JPH0373172B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an acoustic power amplifier circuit, and more particularly to a shock noise prevention circuit when the power is turned off for a single power supply type SEPP (Single Ended Push-Pull) amplifier circuit.

[Technical Background of the Invention] FIG. 5 shows a conventional example of this type of single power supply type SEPP amplifier circuit integrated into an integrated circuit. That is, 1 is a push-pull type output amplifier circuit, and a reference voltage Vref is applied to its input terminal (+).
A negative feedback resistor R _f is connected between the negative feedback (NF) terminal (-) and the output terminal 2, and the DC voltage at the output terminal 2 is biased to half the power supply voltage V _cc . Resistors R ₁ and R ₂ are connected in series between the V _cc power supply terminal 3 and the ground terminal, and an NPN type transistor is connected to the interconnection point of the resistors R ₁ and R ₂ .
The base of transistor Q ₁ is connected, and the collector of this transistor Q ₁ is connected to the power supply terminal 3 and grounded through emitter resistor R ₃ and then through NPN transistor Q ₂ on the input side of current mirror circuit 4. There is. The output side of this current mirror circuit 4
The emitter of NPN transistor _Q3 is grounded,
The collector is connected to the output amplifier circuit 1 via the resistor _R4 .
It is connected to the negative feedback terminal (-) of the terminal and grounded via the capacitor _C1 . 5 is the resistance R ₁ ,
This is a ripple filter terminal connected to the interconnection point of _R2 , and a ripple filter capacitor 6 is externally connected.

Furthermore, a resistor is provided between the power terminal 3 and the ground terminal.
R ₅ and R ₆ are connected in series, and this resistance R ₅ ,
The interconnection point of R ₆ is connected to the base of a PNP type transistor Q ₄ , the emitter of this transistor Q ₄ is connected to the ripple filter terminal 5, and the collector is connected to the base of an NPN type transistor Q ₅ for discharging. has been done. The emitter of this transistor _Q5 is grounded, and the collector is connected to the output terminal 2 of the output amplifier circuit 1. This output terminal 2 is grounded via an output coupling capacitor 7 and a load circuit (for example, a speaker coil) 8. Note that 9 is a power supply stabilizing capacitor.

In the SEPP circuit with the above configuration, if the ripple filter capacitor 6 is charged,
The DC potential of ripple filter terminal 5 is resistor R ₁ ,
It is determined by the resistance value ratio of R ₂ and the power supply voltage V _cc , and if the values of the above-mentioned resistors R ₁ and R ₂ are equal, the ripple filter terminal voltage V ₅ is V ₅ = V _cc /2 ……( 1) becomes. The current I ₁ flowing through the transistor Q ₁ , the resistor R ₃ , and the transistor Q ₂ increases the base-emitter electrical thickness of each of the transistors Q ₁ and Q ₂ .
Expressed in terms of V _BE , I ₁ =V _cc /2−2V _BE /R ₃ (2). The same current as I ₁ above flows through the output side transistor Q ₃ of the current mirror circuit 4, and this current flows through the negative feedback resistor R _f , so the potential difference between both ends of the current flows through the negative feedback resistor R f.
V _f becomes V _f =I ₁ ×R _f =V _cc /2−2V _BE /R ₃ ×R _f (3).

On the other hand, the DC potential at the negative feedback terminal (-) of the output amplifier circuit 1 is equal to the current potential Vref at the input terminal (+). And the voltage V ₂ of output terminal 2 is
Since it is the sum of the DC potential Vref of the negative feedback terminal (-) and the voltage V _f across the negative feedback resistor R _f , V ₂ = Vref + V _f = V _cc /2-2V _BE /R ₃ ×R _f +Vref...(4). If Vref = 2V _BE and R ₃ = R _f in the above equation (4), then V ₂ = V _cc /2 (5), and the output terminal 2 is biased to the midpoint.

Now, when the _Vcc power supply is turned off, the potential at the ripple filter terminal 5 slowly decreases due to the time constant of the ripple filter capacitor 6, and following this, the voltage at the output terminal 2 slowly decreases. In other words,
Since no sudden voltage change occurs at the output terminal 2, no shock noise occurs. Furthermore, if the charge in the output coupling capacitor 7 is not completely discharged when the power is turned off, a DC potential will remain at the output terminal 2. To avoid this, the resistors R ₅ , R ₆ ,
A discharge circuit including a start transistor Q ₄ and a discharge transistor Q ₅ is provided.
That is, the ratio of the values of the resistors R ₅ and R ₆ is the ratio of the values of the resistors R ₁ and
During _normal operation, the base potential and emitter potential of transistor _Q4 are equal, so transistor _Q4 is turned off, and thereby transistor _Q5 is also turned off. When the power is turned off, the emitter potential of the transistor Q ₄ (that is, the ripple filter terminal potential) slowly decreases due to the time constant of the ripple filter capacitor 6 as described above, while the base potential of the transistor Q ₄ As Q4 suddenly drops, this transistor _Q4 becomes operational and supplies base current to transistor _Q5 . This turns on the transistor _Q5 , completely discharging the charge on the output coupling capacitor 7, and the output terminal 2 drops to zero volts.

[Problems with background technology]

Incidentally, in the output amplifier circuit 1, a bias to a drive circuit (for example, a complementary drive circuit) for driving the output power transistor is generated from the voltage at the bootstrap terminal 10. This bootstrap terminal 10
is connected to the output terminal 2 via a bootstrap capacitor 11 and to the power supply terminal 3 via a bias resistor 12. In other words, the bias of the power transistor drive circuit depends on the voltages at the output terminal 2 and the ripple filter terminal 5. Therefore, when the power is turned off, the off operation of the power transistor drive circuit is delayed somewhat, and the operating state of the output amplifier circuit 1 continues for a short time. Therefore, when the output amplifier circuit 1 is performing the amplification operation of the input signal (when the input signal is being input)
When the power is turned off, an output signal appears at the output terminal 2 while the output amplifier circuit 1 continues to operate as described above. At the same time, the DC potential at the output terminal 2 is decreasing, so the output signal is distorted, and a harsh output sound due to its harmonic components is temporarily generated, and then the output disappears. In other words, a residual sound phenomenon occurs. FIG. 6 shows the state of the output voltage when the power is turned off.

Furthermore, the way the DC voltage at the output terminal 2 falls when the power is turned off changes depending on the capacitance value of the ripple filter capacitor 6. Moreover, in a SEPP amplifier circuit that is generally integrated, the ripple filter capacitor 6 is connected externally, and depending on how the SEPP amplifier circuit is applied, the way the voltage at the output terminal 2 falls may vary. It's going to change. When the way the voltage at the output terminal 2 falls changes in this way, the above-mentioned residual sound phenomenon changes.

[Object of the invention] The present invention has been made in view of the above circumstances, and
To provide a shock noise prevention circuit when the power is turned off for an audio output amplification circuit which does not produce a shock sound or a harsh residual sound when the power is turned off.

[Summary of the invention]

That is, the shock noise prevention circuit when the power is turned off in the audio output amplification circuit of the present invention is a single power supply type SEPP equipped with a ripple filter circuit and a bootstrap circuit.
It is provided in the amplifier circuit, and enters the operating state due to the potential difference that occurs when the fall of the ripple filter terminal voltage is delayed due to the time constant of the ripple filter capacitor than the fall of the power supply terminal voltage when the circuit power is turned off. A discharge transistor for discharging the electric charge of the amplifier circuit output coupling capacitor and a bias cutoff transistor for cutting off the bias supply to the output transistor drive circuit are controlled to be on by the start transistor, respectively. It is characterized by:

As a result, when the power is turned off, the potential of the bootstrap terminal can be quickly lowered to quickly put the output amplifier circuit into a non-operating state, and the potential of the output terminal can be lowered quickly to the extent that no shock noise occurs. Therefore, it is possible to prevent the shock noise and sound lingering phenomenon when the power is turned off.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

Compared to the conventional circuit described above with reference to FIG. ₅ , the SEPP amplifier circuit shown in FIG. ₇ is additionally connected, and the rest is the same, so the same parts as in FIG. 5 are given the same reference numerals and their explanation will be omitted.

The emitter of the starting PNP transistor Q ₆ is connected to the ripple filter terminal 5, the base is connected to the interconnection point of the resistors R ₅ and R ₆ , and the collector is connected to the base of the bias cutoff NPN transistor Q ₇ . The emitter of this transistor _Q7 is grounded, and the collector is connected to the bootstrap terminal 10. In addition,
21 is a bias circuit that applies a desired voltage to the output terminal 2 of the output amplifier circuit 1; 22 is a ripple filter circuit connected to the bias circuit 21; and 23 is a ripple filter circuit that is instantly activated when the power is turned off. This is the start circuit that enters. Further, the bootstrap circuit is composed of a capacitor 11 and a resistor 12.

In the SEPP amplifier circuit having the above configuration, in addition to the same operations as in the embodiment described above, the following operations are performed by the added transistors Q ₆ and Q ₇ . That is, during normal operation, the emitter potential and base potential of the starting transistor Q ₆ are equal, so the transistor Q ₆ is in the off state, and as a result, the bias cutoff transistor Q ₇
is also off. When the power is turned off, the emitter potential of the starting transistor Q ₆ slowly drops while its base potential drops suddenly, so the starting transistor Q ₆ becomes operational and the bias cutoff transistor Q Since the base current is supplied to _Q7 , this transistor _Q7 is turned on. As a result, the potential of the bootstrap terminal 10 suddenly drops, and the required bias is no longer applied to the power transistor drive circuit, so that the output amplifier circuit 1 immediately becomes inactive when the power is turned off, and no residual sound occurs. Moreover, by turning on the bias cutoff transistor _Q7 , the bootstrap capacitor 1
1 is discharged, the voltage at the output terminal 2 drops quickly to the extent that no shock noise is generated. Also, the charge in the power supply stabilizing capacitor 9 is discharged via the resistor 12, so the output amplifier circuit 1 is shut down even more quickly. Positive feedback is provided to bring it into operation.

FIG. 2 shows how the output voltage changes when the power is turned off. If this state is compared with the state of output voltage change in the conventional example shown in FIG. 6, it can be seen that the fall is quick and the output signal no longer appears in the middle of the fall.

Figure 3 shows the start circuit 23 of the circuit in Figure 1 above.
This figure shows a first modification example in which an NPN transistor Q 8 is inserted to amplify the base current of the discharging transistor Q ₅ to discharge the charge of the output coupling capacitor ₇ compared to the one shown in Fig. 1. The difference is that The base and emitter of the current amplifying transistor Q ₈ are connected to the collector of the starting transistor Q ₄ and the base of the discharging transistor Q ₅ , respectively, and the collector is connected to the power supply terminal 3.

According to the circuit shown in FIG. 3 above, in addition to the effect obtained by the circuit shown _{in FIG .}
The current flowing from the ripple filter terminal 5 is small, and a small capacitor with a small capacitance value can be used as the ripple filter capacitor 6. Furthermore, the added transistor _Q8 is turned on when the power is turned off. In particular, since the charge in the power supply stabilizing capacitor 9 is discharged, there is an advantage that the fall to the non-operating state when the power is turned off can be further accelerated.

FIG. 4 shows a second modification of the start circuit 23 of the circuit shown in FIG. That is, Q ₉ is an NPN type transistor for starting, and its emitter is connected to the interconnection point of resistors R ₅ and R ₆ .
The base is connected to the ripple filter terminal 5, and the collector is connected to the input side of the current mirror circuit 13.
It is connected to the power supply terminal 3 via a PNP transistor _Q10 . The collectors of the first and second output PNP transistors Q ₁₁ and Q ₁₂ of the current mirror circuit 13 are connected to the bases of the discharge transistor Q ₅ and the bias cutoff transistor Q ₇ .

The circuit shown in FIG. 4 differs from the operation of the circuit shown in FIG. 1 in the following points. That is, during normal operation, the starting transistor _Q9 is in the off state,
As a result, the current mirror circuit 10, the discharge transistor _Q5 , and the bias cutoff transistor _Q7 are also in the off state. When the power is turned off, the starting transistor Q ₉ becomes operational, the current mirror circuit 10 is turned on, and the discharging transistor Q ₅ and the bias cutoff transistor Q ₇ are turned on.

Therefore, even in the circuit shown in Fig. 4 above, when the power is turned off, current flows from the ripple filter terminal 5 to only one starting transistor _Q9 , so the current is small, and the capacitance value of the ripple filter capacitor 6 is reduced. You can use a small one. In addition, when the power is turned off, the current mirror circuit 10
When turned on, the electric charge in the power supply stabilizing capacitor 9 is discharged, so there is an advantage that the fall to the non-operating state when the power is turned off can be further accelerated.

〔Effect of the invention〕

As described above, by using the audio output amplification circuit of the present invention as a shock noise prevention circuit when the power is turned off, it is possible to obtain an effect that no shock noise or harsh sound remains when the power is turned off.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the shock noise prevention circuit when the power is turned off in the SEPP circuit of the present invention, FIG. 4 and 4 are circuit diagrams showing modified examples of the circuit shown in FIG. 1, FIG. 5 is a circuit diagram showing a shock noise prevention circuit when the power is turned off in a conventional SEPP amplifier circuit, and FIG. FIG. 3 is a waveform diagram showing changes in output voltage when the circuit is powered off. 1... Output amplifier circuit, 2... Output terminal, 3...
Power supply terminal, 5... Ripple filter terminal, 6...
Ripple filter capacitor, 7... Output coupling capacitor, 9... Power supply stabilization capacitor, 10
...Bootstrap terminal, 11...Bootstrap capacitor, 12...Bias resistor,
13...Current mirror circuit, _Q4 , _Q6 , _Q9 ...
Start transistor, Q ₅ ... Discharge transistor, Q ₇ ... Bias cutoff transistor,
Q ₈ ... Current amplification transistor, R ₁ to R ₆ ... Resistor.

Claims (1)

[Scope of Claims] 1. An output amplification circuit having an output transistor and a drive circuit for driving the output transistor, a bootstrap circuit that supplies a bias to the drive circuit of the output amplification circuit, and an output terminal of the output amplification circuit. a bias circuit that supplies a bias; a ripple filter circuit that has a ripple filter capacitor and is connected to the bias circuit; and when the power is off, the fall time of the power supply terminal voltage and the ripple filter capacitor A start circuit that operates instantaneously by utilizing the time difference between the fall time of the ripple filter terminal voltage due to the time constant of a discharging transistor that instantly discharges the charge of the output amplifier; and a bias cutoff transistor that is turned on by the operation of the start circuit and instantly cuts off the bias supplied from the bootstrap circuit to the drive circuit of the output amplification circuit. A shock noise prevention circuit when the power is turned off for the audio output amplification circuit. 2. The start circuit includes a first transistor whose collector is connected to the base of the discharge transistor, and a second transistor whose collector is connected to the base of the bias cutoff transistor, and when the power is turned off, A voltage difference is generated between the base and emitter of the first and second transistors due to the time difference, the first and second transistors are turned on, and the start circuit is operated. A shock noise prevention circuit when the power is turned off in the audio output amplification circuit according to item 1. 3. When the audio output amplifier circuit according to claim 1 is powered off, the bias cutoff transistor grounds the bootstrap terminal of the bootstrap circuit by turning on. Shock noise prevention circuit.