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.

Description

[0001] This application claims the benefit of U.S. Provisional Application No. 60 / 713,039, filed on Aug. 31, 2005, entitled “Passive Loop Filter for Switching Amplifiers Providing Low Distortion and Low Output Impedance,” which application is hereby incorporated herein by reference.TECHNICAL FIELD [0002] The present invention relates generally to a system for audio signal amplification, and more particularly to a system for pulse width modulated class D audio amplifiers. BACKGROUND [0003] There are several different types or classes of audio amplifiers. A class A amplifier will conduct a DC current even though there is no audio signal present at the amplifier's input. The class A amplifier will also have a single polarity voltage swing. A class B amplifier is more efficient than a class A amplifier due to the fact that both of the class B amplifier's differential mode voltage swings are not on at the same time. Class AB amplifiers maintain a small bias current through complementary ...

AI Technical Summary

Benefits of technology

[0020] In accordance with a preferred embodiment of the present invention, an amplifier circuit is provided. The amplifier circuit includes a pulse code modulation to pulse width modulation (PCM-PWM) converter, a feedforward filter coupled to the PCM-PWM converter, an adder coupled to the feedforward filter, a pulse width modulator coupled to the adder, a filter coupled to an output of the pulse width modulator, a feedback filter with a first input coupled to an output of the feedforward filter, a second input coupled to an input of the feedforward filter, and an output coupled to the adder. The PCM-PWM converter converts an input pulse code modulated signal into a pulse width modulated signal, the feedforward filter having a transfer function, the adder generates a difference signal responsive to a signal produced by the feedforward signal and a feedback signal, the pulse width modulator compares the difference signal to a reference signal to produce a pulse width modulated signal based on the comparing, the feedforward filter having a transfer function. The feedback filter generates the feedback signal by applying transfer functions to signals at its inputs, where a loop transfer function of the amplifier circuit is minimum aliasing error.

[0021] In accordance with another preferred embodiment of the present invention, an audio system is provided. The audio system includes a coder-decoder coupled to a signal input, an amplifier coupled to the coder-decoder, and a transducer coupled to the amplifier. The coder-decoder converts an audio input signal provided at the signal input into a digital representation, the amplifier provides low output impedance and low distortion amplification of the digitized audio input signal, and the transducer converts the amplified audio input signal into audible sounds. The amplifier includes a pulse code modulation to pulse width modulation (PCM-PWM) converter, a feedforward filter coupled to the PCM-PWM converter, an adder coupled to the feedforward filter, a pulse width modulator coupled to the adder, a filter coupled to an output of the pulse width modulator, a feedback filter with a first input coupled to an output of the feedforward filter, a second input coupled to an input of the feedforward filter, and an output coupled to the adder. The PCM-PWM converter converts an input pulse code modulated signal into a pulse width modulated signal, the feedforward filter having a transfer function, the adder generates a difference signal responsive to a signal produced by the feedforward signal and a feedback signal, the pulse width modulator compares the difference signal to a reference signal to produce a pulse width modulated signal based on the comparing, the feedforward filter having a transfer function. The feedback filter generates the feedback signal by applying transfer functions to signals at its inputs, where a loop transfer function of the amplifier circuit is minimum aliasing error.

[0022] An advantage of a preferred embodiment of the present invention is that the loop filter of the feedback controlled PWM amplifier is passive in nature, so that problems with active components, such as slew-rate distortion present in operational amplifiers, are avoided.

[0023] A further advantage of a preferred embodiment of the present invention is that the feedback loop takes its feedback signal from after the loop filter. This enables a reduction in the output impedance of the feedback controlled PWM amplifier in the frequency range of relevance (e.g. audio band).

[0024] Yet another advantage of a preferred embodiment of the present invention is that the minimum aliasing error (MAE) property of the loop transfer function helps to ensure that the ripple of the loop filter is triangular, which reduces the total harmonic distortion that is induced by the feedback loop.

[0025] A further advantage of a preferred embodiment of the invention is that the feedback controlled PWM amplifier is stable even without load, therefore, eliminating the need for a Zobel network.

Problems solved by technology

A disadvantage of the LC filter 165 is that it has a frequency response that is a function of the load impedance, i.e., the LC filter 165 has a high output impedance that tends towards infinity at the resonant frequency of the LC filter 165.

In practice, the non-feedback PWM amplifier 150 can be susceptible to non-ideal electrical behavior, such as non-harmonic components, noise and distortion arising from switching delays, non-linearities, distortions in the carrier signal, amplitude distortion, power supply noise and ripple, and so forth.

The non-ideal electrical behavior can cause the PWM modulator 100 to introduce noise and distortion into the output signal.

However, the LC filter 165 can add a large phase delay of up to 180 degrees which can make it difficult to achieve a stable loop when the feedback is primarily taken from after the LC 165.

One disadvantage of the prior art is that the feedback loop takes its signal primarily before the LC filter 165, so the output impedance of the feedback controlled PWM amplifier 175 at high frequencies can be high.

Another disadvantage of the prior art is that a typical loop filter 180 will have active components, such as operational amplifiers, which could have slew-rate distortion.

Yet another disadvantage of the prior art is that aliasing error may not be minimized, resulting in higher than necessary distortion levels.

The aliasing error can increase the overall distortion of the feedback controlled PWM amplifier 175.

A further disadvantage of the prior art is that feedback loops that include the LC filter 165 in the loop can be unstable when the feedback controlled PWM amplifier 175 is not loaded.

However, the Zobel network will dissipate a substantial amount of power when a large amplitude audio signal is being reproduced.

This reduces the overall power efficiency of the feedback controlled PWM amplifier 175 and requires bulky components as well as protection schemes to prevent damage to the components of the Zobel network due to overheating.

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