Multi-level Class-D amplifier by means of 2 physical layers

[00002] (1) Field of the Invention

[00003] 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 a two-level amplifier stage to drive an output load, like a loudspeaker.

[00004] (2) Description of the Prior Art

[00005] Class-AB amplifiers are notoriously inefficient and Class-D amplifiers overcome this shortfall. The common concept of Class D amplifiers is to switch the output between the 2 (or 3) 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 as 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 supply voltage.

[00006] 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, in the most common case can only drive +V or −V, thus limiting the pulse generation to 2 levels. An alternative Class-D driver can drive the output to +V, −V or zero, thus providing a maximum of 3 levels.

[00007] 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).

[00008] FIG. 2a shows a simplified diagram of a Complementary-Pair-Driver and FIG. 2b shows the 2 output signal levels and the corresponding states of the output devices. The output level at the load LOAD is “+V” with Transistor T1 closed, T2 open (21); it is “−V” with Transistor T2 closed, T1 open (22).

[00009] U.S. Pat. No. 6,311,046 (to Dent) describes a circuit with an input signal of varying amplitude and varying phase being converted into more than two signals of constant amplitude and controlled phase. Each of the more than two signals of constant amplitude and controlled phase is then separately amplified in separate amplifiers. The separately amplified more than two signals of constant amplitude and controlled phase are then combined to produce an output signal that is an amplification of the input signal at the desired power level. When converting the input signal into more than two signals, the phase of each of the more than two signals of constant amplitude and controlled phase is controlled to produce the output signal that is an amplification of the input signal at the desired power level. According to another aspect, a signal of varying amplitude and varying phase is generated from a plurality of constant amplitude varying phase signals, the sum of which is the signal of varying amplitude and varying phase.

[00010] U.S. Pat. No. 6,232,833 (to Pullen) shows a low jitter dead time circuit which uses one RC combination to set the turn on delay for both the upper and lower MOSFETs in the half bridge. This circuit minimized jitters in the turn on delay and results in matched turn on delays for both MOSFETs in a half bridge. This minimizes noise and distortion. This circuit is further designed to be used in conjunction with shunt regulators to reject ripple from the power supplies.

[00011] In accordance with the objectives of this invention, a circuit to generate virtual multi-level output pulses for a Class-D Amplifier, where the output has only 2 physical levels (+V, −V), if for example a Complementary-Pair-Driver is used, is achieved. The disclosed invention adds circuits and methods to transform the output from the SDM (Sigma Delta Modulator) into pulses with well-defined time-voltage areas. The disclosed invention further adds circuits and methods for a pulse length control mechanism, that produces output pulse patterns, where the sum over the positive and negative portions of a pulse results in a time-voltage area, corresponding to one of multiple discrete digital levels. In this way a multi level output from the SDM is possible even though the output driver is only able to output two discrete voltage levels.

[00012] Multi-level pulse widths allow a better quality output signal. Also, using multi-level pulse widths, in contrast to just a single pulse widths, pulse-sampling rate by the same factor. In addition, better power efficiency is achieved, as well as lower EMI.

[00013] A Class-D Amplifier using PDM (Pulse Density Modulation) normally converts the input signal with a Sigma Delta Modulator (SDM) into high-frequency pulses of equal width. And a Class-D amplifier in most cases uses a Complementary-Pair Driver with its 2 switching levels (+V, −V) to drive an output load through a low-pass filter. Typical loads are a loudspeaker or a servo-motor.

[00014] The fundamental idea of the disclosed invention is to add the methods and mechanism to transform said output from the SDM into pulses with well-defined time-voltage areas.

[00015] Further, the fundamental idea is to reshape said pulses of well-defined time-voltage area values into a form, suitable for the 2 physical levels of the output driver stage. It will produce pulses, where summing of the area of the positive part and the negative part of a pulse within one sampling period results in the desired time-voltage area. (See FIG. 4a and 4b) The absolute area within one sampling period is then A1 minus A2. Said resulting time-voltage area values correspond to 3 or more digital levels. (See FIG. 5a and 5b).

[00016] To achieve this, the Digital Signal Processing Unit, also containing the input signal converter, which typically is a Sigma Delta Modulator, then transforms the SDM output signal into pulses with multiple of discrete values of well-defined time-voltage areas. It further takes said pulse width information, calculates the required pulse ratio, i.e. the length of the positive and negative part of the pulse within one sampling period.

[00017] The Length of Pulse Integration Unit then determines the point in time when to change the output phase, which defines the end of the first pulse portion and defines the start of the second pulse portion. Said “Length of Pulse Integrator” generates the pulse start and stop information, to be provided to the pulse generator. The proposed circuit may contain different techniques to determine the required pulse ratio as well as the correct point for alternating the output pulse phase.

[00018] 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, in the proposed invention, a Complementary Pair Driver with only 2 physical output levels (or an equivalent circuit). Said Complementary Pair Driver then drives voltage into said output load, possibly passing through some form of a low pass filter.

[00019] Further, in accordance with the objectives of this invention, a set of desired pulse area values may not only be of a priori fixed level, but may also be externally controlled.

[00020] 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 a signal representing multiple pulse area values. Another method defines a set of output pulse-area values, one for each step of said multiple pulse-area values. Each of these pulse-areas corresponds to a level of the multi level SDM signal. A further method determines the required pulse width information by selecting one value that corresponds to a specific pulse-area out of said set of defined output pulse-area values. Another method determines the length-ratio of positive and negative pulse portions within one sampling period. Said methods can be implemented in a “Length of Pulse Integrator” function. Said “Length of Pulse Integrator” Function takes said pulse width information, calculates the required pulse ratio, i.e. the positive and negative part of the pulse within one sampling period, and further determines the point in time when to change the output phase, which defines the end of the first pulse portion and defines the start of the second pulse portion. Said “Length of Pulse Integrator” generates the pulse start and stop information, to be provided to the pulse generator. 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, where the load includes some form of a low pass filter.

[00030] The objectives of this invention are to generate virtual multi-level output pulses for a Class-D Amplifier, where the output has only 2 physical levels (+V, −V). In certain environments, only a 2-level driver, like a Complementary-Pair-Driver or a Push-Pull-Driver can be implemented. The disclosed invention adds circuits and methods to transform the output from the SDM (Sigma Delta Modulator) into pulses with well-defined time-voltage areas. The disclosed invention further adds circuits and methods for a pulse length control mechanism, that produces output pulse patterns, where the sum over the positive and negative portions of a pulse results in a time-voltage area, corresponding to one of multiple discrete digital levels. In this way a multi level output from the SDM is possible even though the output driver is only able to output two discrete voltage levels. 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.

[00031] A Class-D Amplifier using PDM (Pulse Density Modulation) normally converts the input signal with a Sigma Delta Modulator into high-frequency pulses low bit rate pulses. And a Class-D amplifier typically uses an H-Bridge with its 3 switching levels (+V, 0, −V) to drive an output load through a low-pass filter. Typical loads are a loudspeaker or a servo-motor.

[00032] Table FIG. 2b, which shows the states a Complementary-Pair-Driver FIG. 2a can take.

[00033] A typical pulse within a single sample period is shown in FIG. 4a. When the area of the negative pulse A2 is subtracted from the area of the positive pulse A1, as shown in FIG. 4b, it results in a signal with the area A1-A2. As an example a 5-level system is shown: the real 2-level pulses in FIG. 5a and their resulting multilevel pulses in FIG. 5b. It visualizes the real 2-level pulses and their corresponding virtual multilevel pulses for said 5 levels: “+2” (50) (51), for “+1” (52) (53), for “0” (54) (55), for “−1” (56) (57) and for “−2” (58) (59).

[00034] The schematic block diagram in FIG. 3 gives a coarse overview on the concept of adding circuits and methods for a pulse transformation and a pulse length control mechanism, that produces output pulse patterns corresponding to 3 or more discrete digital levels on a system with only 2 physical output levels.

[00035] The fundamental idea is to generate pulses with a variety of well-defined time-voltage areas and then to reshape said pulses to make them suitable for the 2 physical levels of the output driver stage. It will produce pulses, where summing of the area of the positive and the negative pulse within one sampling period results in the desired time-voltage area. (See FIG. 4a and 4b) The absolute area within one sampling period is then A1 minus A2. Said time-voltage area values, corresponds to 3 or more digital levels. (See FIG. 5a and 5b).

[00036] FIG. 6 shows a schematic block diagram as disclosed in this invention. To achieve the objectives, the processing unit for the input signal converter (61), typically comprising a Sigma Delta Modulator, converts the input signal into high-frequency low bit rate signal. Then it transforms the SDM output signal into pulses with multiple of discrete values of well-defined time-voltage areas A “Length of Pulse Integrator” Function (63) takes said pulse width signal, calculates the required pulse ratio, i.e. the positive and negative part of the pulse within one sampling period, and further determines the point in time when to change the output phase, which defines the end of the first pulse portion and defines the start of the second pulse portion. Said “Length of Pulse Integrator” generates the pulse start and stop information, to be provided to the pulse generator. The proposed circuit may contain different techniques to determine the required pulse ratio as well as the correct point for alternating the output pulse phase.

[00037] The circuit also comprises a “Pulse Generator Unit” (64) inserted into the signal path and driving the Class-D output power stage, which is, as said before, a Complementary-Pair-Driver or a Push-Pull-Driver or equivalent (65). Said output driver then drives voltage into said output load possibly through some form of a low pass filter.

[00038] The method to achieve the objectives of this invention is illustrated in FIG. 7. First, in (71), a set of desired pulse area values is defined. In step (72) the input signal is converted by the Sigma-Delta-Modulator into high frequency low bit rate signal. In step (73) it generates a signal representing multiple pulse area values. Another step (74) determines the required pulse width information by selecting one value that corresponds to a specific pulse-area out of said set of defined output pulse-area values and then determines the length-ratio of positive and negative pulse portions within one sampling period. Now at pulse start time (75), said Length of Pulse Integrator starts to integrate the time (76). At the same time the Pulse-Generator switches on the control signal to the Complementary Pair Driver (77). Said Length of Pulse Integrator compares said integrated time value with selected pulse area reference and determines the point in time when to change the output phase, which defines the end of the first pulse portion and defines the start of the second pulse portion (78). Once the Stop condition is reached (79), the Length of Pulse Integrator stops integrating and generates the pulse stop signal (80) and therefore the control signal to the Complementary Pair-Driver is alternated again (81).

[00039] 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.