Patent Publication Number: US-9899971-B1

Title: Offset detection circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-184339, filed Sep. 21, 2016, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an offset detection circuit. 
     BACKGROUND 
     Generally, when output of a power amplifier circuit (power amplifier) that amplifies an audio signal is supplied to a speaker, an offset detection circuit may be adopted for removing a DC offset in the output of an amplifier in order to prevent damage to the speaker. 
     However, the offset detection circuit in the related art cannot detect the DC offset at the time of audio reproduction (at the time of presence of signal), but can detect the DC offset only when there is no signal such as when an audio set is activated, and thus the damage of the speaker may not be reliably prevented. 
     Even in the related art, an offset detection circuit can detect the offset at the time of the presence of the signal. However, there is a problem that a low pass filter having a large time constant or a DC shift circuit is required. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing an amplifier in which an offset detection circuit according to a first embodiment of the exemplary embodiment is incorporated. 
         FIG. 2  is a block diagram showing an example of an audio system in which the amplifier of  FIG. 1  is incorporated. 
         FIG. 3  is a circuit diagram showing a DC offset detection circuit adopted in a related art. 
         FIG. 4  is a timing chart showing a detection result in the related art. 
         FIG. 5A  is an explanatory diagram for explaining determination of a positive phase determination unit and a negative phase determination unit. 
         FIG. 5B  is an explanatory diagram for explaining the determination of the positive phase determination unit and the negative phase determination unit. 
         FIG. 6  is a timing chart for explaining a second embodiment. 
         FIG. 7  is a timing chart for explaining the second embodiment. 
         FIG. 8  is a circuit diagram showing the second embodiment of the exemplary embodiment. 
         FIG. 9  is a circuit diagram showing a third embodiment of the exemplary embodiment. 
         FIG. 10  is a timing chart for explaining an operation of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide an offset detection circuit capable of reliably detecting an offset even when an AC signal is input to an amplifier. 
     In general, according to one embodiment, an offset detection circuit includes: a comparison unit connected to an output of an amplifier that amplifies an input signal and configured to generate a first comparison result between the output of the amplifier and a positive detection threshold value, and a second comparison result between the output of the amplifier and a negative detection threshold value; a first determination unit configured to generate a first offset determination result of two values indicating presence or absence of an offset according to a period during which the output of the amplifier exceeds the positive detection threshold value, based on the first comparison result; a second determination unit configured to generate a second offset determination result of two values indicating the presence or absence of an offset according to a period during which the output of the amplifier exceeds the negative detection threshold value, based on the second comparison result; and an output unit configured to generate a determination output of the offset based on the first and second offset determination results. 
     Hereinafter, exemplary embodiments will be described in detail with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a circuit diagram showing an amplifier  1  in which an offset detection circuit according to a first embodiment of the exemplary embodiment is incorporated. In addition,  FIG. 2  is a block diagram showing an example of an audio system in which the amplifier  1  of  FIG. 1  is incorporated. 
     In  FIG. 2 , a control unit  21  can control the entirety of the system. The control unit  21  may be configured with a processor using a CPU (not shown) or the like, and may control each unit by being operated according to a program stored in a memory (not shown). A read device  22  is a driver that drives a storage medium such as a compact disc (CD), a digital versatile disc (DVD), a Blu-ray disc, a cassette tape, and an HDD recorder (not shown), reads audio information stored in the storage medium, and thereby outputs an audio signal to a signal processing unit  23  under control of the control unit  21 . 
     The signal processing unit  23  is configured with an electronic volume, a digital signal processor (DSP), and the like, performs a predetermined signal process such as a high sound quality process on the received audio signal under the control of the control unit  21 , and amplifies the audio signal at a predetermined amplification rate. The audio signal from the signal processing unit  23  is supplied to the amplifier  1 , after a DC component is removed by a coupling capacitor C 1 . The amplifier  1  amplifies the received audio signal, and outputs the amplified audio signal to a speaker  24 . Thus, audio is output from the speaker  24  as a sound. 
     In  FIG. 1 , the amplifier  1  is configured with an amplification unit  2  and a detection unit  3 . The amplification unit  2  and the detection unit  3  may be integrated as an integrated circuit, and the entirety of the amplifier  1  may be configured on one chip. 
     An input terminal IN receives the audio signal that is an AC signal through the coupling capacitor C 1 . The input terminal IN is connected to a non-inverting (positive phase) input terminal of an amplifier OP 1 . The amplifier OP 1  amplifies the received audio signal and obtains the audio signal of the positive phase. A non-inverting input terminal of an amplifier OP 2  that obtains an audio signal of a negative phase is connected to a reference potential point through a negative input capacitor C 2  connected to a terminal  6 . The amplifier OP 2  amplifies the received audio signal, and obtains the audio signal of the negative phase. 
     The non-inverting input terminal of the amplifier OP 1  is connected to a positive polarity terminal of a power source VRB through a resistor R 4 , the non-inverting input terminal of the amplifier OP 2  is connected to the positive polarity terminal of the power source VRB through a resistor R 5 , and a negative polarity terminal of the power source VRB is connected to the reference potential point. This arrangement causes a predetermined bias to be applied to the non-inverting input terminals of the amplifiers OP 1  and OP 2 . 
     A resistor R 2  is connected between an output terminal and an inverting (negative phase) input terminal of the amplifier OP 1 . The inverting input terminal of the amplifier OP 1  is connected to the inverting input terminal of the amplifier OP 2  through a resistor R 1 . A resistor R 3  is connected between an output terminal and the inverting input terminal of the amplifier OP 2 . 
     A DC component of the audio signal that is input through the input terminal IN is removed by an input coupling capacitor C 1 , and the resultant audio signal is supplied to the non-inverting input terminal of the amplifier OP 1 . In addition, the non-inverting input terminal of the amplifier OP 2  is connected to the reference potential point through the capacitor C 2  so that a noise component is removed. The gain of the amplifier OP 1  is determined by resistance values of the resistors R 1  and R 2 , and the gain of the amplifier OP 2  is determined by resistance values of the resistors R 1  and R 3 . 
     The received audio signal is positively amplified by the amplifier OP 1 . By the virtual ground of the non-inverting input terminal and the inverting input terminal of the amplifier OP 1 , the same potential signal as the input audio signal is also applied to the inverting input terminal of the amplifier OP 2 . The received audio signal is negatively amplified by the amplifier OP 2 . Then, the amplifier OP 1  outputs the audio signal with positive phase to a positive phase output terminal OUTP, and the amplifier OP 2  outputs the audio signal with negative phase to a negative phase output terminal OUTM. Although not shown in  FIG. 1 , the audio signals with positive phase and negative phase respectively generated in output terminals OUTP and OUTM are supplied to the speaker  24  as output of the amplification unit  2 . 
     A configuration of the amplification unit  2  is not limited to the configuration of  FIG. 1 . If a signal with positive phase is output to the positive phase output terminal OUTP and a signal with negative phase is output to the negative phase output terminal OUTM by a pair of amplifiers, any configuration may be implemented. 
     Next, before explaining the detection unit  3 , a configuration and a problem of a DC offset detection circuit present in the related art will be described. 
     Typically, in a single-ended output power amplifier, by providing a DC blocking capacitor in an output terminal, a DC offset blocked from the speaker. 
     On the other hand, in a bridged transformer less (BTL) amplifier that has two power amplifiers whose outputs are connected to each other in a bridge manner and that obtains a doubled output due to the difference between the positive and negative outputs, because the same DC bias is applied to the positive and negative outputs, the DC blocking capacitor is not used. 
     However, there is a case where a DC offset that is large is included in output of the power amplifier due to leakage of a coupling capacitor connected to an input terminal of the power amplifier. Then, the offset detection circuit that detects the DC offset is used in the BTL amplifier in order to prevent damage to a speaker when the DC offset is large. 
       FIG. 3  is a circuit diagram showing an amplifier including the DC offset detection circuit used in the related art. In  FIG. 3 , a configuration of the BTL amplifier configured with the amplifier OP 1 , the amplifier OP 2 , and the like has the same configuration as the amplification unit  2  of  FIG. 1 . The audio signal with positive phase and the audio signal with negative phase are generated at output terminals OUTP and OUTM of the BTL amplifier, respectively. The positive phase output terminal OUTP is connected to a non-inverting input terminal of a comparator COM 1 . In addition, the positive phase output terminal OUTP is connected to a negative polarity terminal of the power source VRM that generates a detection threshold value VRM, and a positive polarity terminal of the power source VRM is connected to an inverting input terminal of the comparator COM 2 . An output terminal of the comparator COM 1  is connected to a positive phase detection terminal DET_P. 
     Similarly, the negative phase output terminal OUTM is connected to a non-inverting input terminal of the comparator COM 2 . In addition, the positive phase output terminal OUTP is connected to a negative polarity terminal of a power source VRP that generates a detection threshold value VRP, and a positive polarity terminal of the power source VRP is connected to an inverting input terminal of the comparator COM 1 . An output terminal of the comparator COM 2  is connected to a negative phase detection terminal DET_M. 
     The comparator COM 1  outputs a detection result of the positive phase to the positive phase detection terminal DET_P, which becomes a high level (hereinafter, referred to as an H level) when a value obtained by subtracting the audio signal of the negative phase generated in the negative phase output terminal OUTM from the audio signal of the positive phase generated in the positive phase output terminal OUTP is higher than the detection threshold value VRP, and, otherwise, becomes a low level (hereinafter, referred to as an L level). In addition, the comparator COM 2  outputs a detection result of the negative phase to the negative phase detection terminal DET_M, which becomes an H level when a value obtained by subtracting the audio signal of the negative phase generated in the negative phase output terminal OUTM from the audio signal of the positive phase generated in the positive phase output terminal OUTP is lower than the detection threshold value VRM, and, otherwise, becomes an L level. Outputs of the comparators COM 1  and COM 2  are provided to an OR circuit OR 1 , and the OR circuit OR 1  outputs an output obtained by an OR operation on two inputs, to a detection terminal DET as the detection result. 
       FIG. 4  is a timing chart showing a detection result in the related art. In the following description, by using the signal obtained by subtracting the audio signal generated in the negative phase output terminal OUTM from the audio signal generated in the positive phase output terminal OUTP as a BTL output, an operation according to a waveform of the BTL output will be described. When the BTL output of  FIG. 4  is greater than the detection threshold value VRP in a positive side, an output of the comparator COM 1  becomes the H level, and when the BTL output is greater than the detection threshold value VRM in a negative side, an output of the comparator COM 2  becomes the H level. Then, a detection result from the OR circuit OR 1  becomes a pulse oscillating between H and L showing in  FIG. 4 . 
     The offset period depicted in  FIG. 4  indicates that a DC offset is present in an output of the BTL amplifier due to the leakage of the input coupling capacitor C 1  or the like. At a time t 1  of  FIG. 4 , since a BTL output amplitude exceeds the detection threshold values VRP and VRM in a state where there is no offset, the detection result becomes a signal that frequently changes (oscillates) between the H level and the L level. The waveform at a time t 2  of  FIG. 4  indicates a state where the offset is present when there is no signal. In this case, it is possible to determine that the detection result is stable at the H level and indicates an offset. The waveform at a time t 3  of  FIG. 4  indicates a state where the offset is present when there is a signal. Since the BTL output has a portion which does not exceed the detection threshold values VRP and VRM, the detection result changes between an H level and an L level. The waveform at a time t 4  of  FIG. 4  indicates a state where there is no offset no signal. In this case, it is possible to determine that the detection result is stable at an L level and there is no offset. 
     The detection terminal DET of  FIG. 3  is connected to the reference potential point through a low pass filter configured with a resistor R 6  and a capacitor C 3  that are connected in series. The detection result generated in the detection terminal DET is integrated by the low pass filter, and the integrated result of the detection result is generated in a connection point between the resistor R 6  and the capacitor C 3 . The integrated result is output through a determination output terminal DEC as a determination output of the DC offset. 
     As shown in  FIG. 4 , the determination output generated in the determination output terminal DEC is a waveform that is relatively smooth even when the detection result generated in the detection terminal DET are frequently switched between an H level and an L level, and approximate offset determination can be performed even when there is a signal. 
     However,  FIG. 4  shows an example in a case where an amplitude of the BTL output is a relatively small level slightly exceeding the detection threshold values VRP and VRM. When the amplitude of the BTL output is relatively great, for example, when an audio signal in the vicinity of a clip level is input, since the detection result generated in the detection terminal DET becomes an L level for a short time, and becomes an H level most of time, it is erroneously determined that there is the offset regardless of the presence or absence of the offset. In addition,  FIG. 3  shows an example of the related art in which the low pass filter configured with the resistor R 6  and the capacitor C 3  is adopted. However, the low pass filter needs to be set to have cutoff characteristics sufficiently lower than that of the audio signal, which leads to increase in external components and cost. 
     In the related art of  FIG. 3 , for a process of the DC offsets generated in output of an amplification unit, a positive phase audio signal and a negative phase audio signal are respectively compared with detection threshold values, respective comparison results are combined through the OR circuit OR 1  to obtain one detection result, and the determination output obtained based on the detection result is output through one determination output terminal DEC. That is, the related art of  FIG. 3  has an advantage that it is possible to reduce the number of pins when being implemented as an integrated circuit. 
     However, in the related art of  FIG. 3 , the detection result generated in the detection terminal DET is used for determining both whether or not the BTL output exceeds a positive detection threshold value and a negative detection threshold value, and becomes an H level waveform most of time and an L level for a short time with respect to an input having a great amplitude. Therefore, there is a possibility that there is an erroneous output at the time of the great amplitude. A single-ended amplifier provided with a negative power source may not have a blocking capacitor and has a similar problem. 
     Alternatively, in the embodiment, respective DC offsets are determined based on the detection result obtained by comparing the BTL output and the positive detection threshold value and the detection result obtained by comparing the BTL output and the negative detection threshold value. Respective determination results are combined. Thereby, the determination output is output from one determination output terminal DEC. In this case, the low pass filter is not used in the determination of the DC offset, and this embodiment is improved with respect to circuit integration. 
     As shown in  FIG. 1 , in the embodiment, the detection unit  3  provides outputs of the comparators COM 1  and COM 2  serving as comparison units to the positive phase determination unit  4 P and the negative phase determination unit  4 M, respectively. Configurations of the power sources VRM and VRP and the comparators COM 1  and COM 2  are the same as those of  FIG. 3 , and the comparator COM 1  outputs the detection result of the positive phase which becomes an H level when a signal obtained by subtracting the audio signal of the negative phase generated in the negative phase output terminal OUTM from the audio signal of the positive phase generated in the positive phase output terminal OUTP, that is, the BTL output is higher than the detection threshold value VRP, and, otherwise, becomes an L level. In addition, the comparator COM 2  outputs the detection result of the negative phase which becomes an H level when the BTL output is lower than the detection threshold value VRM, and, otherwise, becomes an L level. 
     In the embodiment, the output of the comparator COM 1  is provided to the positive phase detection terminal DET_P of the positive phase determination unit  4 P. In addition, the output of the comparator COM 2  is provided to the negative phase detection terminal DET_M of the negative phase determination unit  4 M. As configurations of the positive phase determination unit  4 P and the negative phase determination unit  4 M are the same, the configuration of the negative phase determination unit  4 M is not shown in  FIG. 1 . In the positive and negative phase determination units  4 P and  4 M, the same components are denoted by the same reference characters, and P or M is added to the reference characters of each component when it is necessary to distinguish each component of the positive phase determination unit  4 P from each component of the negative phase determination unit  4 M. 
     The detection result of the positive phase is provided to an up/down counter  11 P of the positive phase determination unit  4 P through the positive phase detection terminal DET_P. The detection result of the negative phase is provided to an up/down counter  11 M of the negative phase determination unit  4 M through the negative phase detection terminal DET_M. The detection result of the positive phase is also supplied to the counter control unit  15 P. An oscillator  5  generates a predetermined frequency clock CK, and supplies the clock CK to the counter control units  15 P and  15 M of the positive phase determination unit  4 P and the negative phase determination unit  4 M. The counter control unit  15 P supplies the clock CK to the up/down counter  11 P, generates an up/down control signal UD for controlling incrementing and decrementing based on the detection result of the positive phase, and outputs the generated up/down control signal UD to the up/down counter  11 P. In addition, the counter control unit  15 M supplies the clock CK to the up/down counter  11 M, generates the up/down control signal UD for controlling the incrementing and the decrementing based on the detection result of the negative phase, and outputs the generated up down control signal UD to the up/down counter  11 M. The clock CK will be described below. 
     The up/down counter  11 P increments in response to the clock CK during an H level period in the detection result of the positive phase, and decrements in response to the clock CK during an L level period in the detection result of the positive phase. A counted value obtained from the up/down counter  11 P becomes greater when an H level period in the detection result of the positive phase is longer, and becomes smaller when an L level period is longer. The up/down counter  11 M increments in response to the clock CK during an H level period in the detection result of the negative phase, and decrements in response to the clock CK during an L level period in the detection result of the negative phase. The counted value obtained from the up/down counter  11 M becomes greater when an H level period in the detection result of the negative phase is longer, and becomes smaller when an L level period is longer. The counted value of the up/down counter  11  is supplied to an overflow detection unit  12  and an underflow detection unit  13 . 
     The overflow detection unit  12  outputs an overflow detection result that is changed from an L level to an H level when the counted value of the up/down counter  11  overflows. The overflow detection unit  12  may determine that the overflow is generated, when the counted value of the up/down counter  11  is greater than a predetermined threshold value (hereinafter, referred to as overflow set value), and output the overflow detection result that is changed from the L level to the H level. That is, in the embodiment, the overflow means that the counted value of the up/down counter  11  reaches the overflow set value. 
     The underflow detection unit  13  outputs an underflow detection result that is changed from the L level to the H level when the counted value of the up/down counter  11  underflows. The underflow detection unit  13  may determine that the underflow is generated, when the counted value of the up/down counter  11  is smaller than a predetermined threshold value (hereinafter, referred to as underflow set value), and output the underflow detection result that is changed from an L level to an H level. That is, in the embodiment, the underflow means that the counted value of the up/down counter  11  reaches the underflow set value. 
       FIG. 5A  and  FIG. 5B  are explanatory diagrams for explaining determination by the positive phase determination unit  4 P and the negative phase determination unit  4 M.  FIG. 5A  and  FIG. 5B  show examples in which a level of the BTL output is extremely great.  FIG. 5A  shows the detection result that is output from the detection terminal DET in the related art of  FIG. 3 , and  FIG. 5B  shows the detection result of the positive phase that is output from the positive phase detection terminal DET_P of  FIG. 1 . The detection result in the related art of  FIG. 3  is obtained by the OR operation on outputs of the comparators COM 1  and COM 2 , and, as described above, becomes an H level most of time when the level of the BTL output is great. Therefore, as described above, even during a period in which there is no offset the level of the determination output from the determination output terminal DEC is high, and it is erroneously determined that there is an offset. 
     On the other hand, as shown in  FIG. 5B , if, in the detection result of the positive phase that is output from the positive phase detection terminal DET_P, an L level period is longer on average than an H level period, then a determination of an offset is not generated. However, during the offset period, an H level is correctly maintained most of time. The same is also applied to the detection result of the negative phase that is output from the negative phase detection terminal DET_M. Thus, during a period in which the offset is present, the counted value of the up/down counter  11  reaches the overflow set value, and during a period in which there is no offset, the counted value of the up/down counter  11  does not reach the overflow set value. 
     In the embodiment, the positive phase and negative phase determination units  4 P and  4 M determine that the offset is present when the counted value of the up/down counter  11  is greater than the overflow set value, and the units determine that the offset is not present when the counted value of the up/down counter  11  is smaller than the underflow set value. That is, the overflow detection result from the overflow detection unit  12  indicates determination that the offset is present, and the underflow detection result from the underflow detection unit  13  indicates determination that the offset is not present. 
     The overflow detection result from the overflow detection unit  12  is supplied to an input terminal of an OR circuit OR 2 , also supplied to a set terminal SET of an RS flip-flop (RSFF)  14 . In addition, the underflow detection result from the underflow detection unit  13  is supplied to the input terminal of the OR circuit OR 2 , also supplied to a reset terminal RSET of the RS flip-flop (RSFF)  14 . 
     The RSFF  14 P outputs a determination result of the positive phase of an H level based on the overflow detection result of an H level supplied to the set terminal SET, and outputs the determination result of the positive phase of an L level based on the underflow detection result of an H level supplied to the reset terminal RSET. The determination result of the positive phase of an H level from the RSFF  14 P indicates that the input signal is in an offset period, and the determination result of the positive phase of an L level from the RSFF  14 P indicates that the input signal is not in an offset period. Output of the RSFF  14 P is output from a positive phase determination terminal DEC_P as the determination output of the positive phase. 
     Similarly, the RSFF  14 M outputs the determination result of the negative phase of an H level based on the overflow detection result of an H level supplied to the set terminal SET, and outputs the determination result of the negative phase of an L level based on the underflow detection result of an H level supplied to the reset terminal RSET. The determination result of the negative phase of an H level from the RSFF  14 M indicates that the input signal is in an offset period, and the determination result of the negative phase of an L level from the RSFF  14 M indicates that the input signal is not in an offset period. Output of the RSFF  14 M is provided from the negative phase determination terminal DEC_M as the determination output of the negative phase. 
     When the overflow detection result of an H level or the underflow detection result of an H level is input, the OR circuit OR 2  outputs a stop signal to the count control unit  15  so that the up/down counter  11  does not increment/decrement further. The count control unit  15  stops the clock CK when receiving the stop signal. When the clock CK is stopped by the overflow detection result, the value of the up/down counter  11  is maintained at the overflow set value. In this state, when the detection result of the positive phase or the negative phase that is input to the count control unit  15  is changed to an L level, the count control unit  15  restarts supply of the clock CK and the up/down counter  11  starts decrementing. In addition, when the clock CK is stopped by the underflow detection result, the counted value of the up/down counter  11  is fixed at the underflow set value. In this state, when the detection result of the positive phase or the negative phase that is input to the count control unit  15  is changed to an H level, the count control unit  15  restarts the supply of the clock CK and the up/down counter  11  restarts incrementing. 
     As described above, the detection result of the positive phase becomes an H level during a relative long period and the counted value reaches the overflow set value so that the positive phase determination unit  4 P determines that the offset is present. In addition, the detection result of the negative phase becomes an H level during a relative long period and the counted value reaches the overflow set value so that the negative phase determination unit  4 M determines that the offset is present. In this case, the length of time that an H level needs to be maintained until the counted value reaches the overflow set value is determined based on a cycle of the clock CK, with respect to a BTL output cycle, that is, an audio signal cycle and the overflow set value. 
     In order to reliably perform comparison with the detection threshold value for every one cycle of the BTL output, it is preferable that a frequency of the clock CK from the oscillator  5  is equal to or greater than that of twice of an input audio signal bandwidth. For example, an oscillation frequency (frequency of clock CK) of the oscillator  5  is set to 60 KHz. Meanwhile, since the up/down counter  11  increments in response to the clock CK during a period in which the BTL output exceeds the detection threshold value, even when the DC offset is not generated, the counted value increases as a cycle of the audio signal is longer. When the DC offset is not generated, the number of bits of the up/down counter  11  is determined by considering a case where the BTL output is 20 Hz, which frequency is designated as the lowest value of the audible frequency that does not overflow the counted value of the up/down counter  11 . 
     That is, the up/down counter  11  may be implemented if the counted value of ( 1/20 Hz)/( 1/60 KHz)=3,000 counts or more can be output. For example, as the up/down counter  11 , a 12-bit counter capable of counting 4,096 may be adopted. 
     The clock CK of a frequency equal to or greater than that of twice of the input audio signal bandwidth is not required at all times. The clock frequency may be lowered to perform an under-sampling operation, and it is not necessary that the maximum counts of the up/down counter  11  be set to the counted value equal to or greater than 3,000 counts described above at all times. 
     The determination output of the positive phase that is output from the positive phase determination terminal DEC_P and the determination output of the negative phase that is output from the negative phase determination terminal DEC_M are input to an OR circuit OR 3 . The OR circuit OR 3  serving as an output unit provides, as the determination output, an operation result obtained by performing the OR operation on two inputs, to the determination output terminal DEC. The determination output that is provided to the determination output terminal DEC is supplied to the control unit  21  of  FIG. 2 . 
     The control unit  21  may, for example, shut down a system when it is indicated that the DC offset is present in the output of the amplification unit  2  by the determination output. 
     Next, an operation of the embodiment configured in this manner will be described with reference to timing charts of  FIG. 6  and  FIG. 7 . 
     The input audio signal through the input terminal IN is amplified by the amplifiers OP 1  and OP 2 , and the audio signal of the positive phase and the audio signal of the negative phase are generated in output terminals of operational amplifiers OP 1  and OP 2 . The comparator COM 1  outputs the detection result of the positive phase which becomes an H level when the BTL output is higher than the detection threshold value VRP, and, otherwise, becomes an L level, to the positive phase detection terminal DET_P. As shown in  FIG. 6 , the detection result of the positive phase shown by a solid line with respect to the BTL output can be obtained. 
     In addition, the comparator COM 2  outputs the detection result of the negative phase which becomes an H level when the BTL output is lower than the detection threshold value VRM, and, otherwise, becomes an L level, to the negative phase detection terminal DET_M. As shown in  FIG. 6 , the detection result of the negative phase is shown by a broken line with respect to the BTL output. 
     The detection result of the positive phase is provided to the up/down counter  11 P of the positive phase determination unit  4 P. The up/down counter  11 P starts the incrementing when the BTL output exceeds the detection threshold value VRP and therefore the detection result of the positive phase becomes an H level, and starts the decrementing when the BTL output is lower than the detection threshold value VRP and therefore the detection result of the positive phase becomes an L level. The counted value of the positive phase determination unit  4 P in  FIG. 6  shows the counted value of the up/down counter  11 P in this case. The overflow detection unit  12 P or the underflow detection unit  13 P determine respectively whether or not the counted value of the up/down counter  11 P reaches the overflow set value or the underflow set value. 
     In an example of  FIG. 6 , the overflow detection unit  12 P does not output the overflow detection result of the H level, because the counted value of the up/down counter  11 P does not reach the overflow set value. Meanwhile, the counted value of the up/down counter  11 P reaches the underflow set value, so that the underflow detection unit  13 P outputs the underflow detection result of an H level. In this case, the determination result of the positive phase indicating that the offset is not present is output from the positive phase determination terminal DEC_P. In addition, the counter control unit  15 P stops generation of the clock CK based on the underflow detection result, and the counted value of the up/down counter  11 P maintains the underflow set value. When the BTL output increases and exceeds the detection threshold value VRP, the detection result of the positive phase becomes the H level again, and the counter control unit  15 P restarts the supply of the clock CK, and the up/down counter  11 P restarts incrementing the clock CK. 
     Subsequently, the same operation is repeated. However, the counted value of the up/down counter  11 P does not reach the overflow set value during the entire period of  FIG. 6 . 
     The detection result of the negative phase is provided to the up/down counter  11 M of the negative phase determination unit  4 M. The up/down counter  11 M starts the incrementing when the BTL output exceeds the detection threshold value VRM and therefore the detection result of the negative phase becomes an H level, and starts the decrementing when the BTL output is greater than the detection threshold value VRM and therefore the detection result of the negative phase becomes an L level. The counted value of the negative phase determination unit  4 M of  FIG. 6  shows the counted value of the up/down counter  11 M in this case. The overflow detection unit  12 M or the underflow detection unit  13 M, respectively determine whether or not the counted value of the up/down counter  11 M reaches the overflow set value or the underflow set value. 
     In an example of  FIG. 6 , until the offset is present, the counted value of the up/down counter  11 M does not reach the overflow set value, and the overflow detection unit  12 M does not output the overflow detection result of an H level. Meanwhile, the counted value of the up/down counter  11 M reaches the underflow set value, and the underflow detection unit  13 M outputs the underflow detection result of an H level. In this case, the determination result of the negative phase indicating that the offset is not generated is output from the negative phase determination terminal DEC_M. In addition, the counter control unit  15 M stops the generation of the clock CK based on the underflow detection result, and the counted value of the up/down counter  11 M maintains the underflow set value. When the BTL output decreases and exceeds the detection threshold value VRM, the detection result of the negative phase becomes an H level again, and the counter control unit  15 M restarts the supply of the clock CK, so that the up/down counter  11 M restarts incrementing the clock CK. 
     The offset period of  FIG. 6  indicates a period during which the offset is present in the BTL output. When the offset is present, a period during which the BTL output exceeds the detection threshold value VRM becomes long, and the H level period in the detection result of the negative phase becomes long, so that the counted value of the up/down counter  11 M increases and reaches the overflow set value. The overflow detection unit  12 M then outputs the overflow detection result of an H level. By this overflow detection result, the RSFF  14 M outputs the determination result of the negative phase as an H level to the negative phase determination terminal DEC_M. 
     The OR circuit OR 3  performs the OR operation on the determination result of the positive phase generated in the positive phase determination terminal DEC_P and the determination result of the negative phase generated in the negative phase determination terminal DEC_M, and outputs the operation result as the determination output. Thus, as shown in  FIG. 6 , after a predetermined period (hereinafter, referred to as determination period) elapses from start of the offset period, the determination output of the H level indicating that the offset is present is output. 
     When the counted value reaches the overflow set value, the counter control unit  15 M stops the generation of the clock CK based on the overflow detection result, and the counted value of the up/down counter  11 M maintains the overflow set value. When the BTL output increases and becomes greater than the detection threshold value VRM, the detection result of the negative phase becomes an L level again, and the counter control unit  15 M restarts the supply of the clock CK, so that the up/down counter  11 M restarts decrementing the clock CK. In this case as well, the RSFF  14  maintains the determination result of an H level until the underflow detection result of an H level is input to the reset terminal RSET. When the counted value of the up/down counter  11 M reaches the underflow set value, the RSFF  14  is reset by the underflow detection result of an H level, and changes the determination result to an L level indicating that the offset is not present. 
       FIG. 7  is a timing chart for explaining the determination period. In the embodiment, the counted value exceeds the overflow set value and thereby the determination result indicating that the offset is present is obtained, and the counted value that reaches the overflow set value reaches the underflow set value and thereby the determination result indicating that the offset is not present is obtained. Accordingly, as shown in  FIG. 7 , even though the offset is present, a predetermined determination period is necessary until the determination result indicates the presence of the offset. In addition, similarly, even though the offset is not present, a predetermined determination period is necessary until the determination result indicates absence the offset. The determination period is determined by the frequency of the clock CK and the overflow set value or the underflow set value.  FIG. 7  shows an example in which by changing a count increment amount of the incrementing per one clock CK and a count decrement amount of the decrementing per one clock CK, the determination period for detecting absence of the offset is shorter than the determination period for detecting presence of the offset. 
     In the embodiment, the detection result of the positive phase indicating whether or not the BTL output exceeds the positive detection threshold value, and the detection result of the negative phase indicating whether or not the BTL output exceeds the negative detection threshold value are provided respectively, respective DC offsets are determined and respective determination results are combined so that the determination output is performed from one determination output terminal DEC. During a period in which the offset is not present, a period during which the BTL output exceeds the positive detection threshold value is shorter than other periods. In addition, a period during which the BTL output exceeds the negative detection threshold value is shorter than other periods. Accordingly, since determination of the DC offset is separately performed with respect to the detection result of the positive phase and the detection result of the negative phase, there is no case where an erroneous determination that the offset is present when the offset is not present. In addition, when the offset is present, since a time during which the BTL output exceeds the positive detection threshold value or the negative detection threshold value increases, it is possible to reliably detect the offset. With this, even when an AC signal is being input, it is possible to accurately determine the offset at any time. That is, in an acoustic system, even when the audio signal is being reproduced, it is possible to detect the presence of the offset. With this, it is possible to reliably prevent the speaker to which output of the amplifier is supplied, from damage. Moreover, since the determination of the offset is performed by counting the period during which the positive and negative detection results exceed the detection threshold values and by comparing the counted values, the low pass filter or the like for obtaining the determination result is not necessary, and this improves implementation in an integrated circuit. In addition, the determination results of the positive phase and the negative phase are combined and a combined result is output from one determination output terminal, so that it is possible to prevent increase of the number of pins. That is, since no external filter or the like is required and all components can be provided in one chip IC, it is possible to provide the embodiment at a low cost. 
     Second Embodiment 
       FIG. 8  is a circuit diagram showing a second embodiment of the exemplary embodiment. In the first embodiment, detection of the DC offset is performed in a digital manner by using the counter. However, if each of the DC offsets is determined based on the detection result of the positive phase audio signal and the positive detection threshold value, and the detection result of the negative phase audio signal and the negative detection threshold value, reliable detection of the offset can be performed, and the detection of the offset may be performed in an analog manner. 
     The offset detection circuit of  FIG. 8  is different from the offset detection circuit of  FIG. 1  in that the detection unit  30  is adopted instead of the detection unit  3 . The detection unit  30  is different from the detection unit  3  of  FIG. 1  in that the low pass filter configured with a resistor R 11  and a capacitor C 11 , and the low pass filter configured with a resistor R 12  and a capacitor C 12 , comparators COM 3  and COM 4  and an OR circuit OR 4  are adopted instead of the positive phase determination unit  4 P, the negative phase determination unit  4 M, the oscillator  5 , and the OR circuit OR 3 . 
     The positive phase detection terminal DET_P is connected to the reference potential point through the resistor R 11  and the capacitor C 11 , and a connection point of the resistor R 11  and the capacitor C 11  is connected to the positive phase determination terminal DEC_P. In addition, the negative phase detection terminal DET_M is connected to the reference potential point through the resistor R 12  and the capacitor C 12 , and a connection point of the resistor R 12  and the capacitor C 12  is connected to the negative phase determination terminal DEC_M. 
     The low pass filter configured with the resistor R 11  and the capacitor C 11  integrates the detection result of the positive phase generated in the positive phase detection terminal DET_P. The integrated result is output through the positive phase determination terminal DEC_P as the determination result of the positive phase of the DC offset. In addition, the low pass filter configured with the resistor R 12  and the capacitor C 12  integrates the detection result of the negative phase generated in the negative phase detection terminal DET_M. The integrated result is output through the negative phase determination terminal DEC_M as the determination result of the negative phase of the DC offset. The comparator COM 3  generates a digital output by comparing the determination result of the positive phase with a threshold value voltage VDC, and the comparator COM 4  generates a digital output by comparing the determination result of the negative phase with the threshold value VDC. The OR circuit OR 4  performs the OR operation on the determination results of the positive phase and the negative phase from the comparators COM 3  and COM 4 , and outputs the operation result as the determination output through the determination output terminal DEC. 
     In the embodiment configured in this manner, the determination of the DC offset is performed by the low pass filter. As shown in  FIG. 5B , in the detection result of the positive phase that is output from the positive phase detection terminal DET_P and the detection result of the negative phase that is output from the negative phase detection terminal DET_M, the L level period is longer than the H level period on average if the offset is not present, and the H level is maintained most of time during an offset period. Accordingly, during the period in which the offset is not present, regardless of the level of the BTL output, all of the determination results of the positive phase and the negative phase obtained by the low pass filter by the resistor R 11  and the capacitor C 11  and the low pass filter by the resistor R 12  and the capacitor C 12  do not become values greater than a predetermined threshold value (VDC). Meanwhile, during the period in which the offset is present, one of the determination results of the positive phase and the negative phase becomes a value greater than the predetermined threshold value (VDC). 
     The OR circuit OR 4  outputs an OR operation result, as the determination output, on the determination results of the positive phase and the negative phase from the determination output terminal DEC. Accordingly, the determination output becomes an H level during only a period when the DC offset is present, and thereby it is possible to reliably detect the DC offset. 
     In the embodiment as well, it is possible to accurately perform the offset determination at any time even when the AC signal is being input. 
     Third Embodiment 
       FIG. 9  is a circuit diagram showing a third embodiment of the exemplary embodiment. In a configuration of the amplification unit  2  in the embodiment, the same description as that of  FIG. 1  is not shown. In  FIG. 9 , the same components as those in  FIG. 1  will be denoted by the same reference characters, and descriptions thereof will be omitted. 
     A period during which the BTL output exceeds the detection threshold value VRP or VRM: becomes a period shorter than the half of the BTL output cycle at any time when the offset is not generated in the BTL output. That is, in the first and second embodiments, if the DC offset is not present, since a duty ratio of the period during which the BTL output exceeds the detection threshold value becomes a duty ratio less than 50%, it is possible to prevent erroneous detection of the DC offset. 
     However, the case in which this condition is not satisfied for some reason is now considered. For example, when an amplitude of the audio signal is great, and the BTL output is clipped, or the like, it is assumed that a ratio of the period during which the BTL output exceeds the detection threshold value becomes great, and a margin of the erroneous detection becomes small. Furthermore, when amplification of the amplifiers OP 1  and OP 2  is performed non-linearly, it is assumed that the BTL output is distorted, and a duty ratio between a positive period and a negative period is relatively greatly changed. In this case, it is possible that the duty ratio of the period during which the BTL output exceeds the detection threshold value exceeds 50%, even when the DC offset is not present. 
     Then, in the embodiment, by decreasing the counted value of the up/down counter  11  corresponding to the period during which the BTL output exceeds the detection threshold value, the erroneous detection of the DC offset can be reliably prevented. 
     A detection unit  41  of  FIG. 9  adopts a positive phase determination unit  40 P to which a thinning-out unit  16 P is added and a negative phase determination unit  40 M to which a thinning-out unit  16 M (not shown) is added, instead of the positive phase determination unit  4 P and the negative phase determination unit  4 M of  FIG. 1 . Configurations of the positive phase determination unit  40 P and the negative phase determination unit  40 M are the same, and the thinning-out units  16 P and  16 M are referred to as thinning-out units  16  when the thinning-out units  16 P and  16 M are not distinguished from each other. 
     The clock CK is provided from the count control unit  15  to the thinning-out unit  16 . The thinning-out unit  16  generates a clock CCK by thinning out the clock CK and supplies the generated clock CCK to the up/down counter  11 , during a predetermined period during which the period during which the BTL output exceeds the detection threshold value, that is, the detection result of the positive phase or the negative phase becomes the H level. The up/down counter  11  increments in response to the clock CCK during the period in which the detection result of the positive phase or the negative phase becomes the H level. 
     For example, the thinning-out unit  16  may operate in a period during which the clock CK is thinned out by performing an AND operation on a reduction signal obtained by dividing (for example, dividing by 32) the clock CK and the detection result of the positive phase or the negative phase, and may generate the clock CCK by thinning out the clock CK in the thinning-out period. That is, it is possible to decrease the counted value of the up/down counter  11  by the number of the clocks CK in the thinning-out period. 
     Since the audio signal that is input and the oscillator  5  are operated asynchronously or the number of stages of the up/down counter  11  is sufficiently great, the reduction signal is randomly generated during the period in which the BTL output exceeds the detection threshold value. With this, when the DC offset is not present, although waveform distortion occurs in the BTL output or the like, reaching the overflow set value is reliably prevented by decreasing the counted value of the up/down counter  11 . 
     In a case of an ordinary audio signal, there is little possibility that the input audio signal and the clock CK are continuously synchronized with each other during the H level period in the detection result of the positive phase or the negative phase. However, when the reduction signal is continuously generated during the L level period in the detection result of the positive phase or the negative phase, the thinning-out unit  16  may generate the clock CCK from which the clock CK is thinned out, by forcibly generating a thinning-out period in a next H level period. 
     Next, an operation of the embodiment configured in this manner will be described with reference to a timing chart of  FIG. 10 . 
     The clock CK from the count control unit  15  is supplied to the thinning-out unit  16 . For example, the thinning-out unit  16  generates the reduction signal by dividing the clock CK.  FIG. 10  shows this example, and the reduction signal is generated in a predetermined cycle. 
     The thinning-out unit  16  obtains the thinning-out period indicating a period during which the clock CK is thinned out in the period during which the BTL output exceeds the detection threshold value, by using the reduction signal and the detection result of the positive phase or the negative phase. As shown in  FIG. 10 , the detection result of the positive phase becomes the H level, when the BTL output exceeds the detection threshold value VRP, and the detection result becomes the L level during other periods. The thinning-out unit  16  generates the thinning-out period by the AND operation on the detection result of the positive phase or the negative phase and the reduction signal. 
     Accordingly, the thinning-out period becomes a period shown as UT during periods T 1  to T 3  of  FIG. 10 . The clock CCK obtained by thinning out the clock CK is supplied to the up/down counter  11  during the thinning-out period. A period shown as CT in each of periods T 1  to T 3  of  FIG. 10  is a period during which the incrementing is performed, and a period shown as UT is a period during which the incrementing stops. During a period other than each of periods T 1  to T 3  of  FIG. 10 , the up/down counter  11  decrements in response to the clock CCK. 
     As shown in  FIG. 10 , when the reduction signal is generated twice in succession during the L level period in the detection result of the positive phase or the negative phase, the thinning-out unit  16  forcibly sets the thinning-out period during the next period T 3 , and supplies the clock CCK obtained by thinning out the clock CK, to the up/down counter  11 . With this, even when the reduction signal is randomly generated, it is possible to reliably prevent erroneous detection of the offset. 
     By the thinning-out period, incrementing the up/down counter  11  is reduced, and even when waveform distortion or the like occurs in the BTL output, if the offset is not present, the counted value does not reach the overflow set value. 
     In the embodiment as well, it is possible to obtain the same effect as the first embodiment. Furthermore, in the embodiment, the counted value of the up/down counter is reduced, and even when waveform distortion or the like occurs in the BTL output, if the offset is not present, it is possible to reliably prevent the erroneous detection of the offset. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the exemplary embodiments. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the exemplary embodiments. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the exemplary embodiments. 
     For example, in each of the embodiments described above, an example in which a BTL configuration amplifier is employed as an amplifier has been described, but the same can also be applied to a single-ended amplifier. In addition, the above-described embodiment includes exemplary embodiments at various stages, and various exemplary embodiments can be extracted by an appropriate combination of a plurality of disclosed constitutional requirements. For example, even if some constituent requirements are deleted from all the constituent requirements shown in the embodiment, the problem described in the section of the problem to be solved by the exemplary embodiment can be solved and the effect described in the effect of the exemplary embodiment can be obtained, and a configuration in which the constituent requirements are deleted can be extracted as an exemplary embodiment.