Reduced crosstalk and matched output power class D audio amplifier with oppositely polarized triangle waves

Abstract

A multi-channel Class D audio amplifier is provided to substantially reduce channel-to-channel crosstalk by employing in each channel a local triangle ramp generator controlled by a single global digital timing signal. The noise critical timing/integrating capacitor for the triangle ramp generator resides locally in each channel and adjacent to the PWM comparator of that channel and referenced to the local ground of that channel. The amplifier can also include a duty cycle limitation circuit to limit output power availability depending on the impedance of any attached loads (speakers).

What is claimed is: 
     
       1. A class D amplifier, comprising:
 two or more pulse width modulator comparators that receive a respective channel of analog audio signals received at respective inputs of the class D amplifier, and compare each of the received respective channel of analog audio signals with respective triangle wave signals and generate respective pulse width modulated signals for each of the received channels of analog audio signals; 
 two or more switching stage amplifiers that receive the two or more pulse width modulated signals and amplifying amplify each respectively according to a predetermined gain based on a channel identity of the received audio channel; 
 two or more filter stages that filter each of the two or more amplified pulse width modulated signals; 
 two or more speakers that receive a respective one of the two or more filtered amplified pulse width modulated signals; and 
 a triangle wave generator that generates the triangle wave signals, the triangle wave generator including a DC servo circuit, the DC servo circuit comprising
 a first resistor connected at a first end to the triangle wave output signal of the triangle wave generator, 
 a first amplifier connected to a second end of the first resistor at an inverting input of the first amplifier, 
 a second capacitor connected to the second end of the first resistor and an output of the first amplifier, and wherein 
 
 the output of the first amplifier is connected to a current adjusting input of a first current source, and wherein
 the DC servo circuit monitors a first DC voltage on the triangle wave, and provides a second, oppositely polarized DC voltage to counteract the first DC voltage on the triangle wave, and wherein
 at least two local triangle waves are generated and wherein 
 a positive going portion of the triangle wave and a negative going portion of the triangle wave are kept substantially equal in duration, amplitude, and slope, by the DC servo circuit. 
 
 
 
     
     
       2. The class D amplifier according to  claim 1 , wherein each of the filter stages comprises:
 a low pass filter, one for each audio channel, each of the low pass filters including an inductor connected at a first end of the inductor to an output of the switching stage amplifier and including a second end that is connected to a first end of a capacitor and respective one of the two or more speakers, and further wherein a second end of the capacitor is connected to a ground. 
 
     
     
       3. The class D amplifier according to  claim 2 , wherein
 the inductor and capacitor are selected according to a desired low pass frequency response of the channel that corresponds to a desired frequency response of the speaker connected thereto. 
 
     
     
       4. A direct current (DC) servo circuit for use with a triangle wave generator in a class D amplifier, the DC servo circuit comprising:
 a first end of a first resistor connected to an output of the triangle wave generator;   an inverting input of a first amplifier connected to a second end of the first resistor; and   a first end of a first capacitor connected to the second end of the first resistor, and a second end of the first capacitor connected to an output of the first amplifier, and wherein
 the output of the first amplifier is connected to a current adjusting input of a first current source, and wherein 
 the DC servo circuit monitors a first DC bias voltage on a generated triangle wave, and provides a second, oppositely polarized DC bias voltage to counteract the DC bias voltage on the triangle wave. 
   
     
     
       5. The DC servo circuit according to  claim 4 , wherein
 the triangle wave generator is adapted to generate a plurality of triangle waves, each of the plurality of triangle waves comprising a positive going portion and a negative going portion, and wherein   the positive going portion of the triangle wave and the negative going portion of the triangle wave are kept substantially equal in duration, amplitude, and slope, by the DC servo circuit.   
     
     
       6. The DC servo circuit according to  claim 4 , wherein the Class D audio amplifier further comprises:
 first and second pulse width modulator comparators that receive, respectively, first and second channel of audio signals and compares each of the received respective channel of audio signals with respective triangle wave signals to generate respective pulse width modulated signals for each of the first and second received channels of audio signals;   first and second switching stage amplifiers that receive, respectively, the first and second pulse width modulated signals and amplify each according to a predetermined gain based on a channel identity of the received audio channel;   first and second filter stages that filter, respectively, the first and second amplified pulse width modulated signals; and   first and second sets of speakers that receive, respectively, first and second filtered amplified pulse width modulated signals.   
     
     
       7. The DC servo circuit according to  claim 4 , wherein each of the filter stages comprises:
 a low pass filter. one for each audio channel. each of the low pass filters including an inductor connected at a first end of the inductor to an output of the switching stage amplifier and including a second end that is connected to a first end of a capacitor and respective one of the two or more speakers, and further wherein a second end of the capacitor is connected to a ground.   
     
     
       8. The DC servo circuit according to  claim 7 , wherein
 the inductor and capacitor are selected according to a desired low pass frequency response of the channel that corresponds to a desired frequency response of the speaker connected thereto.