JP6330364B2 - Amplifier circuit protection circuit - Google Patents

Description

  The present invention relates to a protection circuit for an amplifier circuit suitable for application to, for example, a class D amplifier circuit.

  As an amplifier circuit, a configuration including a high-potential side transistor and a low-potential side transistor in the final stage is well known. In such an amplifier circuit, if an overcurrent flows through the transistor, the transistor is destroyed. Therefore, in order to prevent this kind of destruction, a protection circuit is provided in the amplifier circuit.

  As this protection circuit, for example, firstly, even when an overcurrent occurs in one of a pair of transistors constituting an output stage, a technique for controlling both transistors to be in an off state (for example, Patent Documents). 1) and secondly, with one of the two input terminals of the speaker as a load connected to one of the two output terminals of the BTL (Balanced Transformer Less) output circuit, the other of the signal input terminals of the speaker A technique for detecting a negative current flowing in a detection resistor of a BTL output circuit when the signal is grounded, and stopping the operation of the BTL output circuit based on the detection result (for example, see Patent Document 2); When a current exceeding the allowable value flows to the transistor, the mode is changed to a mode in which the current flowing through the transistor becomes a constant current to minimize damage to the transistor (example: For example, see Patent Document 3).

JP 2004-066473 A JP 2004-214933 A JP 2010-226155 A

  By the way, when the output terminal of the amplifier circuit is short-circuited to the ground via a low-resistance wire or the like for any reason, when the transistor on the high potential side is turned on (or in a state where current flows), If the flowing current is larger than the predetermined value, it can be detected that an overcurrent flows through the high potential side transistor.

However, when the output terminal is short-circuited to the ground via a wire having a large inductance (for example, a coil) in a state where the voltage of the output terminal of the amplifier circuit is relatively low, the current flowing through the wire is reduced to the low-resistance conducting wire. Compared with short-circuiting, it does not increase rapidly.
For this reason, it takes time for the current flowing through the transistor to become larger than a predetermined value compared to the case where the low-resistance wire is short-circuited, and as a result, the transistor may be destroyed. Has been.

  The present invention has been made in view of such circumstances, and one of its purposes is a technique for quickly protecting a transistor even when an output terminal is short-circuited through a wire having a large inductance. Is to provide.

  In order to achieve the above object, a protection circuit for an amplifier circuit according to one embodiment of the present invention includes a first transistor that outputs a signal corresponding to a high potential side to an output terminal, and a signal that corresponds to a low potential side. A protection circuit for an amplifier circuit, comprising: a second transistor that outputs to a terminal; and a drive circuit that drives the first transistor and the second transistor based on an input signal, wherein the input signal is equal to or higher than a threshold voltage If there is, a reference signal that outputs a reference signal that defines a first current and outputs a reference signal that defines a second current that is smaller than the first current if the reference signal is less than the threshold voltage; and the first And a detection circuit that detects whether or not a current flowing through the transistor is equal to or greater than a current defined by the reference signal, and is greater than or equal to a current defined by the reference signal by the detection circuit. If it is detected that the driver circuit is characterized in that stops the driving of the first transistor and the second transistor.

In the protection circuit for an amplifier circuit according to one embodiment of the present invention, when the current flowing through the first transistor is detected to be greater than or equal to the current defined by the reference signal, the drive circuit stops driving the transistor. The reference signal output circuit reduces the current defined by the reference signal if the voltage of the input signal is less than the threshold value. Therefore, according to the protection circuit for an amplifier circuit according to one aspect, the output terminal is short-circuited to the ground side through a wire having a large inductance component, for example, and the voltage of the input signal is low and the output terminal Even in a situation where current is difficult to flow because of the low voltage of the transistor, the driving of the transistor can be stopped quickly to prevent the transistor from being destroyed.
The fact that the current flowing through the first transistor is greater than or equal to the current defined by the reference signal can be detected, for example, by determining whether or not the voltage at the output terminal is greater than or equal to the voltage corresponding to the current of the reference signal. it can.

  In the one aspect, the reference signal output circuit includes a comparison circuit that compares the voltage of the input signal and the threshold voltage, and when the voltage of the input signal is equal to or higher than the threshold voltage by the comparison circuit, The first current may be defined by the reference signal, and the second current may be defined by the reference signal when the voltage of the input signal is less than the threshold voltage. According to this configuration, the reference signal output circuit can be realized with a simple configuration such as a comparison circuit.

In the above configuration, if the voltage of the input signal fluctuates in the vicinity of the threshold voltage, the current defined by the reference signal fluctuates frequently, causing malfunctions. Therefore, the reference signal output circuit is connected to the voltage of the input signal. May be changed from the first current to the second current when the state below the threshold voltage continues for a predetermined time. As a result, even if the voltage of the input signal is momentarily less than the threshold voltage, the current specified by the reference signal does not change from the first current to the second current unless the state continues for a predetermined time, thus preventing malfunction. can do.
The reference signal output circuit, when the voltage of the input signal transitions from a state below the threshold voltage to a state equal to or higher than the threshold voltage, a current defined by the reference signal in a time shorter than the predetermined time, The second current may be changed to the first current.

It is a figure which shows the structure of the whole system containing the protection circuit which concerns on 1st Embodiment. It is a figure for demonstrating operation | movement of the reference signal output circuit in a protection circuit. It is a figure for demonstrating operation | movement of a protection circuit. It is a figure which shows the structure of the whole system containing the protection circuit which concerns on 2nd Embodiment. It is a figure which shows an example of the input / output characteristic of the delay block in a protection circuit. It is a figure which shows the structure of the whole system containing the protection circuit which concerns on an application example.

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

FIG. 1 is a diagram illustrating a configuration example of a system including a protection circuit according to the first embodiment.
The example of this figure shows a configuration in which the input signal Vin is amplified and output to the speaker 30, that is, a sound is emitted, and includes an amplifier circuit 10 and a protection circuit 20. The amplifier circuit 10 is a so-called class D amplifier circuit, which inputs an analog input signal Vin to an input terminal In and outputs a switching signal from an output terminal Out.
The amplifier circuit 10 includes a pulse modulation circuit 112, a drive circuit 114, and transistors 121 and 122. The pulse modulation circuit 112 performs, for example, pulse width modulation according to the level (voltage) of the input signal Vin. Note that the present invention is not limited to pulse width modulation, and may be pulse density modulation.

  The drive circuit 114 exclusively turns on and off the transistors 121 and 122 by level-shifting the pulse width modulated signal. However, the drive circuit 114 has a mute terminal (Mute), and is configured to forcibly turn off the transistors 121 and 122 when a signal supplied to the mute terminal becomes, for example, an H level.

The transistors 121 and 122 are, for example, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). Among these transistors, the transistor 121 is supplied with the higher voltage Vdd of the power supply at the drain terminal and the source terminal as the output terminal. Connected to Out. In the transistor 122, the drain terminal is connected to the output terminal Out, and the source terminal is grounded to the ground Gnd which is the lower side of the power supply.
For this reason, the transistor 121 becomes a first transistor that outputs a signal corresponding to the high potential side to the output terminal Out, and the transistor 122 becomes a second transistor that outputs a signal corresponding to the low potential side to the output terminal Out.
Unless otherwise specified in this description, the ground Gnd is used as a reference for zero voltage.

During operation, the output terminal Out is connected to one end of the inductor L, and the other end of the inductor L is connected to one terminal of the speaker 30 as an example of a load and one end of the capacitor C. The other end is grounded to the ground Gnd. That is, the output terminal Out is connected to one terminal of the speaker 30 via an LPF (Low Pass Filter) constituted by the inductor L and the capacitor C. Note that Vdd / 2, which is half the power supply voltage, is supplied to the other terminal of the speaker 30.
With this configuration, the switching signal output from the output terminal Out is converted into an analog signal by the LPF and output from the speaker 30.

Incidentally, for example, as shown by a broken line (a) in FIG. 1, one terminal of the speaker 30 is connected to the grant Gnd for some reason, and the other end of the inductor L is electrically short-circuited (ground fault). Then, an overcurrent exceeding the allowable value may flow through the transistor 121, which may be destroyed in the worst case. As a similar case, as shown by a broken line (b), there is a case where the output terminal Out has a ground fault via a wire having a large inductor component.
In the present embodiment, a protection circuit 20 is provided in order to prevent the destruction of the transistor due to overcurrent.

As shown in FIG. 1, the protection circuit 20 includes a reference signal output circuit 210 and a detection circuit 220.
The reference signal output circuit 210 includes a comparator 212, switches 216 and 218, and resistance elements R1 to R5. The detection circuit 220 includes comparators 223 and 224, an AND circuit 226, and an OR circuit 228. Each of the comparators 212, 223, and 224 has a negative input terminal (−) and a positive input terminal (+), and a voltage supplied to the negative input terminal (−) is a voltage supplied to the positive input terminal (+). When the voltage is less than V, the H level is output, and when the voltage supplied to the negative input terminal (−) is equal to or higher than the voltage supplied to the positive input terminal (+), the L level is output.

  In the reference signal output circuit 210, the input signal Vin is supplied to the negative input terminal (−) of the comparator 212, and the threshold voltage Vth is supplied to the positive input terminal (+). Therefore, the signal Vcmp output from the comparator 212 is at the H level if the input signal Vin is less than the threshold voltage Vth, and is at the L level if the input signal Vin is equal to or higher than the threshold voltage Vth.

The resistance elements R1 to R5 are connected in series with the power supply voltage (Vdd, Gnd). The switch 216 is turned on / off between the power supply voltage Vdd and the connection point of the resistance elements R1 and R2. The switch 218 is turned on / off between the ground Gnd and the connection point of the resistance elements R4 and R5. The switches 216 and 218 are turned on when the signal Vcmp is at the H level and turned off when the signal Vcmp is at the L level.
In the reference signal output circuit 210, the voltage at the connection point between the resistance elements R2 and R3 is output as the reference signal Ref-H, and the voltage at the connection point between the resistance elements R3 and R4 is output as the reference signal Ref-L.

In the reference signal output circuit 210 configured as described above, when the input signal Vin is equal to or higher than the threshold voltage Vth, the signal Vcmp by the comparator 212 becomes L level, so that the switches 216 and 218 are turned off. To simplify the description, if the resistance values of the resistance elements R1 to R5 are R1 to R5 as they are, the voltage of the reference signal Ref-H is
Vdd · (R3 + R4 + R5) / (R1 + R2 + R3 + R4 + R5)
It becomes.
Further, the voltage of the reference signal Ref-L is as follows when the input signal Vin is equal to or higher than the threshold voltage Vth:
Vdd · (R4 + R5) / (R1 + R2 + R3 + R4 + R5)
It becomes.
FIG. 2A is a diagram illustrating a voltage relationship between the reference signals Ref-H and Ref-L when the input signal Vin is equal to or higher than the threshold voltage Vth. Note that this figure shows a voltage relationship assuming that the resistance values R1 to R5 are equal to each other for the sake of explanation, but in practice, as will be described later, in order to prevent erroneous detection during normal operation, the transistor In many cases, the values are set separately so as to have a margin for the maximum current allowed.

On the other hand, when the input signal Vin is less than the threshold voltage Vth, the signal Vcmp becomes H level, so that the switches 216 and 218 are turned on. For this reason, the voltage of the reference signal Ref-H is
Vdd · (R3 + R4) / (R2 + R3 + R4)
Thus, the input signal Vin becomes higher than when the input voltage Vin is equal to or higher than the threshold voltage Vth.
Further, the voltage of the reference signal Ref-L is as follows when the input signal Vin is less than the threshold voltage Vth:
Vdd · (R4) / (R2 + R3 + R4)
Thus, the input signal Vin is lower than when the input voltage Vin is equal to or higher than the threshold voltage Vth.
FIG. 2B is a diagram illustrating a voltage relationship between the reference signals Ref-H and Ref-L when the input signal Vin is less than the threshold voltage Vth.

In the detection circuit 220 of FIG. 1, the negative input terminal (−) of the comparator 223 is connected to the output terminal Out of the amplifier circuit 10, and the reference signal Ref-H is supplied to the positive input terminal (+). The reference signal Ref-L is supplied to the negative input terminal (−) of the comparator 224, and the positive input terminal (+) is connected to the output terminal Out of the amplifier circuit 10.
The AND circuit 226 outputs a logical product signal between the output signals of the comparators 223 and 224, and the OR circuit 228 outputs a logical sum signal of the logical product signal and the mute signal (Mute) from the AND circuit 226 to the drive circuit 114. Supply to the mute terminal. The mute signal here is a signal for instructing the cutoff of the output to the speaker 30 at the H level, and is supplied from the outside.

In the detection circuit 220 configured as described above, if the voltage of the output terminal Out of the amplifier circuit 10 is less than the voltage of the reference signal Ref-H, the output signal of the comparator 223 becomes H level. If the voltage at the output terminal Out is equal to or higher than the voltage of the reference signal Ref-L, the output signal of the comparator 224 becomes H level.
For this reason, in the protection circuit 20,
(1) When the output terminal Out is less than the voltage of the reference signal Ref-H and greater than or equal to the voltage of the reference signal Ref-L, or
(2) When the mute signal (Mute) is at the H level,
Then, the output signal of the OR circuit 228 becomes H level, and the transistors 121 and 122 are forcibly turned off in the amplifier circuit 10.

Incidentally, one of the transistors 121 and 122 is turned on according to the voltage of the input signal Vin, and the other is turned off. When the output terminal Out is grounded, even if the transistor 122 is turned on, the overcurrent does not continue to flow through the transistor 122. In this case, the target to be protected is the transistor 121.
When the transistor 121 is turned on when the output terminal Out is grounded, a current flows through a path of the power supply voltage Vdd feed line → the transistor 121 → the output terminal Out → the ground Gnd. If the ground fault at this time is generated via a wire having a small inductance, the current rises relatively rapidly with the passage of time, as shown in FIG.

Here, the voltage drop at the output terminal Out increases as the current flowing through the transistor 121 increases. Therefore, when the transistor 121 is turned on and a current flows, by detecting that the output terminal Out is less than the voltage of the reference signal Ref-H, it is detected that the current is greater than or equal to the threshold current Ith. be able to.
The voltage of the output terminal Out is either the power supply voltage Vdd or the ground Gnd in normal driving. For this reason, in detecting that the current flowing through the transistor 121 is equal to or greater than the threshold current Ith, the voltage of the output terminal Out is less than the voltage of the reference signal Ref-H in order to eliminate the case where the voltage of the output terminal Out is the ground Gnd. In addition, the condition is that the voltage is equal to or higher than the voltage of the reference signal Ref-L. Note that the voltage of the reference signal Ref-H defines a threshold current Ith used when determining the current flowing through the transistor 121.

As described above, when the current flowing through the transistor 121 becomes equal to or greater than the threshold current Ith, the comparator 223, 224 causes the output terminal Out to be less than the voltage of the reference signal Ref-H and greater than the voltage of the reference signal Ref-L. It is detected by being.
In normal driving, the voltage at the output terminal Out is either the power supply voltage Vdd or the ground Gnd. Therefore, at the time of this switching, the voltage at the output terminal Out is less than the voltage of the reference signal Ref-H. Thus, there is room for entry in a range not less than the voltage of the reference signal Ref-L, although it is a short time. Therefore, the input / output characteristics of the comparators 223 and 224 are adjusted so as to output an H level signal only when the comparison result is satisfied for a certain time or more.

  Incidentally, when the threshold current Ith is fixed, there are the following problems. That is, when the transistor 121 is initially turned off and the transistor 122 is turned on (the voltage at the output terminal Out is low), the output terminal Out is grounded through a wire having a large inductance, and the transistors 121 and 122 are connected. When the on / off state is switched, as shown in FIG. 3B, since the current hardly flows, the current rises relatively slowly with time. For this reason, the time Tb until the current flowing through the transistor 121 becomes equal to or greater than the threshold current Ith takes longer than the time Ta in FIG. As is well known, the amount of heat generated when a current flows through a transistor is proportional to the on-resistance of the transistor, proportional to time, and proportional to the square of the current. Therefore, when a ground fault occurs through a wire having a large inductance, it takes a long time to protect the transistor 121, that is, a time until the output signal of the OR circuit 228 becomes H level. There is a problem that the transistor 121 is likely to be thermally destroyed.

When the transistor 121 is initially turned on and the transistor 122 is turned off (the voltage of the output terminal Out is high), if the output terminal Out is grounded via a wire having a large inductance, the current flowing through the transistor 121 is The above problem does not need to be considered because it rises relatively rapidly over time.
Conversely, in terms of preventing the transistor from being destroyed by the protection circuit 20, a state where the voltage at the output terminal Out is low is important.

  As is well known, in the class D amplifier circuit, as the voltage of the input signal Vin decreases, the ratio of the on time of the high potential side transistor decreases (the ratio of the on time of the low potential side transistor increases). In other words, the state where the voltage at the output terminal Out is low means that the voltage of the input signal Vin is low.

Therefore, in this embodiment, the state where the voltage at the output terminal Out is low is detected as the input signal Vin being less than the threshold voltage Vth. Specifically, when the input signal Vin becomes equal to or higher than the threshold voltage Vth and lower than the threshold voltage Vth, the threshold signal Ith defined by the reference signal Ref-H is increased by increasing the voltage of the reference signal Ref-H. , Pulled down as shown in FIG.
Note that the reference signal Ref-H defines a relatively large first current as the threshold current Ith if the input signal Vin is the first voltage equal to or higher than the threshold voltage Vth, and the second signal whose input signal Vin is lower than the threshold voltage Vth. In the case of a voltage, there is a relationship that a relatively small second current is defined as the threshold current Ith.

  As described above, when the voltage at the output terminal Out is low, when the output terminal Out is grounded via a wire with a large inductance, the current flowing through the transistor 121 is relatively slow over time. However, according to the present embodiment, the time Tc until the current flowing through the transistor 121 becomes equal to or greater than the threshold current Ith can be made shorter than the time Tb when the threshold current Ith is constant. Therefore, according to the present embodiment, even when the voltage of the output terminal Out is low and the output terminal Out is grounded via a wire having a large inductance, the possibility that the transistor 121 is thermally destroyed is reduced. Can do it.

  A configuration in which the voltage of the output voltage Out is directly compared with a voltage corresponding to the threshold current Ith is also possible. However, since the output voltage Out fluctuates from the zero voltage ground Gnd to the power supply voltage Vdd (since it has a large amplitude), it is required that the constituent elements have a sufficient withstand voltage to be determined by the voltage of the output voltage Out. On the other hand, since the input signal Vin is a signal before being amplified, it has a small amplitude. Therefore, according to the configuration for determining the voltage of the input signal Vin as in the present embodiment, no breakdown voltage is required for the constituent elements, and thus the configuration can be simplified.

  Further, the threshold voltage Vth and the resistance values of the resistance elements R1 to R5 are set so as not to be erroneously detected during normal operation and to have a margin for the maximum current allowed for the transistor.

Next, a second embodiment of the present invention will be described.
FIG. 4 is a diagram illustrating a system including a protection circuit according to the second embodiment.
In the second embodiment, in the protection circuit 20, the signal Vcmp that is the comparison result of the comparator 212 is input to the delay block 214, and the switches 216 and 218 are turned on / off by the signal Bout output from the delay block 214. It has become.

FIG. 5 is a diagram showing the relationship between the signal Vcmp that is the input of the delay block 214 and the signal Bout that is the output, that is, the input / output characteristics.
As shown in this figure, when the signal Vcmp changes from the L level to the H level, that is, when the input signal Vin becomes less than the threshold voltage Vth, the signal Bout becomes L when the time Tda elapses from the change point. It changes from level to H level. That is, the delay block 214 delays the output of the signal Vcmp by the time Tda and outputs the delayed signal.
As a result, even if the voltage of the input signal Vin gradually decreases and becomes less than the threshold voltage Vth, the switches 216 and 218 are not immediately turned on, and when the state less than that continues for the time Tda, 218 is turned on and the threshold current Ith is lowered. For this reason, when the voltage of the input signal Vin tends to decrease and fluctuates in the vicinity of the threshold voltage Vth, the threshold current Ith is not switched unless the state of being less than the threshold voltage Vth continues for the time Tda. Therefore, switching of the threshold current Ith is suppressed, and erroneous detection is prevented.
Even when the signal Vcmp changes from the L level to the H level, when the time Tdc shorter than the time Tda elapses from the change point, the signal Bout does not change to the H level but changes to the L level. Maintained at level.

On the other hand, when the signal Vcmp changes from the H level to the L level, that is, when the input signal Vin becomes equal to or higher than the threshold voltage Vth, the signal Bout changes from the H level to the L level when the time Tdb elapses from the change point. To do. Here, as shown in the figure, Tda> Tdb is set. Specifically, the delay block 214 is set to have the shortest response speed.
As a result, when the input signal Vin gradually rises and becomes equal to or higher than the threshold voltage Vth, the signal Bout immediately becomes L level, the switches 216 and 218 are turned off, and the threshold current Ith is raised.
In the state where the voltage of the input signal Vin is high, the ratio of the time during which the transistor 121 on the high potential side is turned on is large, that is, there is a high possibility of being turned on. For this reason, in a state where the voltage of the input signal Vin is high, a relatively large current instantaneously flows through the transistor 121 even in a normal state (a state where no short circuit occurs). In this situation, if the threshold current Ith is in a lowered state, there is a high possibility that a false detection will occur even though it is normal. In the second embodiment, when the voltage of the input signal Vin becomes equal to or higher than the threshold voltage Vth, the threshold current Ith is immediately increased, so that such erroneous detection is prevented.

  The delay block 214 can be realized by a combination of a delay circuit and an SR (set / reset) flip circuit, a general-purpose timing device called a noise discriminator, or the like.

  The present invention is not limited to the above-described embodiments, and various applications and modifications as described below are possible, for example. In addition, one or more arbitrarily selected aspects of application / deformation described below can be appropriately combined.

The output signal may be fed back and pulse width modulated.
FIG. 6 is a diagram illustrating an example of the configuration.
In the example of this figure, the output signal of the LPF is fed back to the pulse modulation circuit 112 in the amplifier circuit 10. The pulse modulation circuit 112 multiplies the output signal of the LPF having a large amplitude by a coefficient smaller than “1” to match the small amplitude of the input signal Vin. Thereafter, for example, the pulse modulation circuit 112 subtracts the input signal from the feedback signal converted into a small amplitude, and the drive circuit 114 turns off the transistor 121 (turns on the transistor 122 if the subtraction result is positive). If the subtraction result is negative, the transistor 121 may be turned on (the transistor 122 is turned off). In other words, the drive circuit 114 may be configured to drive the transistors 121 and 122 in consideration of not only the input signal Vin but also the output signal.
In the configuration using feedback as shown in FIG. 6, the delay block 214 shown in FIG. 4 may be provided.

  The input signal Vin is not limited to an analog signal, and may be a digital signal. If the input signal Vin is a digital signal, the switches 216 and 218 may be turned on / off according to the bit of the input signal Vin.

  The reference signal output circuit 210 is configured to switch the reference signal Ref-H between high and low depending on whether the input signal Vin is less than the threshold voltage Vth. However, the reference signal output circuit 210 may be switched in three or more stages, or the voltage of the input signal Vin. The reference signal Ref-H may be gradually increased as the signal becomes lower.

  As long as the amplifier circuit has a pair of a high-potential side transistor and a low-potential side transistor, the amplifier circuit to which the protection circuit according to the present invention is applied is not limited to a class D amplifier.

DESCRIPTION OF SYMBOLS 10 ... Amplifier circuit, 20 ... Protection circuit, 30 ... Speaker, 114 ... Drive circuit, 121, 122 ... Transistor, 210 ... Reference signal output circuit, 212 ... Comparator, 214 ... Delay block, 220 ... Detection circuit.

Claims (4)

A first transistor that outputs a signal corresponding to the high potential side to an output terminal;
A second transistor that outputs a signal corresponding to a low potential side to the output terminal;
Turn on one of the first transistor or the second transistor based on the input signal, a drive circuit for driving off the other,
A protection circuit for an amplifier circuit having
If the input signal is equal to or higher than a threshold voltage, a reference signal that defines a first current is output. If the input signal is less than the threshold voltage, a reference signal that defines a second current that is smaller than the first current is output. A reference signal output circuit;
A detection circuit for detecting whether or not a current flowing through the first transistor is equal to or greater than a current defined by the reference signal;
Comprising
When the detection circuit detects that the current is greater than or equal to the current defined by the reference signal, the drive circuit stops driving the first transistor and the second transistor. circuit. The reference signal output circuit is
A comparison circuit for comparing the voltage of the input signal and the threshold voltage;
When the voltage of the input signal is equal to or higher than the threshold voltage by the comparison circuit, the first current is defined by the reference signal;
2. The protection circuit for an amplifier circuit according to claim 1, wherein the second current is defined by the reference signal when the voltage of the input signal is less than the threshold voltage. 3. The reference signal output circuit is
The current defined by the reference signal is changed from the first current to the second current when a state where the voltage of the input signal is lower than the threshold voltage continues for a predetermined time. Amplifying circuit protection circuit. The reference signal output circuit is
When the voltage of the input signal transitions from a state below the threshold voltage to a state above the threshold voltage, the current defined by the reference signal is changed from the second current to the first in a time shorter than the predetermined time. The protection circuit for an amplifier circuit according to claim 3, wherein the protection circuit is changed to a current.

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