Abstract:
A distortion compensation method for a class-D audio amplifier has the steps of connecting a compensation circuit to the class-D audio amplifier; providing a feed-forward signal from the compensation circuit to the loop filter, wherein the feed-forward signal contains replicate high-frequency components replicated from original high-frequency components of the class-D audio amplifier; and adding the feed-forward signal to those of the class-D audio amplifier. By adding the feed-forward signal, PWM-intermodulated distortion resulted from the high-frequency components is reduced. The class-D audio amplifier still maintains loop stability without affecting its phase margin.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a distortion compensation method for a class-D audio amplifier, and more particularly to a distortion compensation method using an extra-added feed-forward signal to reduce PWM-intermodulated distortion of a class-D audio amplifier. 
     2. Description of the Prior Art 
     Audio amplifier ICs usually require high efficiency, and class-D audio amplifiers can meet this requirement. For many high-quality applications, superior audio performance is demanded, and distortions of such class-D audio amplifiers would be a serious concern. 
     With reference to  FIG. 8 , a conventional open-loop class-D amplifier for driving a load comprises a pulse-width-modulation (PWM) generator  81 , a power stage  82  and an optional LC filter  83 . The PWM generator  81  generates a PWM signal by comparing an audio signal with a triangular wave. Due to the inherent PWM property, the generated PWM signal contains the audio signal and large high-frequency components HF. The power stage  82  uses the PWM signal to drive the load. 
     With reference to  FIG. 9 , a conventional closed-loop class-D audio amplifier has elements similar to the open-loop class-D audio amplifier mentioned above and further has a loop filter  91 , wherein the power stage  93  has its output connecting to an input of the loop filter  91  to form a feedback path. The in-band harmonic distortion components at the output of the power stage  93  are fed back and processed by the loop filter  91  with high in-band gain; then, the output of the PWM generator  92  can be correspondingly adjusted to suppress the in-band harmonic distortion components, where the in-band distortion suppression capability increases with the increasing in-band loop gain. However, in addition to the in-band harmonic distortion components, the PWM high-frequency components HF at the output of the power stage  93  are also fed back into the loop. Such unwanted high-frequency components HF should be filtered by the loop filter  91  with high out-of-band attenuation. Nevertheless, if the out-of-band loop attenuation is insufficient, the residuals of high-frequency components HF will intermodulate with the triangular wave of the PWM generator  92 . Subsequently, the intermodulation products will alias back into the audio band, resulting in the PWM-intermodulated distortion at the output of the power stage  93 . To further reduce the PWM-intermodulated distortion, the out-of-band loop attenuation of the original loop gain design in  FIG. 10  can be increased by designing the loop filter to obtain a shifted loop gain design in  FIG. 10 ; however, its in-band loop gain is decreased. Hence, the THD+N is usually limited by the trade-off between high in-band loop gain and high out-of-band loop attenuation. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to provide a distortion compensation method for a class-D audio amplifier for reducing the PWM-intermodulated distortion without degrading the loop stability of the class-D audio amplifier. 
     The class-D audio amplifier has a main loop formed by a loop filter, a PWM generator, a power stage, and an optional LC filter sequentially connected, and a feedback path is established from the output of the power stage to an input of the loop filter. The distortion compensation method of the present invention has the steps of: 
     providing a feed-forward signal from a compensation circuit to the loop filter, wherein the feed-forward signal contains replicate high-frequency components replicated from original high-frequency components existing in the main loop of the class-D audio amplifier; and 
     adding the feed-forward signal to the main loop of the class-D audio amplifier. 
     The compensation circuit provides a feed-forward signal carrying replicate high-frequency components to cancel those in the class-D audio amplifier. As a result, the PWM-intermodulated distortions resulted from the original high-frequency components of the class-D audio amplifier can be reduced. Further, total harmonic distortion plus noise (THD+N) is improved. The class-D audio amplifier still maintains its loop stability without affecting its phase margin. 
     Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram of a class-D audio amplifier with a compensation circuit for implementing the compensation method of the present invention; 
         FIG. 2  shows a first embodiment of the compensation circuit in  FIG. 1  of the present invention; 
         FIG. 3  shows a second embodiment of the compensation circuit in  FIG. 1  of the present invention; 
         FIG. 4  shows transfer function blocks of two filters shown in  FIG. 3 ; 
         FIG. 5  shows transfer functions of the two filters shown in  FIG. 4 ; 
         FIG. 6  is a detailed circuit diagram of  FIG. 3 ; 
         FIG. 7  shows a third embodiment of the compensation circuit in  FIG. 1  of the present invention; 
         FIG. 8  is a circuit block diagram of a conventional open-loop class-D audio amplifier; 
         FIG. 9  is circuit block diagram of a conventional closed-loop class-D audio amplifier; and 
         FIG. 10  shows two different loop gain designs of the class-D audio amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , the present invention proposes a distortion compensation method for a class-D audio amplifier  10 , which has a main loop formed by a loop filter  11 , a pulse-width-modulation (PWM) generator  12 , a power stage  13  and an optional LC filter  14 . A feedback path is established from the output of the power stage  13  to an input of the loop filter  11 . As described above, the high-frequency components generated by the PWM generator  12 , the original high-frequency components hereinafter, will return to the input of the loop filter  11  via the feedback path and become a part of an input signal of the loop filter  11 . Such high-frequency components may cause PWM-intermodulated distortion in the class-D audio amplifier  10 . 
     To compensate or reduce the original high-frequency components existing in the class-D amplifier  10 , a compensation circuit  20  is connected to the loop filter  11  and outputs a feed-forward signal to the loop filter  11 . The feed-forward signal itself carries analogue high-frequency components similar to the original high-frequency components at around switching frequency and its harmonics. By adding the feed-forward signal to cancel the original high-frequency components from negative feedback, the PWM-intermodulated distortion of the class-D audio amplifier  10  will be effectively reduced. 
     With reference to  FIG. 2 , a first embodiment of the compensation circuit  20  is shown. The compensation circuit  20  generates an analogue signal V out2  similar to an output signal V out1  of the power stage  13  of the class-D audio amplifier  10 , and filters the analogue signal V out2  by high-pass filtering  24  to keep the high-frequency components. In more detail, the compensation circuit  20  comprises a replica unit and the high-pass filtering  24 . The replica unit has a loop filter  21 , a PWM generator  22  and a power stage  23  all respectively duplicated from the loop filter  11 , the PWM generator  12  and the power stage  13  of the class-D audio amplifier  10  and connected in the same configuration. Therefore, the analogue signal V out2  will include the analogue high-frequency components similar to the original high-frequency components existing in the class-D audio amplifier  10 . 
     With the high-pass filtering  24 , only the analogue high-frequency components from the replica unit are allowed to enter the class-D audio amplifier  10 . The feed-forward signal containing the analogue high-frequency components is added to a summing node  100  in the main loop of the class-D audio amplifier  10  to cancel the original high-frequency components of V out1  from negative feedback. In this embodiment, the feed-forward signal is added to the input of the loop filter  11 . 
     With reference to  FIG. 3 , a second embodiment of the compensation circuit  20  is shown. The compensation circuit  20  comprises a loop filter  21  and a PWM generator  22  respectively duplicated from the loop filter  11  and the PWM generator  12  of the class-D audio amplifier  10  and connected in the same configuration. Therefore, an analogue signal V out2  of the PWM generator  22  will contain the analogue high-frequency components similar to the original high-frequency components existing in the class-D audio amplifier  10 . 
     With further reference to  FIG. 4 , the two loop filters  11  and  21  can be expressed by their transfer function blocks. Each loop filter  11  and  21  comprises two stages. For the loop filter  21  of the compensation circuit  20 , the first stage and the second stage use respective transfer functions A 1  and A 2  as shown in  FIG. 5  to process the audio signal. For the loop filter  11  of the class-D audio amplifier  10 , the first stage and the second stage use respective transfer functions A 1  and A 2  as shown in  FIG. 5  to process the audio signal. However, the signal output from the first stage of the loop filter  21 , i.e. the feed-forward signal, will be further input to the second stage of the loop filter  11  and processed by a new transfer function Af 2  instead of the original transfer function A 2 , where A 2  is the transfer function from the original signal of the main loop to the summing node of the main loop, and Af 2  means the transfer function from the analogue signal of the compensation circuit  20  to the summing node of the main loop. Consequently, the loop filter  11  of the class-D audio amplifier  10  provides a transfer function Af 2 /A 2  as shown in  FIG. 5  to realize high-pass filtering. The transfer function Af 2 /A 2  is an equivalent high-pass filtering without additional operational amplifiers OPs to save chip area and power consumption. 
     With reference to  FIG. 6 , a detailed circuit diagram of the compensation circuit  20  of  FIG. 3  is shown. In the loop filter  11  of the class-D audio amplifier  10 , the signal output from the first stage is input to an operational amplifier OP of the second stage through resistors R 1 , R 1 ′. The two resistors R 1 , R 1 ′, and the amplifier OP accompanied by other elements collectively determine the transfer function A 2 . In another aspect, the feed-forward signal from the first stage of the loop filter  21  of the compensation circuit  20  is input to the same operational amplifier OP through resistors R 2 , R 2 ′ and capacitors C, C′. The resistors R 2 , R 2 ′, the capacitors C, C′ and the amplifier OP accompanied by other elements collectively determine the transfer function Af 2 . In the embodiment of  FIG. 6 , the high-pass filtering is implemented with a part of circuit, i.e. the elements within the block representing the transfer function A 2 , sharing with the main loop. 
     With reference to  FIG. 7 , a detailed circuit diagram of a third embodiment of the compensation circuit  20  is shown. This compensation circuit  20  comprises a PWM generator  22  duplicated from the PWM generator  12 . The PWM generator  22  generates the feed-forward signal and then input to the input of the first operational amplifier OP in loop filter  11  via two resistors R 3 , R 3 ′ and two capacitors C 2 , C 2 ′. In this embodiment, the summing node  100  is the input of the first operational amplifier OP in loop filter  11 . In the embodiment of  FIG. 7 , the high-pass filtering is implemented with a part of circuit, i.e. the elements within the block representing the transfer function A 1 , sharing with the main loop. 
     In the present invention, the compensation circuit  20  provides a feed-forward signal carrying high-frequency components to cancel those in the class-D audio amplifier. As a result, the invention significantly improves the total harmonic distortion plus noise (THD+N) and maintains the stability of the class-D audio amplifier since the PWM-intermodulated distortions are suppressed by the feed-forward signal. 
     Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.