Multi-level class-D amplifier by means of 3 physical levels

[0002] This application is related to U.S. patent applications Ser. No. 10/336,967, filed on Jan. 6, 2003, and Ser. No. 10/350,836, filed on Jan. 24, 2003, and assigned to the same assignee as the present invention.

[0003] (1) Field of the Invention

[0004] The invention relates to a Class-D Power Amplifier, and more particularly, to a Class-D Power Amplifier having a pulse coded digital input signal and typically using an H-Bridge to drive an output load, like a loudspeaker.

[0005] (2) Description of the Prior Art

[0006] Class-AB amplifiers are notoriously inefficient and Class-D amplifiers overcome this shortfall. With Class D amplifiers, the output is made to switch between the two output levels at a very high frequency—substantially higher than the highest audible frequency, which is done by feeding high-frequency pulses to the power amplification stage. Either the pulse-width ratio of the driving signal can be varied at a constant frequency or the pulse density of the driving signal can be varied at a constant pulse width in order to make the averaged (filtered) output signal follow the (amplified) input signal very closely. Such amplifier is referred to as Pulse Width Modulated (PWM) or Pulse Density Modulated (PDM). The output voltage at the load, after passing a low pass filter, represents the input under the assumption of a constant the supply voltage.

[0007] In the case of Pulse Density Modulation, the pulse width is always constant, where the high frequency pulses can be generated by for example a Sigma Delta Modulator. The output device, a Class-D driver, can only provide +V or −V or zero, thus limiting the pulse generation to a maximum of 3 levels. This limited number of levels also limits the signal accuracy.

[0008] FIG. 1 shows a schematic block diagram of a state-of-the-art PDM Class-D Amplifier. It typically comprises a Sigma Delta Modulator (11) to generate the driving signal for the Class-D power output stage, which is typically an H-Bridge (12) and the output load, often a loudspeaker (13).

[0009] FIG. 2ashows a simplified diagram of an H-Bridge and FIG. 2bshows the 3 output signal levels and the corresponding states of the output devices. The output level at the load LOAD is “+V” with Transistor T2 and T3 closed, T1 and T4 open (21); it is “−V” with Transistor T1 and T4 closed, T2 and T3 open (22) and it is “±0” with Transistor T1 and T4 closed, T2 and T3 open (23).

[0010] U.S. Pat. No. (5,949,282 to Nguyen, Huey, Takagishi, Hideto) describes circuit for, first, generating an accurate reproduction of the output of a Class D amplifier for error-correction purposes, and then, second, comparing the reference signal to the original signal input to the amplifier for error-correcting purposes.

[0011] U.S. Pat. No. (5,847,602 to Su, David) shows a delta-modulated magnitude amplifier which is used to amplify the magnitude component of an RF power amplifier that employs envelope elimination and restoration. The delta-modulated amplifier introduces a smaller amount of non-linearity than traditional approaches, which are based upon pulse-width modulation. The disclosed technique can be implemented using switched-capacitor circuits in a standard MOS technology with only two external components, i.e., an inductor and a capacitor. Thus, the disclosed technique allows the implementation of an efficient and yet linear RF power amplifier using low-cost MOS technology.

[0012] U.S. Pat. No. (5,974,089 Tripathi, et al) describes an oversampled, noise shaping signal processor having at least one integrator stage in a feedback loop. A sampling stage in the feedback loop is coupled to the at least one integrator stage. The sampling stage samples an analog signal at a sample frequency. Qualification logic coupled to the sampling stage receives a pulse waveform therefrom, and ensures that signal transitions in the pulse waveform occur more than a first time period apart and that the waveform can therefore be handled by, for example, a power switching device. A switching stage in the feedback loop is coupled to the qualification logic. The signal processor has a feedback path from the output of the switching stage to the input of the at least one integrator stage thereby closing the feedback loop.

[0013] In accordance with the objectives of this invention, a circuit to generate virtual multi-level output pulses for a Class-D Amplifier, where the time-voltage-area corresponds to a multiple of digital levels, is achieved. Multi-level pulse widths allow a better quality output signal. Also, using multi-level pulse widths, in contrast to just a single pulse width, allows the reduction of the pulse-sampling rate by the same factor.

[0014] A Class-D Amplifier using PDM (Pulse Density Modulation) normally converts the input signal with a Sigma Delta Modulator into high-frequency pulses of equal width. And a Class-D amplifier typically uses an H-Bridge with its 3 switching levels (+V, −V, 0) to drive an output load through a low-pass filter. Typical loads are a loudspeaker or a servo-motor.

[0015] The fundamental idea of the disclosed invention is to add the circuits and methods to produce pulses with a multiple of discrete values-of-width and to provide the method to generate a pulse length select signal for these variable-width-pulses. Now the output signal has virtual multi-level pulses with only 3 physical levels.

[0016] To achieve this, the processing unit for the input signal converter, typically containing a Sigma Delta Modulator, not only generates the digital signals “Pulse active” and “Pulse pos/neg”, but also a digital “Pulse length select” signal. A “Length of Pulse Integrator” Function then takes the pulse start information and starts integrating. When the integrated value reaches a specific reference level, selected by the pulse-length-select signal out of a set of pulse area reference values, the integration stops the output signal pulse and generates the pulse stop information. The proposed circuit may contain different techniques to define a set of output pulse area reference values, one for each step of said multi-level output.

[0017] The circuit also comprises a “Pulse Generator Unit” inserted into the signal path between said converter of PCM signals and the Class-D output power stage, which is, as said before, typically an H-Bridge. Said H-Bridge then drives voltage into said output load.

[0018] Further, in accordance with the objectives of this invention, said pulse area reference levels may not only be of fixed level, but may also be externally controlled.

[0019] In accordance with the objectives of this invention, a method to generate virtual multi-level output pulses for a Class-D Amplifier, where the time-voltage-area corresponds to a multiple of digital levels, is achieved. First it converts said input signal into ideal PCM control pulses. In addition to said “pulse active” and “pulse polarity” signal, it also generates said “pulse length select” signal. The proposed method defines said set of output pulse area reference values, one for each step of said multi-level output. The “Length of Pulse Integrator” determines said pulse stop time, based on said pulse start time (clock), on said digital “pulse length select” signal and based on said output pulse area reference values. The “Pulse Generator Unit” generates said multi-level output pulses using said pulse start and pulse stop signals and applies said power driver control pulses to said Class-D power driver. Finally said power driver feeds the output voltage to said output load.

[0020] In the accompanying drawings, forming a material part of this description, there is shown.

[0021] FIG. 1 (Prior Art) shows the schematic block diagram for a prior art solution.

[0022] FIGS. 2aand 2b(Prior Art) shows a principal circuit of an H-Bridge and the output level states.

[0023] FIG. 3 shows the schematic block diagram for a solution, in accordance with an embodiment of this invention.

[0024] FIG. 4 shows the concept of multi-level pulses; the example shows a 5 level system.

[0025] FIG. 5 shows a method to generate virtual multi-level output pulses for a Class-D Amplifier.

[0026] The objectives of this invention are to generate virtual multi-level output pulses for a Class-D Amplifier, where the time-voltage-area corresponds to a multiple of digital levels. A Class-D Amplifier using PDM (Pulse Density Modulation) normally converts the input signal with a Sigma Delta Modulator into high-frequency pulses of equal width. And a Class-D amplifier typically uses an H-Bridge with its 3 switching levels (+V, −V, 0) to drive an output load through a low-pass filter. Typical loads are a loudspeaker or a servo-motor.

[0027] The fundamental idea of the disclosed invention is to add the circuits and methods to produce pulses with a multiple of discrete values-of-width and to provide the method to generate a pulse length select signal for these variable-width-pulses. Now the output signal has virtual multi-level pulses with only 3 physical levels.

[0028] Multi-level pulse widths allow a better quality output signal. Also, using multi-level pulse widths, in contrast to just a single pulse width, allows the reduction of the pulse-sampling rate. In addition, better power efficiency is achieved, as well as lower EMI.

[0029] FIG. 3 shows a schematic block diagram as disclosed in this invention.

[0030] To achieve this, the processing unit (31) for the input signal converter, typically comprising a Sigma Delta Modulator, not only generates the digital signals “Pulse active” and “Pulse pos/neg”, but also a digital “Pulse length select” signal. A “Length of Pulse Integrator” Function (32) then takes the pulse start information and starts integrating. When the integrated value reaches a specific reference level, selected by the pulse-length-select signal out of a set of pulse area reference values, the integration stops the output signal pulse and generates the pulse stop information. The proposed circuit may contain different techniques to define a set of output pulse area reference values, one for each step of said multi-level output.

[0031] The circuit also comprises a “Pulse Generator Unit” (33) inserted into the signal path between said converter of PCM signals and the Class-D output power stage, which is, as said before, typically an H-Bridge (34). Said H-Bridge then drives voltage into said output load.

[0032] Said H-Bridge finally drives the output load (35), typically a loudspeaker.

[0033] FIG. 4 shows the pulse forms for 5 level system. It shows the pulse for the 5 levels “+2” (41), for the “+1” (42), for the “0” (43), for the “−1” (44) and for the “−2” (45).

[0034] The method to achieve the objectives of this invention is illustrated in FIG. 5.

[0035] First, in step 51, the input signal is converted by the Sigma-Delta-Modulator into high frequency Multi-Level-PDM pulses. At the same time, the converter generates the pulse length select signal (52). Now at pulse start time (53), said Pulse-Length-Integrator starts to integrate the time and generates the start pulse signal (54). Now the Pulse-Generator Switch on the control signal to the H-Bridge (55). The pulse area reference value is selected through the pulse length select signal (56). Said Pulse-Length-Integrator comprises the integrated time value with selected pulse length reference value. As soon as the Stop condition reached (58), the Length-of-Pulse integrator stops integrating and generates the pulse stop signal (59) and the control signal to the H-Bridge is switched off (60

[0036] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.