Abstract:
A power amplifier with noise shaping function is provided. The power amplifier includes a differential mode integrator, an integration and adjustment unit and a switch unit. The differential mode integrator is used for receiving a differential mode input signal and a differential mode output signal, and outputting a differential mode first signal. The integration and adjustment unit is coupled to the differential mode integrator for receiving the first signal and an output signal and outputting a single-end mode second signal. The switch unit is used for receiving the second signal and outputting the differential mode output signal to drive the load. The present invention uses a common mode input signal instead of the single-end input signal to eliminate the common mode noise, and uses a 2 nd -order (or more than that) integrating circuit instead of the conventional filter to achieve a preferred effect of reducing the off band noise.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a power amplifier, and more particularly, to a power amplifier with noise shaping function. 
         [0003]    2. Description of Related Art 
         [0004]    Referring to  FIG. 1 , a common power amplifier, particularly, an audio power amplifier, adopts a negative feedback design to achieve a stable gain. Meanwhile, a low-pass filter is further connected in series to eliminate the high-frequency noise, wherein the low-pass filter is usually an integrating circuit. As shown in  FIG. 1 , a feedback circuit  110  converts output signals Vo 1  and Vo 2  into a single-end mode feedback signal Vsum_P. The feedback signal Vsum_P and the input signal VI pass through the integrator  120  to filter the noise. 
         [0005]    However, since the amplifier has evolved from the vacuum tube to the transistor, noise interference has always been the largest problem suffered by the audio power amplifier. The main source of noises is divided into two types, wherein one is power supply hum. The output power amplification stage  130  withdraws a high current from the power supply end VDD, but the power supply end VDD cannot be effectively filtered. Therefore, when the gate signal is triggered, and requires a pure high current to passes through the power supply end VDD, the high current contains noises and is not pure, such that the noises are fed into the whole circuit through the feedback circuit  110 . The above content can be obtained with reference to Paragraph 2 of Chapter 8 of “Microelectronic Circuit” (fourth version) written by Adel S. Sedra &amp; Kenneth C. Smith. Therefore, the common solution is to add a filter in the feedback circuit  110  to eliminate noises. However, in such design, as shown in  FIG. 1 , the effect of eliminating noises by a conventional filter is not desirable. 
         [0006]    The second source of noises is radio frequency interference. When a radio frequency signal gets close to the audio amplifier, the circuit with a relatively poor common mode rejection ratio receives the radio frequency together as the noise interference of the audio amplifier itself, for example, the buzz sound produced when a mobile phone gets close to a speaker. However, the conventional audio amplifier does not use a fully differential architecture to increase the common mode rejection ratio. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to providing a power amplifier with noise shaping function, so as to achieve a maximum signal to noise ratio (SNR) for the amplifier. The present invention uses a delta-sigma circuit to connect a 2 nd -order (or more) integrator in series, to achieve a preferred effect in reducing the off band noise, and replaces the manner for a conventional audio amplifier to dispose a filter in the feedback circuit. Moreover, a common mode input signal is further used to replace the conventional single-end input signal, and a fully differential architecture is used to increase the common mode rejection ratio and enhance the capability of the circuit for resisting the radio frequency interference. 
         [0008]    The present invention provides a power amplifier with noise shaping function. The power amplifier comprises a differential mode integrator, an integration and adjustment unit and a switch unit. The differential mode integrator is used for receiving a differential mode input signal and a differential mode output signal, and performing an integration operation to generate a differential mode first signal. The integration and adjustment unit is coupled to the differential mode integrator for receiving the first signal and an output signal, and then performing an integration operation to adjust the first signal into a single-end mode second signal. The second signal is inputted into the switch unit. The working state of the switch unit is determined in accordance with the second signal, and the switch unit outputs the differential mode output signal to drive the load. 
         [0009]    The present invention uses a fully differential mode design, which uses a common mode input signal instead of a conventional single-end input signal to increase the common mode rejection ratio, so as to eliminate the common mode noises and improve the capability of the circuit for resisting the radio frequency interference. Moreover, the differential mode integrator and the integration and adjustment unit both comprises a delta-sigma circuit, and they are connected with each other to form a 2 nd -order high-pass filter circuit, which achieves a stronger rejection effect of the off band noise than the conventional combination of a negative feedback circuit and a filter, so that a better filtering effect is achieved and a preferred SNR is obtained. However, the present invention is not limited to this, persons of ordinary skill in the art may serially connect more integrating circuits to form a 3 rd -order filter circuit, according to the actual requirement. 
         [0010]    In order to make the aforementioned and other objectives, features and advantages of the present invention comprehensible, preferred embodiments accompanied with figures are described in detail below. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0013]      FIG. 1  is a circuit diagram of an audio amplifier in a conventional art. 
           [0014]      FIG. 2  is a circuit diagram of a power amplifier with noise shaping function according to an embodiment of the present invention. 
           [0015]      FIG. 3  is a circuit diagram of the power amplifier in  FIG. 2  according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0016]    A class-D audio power amplifier is taken as an example below in an embodiment of the present invention. However, the present invention is not limited to this, and persons of ordinary skill in the art can apply it in various power amplifiers, such as class-A, class-B, class-AB power amplifiers, according to the spirit of the present invention. 
         [0017]      FIG. 2  is a circuit diagram of a power amplifier with noise shaping function according to an embodiment of the present invention. Referring to  FIG. 2 , the power amplifier includes a differential mode integrator  210 , an integration and adjustment unit  220  and a switch unit  230 . The differential mode integrator  210  performs an integration operation on the received audio signals (the differential mode input signals VIN and VIP herein) and differential mode input signals VON and VOP, so as to generate first signals VIN′ and VIP′. The integration and adjustment unit  220  receives the differential mode first signals VIN′ and VIP′ and differential mode output signals VON and VOP, and performs an integration operation. Moreover, the integration and adjustment unit  220  further adjusts the first signals VIN′ and VIP′ into a single-end mode second signal Vo, and outputs the second signal Vo to the switch unit  230 . The working state of the switch unit  230  is determined in accordance with the second signal Vo, and the switch unit  230  outputs the differential mode output signals VON and VOP to drive the load  240 . In this embodiment, the load  240  is a speaker or another load. 
         [0018]      FIG. 3  shows an embodiment of the power amplifier in  FIG. 2  of the present invention. Referring to  FIG. 3 , the differential mode integrator  210  includes a first resistor  311 , a second resistor  312 , a third resistor  313 , a fourth resistor  314 , a first operational amplifier  315 , a first capacitor  316  and a second capacitor  317 . 
         [0019]    The first terminals of the resistors  311  and  312  respectively receive a first-end signal VIN and a second-end signal VIP of the input signal. The first terminals of the resistors  314  and  312  respectively receive a first-end signal VON and a second-end signal VOP of the output signal. The first operational amplifier  315  has a first input terminal (for example, a positive input terminal) coupled to a second terminal of the second resistor  312  and a second terminal of the third resistor  313 , and has a second input terminal (for example, a negative input terminal) coupled to a second terminal of the first resistor  311  and a second terminal of the fourth resistor  314 . The first output terminal (for example, a positive output terminal) and the second output terminal (for example, a negative output terminal) of the first operational amplifier  315  respectively output the first-end signal VIN′ and the second-end signal VIP′ of the first signal. A first terminal and a second terminal of the first capacitor  316  are respectively coupled to the first output terminal and the second input terminal of the first operational amplifier  315 . A first terminal and a second terminal of the second capacitor  317  are respectively coupled to the second output terminal and the first input terminal of the first operational amplifier  315 . Therefore, a differential input and output integrating circuit is formed. Persons of ordinary skill in the art can adjust the resistance value of the resistors  311 - 314  and the capacitance value of the capacitors  316 - 317 , i.e., adjust the impedance match, so as to control the low-pass bandwidth of the differential mode integrator  210 . In this embodiment, the resistance value of the first resistor  311  and the second resistor  312  is, for example, R 1 , and the resistance value of the third resistor  313  and the fourth resistor  314  is, for example, G*R 1  (G and R 1  are real numbers). The capacitance value of the first capacitor  316  and the second capacitor  317  is, for example, C1 (C1 is a real number). 
         [0020]    The integration and adjustment unit  220  includes a first impedance device  321 , a second impedance device  322 , a third impedance device  323 , a fourth impedance device  324 , a second operational amplifier  325 , a fifth impedance device  326  and a sixth impedance device  327 . The first terminals of the impedance devices  321  and  322  are respectively coupled to the first output terminal and the second output terminal of the differential mode integrator  210  for receiving the first-end signal VIN′ and the second-end signal VIP′ of the first signal. The first terminal of the third impedance device  323  receives the first-end signal VON of the output signal. The first terminal of the fourth impedance device  324  receives the second-end signal VOP of the output signal. The second operational amplifier  325  has a first input terminal (for example, a positive input terminal) coupled to the second terminals of the first impedance device  321  and the third impedance device  323 , and has a second input terminal (for example, a negative input terminal) coupled to the second terminals of the second impedance device  322  and the fourth impedance device  324 . The output terminal of the first operational amplifier  315  outputs a single-end mode second signal Vo. A first terminal of the fifth impedance device  326  is coupled to a first input terminal of the second operational amplifier  325 , and the second terminal receives a first voltage. In this embodiment, the above first voltage is, for example, a ground voltage. A first terminal and a second terminal of the sixth impedance device  327  are respectively coupled to the output terminal and the second input terminal of the second operational amplifier  325 . 
         [0021]    Persons of ordinary skill in the art can implement the first impedance device to the sixth impedance device in any way according to the actual demand, and determine the impedance value of the impedance devices, so as to obtain the required circuit characteristics according to the actual demand. In this embodiment, the first impedance device  321  and the second impedance device  322  are all implemented as a resistor with a resistance value of R 2 , and the third impedance device  323  and the fourth impedance device  324  are all implemented as a resistor with a resistance value of F*R 2  (F is a real number). Moreover, in this embodiment, the fifth impedance device  326  and the sixth impedance device  327  are all implemented as a capacitor with a capacitance value of C 2 . The first voltage is a reference voltage level, which includes a ground voltage, a level voltage adopted by the frame grounding or according to the actual design requirement. 
         [0022]    The switch unit  230  includes a pulse width modulation (PWM) stage  331  and a bridge circuit. The PWM stage  331  is used for generating at least a PWM signal (a first PWM signal PWMS 1  and a second PWM signal PWMS 2  in this embodiment). The PWM stage  331  modulates the pulse widths of the PWM signals PWMS 1  and PWMS 2  in accordance with the single-end mode second signal Vo. A class-D output amplification stage is taken as, but not limited to, an example for the bridge circuit in this embodiment. The bridge circuit includes a plurality of switches (a first switch  332 , a second switch  333 , a third switch  334 , and a fourth switch  335  in this embodiment) controlled by the PWM signals PWMS 1  and PWMS 2 . In the bridge circuit, a first terminal of the first switch  332  receives a second voltage (e.g., a power supply voltage VDD), and a second terminal outputs the first-end signal VON of the output signal to a first terminal of the load  240 . A first terminal of the second switch  333  receives a third voltage (e.g., a ground voltage), and a second terminal is coupled to the first terminal of the load  240 . The switches  332  and  333  are all controlled by the first PWM signal PWMS 1 . 
         [0023]    A first terminal of the third switch  334  receives a second voltage, and a second terminal outputs the second-end signal VOP of the output signal to a second terminal of the load  240 . A first terminal of the fourth switch  335  receives a third voltage, and a second terminal is coupled to the second terminal of the load  240 . The switches  334  and  335  are all controlled by the second PWM signal PWMS 2 . 
         [0024]    In this embodiment, the above first switch  332  and the third switch  334  are a P-type transistor, and the second switch  333  and the fourth switch  335  are an N-type transistor, and the four switches form a class-D output amplification stage. The PWM stage  331  controls the switches  332 - 335  of the bridge circuit by modulating the pulse width and phase of the PWM signals PWMS 1  and PWMS 2 . Therefore, the switch unit  230  outputs an audio signal (i.e., output signals VON and VOP) in accordance with the second signal Vo to drive the load  240 . In this embodiment, the second voltage is a power supply voltage, which indicates the voltage for driving the output amplification stage, and it is not limited to the power supply voltage of the whole circuit. The third voltage is also a voltage reference level, which is not limited to the ground voltage. 
         [0025]    In this embodiment, after being processed through two layers of integrator, the differential input signal forms an output signal Vo=(Vip−Vin)/{S 2 R 2 C 2 +SRC/F+1/G}+S 2 R 2 C 2 *(THD+Noise)/{S 2 R 2 C 2 +SRC/F+1/G}, wherein, S=j□=j2□f, f indicates the frequency. The THD in the above formula indicates total harmonic distortion, which is calculated by dividing the root mean square (rms) of the output signal harmonic component by the basic harmonic, so as to indicate a percentage of the basic harmonic root mean square. An evaluation on the output design is usually based upon the THD parameter. Usually, the THD of a high-fidelity audio amplifier has a stage of about less than one percentage. Based on the above formula, in the audio signal, SRC&lt;&lt;1, the noise is greatly reduced to obtain a preferable SNR. 
         [0026]    Though the present invention has been disclosed above by the preferred embodiments, they are not intended to limit the present invention. Anybody skilled in the art can make some modifications and variations without departing from the spirit and scope of the present invention. Therefore, the protecting range of the present invention falls in the appended claims and their equivalents.