Patent Publication Number: US-2010124334-A1

Title: Amplification circuit and audio output equipment

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-292029, filed on Nov. 14, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates to an amplification circuit to amplify an audio signal and to an audio output equipment provided with an amplification circuit. 
     DESCRIPTION OF THE BACKGROUND 
     Many of car audio systems employ an audio output equipment of a BTL (Bridged Tied Load) connection system. The audio output equipment drives one speaker with two amplifiers. The two amplifiers are first and second amplifiers composing an amplification circuit. The first amplifier amplifies a non-inverted signal of an audio signal. The second amplifier amplifies an inverted signal of the audio signal. The first amplifier is connected to a terminal of the speaker, while the second amplifier is connected to another terminal of the speaker. 
     The potential difference is 2Vo between the terminals of the speaker, when the output voltage of the first amplifier is +Vo, and the output voltage of the second amplifier is −Vo. An output power Pout, which is generated between both ends of a load resistance of the speaker, is expressed by the following formula where the load resistance is RL. 
         P out=(2 Vo ) 2   /RL= 4 Vo   2   /RL    
     The output of the BTL connection system is four times the output of an audio output equipment which drives one speaker with one amplifier. 
     When two speakers are used to receive stereo audio signals for two right and left channels One amplification circuit of such a BTL connection system needs four amplifiers totally, in order to connect two amplifiers to each of two speakers to output stereo audio signals for two right and left channels. The car audio systems are required to be installed in a limited space. Therefore, the size of a semiconductor chip of the amplification circuit becomes large since the semiconductor chip contains the four amplifiers. 
     On the other hand, an audio output equipment of a SEPP (Single Ended Push Pull) connection system is also known. The audio output equipment drives one speaker with one amplifier. In the SEPP connection system, an amplifier is connected to a terminal of a speaker, and another terminal of the speaker is connected to a ground terminal. 
     Though the output power of the SEPP connection system is inferior to that of the BTL connection system, the former system performs less power consumption because the former system contains a smaller number of amplifiers. 
     Japanese Patent Application Publication No. 2001-168658 (PP. 3-4,  FIG. 1 ) discloses an audio output equipment which is provided with a BTL connection system and a SEPP connection system together. In the audio output equipment, the BTL connection system and the SEPP connection system are switched according to the levels of an input audio signal. During a low output mode of the audio output equipment, the SEPP connection system is used to drive a speaker in order to reduce power consumption of the audio output equipment. 
     However, the audio output equipment disclosed in the patent publication needs to have four amplifiers for two right and left channels to be used for the BTL connection system. The size of a semiconductor chip of the audio output equipment is not reduced, because the four amplifiers are formed on the semiconductor chip as in the audio output equipment which is provided with only a BTL connection system. 
     In order to make a chip size of an amplification circuit smaller, an audio output equipment containing amplification circuit may be composed of a SEPP connection system. When the amplification circuit is operated by a single power supply, an audio signal of the audio output equipment varies up and down with an intermediate voltage of the power supply centered. 
     A capacitor is needed to be inserted between an output terminal of the amplification circuit and a terminal of a speaker to remove direct current ingredient, for the purpose of performing an alternating current drive of the speaker. As a result, the audio output equipment becomes complicated. The cost of the audio output equipment rises as a whole. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides an amplification circuit including a first amplifier to amplify a first audio signal and to output a first output signal, an inverter to invert a second audio signal and to output an inverted signal, a second amplifier to amplify the inverted signal and to output a second output signal, and a third amplifier to output a reference voltage corresponding to a central voltage of the amplitude of the first and the second output signals. 
     Another aspect of the present invention provides an audio output equipment including a first amplifier to amplify a first audio signal and to output a first output signal, an inverter to invert a second audio signal and to output an inverted signal, a second amplifier to amplify the inverted signal and to output a second output signal, a third amplifier to output a reference voltage corresponding to a central voltage of the amplitude of the first and the second output signals, and first and second speakers, wherein an output terminal of the first amplifier is connected to a terminal of a first polarity provided at the first speaker, an output terminal of the second amplifier is connected to a terminal of a second polarity provided at the second speaker, and an output terminal of the third amplifier is connected to another terminal of the second polarity provided at the first speaker and to another terminal of the first polarity provided at the second speaker. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an amplification circuit and an audio output equipment according to an embodiment of the invention. 
         FIG. 2  is a view to explain an operation to output stereo signals in the audio output equipment according to the embodiment. 
         FIG. 3  is an output waveform of the stereo signals which is produced in the amplification circuit according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereafter, an embodiment of the invention will be described with reference to the drawings. In the drawings, the same numerals designate the same portions, respectively. 
       FIG. 1  is a block diagram showing an amplification circuit and an audio output equipment according to the embodiment. 
     As shown in  FIG. 1 , an amplification circuit  1  of the embodiment is provided with an amplifier  11 . The amplifier  11  amplifies an inputted audio signal IN 1  and outputs an amplified output signal OUT 1  to an output terminal OT 1 . The amplification circuit  1  is provided with an inverter  14 . The inverter  14  inverts an inputted audio signal IN 2  and outputs an inverted audio signal. 
     The amplification circuit  1  is provided with amplifier  12  which amplifies the output signal of the inverter  14  and transmits an amplified output signal OUT 2  to an output terminal OT 2 . The Amplification circuit  1  is further provided with an amplifier  13  which receives a predetermined voltage Vin and outputs a reference voltage, which corresponds to a central voltage of the amplitude of the output signal OUT 1  and the output signal OUT 2 , to an output terminal OT 3 . The amplification circuit  1  is formed in a semiconductor chip. 
     An audio output equipment  100  is provided with the above amplification circuit  1  and speakers  101 ,  102  for two right and left channels respectively. 
     The output terminal OT 1  of the amplification circuit  1  is connected to a terminal T 11  of the speaker  101  which has a plus (+) polarity. The output terminal OT 2  of the amplification circuit  1  is connected to a terminal T 22  of the speaker  102  which has a minus (−) polarity. The output terminal OT 3  of the amplification circuit  1  is connected to a terminal T 12  of the speaker  101  which has the minus (−) polarity and to a terminal T 21  of the speaker  102  which has the plus (+) polarity. 
     The amplifier  13  is connected to the minus (−) polarity terminal T 12  of the speaker  101  and to the plus (+) polarity terminal T 21  of the speaker  102 . The amplifier  13  has a function to continue keeping a reference voltage Vref at a constant level by performing push-pull operation. The amplifier  13  absorbs current when the voltages of the terminal T 11 , T 22  of the speaker  101 ,  102  increase. The amplifier  13  releases current when the voltages of the terminal T 11 , T 22  of the speaker  101 ,  102  decrease. 
     Operation of audio output equipment  100  according to the embodiment will be explained with reference to  FIGS. 2 and 3 .  FIGS. 2 and 3  show operation when stereo signals are reproduced by the audio output equipment  100 . 
     When stereo signals are reproduced, as shown in  FIG. 2 , an audio signal Lch of an L channel is inputted into the amplifier  11 . An audio signal Rch of an R channel is inverted by the inverter  14 . The inverted signal is inputted into the amplifier  12 . 
     The reference voltage Vref, which is outputted from the amplifier  13 , is expressed by the following formula, assuming that the amplitudes of an output signal Lout of the amplifier  11  and an output signal Rout of the amplifier  12  are equal to a power supply voltage VCC of the amplification circuit  1  or of the amplifiers  11  and  12 . 
       Vref=½ VCC 
     A current I 1 , which flows in the speaker  101 , is expressed by the following formula, when both of the load resistances of the speakers  101  and  102  are RL. 
         I 1=( L out−½ VCC)/ RL    
     Further, a current I 2  which flows in the speaker  102  is expressed by the following formula. 
         I 2=(½ Vcc− R out)/ RL    
     Generally, a remarkable correlativity exists between the audio signal Lch of the L channel and the audio signal Rch of the R channel which constitute stereo signals respectively. The audio signal Lch contains first signal components, while the audio signal Rch contains second signal components. Lots of the first signal components are in-phase with Lots of the second signal components. The sound volumes based on the audio signals Lch, Rch need to be same substantially in order to have an acoustic image positioned at the center between the speakers  101 ,  102 . 
     As the correlation exists, the inverted signal of the audio signal Rch may be deemed to be inputted to the amplifier  12  with the phase reversed to the audio signal Lch to be inputted to the amplifier  11 . 
     Thus, as shown in  FIG. 3 , the output signal Lout and the output signal Rout are outputted from the amplifiers  11 ,  12  with th phases reversed to each other substantially. 
     The output signal Lout and the output signal Rout vary up and down and symmetrically with the reference voltage ½ VCC centered. 
     The level of the output signal Lout produced at a certain time is expressed by the following formula. V0 indicates a voltage level deviated from the reference voltage ½ VCC. 
         L out=½ VCC+ V 0 
     The level of the output signal Rout produced at the same time is expressed by the following formula. 
         R out=½ VCC− V 0 
     A current I 1 , which flows in the speaker  101  at the time, is expressed by the following formula. 
         I 1=(½ VCC+ V 0−½ VCC)/ RL=V 0/ RL    
     A current I 2 , which flows in the speaker  102  at the time, is expressed by the following formula. 
         I 2={½ VCC−(½ VCC− V 0)}/ RL=V 0/ RL    
     As a result, as shown below, the currents I 1  and I 2 , which flow in the speakers  101 ,  102 , are equal. 
       I1=I2 
     As mentioned above, currents of substantially the same value flow in the speakers  101 ,  102 , when the audio signal Lch, Rch of the L and R channels, which constitute the stereo signals respectively, are in-phase substantially. A current value same as that being provided to the speaker  101  is provided to the speaker  102 . 
     According to the embodiment, the amplification circuit  1  is composed of the three amplifiers  11  to  13  so that the power consumption in the amplification circuit  1  can be reduced by half, compared with a conventional audio output equipment which has a BTL connection system provided with four amplifiers. 
     Further, according to the embodiment, since the number of amplifiers decreases, the circuit scale of the amplification circuit  1  may be reduced so that the chip size of the amplification circuit can be smaller. 
     The audio output equipment of the embodiment does not need a capacitor which is required in a SEPP system. Thus, the structure of the audio output equipment is simplified. 
     A conventional audio output equipment of a BTL connection system may be exchanged for the audio output equipment of the embodiment easily, because the latter is compatible with the BTL connection system. 
     Other embodiments or modifications of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and example embodiments be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following.