Abstract:
A system and method for compensating switched mode amplifiers. The amplifier system includes an adaptive filter coupled to the output of circuitry that recovers the analog signal from the output of the switching amplifier circuitry. The adaptive filter derives an inverse transfer function of the forward path through the system. The derived inverse transfer function is used to generate a coefficient map which a compensation filter responsively compensates for from variations in the output load of the amplifier system. In the disclosed method, the deriving the inverse transfer function may further include calibrating the adaptive filter continuously, at system startup or in response to user demand.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates in general to switched mode electronic amplifiers, and in particular to circuits and methods for compensating for switched mode amplifiers. 
     2. Background of Invention 
     Class D audio power amplifiers (APAs) have been used for many years in systems, such as wireline telephony, where high bandwidth is not critical. More recently however, new fabrication techniques, and in particular, new techniques for fabricating power transistors, have made integrated Class D APAs possible. This has extended their potential applications to lower-power, higher-bandwidth systems, including battery-powered portable music players and wireless communications devices. 
     One major advantage of Class D amplifiers is their efficiency. Generally, an audio signal is converted into a relatively high frequency stream of pulses varying in width with the amplitude of the audio signal. This pulse width modulated (PWM) signal is used to switch a set of power output transistors between cutoff and saturation which results in efficiencies above ninety percent (90%). In contrast, the typical Class AB push-pull amplifier, using output transistors whose conduction varies linearly during each half-cycle, has an efficiency of around sixty percent (60%). The increased efficiency of Class D amplifiers in turn reduces power consumption and consequently lowers heat dissipation and improves battery life in portable systems. 
     One problem that must be addressed in the Class D amplifier design is the non-ideal response of the output load. Specifically, some accommodation must be made for the frequency response of reactive loads. Moreover, the amplifier should be capable of driving a range of different output devices coupled to the system output. 
     SUMMARY OF INVENTION 
     According to the principles of the present invention, an amplifier system is disclosed which includes a switching amplifier unit and analog conversion circuitry configured to recover an analog signal from an output signal of the switching amplifier. An adaptive filter coupled to an output of the conversion circuitry is configured to derive an inverse transfer function of a forward path through the amplifier system, which includes the analog conversion circuitry, the derived inverse transfer function is used to generate a coefficient map from which a compensation filter responsively compensates for variations in the output load of the amplifier unit. 
     The inventive concepts address a significant problem in the application of switched mode amplifiers generally, and particularly in applications in which faithful reproduction of the input signals is desired, namely the sensitivity of the frequency response of the system to the non-ideal conditions represented by realistic loads. Among other things, these concepts allow for the implementation of compensation mechanisms which equalize the frequency response of the amplifier system in the presence of reactive loads, as well as loads with nominal impedances that span the range of values presented by different output devices that may be connected to the amplifier system. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B illustrates, in block diagram form, a switching amplifier system in accordance with the prior art; 
     FIG. 2 illustrates, in graphical form, the frequency response of a switching amplifier system in accordance with FIG. 1; 
     FIG. 3 illustrates a switching amplifier system in accordance with an embodiment of the present invention; and 
     FIG. 4 illustrates an adaptation methodology in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-4 of the drawings in which like numbers represent like parts. 
     As previously described, in a Class D amplifier, efficiency is gained by switching the power devices hard between the power supply rails. The high frequency noise is then filtered with a low pass filter. Typically, the low pass filter is of the passive type, including inductive and/or capacitive reactive elements to smooth the signal. 
     FIG. 1A illustrates, in block diagram form, a typical Class D amplifier system  100 . Amplifier system  100  includes Class D amplifier  102  containing MOSFET switches  104 , along with their associated gates and drivers, and PWM controller  106 . PWM controller  106  receives a digitized audio input signal (Audio In), which constitutes the signal to be amplified. MOSFET switches  104  may constitute a full bridge amplifier. Alternatively other circuit designs may be used, including a half-bridge switch. PWM controller also receives two feedback signals. Feedback path  109  from the output of MOSFET switches  104  may be used to correct the errors introduced by non-ideal switch behavior. PWM signal generation techniques are discussed in coassigned U.S. Pat. No. 5,815,102 to Melanson, entitled “Delta Sigma PWM DAC to Reduce Switching”, and feedback from Class D amplifier switches is discussed in the coassigned U.S. Pat. No. 6,150,969 to Melanson entitled “Correction of Non-linear Output Distortion in a Delta Sigma DAC”, which references are hereby incorporated herein by reference. 
     The amplified audio is recovered via low pass filter (LPF)  110 , which provides the audio output to a load, Z. (LPF  110  is thus preferably a simple digital to analog converter.) 
     One possible application of audio amplifier  100  is in a digital radio  110 , such as that shown in FIG.  1 B. 
     Digital radio  110  includes an analog section or front-end  111 -which receives radio frequency (RF) signals from an associated antenna  112 . Analog front-end  111  is preferably a conventional RF down-converter including a low noise amplifier (LNA)  113  for setting the system noise figure, a bandpass filter  114  and mixer  115  driven by an analog local oscillator  116 . The mixed-down analog signal is then converted into digital form by analog to digital converter  117 . 
     The digitized data output from A D converter  117  is passed to digital processing section  118 . A pair of mixers  119   a,b  generate in-phase (I) and quadrature (Q) signals from a corresponding pair of clock phases from crystal oscillator  120 . The I and Q signals are next passed through bandpass filters  121   a  and  121   b  on to digital baseband processor  122 . The processed digital signal is then re-converted to analog (audio) form by D/A converter  123 . 
     A switched mode (Class D) audio power amplifier (APA)  124  is used to drive an external set of speakers or a headset. Preferably, at least some of the components of digital radio  110  are powered by a switched mode power supply (SMPS)  124 . 
     To effectively attenuate out-of-band noise, the corner frequency of LPF  110  should be just above the passband of the amplifier system. However, this increases the error in the frequency response in the passband. Moreover, the frequency response is significantly affected by the load impedance, Z. 
     FIG. 2 illustrates typical frequency response curves. Curves  200  and  202  are typical frequency responses with eight and sixteen ohm resistive loads, respectively. With a significant inductive component, which may be typical of audio amplification into speakers, a response of the form in curve  204  may result. Curve  204  exhibits a significant passband ripple. (The frequencies and gain levels shown in FIG. 2 are approximate and are provided as general reference to the depicted curves.) 
     A feedback path  112  from the output of LPF may be provided to compensate for frequency response modifications introduced by LPF  110 . However, it may be difficult to make feedback path  112  effective and concomitantly guarantee the stability of the feedback loop. Typically, the load, Z, has a significant reactive component. Consequently, the phase shift of the filter is not well known, which exacerbates the question of the stability of the feedback loop. Moreover, with a digital PWM controller  106 , feedback path  112  must include, an analog to digital (A/D) converter (not shown in FIG. 1.) This adds additional phase delay, further complicating the performance/stability trade off. 
     Refer now to FIG. 3 illustrating a switch mode amplifier system  300  in accordance with the principles of the present invention. Amplifier system  300  includes switching amplifier  102  and LPF  110  coupled to the load (not shown in FIG.  3 ). Amplifier system  300  includes adaptive filter  302 , which may be a digital filter. The input to adaptive filter  302  is provided by the output of LPF  110  via analog-to-digital converter (A/D)  304 . A/D  304  outputs a digital representation of the analog audio signal from LPF  110 . 
     Adaptive filter  302  may be a finite impulse response (FIR) filter that “learns” the inverse of the transfer function of the forward path through amplifier system formed by switching amplifier  102 , LPF  110  and the load (not shown in FIG.  3 ). Adaptive filter  302  derives the inverse transfer function by adjusting the FIR coefficients, as discussed below. Because the transfer function of LPF  110  is substantially all pole in nature, a FIR implementation of adaptive filter  302  is a good inverse, the FIR being an all zero filter. Filter  302  is adapted such that the combined forward path, including switching amplifier  102 , LPF  110 , the load, and adaptive filter  302  is equivalent to a pure delay. The signal input to switching amplifier  102  is equivalently delayed in delay  306 . The frequency response of delay element  306  is flat. The output of delay element  306  is differenced with the output of adaptive filter  302  in adder  308 . The output of adder  308  provides error signal  310  to adaptive filter  302 . 
     Adaptive filter  302  adjusts the coefficients of the FIR in response to error signal  310 . The adaptive filter adjusts the coefficients so as to drive the error signal towards zero. One such algorithm for adjusting the coefficients, known in the signal processing art, is the least-mean-square (LMS) algorithm; however, it should be recognized that other adaptation algorithms may also be used in alternative embodiments. For typical output filters  110 , and loads that are primarily resistive, the order of compensating filter  312  may be in the range of from four (4) to ten (10). This range is exemplary, and longer filters may be implemented, in alternative embodiments of the present invention, to accommodate loads that have substantial reactive components, or to accommodate embodiments having more complex output filters, that is, filters having increased numbers of poles. Generally, the order of compensation filter depends on the desired flatness of the filter response; the longer the filter the more compensation that is possible. MATLAB simulations have shown that an audio load of 4-16 ohms plus a reactive component can be suitably compensated for with a 6 th  order filter. 
     The signal to be amplified is input to compensation filter  312 . The coefficients from adaptive filter  302  are mapped to the coefficients of compensation filter  312 , indicated by the dashed path coefficient, coefficient map  314 , in FIG.  3 . Thus, the present invention provides a closed loop feedback compensation in which the loop is closed by the coefficient map  314 . Compensation filter  312 , thus, also has a transfer function that is the inverse of the forward path through switching amplifier system  300 . Consequently, the output signal of the loaded amplifier system is flat over the bandwidth of the loop. Compensation filter  312  may be, in an embodiment of the present invention a FIR. In such an embodiment, compensation filter  312  coefficient map  314  may be a one-one copying of the coefficients of adaptive filter  302  to filter  312 . However, filter  312  may also be implemented as in infinite impulse response (IIR) digital filter, and in yet another alternative embodiment implemented in the analog domain, as, for example, a switched capacitor filter. 
     System  300  also includes noise source  314  and switch  316 . The input to switching amplifier  102  may be switched between noise source  314  and the output of compensation filter  312 , as discussed further below. In normal operation, the switch selects the output from compensation filter  312 . 
     FIG. 4 is a flow chart of an adaptation (or calibration) methodology  400  in accordance with an embodiment of the present invention. In Step  402 , a determination is made if the system is initializing on power-up, or if user initiated adaptation has been requested. The user may, for example, elect to train the compensation filter when the system is installed, or, if the load is changed. 
     If, in Step  402 , a system power up has occurred, or user elected calibration is to be performed, in Step  404  process  400  switches to the noise source. Thus, in an embodiment of an amplifier system in accordance with FIG. 3, switch  316  couples the input of switching amplifier  102  to noise source  314 . In Step  407 , the coefficients of the adaptive filter, such as filter  402 , FIG. 3, are adjusted, as long as the calibration cycle has not expired. After the calibration cycle, Step  406 , the input to the switching amplifier, such as switching amplifier  102 , is switched to the signal source, Step  408 . The signal source, in accordance with system  300 , is the output of compensation filter  312 . Additionally, in Step  410 , the coefficients of the adaptive filter are mapped into the compensation filter. Process  400  then returns to step  402 . 
     Adaption process  400  may also continuously adapt the switch mode amplifier system. If, in Step  402 , neither a system power up event nor user demanded calibration event has occurred, then in Step  412 , a determination is made as to continuous adaptation is enabled. If not, process  400  loops back to Step  402 . Otherwise, process  400  proceeds to Step  413  and adjusts the coefficients of the adaptive filter, such as filter  302 . In Step  410 , the adaptive filter coefficients are mapped to the compensation filter. Process  400  then continuously loops between Steps  410 ,  402  and  412  to continuously adapt the switch mode amplifier system. The continuous adaptation may be interrupted by a user elected calibration at Step  402 , as process  400  loops. 
     Although the invention has been described with reference to a specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.