Amplifiers – Signal feedback – Phase shift means in loop path
Reexamination Certificate
1999-11-02
2001-05-08
Pascal, Robert (Department: 2817)
Amplifiers
Signal feedback
Phase shift means in loop path
C330S251000, C330S20700P
Reexamination Certificate
active
06229390
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to signal processing techniques for providing high fidelity signal amplification. More specifically, the present invention provides techniques by which mixed signal amplification is employed with noise-shaping to generate an output signal with very low distortion.
Both switching and analog amplifiers have applications for which they are considered preferable. For example, because of power dissipation advantages, switching amplifiers are often employed for applications in which the fidelity of the output signal is not the greatest concern. That is, switching amplifiers generally exhibit lower power dissipation when outputting power of an appreciable level, but do not typically match the fidelity of analog amplifiers. Exceptions to this general rule are switching amplifiers provided by Tripath Technology Inc. of Santa Clara, Calif. Signal degradation due to increased harmonic distortion becomes especially pronounced for both switching and analog amplifiers as the output signal swing approaches the power supply rails, although some analog techniques allow rail-to-rail operation. The graph of
FIG. 1
illustrates the effect of output signal swing on the total harmonic distortion of a typical switching amplifier.
The advantages of lower power dissipation are well known and include such things as, for example, smaller heat sinks and power supplies, reduced battery drain and operating temperature, and smaller product size. These significant advantages have led to the widespread use of switching amplifiers in a variety of applications. However, despite the design of some analog amplifiers, there are circumstances in which an analog amplifier may be designed with significantly less power dissipation than an equivalent switching amplifier, e.g., a class AB amplifier with a small bias. This typically occurs at or near quiescence, i.e., when there is little or no input signal but the amplifier remains active. This is due to the fact that, at quiescence, a switching amplifier must still produce a large switching voltage signal while an analog amplifier can “rest.” Thus, for applications in which there is a considerable amount of idle or low power time, the use of analog amplifiers may be preferable. Of course, if the output swing for such an application at any time exceeds a certain level, and thus the power dissipation of the analog amplifier exceeds that of a comparable switching amplifier, the size of the heat sink and power supply must still be such that they could support such a power level on a consistent basis and these advantages are not realized. Furthermore, when a low bias is used, distortion problems can be exacerbated.
One approach to solving this dilemma will now be discussed with reference to the block diagram of FIG.
2
. According to this technique, analog amplifier
202
is employed when there is little or no input signal to take advantage of its low quiescent current. When the output signal swing reaches a certain level, switching amplifier
204
is employed to take advantage of its lower power dissipation for higher output power levels.
Unfortunately, the approach of
FIG. 2
is not feasible for high fidelity applications in which only very low levels of distortion are acceptable. This is because of the distortion in the output signal introduced by the transition between the analog and switching amplifiers. Moreover, such an approach does not address the fact that the fidelity of switching amplifiers drops off dramatically as the output signal swing approaches the power supply rails. In addition, with such an approach, the distortion of the analog amplifier goes uncorrected.
It is therefore desirable to provide a signal processing technique which exhibits the advantages of both switching and low power, high-signal-swing analog amplifiers while maintaining low distortion levels for high fidelity applications.
SUMMARY OF THE INVENTION
According to the present invention, methods and apparatus are provided by which switching and analog signal processing techniques are combined in a signal processing circuit to provide lower power dissipation, increased dynamic range, and lower distortion during different modes of operation. The signal processing circuit of the present invention has both a switching amplifier and an analog amplifier in its power stage which alternate operation depending on the level of the input signal.
When there is little or no signal input, i.e., quiescent or near-quiescent conditions, the analog amplifier is enabled thereby resulting in lower power dissipation by taking advantage of the fact that analog amplifiers typically have lower quiescent currents than switching amplifiers.
However, when the input signal reaches a threshold value, the analog amplifier is disabled and the switching amplifier is enabled to take advantage of the fact that, during periods in which the circuit is outputting power above a certain level, the power dissipation in switching amplifiers is lower than in equivalent analog amplifiers.
According to a specific embodiment, when the input signal level is such that the output of the amplification stage is approaching the power supply rails, the switching amplifier is disabled and the analog amplifier is enabled to take advantage of the lower distortion characteristics of the analog amplifier at such signal levels. This also takes advantage of the fact that, due to the low voltage drop across the output transistors, power dissipation at these signal levels again drops to a level similar to that of a comparable switching amplifier. In this way, various embodiments of the invention enjoy the advantages of both switching and analog signal processing techniques during different stages of operation of the signal processing circuit.
According to a specific embodiment, switching artifact due to the transition between the switching and analog amplifiers is minimized because the signal processing circuit is configured in a feedback loop which employs noise shaping techniques which push the transition distortion out of the band of interest. More specifically, the continuous-time output of the signal processing circuit is fed back to a frequency selective network in the loop for noise and distortion correction. This allows the technique of alternately enabling switching and analog amplifiers to be used even in applications where extremely high fidelity is required. In addition, the fidelity of the output signal during operation of both the switching and analog amplifiers is much higher than a typical switching amplifier or a typical low-power analog amplifier because of the noise shaping introduced by the frequency selective network and the continuous-time feedback.
Thus, according to a specific embodiment, the present invention provides a signal processing circuit which includes a frequency selective network, an amplification stage coupled to the frequency selective network, and at least one continuous-time feedback path from the output terminal of the amplification stage to the frequency selective network. The amplification stage comprises a switching amplifier and an analog amplifier. Switching circuitry alternately enables the switching and analog amplifiers for processing of an input signal.
According to another specific embodiment, the present invention provides a method for processing an input signal using a signal processing circuit which includes a frequency selective network, an amplification stage having a switching amplifier and an analog amplifier, and at least one continuous-time feedback path from the output terminal of the amplification stage to the frequency selective network. The input signal is monitored to determine an input signal level. A noise characteristic associated with the input signal is shaped using the frequency selective network and feedback from the continuous-time feedback path. For a first input signal level, the input signal is processed with the analog amplifier. For a second input signal level, the input signal is processed with the switching amplif
Delano Cary L.
Tripathi Adya S.
Beyer Weaver & Thomas LLP
Nguyen Patricia
Pascal Robert
Tripath Technology Inc.
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