Abstract:
A switching amplifier includes first and second output terminals that may be connected to a load. A pulse-width modulator receiving an input signal to obtain respective positive and negative values of the input signal. The modulator is connected to first and second switching circuits. The first switching circuit applies a plurality of pulses to the first output terminal that, in response to the positive samples, have a constant frequency and are pulse-width modulated, and, in response to the negative samples, have a varying frequency and a constant width. Similarly, the second switching circuit applies a plurality of pulses to the second output terminal that, in response to the negative samples, have a constant frequency and are pulse-width modulated, and, in response to the positive samples, have a varying frequency and a constant width. The varying phase of the constant width pulses disperses RF interference across a wider spectrum.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/887,394, filed Jan. 31, 2007. The entire disclosure of the prior application is considered to be part of the disclosure of the instant application and is hereby incorporated by reference therein. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to switching amplifiers and methods, and, more particularly, to a system and method for reducing the electromagnetic interference of switching amplifiers. 
       BACKGROUND OF THE INVENTION 
       [0003]    Switching amplifiers provide far greater efficiency than their analog counterparts, primarily because transistors used to switch voltages to a load are either turned ON, so that the voltage across the transistor is relatively low, or turned OFF, so that the current through the transistor is relatively low. With either a low voltage across the transistor or a low current through the transistor, the power dissipated by the transistor is relatively low. 
         [0004]    Although conventional switching amplifiers are widely used, they can produce excessive distortion in their output signals because of capacitive coupling between the gates of respective switching transistors used by such amplifiers. Another limitation of conventional switching amplifiers is that they can sometimes generate excessive electromagnetic radio frequency (“RF”) interference, that can interfere with the operation of the amplifier as well as with other electronic devices in the vicinity of the amplifier. This RF interference can be attenuated to some extent by coupling the load driven by the amplifier to filters formed by inductors and/or capacitors. However, the remaining RF interference can still be too high in some applications. 
         [0005]    Attempts have been made to minimize the distortion of signals output from switching amplifiers by operating them in a balanced manner so that capacitive coupling to one side of a load is matched by capacitive coupling to the other side of the load. While this approach is successful in minimizing signal distortion, it actually increases the RF interference generated by the amplifier since the number of transistors switching must be increased. 
         [0006]    Attempts have been made to minimize the frequency at which the peak amplitude of the RF interference occurs by varying to “dithering” the switching times of the transistors, but doing so tends to increase the distortion of the output signal since a signal input to the amplifier is not sampled at regular intervals. In addition, spread spectrum EMI reduction is limited by audible distortion products at higher deviations. This distortion is normally greater at higher output amplitudes, so it clearly is a distortion since it increases with amplitude. 
         [0007]    There is therefore a need for a system and method for operating switching amplifiers in a manner that minimizes the magnitude of RF interference generated by the amplifier, and does so without introducing significant distortion. 
       SUMMARY 
       [0008]    A switching amplifier and method includes first and second output terminals to which a load may be connected. An input signal is applied to switching amplifier. The switching amplifier applies a plurality of periodic first pulses to the first output terminal and it adjusts the widths of the first pulses as a function of the magnitude of the input signal. The switching amplifier also applies a plurality of periodic second pulses to the second output terminal. The second pulses are asynchronous with, but substantially equal in number to, the first pulses. If the input signal has positive and negative polarities, the switching amplifier may obtain positive and negative samples, respectively. In response to the positive samples, the first pulses are periodic and their widths are modulated, and the second pulses have a constant width. In response to the negative samples, the second pulses are periodic and their widths are modulated, and the first pulses have a constant width. The widths of the modulated pulses may be greater than the width of the unmodulated pulses by a magnitude corresponding to the amplitudes of the input signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of a prior art switching amplifier that can be operated according to embodiments of the invention. 
           [0010]      FIG. 2  is a timing diagram showing an example of how the switching amplifier of  FIG. 1  has been operated in the prior art. 
           [0011]      FIG. 3  is a timing diagram showing an example of how the switching amplifier of  FIG. 1  has been operated in the prior art in an attempt to minimize output signal distortion. 
           [0012]      FIG. 4  is a timing diagram showing an example of how the switching amplifier of  FIG. 1  has been operated in the prior art in an attempt to minimize electromagnetic RF interference. 
           [0013]      FIG. 5  is a timing diagram showing the operation of the switching amplifier of  FIG. 1  according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    A typical prior art switching amplifier  100  is shown in  FIG. 1 . The switching amplifier  100  includes a pulse-width modulation (“PWM”) modulator  102  that receives a signal through an input line  101 . The PWM modulator  102  generates four outputs A-D, which drive the gates of respective transistors  103 ,  104 ,  105 ,  106 , which are arranged in a bridge or H configuration. The transistors  103 ,  104  are connected in series between a supply voltage V+ and ground, and the transistors  105 ,  106  are similarly connected in series between V+ and ground. A load  114  is connected between a first output node E formed by the junction between the transistors  103 ,  104  and a second output node F formed by the junction between the transistors  105 ,  106 . 
         [0015]    As shown in  FIG. 1 , the load  114  is formed by a resistive load  110  connected in parallel with a capacitor  109 , both of which are connected between a pair of inductors  107 ,  108 . As is well known in the art, the inductors  107 ,  108  and capacitor  109  form an LC filter that reduces the amount of electromagnetic energy generated by switching the transistors  103 - 106  ON and OFF. Although  FIG. 1  shows the load  114  formed by a resistive load  110  and LC filter, the load  114  may instead be inductive, such as a MRI coil, capacitive, such as a piezoelectric acoustic transducer, or some combination on these impedance elements. Also, although transistors  103 - 106  are shown as being the devices used to switch various voltages to the load  114  at various times, it will be understood that other switching devices may be used. Finally, configurations of switching devices other than that shown in  FIG. 1  may also be used to apply various voltages to the load  114  at various times. 
         [0016]    The operation of the amplifier  100  is shown in  FIG. 2  in which six waveforms A-F have been labeled to show an example of the signals present at the corresponding nodes A-F when operating the amplifier  100  in a conventional manner. The signal A is initially driven high to turn ON the transistor  103  for a period corresponding to the amplitude of the signal applied to the PWM modulator  102 . At the same time and for the same period, the signal B is driven low to turn OFF the transistor  104 . The transistor  106  is ON during this time. As a result, the voltage V+ is connected to ground through the inductor  108 , resistive load  110  and inductor  107 , thereby causing current to start flowing through the resistive load  110 . Were it not for the filter formed by the inductors  107 ,  108  and capacitor  109 , the sudden increase in current through the transistors  103 ,  106  resulting from turning the transistor  103  ON would result in substantial RF interference. However, this RF interference is attenuated to some extent by the filter formed by the inductors  107 ,  108  and capacitor  109 . 
         [0017]    At the end of the period corresponding to the amplitude of the signal applied to the PWM modulator  102 , the signal A transitions low to turn OFF the transistor  103 , and the signal B transitions high to turn ON the transistor  104 . Again, if the inductors  107 ,  108  and capacitor  109  were not present, substantial RF interference might be generated by switching the transistor  103  OFF. 
         [0018]    A predetermined period later, the above sequence is repeated except that the transistor  103  is turned ON and the transistor  104  is turned OFF for a period that is shorter than the period that the transistor  103  was previously turned ON because the amplitude of the signal applied to PWM modulator  102  is lower. As a result, a current flows through the resistive load  110  for a shorter period. The above sequence is then repeated again for an even shorter period corresponding to the lower amplitude of the signal applied to the PWM modulator  102 . 
         [0019]    Next, the polarity of the signal applied to the resistive load  110  is reversed by repeating the above sequence except that the transistor  105  is turned ON instead of the transistor  103 , and the transistor  106  is turned OFF instead of the transistor  104 . Thus, when the input signal to the PWM modulator  102  is positive, the pulse width of the signals applied to the transistors  103 ,  104  are modulated, and when the input signal to the PWM modulator  102  is negative, the pulse width of the signals applied to the transistors  105 ,  106  are modulated. 
         [0020]    The switching amplifier  100  operating as described above provides adequate performance in may cases, but it results in distortion that can be excessive when low distortion amplification is required. The reason for this distortion is essentially the capacitive coupling between the gates of the transistors  103 - 106  and the nodes E, F, which effectively distorts the width of the pulses generated at the nodes E, F. When this capacitive coupling occurs, the width of these pulses no longer correspond to the amplitude of the signal applied to the PWM modulator  102 . 
         [0021]    One approach to reducing the distortion of the amplifier  100  operating as described above is to operate the amplifier  100  as shown in  FIG. 3  in which the same signal is applied to the PWM modulator  102  as in the example shown in  FIG. 2 . As shown in  FIG. 3 , when the input signal to the PWM modulator  102  is positive, the pulse width of the signals applied to the transistors  103 ,  104  are still modulated, and when the input signal to the PWM modulator  102  is negative, the pulse width of the signals applied to the transistors  105 ,  106  are still modulated. However, when applying pulsewidth modulated signals to a first side of the load  114 , unmodulated signals are applied to the first side of the load as well as second side of the load  114 . Applying unmodulated signals to both sides of the load, mitigates differential charge injection. The transistors  103 ,  104  are switched ON and OFF respectively for a period that is constant when the input signal to the PWM modulator  102  is positive, and the transistors  105 ,  106  are switched ON and OFF, respectively, for a period that is constant when the input signal to the PWM modulator  102  is negative. When both transistors  103 ,  105  are turned ON, the effect is as if neither transistor  103 ,  105  was turned ON. However, the capacitive coupling from the gate of the transistor  103  to the node E is matched by the capacitive coupling from the gate of the transistor  105  to the node F. The periods during which the transistor  103  is turned ON while the transistor  105  is OFF are identical to the periods during which the transistor  103  is turned ON in the example of  FIG. 2 , as can be seen by the signal E′-F′. As a result, the currents through the resistive load when operating as in  FIG. 3  are the same and of the same duration as when operating as shown in  FIG. 2 . Moreover, the capacitive coupling of the signal A to the node E occurs the same number of times that the signal C is capacitively coupled to the node F. Similarly, the capacitive coupling of the signal B to the node E occurs the same number of times that the signal D is capacitively coupled to the node F. As a result, the capacitive coupling to the node E cancels out the capacitive coupling to the node F, thereby preserving the width of the pulses generated at the nodes E, F. Therefore, operating the amplifier  100  as shown in  FIG. 3  results in very little distortion. 
         [0022]    An additional problem with operating the amplifier  100  as shown in  FIG. 2  is the electromagnetic RF interference resulting from switching the transistors  103 - 108  as explained therein. Operating the amplifier  100  as shown in  FIG. 3  does not solve this problem. To the contrary, operating the amplifier  100  as shown in  FIG. 3  can actually exacerbate the problem of RF interference because both transistors  103 ,  105  switch at the same time compared to the operation as shown in  FIG. 2  in which only one of the transistors,  103  or  105 , switch. Similarly, in  FIG. 3 , both transistors  104 ,  106  switch at the same time. Since RF interference is generated each time one of the transistors  103 - 106  is switched, doubling the number of transistors being switched increases the magnitude of the RF interference. 
         [0023]    One approach to reducing the magnitude of the RF interference is to vary or “dither” the timing (but not the duration) at which the transistors  103 ,  105  are turned ON as shown in  FIG. 4  in which the signal applied to the PWM modulator  102  is shown in the top line, the signal at the node E is shown in the next line, and the signal at the node F is shown in the bottom line. As shown therein, the operation of the circuit is similar to the operation shown in  FIG. 3 . The unmodulated pulses again have a constant width, and they again are provided to compensate for the capacitive coupling through the transistors  103 ,  105  receiving the pulse-width modulated signals from the PWM modulator  102 . The operation of the switching amplifier  100  shown in  FIG. 4  differs from the operation shown in  FIG. 3  in that the time between switching the transistors  103 ,  105  is not constant. Instead, for example, the duration of the period between the transistor  103  first being turned ON and the transistor  103  being turned ON a second time is different from the period between with transistor  103  being turned ON a second time and then turning ON a third time. By varying the conductive times of the transistors  103 - 106  in this manner, the frequency at which the peak amplitude of the RF interference spectrum occurs is varied from pulse-to-pulse, thereby distributing the RF interference over a range of frequencies. In contrast, the frequency at which the peak amplitude of the RF interference spectrum occurs when operating as shown in  FIG. 3  is the same from pulse-to-pulse. As a result, the peak amplitude of the RF interference at output sample rate is significantly higher when operating the amplifier  100  as shown in  FIG. 3  compared to operating as shown in  FIG. 4 . 
         [0024]    Although operating the amplifier  100  as shown in  FIG. 4  significantly reduces the magnitude of the RF interference, the amplifier  100  can nevertheless generate RF interference that can be excessive in some instances. Additionally, operating the amplifier  100  as shown in  FIG. 4  can produce excessive distortion because the signal applied to the PWM modulator  102  is not sampled at a constant rate. The technique disclosed in this embodiment causes low level constant distortion, such as tape hiss, which is not perceived to be distortion by most listeners. Dithering can therefore be pushed higher (reducing measured EMI) before encountering customer complaints. 
         [0025]    According to one embodiment of the invention, the switching amplifier is operated as shown in  FIG. 5 . The signal applied to the PWM modulator  102  is again shown in the top line, the signal at the node E is shown in the next line, and the signal at the node F is shown in the bottom line. When the input signal is positive, the width of the pulses at node E are again modulated, and when the input signal is negative, the width of the pulses at node F are again modulated. The unmodulated pulses continue to have a constant width, and they again are provided to compensate for the capacitive coupling through the transistors  103 ,  105  receiving the pulse-width modulated signals from the PWM modulator  102 . 
         [0026]    The operation of the switching amplifier  100  according to one embodiment of the invention as shown in  FIG. 5  differs from the operation shown in  FIG. 4  by varying or “dithering” the times between only the unmodulated pulses. The times between the PWM modulated pulses are constant. As a result, the sampling times of the input signal can be constant, thus avoiding distortion in the output signal from the amplifier  100 . Yet, by varying or “dithering” the switching times of the unmodulated pulses used to compensate for the capacitive coupling of the modulated pulses, the frequency of the peak amplitude of the RF interference is varied, thus minimizing the peak amplitude of the RF interference. Operating the amplifier  100  as shown in  FIG. 5  thus produces relatively little RF interference in a manner that does not result in output signal distortion. The dithering of the unmodulated pulse switching times may vary in a pseudo-random manner. 
         [0027]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.