Abstract:
A BTL power amplifier circuit with substantially zero distortion is disclosed. A distortion detecting circuit is provided which employs a feed forward technique for sensing the distortion generated in one of two amplifiers constituting the BTL power amplifier circuit and then applying to the other of the two amplifiers so as to output the distortion having the same amplitude and polarity as the sensed distortion. The distortions derived from the two amplifiers are fully cancelled out across a load to each other.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to amplifier circuits with low distortion, and more specifically BTL (Balanced Transformerless) amplifier circuits with low distortion in which the principle of a feed forward technique is incorporated. 
     Among those known amplifier circuits, BTL amplifier circuits are extensively used particularly in the field of such as audio equipments due to certain inherent advantages over other conventional amplifier circuits such as SEPP (Single Ended Push-pull) amplifier circuits. A BTL amplifier circuit, when compared with a SEPP amplifier circuit, has an advantage that the former can obtain, with the same power source voltage as applied to the latter, about two times more load current and four times more output power than those of the SEPP amplifier circuit. 
     The principle of BTL amplifier circuits can be better understood by referring to FIG. 1. FIG. 1 shows a block diagram of one example of a prior BTL amplifier circuit comprising two amplifiers, designated by reference numerals 1 and 2 respectively and having gains which are equal in absolute value and opposite in phase to each offer. An input signal is applied via an input terminal T 1  to the amplifiers 1 and 2. The amplifier 1 effects amplification of the input signal and develops at an output terminal T 2  an output signal same in phase as the input signal, while the amplifier 2 amplifiers the input signal and develops at an output terminal T 3  an output signal opposite in phase to the input signal. A load, e.g. a loudspeaker L, connected between the terminals T 2  and T 3  is driven by the two output signals each having the same amplitude and opposite polarity to each other. 
     It should be noted here that distortions generated in the respective amplifiers 1 and 2 are mostly opposite in phase to each other as is similar in the case of the output signals appearing at the output terminals T 2  and T 3 . The distortions are accordingly superposed additively at the load. This is particularly true when the amplifiers 1 and 2 have substantially the same characteristics and hence the generated distortions within the amplifiers 1 and 2 have substantially the same amplitude and are opposite in phase to each other. In such a case the addition of the distortions across the load is enhanced to some degree. 
     In order to decrease the distortions of the prior BTL power amplifier circuit, therefore, it has been requisite to use the amplifiers 1 and 2 both with a low distortion factor, for example, class-A amplifiers. In result, this had led to a high cost of the circuit arrangement and moreover such disadvantage has been brought that a decrease of the power efficiency occurs in the amplifier circuits. 
     SUMMARY OF THE INVENTION 
     Accordingly, and object of the invention is to provide an amplifier circuit with low distortion, particularly applicable to a BTL power amplifier circuit. 
     It is another object of the invention to provide an amplifier circuit as described above, in which a high power efficiency is obtained with a low cost. 
     Briefly, in an amplifier circuit according to the invention, a feed forward technique is incorporated in order to cancel out the distortions generated within one of the amplifiers constituting a BTL amplifier circuit. The distortions developed at one of the two amplifiers are detected by a distortion detecting circuit, and then are supplied to the other of the two amplifiers so as to obtain at the output of the other amplifier the distortion signal having the same amplitude and polarity as the distortions derived at the one amplifier. Then, the distortions derived at the outputs of the two amplifiers is mutually cancelled out across a load. 
     This invention is applicable to an amplifier circuit of a type which comprises a first amplifier for amplifying an input signal to develop at the output thereof an output signal same in phase as the input signal, a second amplifier for amplifying the input signal to develop at the output thereof an output signal opposite in phase to the input signal, and a load connected between the outputs of the first and second amplifiers. An amplifier circuit according to the invention comprises a distortion detecting circuit for detecting the distortion generated in one of the first and second amplifiers and supplying the detected distortion into the other of the first and second amplifiers so as to develop at the output the distortion having the same amplitude and polarity as the distortion derived at the output of the one of the amplifiers. 
     Other objects and features of the invention will be apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a prior BTL amplifier circuit; 
     FIG. 2 is a schematic diagram showing a first embodiment of an amplifier circuit according to the invention; 
     FIG. 3 is a schematic diagram showing a second embodiment of an amplifier circuit according to the invention; and 
     FIG. 4 is a more detailed schematic diagram showing the second embodiment in which power amplifying stages of two amplifiers constituting a BTL amplifier circuit and a power supply system are illustrated in more detail. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a BTL power amplifier circuit according to the invention is illustrated in FIG. 2 wherein corresponding parts to those in FIG. 1 are designated by identical reference characters. In the embodiment shown in FIG. 2, the signals of input signal sources 4 a  and 4 b  have equal amplitudes and opposite polarities to each other, the reference terminals of the both input signal sources 4 a  and 4 b  being grounded. A voltage signal V i  of the input signal source 4 a  is applied to the non-inverting input terminal of an amplifier 1 (which may be an power amplifier allowed to develop distortions to some extent), while a voltage signal -V i  of the input signal source 4 b  is applied to the non-inverting input terminal of an amplifier 2 (which is in this embodiment an power amplifier with low distortion characteristics such as a class-A power amplifier). The voltage signal V i  of the input signal source 4 a  is also applied to the non-inverting input terminal of an amplifier 3 (which is a voltage amplifier with low distortion characteristics including an operational amplifier, and functions as a distortion detecting circuit in this embodiment). A resistor 7 (resistance value R 2 ) is coupled between the output terminal and inverting input terminal of the amplifier 1, the inverting input terminal being grounded via a resistor 6 (resistance value R 1 ). The output of the amplifier 1 is fed to the inverting input terminal of the amplifier 3 via a resistor 8 (resistor value R 3 ). A resistor 9 (resistance value R 4 ) is coupled between the inverting input terminal and output terminal of the amplifier 3, the output of the amplifier 3 being fed to the inverting input terminal of the amplifier 2 via a resistor 10 (resistance value R 5 ), and a resistor 11 (resistance value R 6 ) being coupled between the inverting input and output terminal of the amplifier 2. The outputs of both the amplifiers 2 and 3 are respectively developed across output terminals T 2  and T 3  between which a load L such as a loudspeaker is connected. 
     In the BTL power amplifier circuit thus constructed, the following relation is obtained: ##EQU1## wherein the output voltage of the amplifier 1 appearing at the output terminal T 2  is represented as V 1  and V d  represents a voltage of distortion derived at the output of the amplifier 1. Representing the output voltage of the amplifier 3 as V 2  and utilizing the equation (1) above, the voltage V 2  is expressed as follows: ##EQU2## 
     Further assigning V 3  as to the output voltage of the amplifier 2 appearing at the output terminal T 3 , and utilizing the equation (2) above, and with the distortion developed in the amplifier 2 being neglected, then the following equation holds: ##EQU3## If the values of resistors 6 to 11 are set so as to meet the condition of R 2  /R 1  =R 3  /R 4  =R 6  /R 5 , 
     the above equation (3) is modified to indicate: ##EQU4## From the equations (1) and (4), a voltage V 4  developed across the load L, which is connected between the output terminals T 2  and T 3 , is given by: ##EQU5## As is readily understood from the equation (5), the voltage V d  of the distortion generated in the amplifier 1 is fully cancelled out across the load L. 
     Referring now to FIG. 3, there is shown a schematic diagram of a BTL power amplifier circuit of a second embodiment in accordance with the invention. The voltage signal of an input signal source 4 has a voltage value of V i . The voltage signal V i  is applied through a resistor 5 (resistance value R 1 ) to the inverting input terminal of an amplifier 1 (which may be a power amplifier allowed to develop distortions to some degree), and through a resistor 9 (resistance value R 5 ) to the inverting input terminal of an amplifier 3 (which is a voltage amplifier with low distortion characteristics including an operational amplifier), and further directly to the non-inverting input terminal of an amplifier 2 (which is an power amplifier with low distortion characteristics such as a class-A power amplifier). The non-inverting input terminal of the amplifier 1 is grounded and between the inverting input terminal and output terminal of the amplifier 1 a resistor 6 (resistance value R 2 ) is connected. The output of the amplifier 1 is fed through a resistor 7 (resistance value R 3 ) to the inverting input terminal of the amplifier 3, the non-inverting input terminal thereof being grounded and a resistor 8 (resistance value R 4 ) being connected between the inverting input terminal and output terminal of the amplifier 3. The output of the amplifier 3 is fed through a resistor 10 (resistance value R 6 ) to the inverting input terminal of the amplifier 2, a resistor 11 (resistance value R 7 ) being connected between the inverting input terminal and output terminal of the amplifier 2. The outputs of the both amplifiers 1 and 2 are respectively developed across output terminals T 2  and T 3  between which a load L such as a loudspeaker is connected. 
     In the BTL power amplifier circuit thus constructed, an output voltage V 1  of the amplifier 1 appearing at the output terminal T 2  may be expressed in the following: ##EQU6## wherein V d  represents a voltage of distortion generated in the amplifier 1. Representing the output voltage of the amplifier 3 (distortion detecting circuit) as V 2  and utilizing the equation (6) above, the voltage V 2  is expressed as follows: ##EQU7## Further assigning V 3  as to the output voltage of the amplifier 2 appearing at the output terminal T 3 , and utilizing the equation (7) above, and with the distortion in the amplifier 2 being neglected, then the following equation holds: ##EQU8## If the values of resistors 5 to 11 are set so as to meet the condition of both ##EQU9## the equation (8) is expressed as: ##EQU10## From the equations (6) and (9), the voltage V 4  appearing across the load L connected between the output terminals T 2  and T 3  is given by: ##EQU11## As evident from the equation (10) and similar to the first embodiment, the voltage V d  of the distortion generated in the amplifier 1 disappears completely across the load L. 
     As discussed in the above first and second embodiments, it is to be noted that since the distortions generated in the amplifiers 1 and 1 are detected by the amplifiers 3 and 3, i.e., distortion detecting circuits, and then applied to the amplifiers 2 and 2, so as to produce at the respective output terminals thereof the distortion having the same amplitude and opposite polarity as those of the respective amplifiers 1 and 1, the distortions from the amplifiers 1 and 1 are completely cancelled out at the load L. Therefore, even if the power amplifiers 1 and 1 are used having relatively high distortions, such as for example class-B, class-D or class-E amplifier, the BTL power amplifier circuit as a whole affords a relatively low distortion so long as the amplifiers 2 and 2 are used having relatively low distortions, such as class-A amplifier. It is also appreciated that since the amplifiers 3 and 3 operate only to provide voltage amplification in the BTL power amplifier circuit the amplifiers 3 and 3 may be implemented easily and with low cost and low distortion by using such as a known operational amplifier. 
     Referring to FIG. 4, there is shown a more concrete and detailed schematic representation of the BTL power amplifier circuit particularly adopting the principle of the second embodiment of the invention, in which corresponding elements to those in FIG. 3 are designated by the identical reference characters and the description thereof is omitted for brevity. In FIG. 4, an input signal to be amplified is applied to an input terminal T 1 . In this embodiment, an amplifier 1 is comprised of an operational amplifier 1 a  and a push-pull power amplification circuit (operating in a class-B mode) made of a combination of an NPN transistor 1 b  and a PNP transistor 1 c , and an amplifier 2 is comprised of an operational amplifier 2 a  and a push-pull power amplification circuit (operating in a class-A mode) made of a combination of an NPN transistor 2 d  and a PNP transistor 2 e  and vias voltage sources 2 b  and 2 c . The amplifiers 1 to 3 are supplied with a positive and negative power voltages +B and -B, respectively from power sources 20 a  and 20 b . The other circuit configuration and its operation is identical with those of the BTL power amplifier circuit shown in FIG. 3, and the values of resistors 5 to 11 is set so as to meet the condition of both ##EQU12## In this embodiment, the amplifier 1 which operates in a class-B mode may generate distortion at an output terminal T 2 . However, since the distortion voltage having the same amplitude and opposite polarity is also delivered at an output terminal T 3 , the distortions appearing at the output terminals T 2  and T 3  will be cancelled out across an load L to each other. The amplifier 2 operates in a class-A mode so that it inherently generates little distortion, and thus the BTL power amplifier circuit as a whole has substantially no distortion at the load L. 
     While there have been shown and described some preferred embodiments of the present invention, it is to be understood that the present invention is not limited thereto but may be variously modified and practiced within the scope set forth in the attached claims.