System for amplifiers with low distortion and low output impedance

Abstract

System for pulse-width-modulated class D audio amplifiers. In one preferred embodiment an adder is described to generate a difference signal responsive to an input signal and a feedback signal, a pulse-width-modulator coupled to the adder to compare the difference signal to a reference signal and produce a pulse-width-modulated signal based on the comparing, a filter coupled to an output of the pulse-width-modulator, and a loop filter having a first input coupled to the output of the filter and a second input coupled to the input of the filter, the loop filter to generate a feedback signal by applying transfer functions to signals at its inputs. The loop transfer function of the amplifier is minimum aliasing error transfer function. The minimum aliasing error properties provide low distortion and taking the feedback from the output of the filter reduces high frequency output impedance.

1 . An amplifier circuit comprising: 
 an adder configured to generate a difference signal responsive to an input signal and a feedback signal;    a pulse width modulator coupled to the adder, the pulse width modulator configured to compare the difference signal to a reference signal and to produce a pulse width modulated signal based on the comparing;    a filter coupled to an output of the pulse width modulator, the filter having a transfer function; and    a loop filter having a first input coupled to an output of the filter and a second input coupled to an input of the filter, and an output coupled to the adder, the loop filter configured to generate the feedback signal by applying transfer functions to signals at its inputs, wherein a loop transfer function of the amplifier circuit is minimum aliasing error.    
   
   
       2 . The amplifier circuit of  claim 1 , wherein the loop filter has poles located at frequencies P 0 , P 1 , and P 2 , and wherein the loop transfer function has a second order behavior between frequencies P 0  and P 2  and a first order behavior at frequencies above P 2 .  
   
   
       3 . The amplifier circuit of  claim 1 , wherein the loop filter comprises a first feedback loop coupled to the first input, the first feedback loop comprising a first first-order high pass filter with a pole at frequency P 1  and a second first-order high pass filter with a pole at frequency P 2 , wherein frequency P 2  is higher than frequency P 1 , wherein the second first-order high pass filter terminates at the adder, and the loop filter further comprises a second feedback loop coupled to the second input, the second feedback loop comprising a first resistor.  
   
   
       4 . The amplifier of  claim 3 , wherein the loop filter further comprises a third feedback loop coupled to the second input, the third feedback loop comprising a second resistor.  
   
   
       5 . The amplifier circuit of  claim 1 , wherein the pulse width modulator has a low impedance input, and wherein the loop filter comprises: 
 a fourth feedback loop coupled to the first input, the fourth feedback loop comprising a third resistor; and    a fifth feedback loop coupled to the first input, the fifth feedback loop comprising a first-order high pass filter and a capacitor arranged serially.    
   
   
       6 . The amplifier circuit of  claim 5 , wherein the first-order high pass filter is designed to form a pole at P 1  and the third resistor and the capacitor are designed to form a pole at P 1 .  
   
   
       7 . The amplifier circuit of  claim 5 , wherein the low impedance input of the pulse width modulator comprises a current conveyor.  
   
   
       8 . The amplifier circuit of  claim 5 , wherein the low impedance input of the pulse width modulator comprises an operational amplifier arranged in an inverting configuration.  
   
   
       9 . An amplifier circuit comprising: 
 a pulse code modulation to pulse width modulation (PCM-PWM) converter, the PCM-PWM converter configured to convert an input pulse code modulated signal into a pulse width modulated signal;    a feedforward filter coupled to the PCM-PWM converter, the feedforward filter having a first transfer function;    an adder coupled to the feedforward filter, the adder to generate a difference signal responsive to a signal produced by the feedforward filter and a feedback signal;    a pulse width modulator coupled to the adder, the pulse width modulator configured to compare the difference signal to a reference signal and to produce a pulse width modulated signal based on the comparing;    a filter coupled to an output of the pulse width modulator, the filter having a second transfer function; and    a feedback filter having a first input coupled to an output of the feedforward filter and a second input coupled to an input of the feedforward filter, and an output coupled to the adder, the feedback filter configured to generate a feedback signal by applying transfer functions to signals at its inputs, wherein a loop transfer function of the amplifier circuit is minimum aliasing error.    
   
   
       10 . The amplifier circuit of  claim 9 , wherein the feedforward filter has a pole at frequency P 1 , wherein the feedback filter has a pole at frequency P 1 *, and wherein the frequency P 1  is slightly lower than the frequency P 1 * to increase a magnitude of the transfer function of the feedforward filter at a switching frequency of the amplifier circuit.  
   
   
       11 . The amplifier circuit of  claim 9 , wherein the feedback filter and the feedforward filter have substantially the same transfer function.  
   
   
       12 . The amplifier circuit of  claim 9  further comprising: 
 an input H-power bridge coupled in between the PCM-PWM converter and the feedforward filter; and    an output H-power bridge coupled in between the pulse width modulator and the filter.    
   
   
       13 . The amplifier circuit of  claim 9 , wherein the feedback filter comprises a first feedback loop coupled to the first input, the first feedback loop comprising a first first-order high pass filter with a pole at frequency P 1  and a second first-order high pass filter with a pole at frequency P 2 , wherein frequency P 2  is higher than frequency P 1 , wherein the second first-order high pass filter terminates at the adder.  
   
   
       14 . The amplifier circuit of  claim 13 , wherein the feedback filter further comprises: 
 a second feedback loop coupled to the first input, the second feedback loop comprising a first resistor; and    a third feedback loop coupled to the second input, the third feedback loop comprising a second resistor.    
   
   
       15 . The amplifier circuit of  claim 9 , wherein the pulse width modulator has a low impedance input, and wherein the feedback filter comprises: 
 a fourth feedback loop coupled to the first input, the fourth feedback loop comprising a third resistor; and    a fifth feedback loop coupled to the first input, the fifth feedback loop comprising a first-order high pass filter and a capacitor arranged serially.    
   
   
       16 . An audio system comprising: 
 a coder-decoder coupled to a signal input, the coder-decoder configured to convert an audio input signal provided at the signal input into a digital representation;    an amplifier coupled to the coder-decoder, the amplifier to provide a low output impedance and low distortion amplification of the digital representation of the audio input signal, the amplifier comprising 
 a pulse code modulation to pulse width modulation (PCM-PWM) converter, the PCM-PWM converter configured to convert the audio input signal into a pulse width modulated signal;  
 a feedforward filter coupled to the PCM-PWM converter, the feedforward filter having a first transfer function;  
 an adder coupled to the feedforward filter, the adder configured to generate a difference signal responsive to a signal produced by the feedforward filter and a feedback signal;  
 a pulse width modulator coupled to the adder, the pulse width modulator configured to compare the difference signal to a reference signal and to produce a pulse width modulated signal based on the comparing;  
 a filter coupled to an output of the pulse width modulator, the filter having a second transfer function;  
 a feedback filter having a first input coupled to the output of the feedforward filter and a second input coupled to the input of the feedforward filter, and an output coupled to the adder, the feedback filter configured to generate a feedback signal by applying transfer functions to signals at its inputs, wherein a loop transfer function of the amplifier circuit is minimum aliasing error (MAE); and  
   a transducer coupled to the amplifier, the transducer to convert the amplified audio input signal into audible sounds.    
   
   
       17 . The audio system of  claim 16 , wherein the feedback filter comprises: 
 a first feedback loop coupled to the first input, the first feedback loop comprising a first first-order high pass filter with a pole at frequency P 1  and a second first-order high pass filter with a pole at frequency P 2 , wherein frequency P 2  is higher than frequency P 1 , wherein the second first-order high pass filter terminates at the adder;    a second feedback loop coupled to the first input, the second feedback loop comprising a first resistor; and    a third feedback loop coupled to the second input, the third feedback loop comprising a second resistor.    
   
   
       18 . The audio system of  claim 16 , wherein the pulse width modulator has a low impedance input, and wherein the feedback filter comprises: 
 a fourth feedback loop coupled to the first input, the fourth feedback loop comprising a third resistor; and    a fifth feedback loop coupled to the first input, the fifth feedback loop comprising a first-order high pass filter and a capacitor arranged serially.    
   
   
       19 . The audio system of  claim 16  further comprising a preamplifier coupled between the coder-decoder and the amplifier, the preamplifier configured to increase the power level of the audio input signal to a signal level compatible with the audio system.  
   
   
       20 . The audio system of  claim 16 , wherein the amplifier is a THX Ultra-II compliant class D amplifier.