in FIG. 4 is applied to the speaker 18. Therefore, only the necessary audio signals are reproduced from the speaker 18 without generating transient sounds such as pumping noise when the power source 6 is turned on. In particular, when a headphone is connected instead of the speaker 18, prevention of transient sound generation is more effective.

Next, FIG. 5 shows a specific embodiment of the amplifier circuit of the present invention. In the figure, the potential detector 44 is composed of a multi-collector transistor 50 and resistors 52 and 54, and in this embodiment, the transistor 50 also serves as the charging circuit 48. That is, the current output flowing to the resistor 54 through the collector _C1 of the transistor 50 is added to the detection output which becomes the control input of the switch circuit 46, while the collector
The detection output applied from C ₂ is applied to the feedback terminal 22 as a charging current. In addition, the charging circuit 4
8 is the emitter/collector C ₂ of the transistor 50
formed through. In addition, the switch circuit 4
Reference numeral 6 comprises a transistor 56 as a switching element corresponding to the equivalent contact 46P of the above embodiment, and a switching control circuit 66 consisting of transistors 58, 60 and resistors 62, 64 for on/off control of this transistor 56. There is.

According to such a configuration, when the power supply 6 is turned on, the transistor 50 is turned on for a certain period of time during which a potential difference is generated in the resistor 36, and the detection output, that is, the charging current supplied from the emitter collector _C2. The capacitor 30 is charged. On the other hand, while the transistor 50 is in the on state, the transistor 58 is in the on state and the transistor 60 is in the off state.
6 is turned off, and the supply of power supply voltage V _CC to the bootstrap circuit is stopped. These operating states continue until the charged potential of the capacitor 38 shifts to the power supply voltage V _CC , and this is the muting time, during which the generation of transient noise can be effectively prevented.

Moreover, according to such a configuration, the potential detector 4
4. The switch circuit 46 and the charging circuit 48 can be configured with a single IC together with the power amplifier 2, and the elements for external connection can also be used from the existing ripple filter circuit 40, so the circuit configuration can be simplified. .

As explained above, according to the present invention, when the power is turned on, the application of power supply voltage to the bootstrap capacitor is cut off, and the capacitor in the feedback circuit is precharged. It is possible to prevent changes in potential due to rapid charging of the capacitor in the device, and it is possible to reliably prevent the generation of transient noise such as pumping noise when the power is turned on.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional amplifier circuit, FIG. 2 is a diagram showing the operation of the amplifier circuit shown in FIG. 1 when the power is turned on, and FIG. 3 is a block diagram showing an embodiment of the amplifier circuit of the present invention. 4 is a diagram showing the operation of the amplifier circuit shown in FIG. 3 when power is turned on, and FIG. 5 is a circuit diagram showing a specific embodiment of the amplifier circuit of the present invention. 2... Power amplifier, 6... Drive power supply, 24...
Bootstrap capacitor, 30... Capacitor (first capacitor), 36... Resistor, 38...
... Capacitor (second capacitor), 44 ... Potential detector, 46 ... Switch circuit, 48 ... Charging circuit.

Claims (1)

[Scope of Claims] 1. An amplifier in which a bootstrap circuit is installed and a first capacitor for removing a DC component included in an output signal is installed in a feedback circuit installed between input and output terminals; a second capacitor connected via a resistor to a drive power source for driving the drive power source to remove a ripple component from the drive power source; a potential detector for detecting a potential change occurring in the resistor; and a potential detector installed between the bootstrap capacitor of the bootstrap circuit and the driving power source, and the voltage detector detects a potential change occurring in the resistor, and is installed between the bootstrap capacitor of the bootstrap circuit and the drive power source, and is configured to change the voltage from the start of supply of the drive power to the drive power source according to a detection output of the potential detector. a switch circuit that blocks charging of the bootstrap capacitor until the potential of the resistor shifts to a specific potential; and a switch circuit that specifies the potential of the resistor from the start of supply of the driving power according to the detection output of the potential detector. and a charging circuit that charges the second capacitor for a period until the potential changes to the potential of the second capacitor.