Abstract:
A beat frequency cancellation circuit, for an amplifier, includes a coupling device connected between two signal processing paths of the amplifier for compensating for beat frequency effects of output signals between the signal processing paths.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 100204056, filed on Mar. 8, 2011, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a multi-channel amplifier circuit, and more particularly to a multi-channel amplifier circuit with a beat frequency cancellation circuit capable of cancelling beat frequency effects between different channels. 
     2. Description of the Related Art 
     As technologies advance, the variety of functions provided by portable electronic devices (such as mobile phones, personal digital assistant, tablet computer, or others) has been greatly increased. Among them, recently, multimedia services have become one of the fundamental features that are provided with portable electronic devices. In order to generate high quality audio signals, a power-saving and high efficiency power amplifier is required. A class D amplifier is a preferable choice for small-sized electronic devices since the circuit area of the class D amplifier is small, and amplifying efficiency of the class D amplifier can be over 90%. Therefore, the class D amplifier has replaced the class AB amplifier and has become main stream when applying audio power amplifiers. 
     The class D amplifier is also called a digital power amplifier, which modulates and amplifies input analog signals and generates digital amplified output signals.  FIG. 1  shows a circuit diagram of a basic class D power amplifier, in which a pulse width modulation (PWM) modulator  101  modulates an input signal Vin to generate a digital signal. The digital signal is transmitted to the driver circuits  102  and  103 , and amplified and output by the power transistors Q 1  and Q 2 . Finally, a low pass filter  104  extracts the original audio signal at the input terminals and transmits the extracted signal to the loud speaker  105 . 
     Because the inputs of the power transistors are digital signals, the power transistors Q 1  and Q 2  work in the saturation and cut-off states. For this reason, the power consumed by the power transistors Q 1  and Q 2  is very small, which not only increases overall efficiency of the power amplifier, but also allows the size of a heat dissipation device to be small. Hence, the size of the electronic components of a class D amplifier can be small. Moreover, while, in general, the amplifying efficiency of a class AB amplifier is only about 50%, the amplifying efficiency of the class D amplifier can be over 90%, and even approach 100%. Therefore, the class D amplifier is greatly used in the audio power amplifying fields. 
     However, for multi-channel audio signal amplifying applications, the different channel digital signals may have different oscillation frequencies after being modulated. The frequency differences between the audio signals in different channels cause a beat frequency effect. When the frequency difference falls to a frequency band perceptible by human ears, a beat tone may become noise heard by a user when there is no audio signal being input. 
     Therefore, the noise caused by the beat frequency in multi-channel audio amplifier is a problem desired to be solved. 
     BRIEF SUMMARY OF THE INVENTION 
     Beat frequency cancellation circuits, pulse width modulation (PWM) signal generating circuits and amplifier circuits are provided. According to an embodiment of the invention, a beat frequency cancellation circuit for an amplifier comprises a coupling device coupled between two signal processing paths of the amplifier to compensate for beat frequency effects of output signals between the signal processing paths. 
     According to another embodiment of the invention, a PWM signal generating circuit comprises at least two PWM modulators and at least one coupling device. Each PWM modulator receives an input signal through an input terminal and generates a PWM signal according to the input signal. The coupling device is coupled between the input terminals for synchronizing oscillating frequencies of the input signals at the input terminals. 
     According to yet another embodiment of the invention, an amplifier circuit comprises at least two signal processing paths and a coupling device. Each signal processing path comprises a pulse width modulation (PWM) modulator, a driving circuit and an output circuit. The PWM modulator generates a PWM signal according to an input signal from an input terminal of the signal processing path. The driving circuit is coupled to the PWM modulator for generating a driving signal according to the PWM signal. The output circuit is coupled to the driving circuit for generating an amplified output signal according to the driving signal. The coupling device is coupled between the at least two signal processing paths for synchronizing oscillating frequencies of the input signals. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a block diagram of a basic class D power amplifier; 
         FIG. 2  shows an amplifier circuit according to an embodiment of the invention; 
         FIG. 3  shows an amplifier circuit according to another embodiment of the invention; 
         FIG. 4  shows an amplifier circuit according to yet another embodiment of the invention; 
         FIG. 5  shows an amplifier circuit according to yet another embodiment of the invention; and 
         FIG. 6  shows an amplifier circuit according to yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 2  shows an amplifier circuit according to an embodiment of the invention. In the embodiments of the invention, the amplifier circuit  200  may be applied in the dual-channel or multi-channel audio signal amplifying fields. Therefore, the amplifier circuit  200  may comprise at least two signal processing paths, wherein each signal processing path is arranged to output an amplified output signal. As shown in the dual-channel embodiment in  FIG. 2 , the amplifier circuit  200  may generate two amplified output signals Vout 1  and Vout 2  respectively on the two signal processing paths according to the differential input signals Vin+ and Vin−. The amplified output signals Vout 1  and Vout 2  may further be transmitted to the loud speaker (not shown in  FIG. 2 ) so as to generate a stereo sound effect. 
     As shown in  FIG. 2 , the amplifier circuit  200  may comprise a coupling device  202 , a pulse width modulation (PWM) signal generating circuit  204 , a driving stage circuit  206  and an output stage circuit  208 . The PWM signal generating circuit  204  may comprise PWM modulators  242  and  244 , where each PWM modulator may comprise an integrator  246  or  248  and a comparator  250  or  252 . The integrators  246  and  248  may receive the differential input signals Vin+ and Vin− at the input terminals, a reference voltage level ref and a feedback of the amplified output signals Vout 1  and Vout 2 . The integrators  246  and  248  may further integrate the received signals and generate the integration signals S Int1  and S Int2 , respectively. The comparators  250  and  252  may receive the integration signals S Int1  and S In2  and reference signals S Tri1  and S Tri2 , and compare signal levels of the integration signals S Int1  and S Int2  and the reference signals S Tri1  and S Tri2  to generate the PWM signals S PWM1  and S PWM2 , respectively. 
     The driving stage circuit  206  may comprise driving circuits  262  and  264 , for generating the driving signals S Dri1  and S Dri2  according to the PWM signals S PWM1  and S PWM2 , respectively. The output stage circuit  208  may comprise the output circuits  282  and  284 , which are driven by the driving signals S Dri1  and S Dri2  for generating the amplified output signals Vout 1  and Vout 2  to drive the load of the loud speaker (not shown). 
     Note that, conventionally, dual-channel or multi-channel amplifier circuits are carried out by coupling two or more single channel amplifier chips to generate stereo sound effect, wherein each single channel amplifier chip is arranged to generate one amplified output signal. In other words, for example, in the conventional design, a dual channel amplifier circuit is implemented by coupling two single channel amplifier chips together, wherein each amplifier chip is arranged to process a signal in one signal processing path so as to generate an amplified output signal. However, according to an embodiment of the invention, the dual-channel amplifier circuit  200  as shown in  FIG. 2  may be integrated in one single chip. In the conventional design, when using different amplifier chips to process signals on different signal processing paths, the reference signals of different amplifier chips are independent from each other. Therefore, there is no interference generated between different channels. However, when integrating two or more signal processing paths in a single chip as shown in  FIG. 2 , the oscillating frequencies of the reference signals S Tri1  and S Tri2  may be different from each other due to inter-chip interference, causing the beat frequency effect in the output audio signals. When the frequency difference falls to a frequency band perceptible to human ears, a noisy beat tone may be heard by a user when there is no audio signal being input. 
     Therefore, according to an embodiment of the invention, in order to cancel the frequency difference between the amplified output signals, the amplifier circuit  200  may further comprise a coupling device  202  coupled between the input terminals to compensate for the beat frequency effect between the amplified output signals on different signal processing paths, by synchronizing oscillating frequencies of the input signals at the input terminals. Thus, after synchronization, the oscillating frequencies of the input signals can be substantially the same. 
     According to an embodiment of the invention, the coupling device  202  is utilized for alternating current (AC) coupling the input signals at the input terminals at high frequencies, so as to compensate for the above-mentioned frequency differences between input signals. Therefore, the oscillating frequencies of the input signals can be synchronized and the beat frequency effect can be cancelled. For example, according to one embodiment of the invention, before synchronization, the oscillating frequencies of the amplified output signals Vout 1  and Vout 2  are 574.2 kH and 564.7 kHz, respectively. The frequency difference is 9.5 Hz. After synchronization by the coupling device, both of the oscillating frequencies of the amplified output signals can be adjusted to 573.7 kHz. 
     According to an embodiment of the invention, the coupling device  202  may comprise capacitors C 1  and C 2  coupled between the input terminals of the amplifier circuit  200  for AC coupling the signals at the input terminals. In this manner, even if the oscillating frequencies of the reference signals S Tri1  and S Tri2  of the amplifier circuit  200  are different, the frequency difference between the PWM signals S PWM1  and S PWM2  and the amplified output signals Vout 1  and Vout 2  may be reduced, or cancelled. 
       FIG. 3  shows an amplifier circuit according to another embodiment of the invention. As shown in  FIG. 3 , most of the elements of the amplifier circuit  300  are the same as the elements of the amplifier circuit  200  as shown in  FIG. 2 . Therefore, introductions of the same elements are omitted here for brevity. Reference may be made to the corresponding descriptions of  FIG. 2 . In the embodiment, the coupling device  302  may comprise serially coupled capacitors and resistors, such as the capacitor C 3  and the resistor R 1  and the capacitor C 4  and the resistor R 2 , respectively coupled between the input terminals of the amplifier circuit  300 . The capacitors are utilized for AC coupling the signals at the input terminals, and the resistors are utilized to hinder the signals from leaking from one signal processing path to another via the capacitors C 3  and C 4 . 
       FIG. 4  shows an amplifier circuit according to yet another embodiment of the invention. As shown in  FIG. 4 , most of the elements of the amplifier circuit  400  are the same as the elements of the amplifier circuit  200  as shown in  FIG. 2 . Therefore, introductions of the same elements are omitted here for brevity. Reference may be made to the corresponding descriptions of  FIG. 2 . In the embodiment, the coupling device  402  may comprise serially coupled capacitors and resistors, such as the capacitor C 5  and the resistor R 3  and the capacitor C 6  and the resistor R 4 , respectively coupled between the input terminals of the amplifier circuit  400 . The capacitors are utilized for AC coupling the signals at the input terminals, and the resistors are utilized to hinder the signals from leaking from one signal processing path to another via the capacitors C 5  and C 6 . 
       FIG. 5  shows an amplifier circuit according to yet another embodiment of the invention. As shown in  FIG. 5  most of the elements of the amplifier circuit  500  are the same as the elements of the amplifier circuit  200  as shown in  FIG. 2 . Therefore, introductions of the same elements are omitted here for brevity. Reference may be made to the corresponding descriptions of  FIG. 2 . In the embodiment, the coupling device  502  may comprise serially coupled capacitors and resistors, such as the resistor R 5 , the capacitor C 7  and the resistor R 7 , and the resistor R 6 , the capacitor C 8  and the resistor R 8 , respectively coupled between the input terminals of the amplifier circuit  500 . The capacitors are utilized for AC coupling the signals at the input terminals, and the resistors are utilized to hinder the signals from leaking from one signal processing path to another via the capacitors C 7  and C 8 . 
       FIG. 6  shows an amplifier circuit according to yet another embodiment of the invention. As shown in  FIG. 6 , most of the elements of the amplifier circuit  600  are the same as the elements of the amplifier circuit  200  as shown in  FIG. 2 . Therefore, introductions of the same elements are omitted here for brevity. Reference may be made to the corresponding descriptions of  FIG. 2 . In the embodiment, the amplifier circuit  600  comprises a half-bridge coupled class D amplifier on each signal processing path. The coupling device  602  may comprise a capacitor C 9  coupled between two input terminals for AC coupling the input signals Vin received at the input terminals. Note that the coupling device  602  may further comprise the current limiting resistors as shown in  FIGS. 4-6 , to hinder the signals from leaking from one signal processing path to another via the capacitor. In addition, in order to clarify the concept of the invention, dual-channel amplifier circuit embodiments were illustrated above. However, the invention is not limited to dual-channel amplifier circuits, and the concept may also be applied to multi-channel audio signal amplifying fields. Similarly, for the multi-channel audio signal amplifier circuit, by adding the proposed coupling device for synchronization of the oscillating frequencies, the oscillating frequencies of the signals at different channels may be synchronized to be substantially the same. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.