Over the past few years, significant advancements have been made in the field of audio amplifiers. Improvements have been made in areas such as size, weight, energy efficiency, cost, sound quality, and product life. Advancements in the area of energy efficiency have been particularly sought after, since the efficiency of the amplifier significantly impacts other circuit and system characteristics such as size, weight and cost of manufacture.
One development that has significantly improved audio amplifier efficiency and thus provided circuits of reduced weight and size is the use of arrangements in which the amplifier operating potentials (typically referred to as "rail voltages") are provided by a "tracking power supply." That is, the output of the power supply varies in accordance with the audio input signal so that the magnitude of the power supply output voltage exceeds the voltage needed by the amplifier by a predetermined voltage offset. Thus, the instantaneous power provided to the amplifier by the power supply exceeds the amplifier output power by a relatively small amount. As a result, the energy dissipated by the amplifier is minimized which allows both the amplifier and the tracking power supply to be mounted in a small enclosure or packaged as a single printed circuit board.
A general discussion of audio amplifier tracking power supplies is provided in U.S. Pat. No. 5,396,194, to Williamson et al., FIGS. 1A, 1B and 1C of which are included herein as FIGS. 1A, 1B and 1C. These figures illustrate the positive and negative amplifier operating potentials (referred to in Williamson et al. as "source" and "sink" voltages) of three types of tracking power supplies referred to as: (1) envelope trackers; (2) rail-to-ground trackers; and (3) rail-to-rail trackers. In FIGS. 1A-1C, the middle signal depicts the amplifier output signal, the upper signal is the positive power input to the audio amplifier and the lower signal is the negative power input to the audio amplifier.
As illustrated in FIG. 1A, in an envelope tracker the magnitude of both the positive and negative power inputs for the audio amplifier increase and decrease in accordance with the magnitude of the audio output signal. That is, the positive operating potential supplied by an envelope tracker is a positive voltage that is equal to the magnitude of the audio amplifier output signal plus a voltage offset (V.sub.OFFSET in FIG. 1A) and the negative operating potential is a negative voltage that is equal to the magnitude of the audio output signal minus a voltage offset. Thus, the maximum differential between the voltages supplied to the positive and negative power inputs occurs when the audio signal is at maximum amplitude, with the difference between the potential supplied to the amplifier positive power input and negative power input being twice the amplitude of the amplifier output signal plus twice the voltage offset.
As illustrated in FIG. 1B, in a rail-to-ground tracker, the positive power input for the audio amplifier tracks the positive portions of the audio signal and the negative power input for the audio amplifier tracks the negative portions of the audio signal. The resulting difference between the positive and negative power inputs at any point in time is thus equal to the instantaneous output of audio amplifier plus two times the voltage offset.
As illustrated in FIG. 1C, a rail-to-rail tracker supplies the positive and negative power inputs to the audio amplifier so that the positive and negative supply voltages both track the audio output signal. That is, the positive supply voltage is equal to the amplifier output voltage plus a voltage offset and the negative supply voltage is equal to the amplifier output voltage minus a voltage offset. Thus, the differential between the two supply voltages is substantially equal to two times the voltage offset for all values of an audio input signal.
With respect to efficiency, envelope trackers are generally less efficient than rail-to-ground and rail-to-rail trackers with rail-to-rail trackers generally being the most efficient. However, as noted in the Williamson et al. patent, rail-to-rail trackers are not commercially available, thus making the rail-to-ground trackers generally the most efficient type of tracking power supply on the market.
An exemplary envelope tracker is illustrated in U.S. Pat. No.4,218,660 to Robert W. Carver, FIGS. 1, 2 and 4 of which are included herein as FIGS. 2, 3A, and 3B, respectively.
As illustrated in FIG. 2, the arrangement of U.S. Pat. No. 4,218,660 includes a speaker 10, which is driven by an amplifier 12 having two signal input terminals 20 and 22. The power supply circuitry in FIG. 2 includes a transformer 14 and a switch 26 that controls the flow of current through the primary winding of the transformer 14. Switch 26 is controlled by a ramp time modulator 40. The ramp time modulator 40 receives a periodic triangular waveform from a square wave to triangular wave converter 38, which converts a constant series of pulses from a pulse generator 36. Ramp time modulator 40 also receives an input from a comparator 32, which compares a feedback signal from the amplifier 12 to a signal derived from the audio signal being amplified. A power output feedback circuit 34 provides the feedback signal to one input terminal of the comparator 32, while an absolute value detector 28 and a non-linear peak detector 30 provide a signal representative of the audio signal to the second input terminal of the comparator 32.
FIGS. 3A-3I illustrate typical signals for the circuitry of FIG. 2. FIG. 3A depicts a typical audio signal that is supplied both to the amplifier input terminals 20 and 22 of the amplifier 12, and to the absolute value detector 28. In FIG. 3B, the audio signal is shown as it appears at the output of the absolute value detector 28. The output of the non-linear peak detector 30 is illustrated as the solid line in FIG. 3C and can be seen to track the audio signal of FIG. 3B. The signal from the non-linear peak detector 30 is compared with a signal from the power output feedback 34 (the dotted line in FIG. 3D) by the comparator 32 to determine the required operating potential for the amplifier 12. The solid line in FIG. 3E represents the output of the comparator 32 and illustrates the manner in which the power supply circuitry in effect "tracks" the audio input signal and maintains the power inputs above required levels by a predetermined amount. In practice, the quiescent voltage drop across each of the transistors 16 and 20 is typically on the order of approximately 5 volts.
FIG. 3F depicts the output of pulse generator 36 which functions to generate a rectangular pulse signal at a predetermined pulse frequency and constant voltage. This signal essentially serves as a clock signal that is supplied to the square wave to triangular wave converter 38. As illustrated by the solid line of FIG. 3G, square wave to triangular wave converter 38 converts the signal shown in FIG. 3F to a periodic triangular signal in which the magnitude of the time rate of change for positive and negative-going portions of the signal is substantially identical.
The output of the square wave to triangular wave converter 38 is coupled to the ramp time modulator 40, which also receives the control signal from the comparator 32. Shown in FIG. 3G is a portion of the control signal from the comparator 32 (illustrated in broken lines) superimposed with the triangular waveform supplied by the converter 38. In effect, the interception of the control signal supplied by the comparator 32 with the triangular wave supplied by square wave to triangular wave converter 38 define switching points that cause the ramp time modulator 40 to produce a pulse-width modulated signal of the type shown in FIG. 3H. As can be seen in FIG. 3H, the pulse frequency of the pulse-width modulated signal is the same as the signal supplied by the pulse generator 36 and the duration of each pulse is determined by the interception of the control signal from comparator 32 with the triangular wave supplied by converter 38. Specifically, the width of each pulse is equal to the time that elapses between intercept points on adjacent negative and positive slopes of the triangular wave.
In the circuit of FIG. 2, the pulse-width modulated signal of FIG. 3H activates the switch 26 to cause current pulses in the primary winding 14a of the transformer 14 such as those illustrated in FIG. 3I. In that regard, it can be seen that a pulse of relatively short duration (e.g., the pulse indicated by reference numeral 42 in FIG. 3H) produces a corresponding transformer current pulse 44 of relatively a small magnitude, since a relatively short time period is available for the current to build or "ramp up". As the voltage pulses of FIG. 3H increase in duration, the magnitude of the current pulses in the transformer primary winding 14a increase correspondingly, with pulse 46 (the longest duration in FIG. 3H) producing a current pulse 48 of the largest magnitude of those shown in FIG. 3I. Rectification and filtering on the secondary side of transformer 14 provides the output transistors with positive and negative operating potentials that track the audio input signal.
More specifically, in the arrangement of FIG. 2, transistors 16 and 18 are connected in a conventional push-pull configuration in which the collector of transistor 16 is connected for receiving a positive operating potential and the collector of transistor 18 is connected for receiving a negative operating potential. Connected to the collectors of transistors 16 and 18 are half wave rectifier circuits that receive a stepped-up input signal of the type shown in FIG. 3I. Because the signals supplied to the half wave rectifier circuits are provided by a grounded center tap secondary winding of transformer 14 and because of the manner in which the rectifier diodes are poled, positive operating potential is supplied to the collector of transistor 16 and negative operating potential is supplied to the collector of transistor 18. Moreover, since the energy in each pulse of current in the primary winding of transformer 14 is representative of the amplitude of the applied audio signal, the positive and negative operating potentials supplied to transistors 16 and 18 substantially track the audio input signal with the magnitude of the operating potentials being primarily determined by the turns ratio of transformer 14.
Another type of tracking power supply is illustrated in U.S. Pat. No. 4,484,150, to Carver in which the voltage levels provided to the power inputs of the audio amplifier increase and decrease in discrete steps in accordance with the amplitude of the audio input signal and, hence, the amplitude of the audio output signal. In one embodiment, a 12-step system is used to approximate the shape of the audio input signal. U.S. Pat. No. 5,748,753, to Carver discloses another type of tracking power supply that is powered by conventional AC power (e.g., 115 volts, 60 Hertz). In the arrangement of U.S. Pat. No. 5,748,753, the AC power signal is stepped down, rectified and controlled to provide operating potentials that track the audio input.
Rail-to-ground tracking power supplies are disclosed in U.S. Pat. No. 4,054,843 to Hamada, U.S. Pat. No. 4,409,559 to Amada, and U.S. Pat. No. 4,507,619 to Dijkstra. The Dijkstra patent discloses an exemplary rail-to-ground tracking power supply that will be discussed in more detail below. The sole figure of the patent to Dijkstra is included herein as FIG. 4.
As illustrated in FIG. 4, the signal from a signal source 101 is supplied to the noninverting input of a control amplifier 102; the power supply terminals of which are connected to a terminal 103 for receiving a positive operating potential and a terminal 104 for receiving a negative operating potential. The complementary outputs 102-1 and 102-2 of the control amplifier 102 provide input signals to transistor output stages 105 and 106, which are of a conventional configuration. A loudspeaker 107 is connected between the output of transistor output stages 105 and 106 and circuit common (ground).
As is typical with rail-to-ground power supplies, separate circuits are used to provide the positive and negative operating potential for the control amplifier 102. In the arrangement shown in FIG. 4, the audio input signal is amplified by an amplifier 109. Positive portions of the signal supplied by the amplifier 109 are coupled through a diode 112 and provided to a series of components 116, 119, 120, 121, 122, and 123, which provide the positive supply voltage for the output stage 105. The negative portion of the output signal of the amplifier 109 is coupled through a diode 113 and processed by the circuit shown in the lower portion of FIG. 4, the components of which correspond on a one-to-one basis with the circuit shown in the upper portion of FIG. 4. Although arrangement of FIG. 4 provides the positive and negative operating potentials required by the depicted transistor output stages 105 and 106, it should be noted that the magnitude of the supplied positive and negative voltages cannot exceed the positive and negative operating potentials of amplifiers 102, 109, and 116 (i.e., the positive and negative voltages supplied to terminals 103 and 104 in FIG. 4). Thus, the arrangement disclosed by the Dijkstra patent is not directly suitable for use in automotive sound systems and other arrangements in which the voltage provided by a battery or other power source is less than the peak-to-peak voltage of the audio output signal required for the desired maximum output power.
Prior art rail-to-ground tracker arrangements such as those disclosed in the previously mentioned Hamada and Amada patents provide amplifier operating potentials that track the audio input and are capable of providing operating potentials that exceed the operating potential supplied to the tracker arrangement. However, transformers are used to obtain the increased operating potentials, thus adding substantial weight, size and cost to the overall arrangement. Moreover, such arrangements require relatively complex circuit arrangements to control and process the signal supplied to and provided by the transformer. Largely because of size, weight and cost, tracking power supplies (and, in particular rail-to-ground trackers) have not been incorporated in automotive sound systems or other applications that are powered by a battery or other relatively low voltage supply.
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