Patent Description:
The present application relates to amplifiers. More specifically, embodiments of the present invention relate to modulation of the power supply rails for linear amplifiers.

Amplification of audio signals for the purpose of driving monitors or loudspeakers may be accomplished using a variety of amplifier topologies. Linear topologies, particularly class A topologies, are characterized by high fidelity (the high quality reproduction of the audio input signal). However, class A amplifiers can be inefficient, particularly at low signal levels. On the other hand, switching topologies, e.g., class D topologies, can achieve high levels of efficiency for a broad range of signal levels but are typically characterized by lower fidelity sound reproduction.

Attempts to improve the efficiency of some linear topologies (e.g., class A, B and class AB) involve manipulation of the rail voltages between discrete levels (class G) or continuously (class H) based on the input signal level.

<CIT> describes an audio amplifier for amplifying an audio signal, the audio amplifier comprising a first amplifier, a second amplifier, and a selector control, wherein the selector control is configured to select which one of the first amplifier and the second amplifier is used to amplify the audio signal, based upon an indicator signal that specifies a type of audio carried by the audio signal.

<CIT> describes audio power amplifier circuits that are arranged to operate in either a class AB or a class D operating mode. A drive circuit is configured to share control between a class AB driver and a class D modulator/driver circuit. The input signal level is monitored to determine their levels. When the input signal level is below a minimum signal level, the audio power amplifier circuits are operated in a standby mode. The power amplifier circuits are operated in a class AB mode when the input signal levels are in a defined operating range that exceeds the minimum signal level. When the input signal levels exceed a maximum signal threshold, the power amplifier circuits are operated in a class D mode. Hysterisis can be employed to minimize oscillation conditions about any one particular trip-point for the operating modes.

<CIT> describes an audio amplifier having an amplifier section and a power supply. The power supply comprises a positive power supply section and a negative power supply section, each of which provides voltage to the positive and negative power inputs to the amplifier. The control circuitry for the power supply sections causes the voltage output from the power supply sections to vary in accordance with the amplitude of the audio signal so that the power supply voltages track the positive and negative components of the audio signal so that the supply voltages are at a predetermined level above and below the positive and negative components of the audio signal, respectively. Each of the power supply sections has a pair of switches which open and close alternately to transmit power pulses to opposite ends of the primary winding, and the secondary winding in turn transmit power pulses alternately through a filter component to its respective input of the amplifier section.

<CIT> describes a linear power amplifier having a pulse density modulated switching power supply including a power supply providing at least a relatively high DC voltage output; a voltage amplifier connected to an external signal source to amplify a relatively low voltage signal received from the external signal source into a relatively high voltage signal; a current amplifier connected to the voltage amplifier to increase the current flow associated with the relatively high voltage signal, as needed, in order to properly drive a load, wherein the current amplifier is powered by a pulse generator. The pulse generator is connected to a first line carrying the relatively high voltage signal to the load, and to a second line supplying power to the current amplifier. The pulse generator (i) compares the instantaneous voltage amplitude of the relatively high voltage signal with the instantaneous voltage level powering the current amplifier, (ii) provides fixed duration pulses, at the relatively high DC voltage, to power the current amplifier whenever, and so long as, the instantaneous voltage amplitude of the relatively high voltage signal rises high enough relative to the instantaneous voltage level powering the current amplifier to cause the current amplifier to approach saturation, (iii) compares the instantaneous voltage level powering the current amplifier with an internal reference voltage standard, and (iv) provides fixed duration pulses at the relatively high DC voltage to power the current amplifier whenever, and for so long as, the instantaneous voltage level powering the current amplifier is less than the internal reference voltage standard.

Reference will now be made in detail to specific implementations. Examples of these implementations are illustrated in the accompanying drawings. It should be noted that these examples are described for illustrative purposes and are not intended to limit the scope of this disclosure. In addition, specific details may be provided in order to promote a thorough understanding of the described implementations. Some implementations within the scope of this disclosure may be practiced without some or all of these details. Further, well known features may not have been described in detail for the sake of clarity.

The present disclosure describes configurable amplifier systems in which the power supply rail(s) of a linear amplifier, e.g., a class A amplifier, is/are modulated by a switching amplifier(s), e.g., a class D amplifier that may also be configured to operate independently of the linear amplifier. A particular class of such amplifier systems are designed for the amplification of audio signals. Systems enabled by the present disclosure include configurable, multi-channel systems in which some channels may be configured to employ a switching amplifier to drive a load, e.g., a loudspeaker, while others may be configured to drive a load using a linear amplifier, the power supply rail(s) of which are modulated by one or more switching amplifiers. The present disclosure also describes various refinements by which the efficiencies with which various amplifiers operate may be enhanced.

<FIG> is a block diagram of an example of a configurable amplifier system implemented according to a particular embodiment. Amplifier system <NUM> includes a linear amplifier <NUM> and switching amplifiers <NUM> and <NUM>. Linear amplifier <NUM> (which may be, for example, a class A amplifier) is configured to amplify the input signal and drive a load (represented by loudspeaker <NUM>). Switching amplifiers <NUM> and <NUM> are dual-mode amplifiers that may be configured (via the depicted config/comm link) in a first mode (referred to herein as rail modulation mode) to modulate positive power rail V+ and negative power rail V-, respectively, of linear amplifier <NUM>. In a second mode (referred to herein as signal amplification mode), switching amplifiers <NUM> and <NUM> may each be configured to operate independently of linear amplifier <NUM>. For example, each of the switching amplifiers might be configured for driving its own load as represented by loudspeakers <NUM> and <NUM> in dashed lines. In another example, the same load is driven in both modes, e.g. loudspeaker <NUM> is driven by the switching amplifier when it is in signal amplification mode and by the linear amplifier when the switching amplifier is in rail modulation mode.

In an example, when the switching amplifier operates in rail modulation mode, the input signal is amplified by the linear amplifier, wherein the power rails of the linear amplifier are modulated by the switching amplifier. In this example, when the switching amplifier operates in signal amplification mode, the input signal is amplified by the switching amplifier, and the linear amplifier is not used.

In rail modulation mode, switching amplifier <NUM> receives the same digital audio input signal received by linear amplifier <NUM> and generates positive rail V+ through positive peak rectification. Switching amplifier <NUM> generates negative rail V- based on the same input signal through negative peak rectification. Both enforce a nonzero DC bias at their outputs in this mode and may also have a minimum allowable voltage below which (in absolute value) the corresponding rail is not allowed to go. In this way, the supply rails for linear amplifier <NUM> track the input signal, providing sufficient supply voltages to handle large instantaneous signal levels with a low risk of clipping, while greatly reducing power consumption for low signal levels relative to designs with fixed rails.

Linear amplifier <NUM> includes (or is preceded by) delay circuitry <NUM> which delays the input signal before amplification by linear amplifier <NUM> to ensure proper synchronization with the modulation of the supply rails. The delay introduced by delay circuitry <NUM> ensures that the point in the input signal being amplified by the output stage of linear amplifier <NUM> (after conversion by digital-to-analog (D/A) converter <NUM>) is sufficiently aligned with the corresponding point in the modulated rail voltages V+ and V-. The proper delay ensures desired efficiency gains due to the appropriate bias voltage levels being available while also reducing the likelihood of clipping events that might occur if the signals aren't properly aligned. A signal may be provided from linear amplifier <NUM> that indicates an amount of delay for synchronizing with other audio channels or accompanying video.

According to some implementations, the delay represented by delay circuitry <NUM> may be characterized at design time and so be fixed. Alternatively, the delay represented by delay circuitry <NUM> may be programmable. This might enable, for example, system designers and/or installers to test for and set the optimal delay for a given system configuration. For example, during the setup of an audio system, a tone might be used as input to the channel amps and the delay adjusted until the amplified tone is correct. Implementations are contemplated in which delay of the input signal to the linear amplifier is not necessary. In such cases, delay circuitry <NUM> may be configured or programmed to introduce delays that are greater than or equal to zero. Alternatively, delay circuitry <NUM> may have accompanying switching circuitry (not shown) that allows for delay circuitry <NUM> to be bypassed.

In addition, implementations are contemplated in which independently programmable delay circuitry is also provided with the switching amplifiers (not shown). This would provide additional flexibility in manipulating the relative delay between the two signal paths to ensure proper alignment between the rail voltages and the signal being amplified. In any of these cases, a signal may be provided from linear amplifier <NUM> and/or switching amplifiers <NUM> (e.g., via their respective config/comm links) that indicates the current delay(s) for synchronizing with other audio channels or accompanying video. As will be appreciated, delay circuitry may also need to be provided with such other audio channels and/or the accompanying video to support synchronization.

The attack and decay times that characterize the operation of switching amplifiers <NUM> and <NUM> in rail modulation mode may vary depending on the implementation. The attack time should be sufficiently fast in rail modulation mode to ensure proper alignment of the rail voltage and the amplified signal. On the other hand, it is desirable that the rail voltages move as slowly as practicable to avoid an unacceptable level of switching artifacts in the output of the linear amplifier.

There is more flexibility on the decay times which can be slower but at the expense of efficiency. That is, if the signal level drops faster than the rail voltage can follow, the rail voltage (and therefore the power dissipated) will be higher than is needed until the rail voltage catches up. Each system can be designed to achieve a desired balance between efficiency and fidelity. In addition, some implementations may make these parameters programmable by the system installer or end user.

In the example depicted in <FIG>, linear amplifier <NUM> is a class A amplifier, the output stage of which is implemented with output transistors <NUM> and <NUM> in a push-pull configuration between positive and negative power supply rails. However, it should be noted that implementations are contemplated that employ other linear amplifier types and configurations. For example, linear amplifier <NUM> might be implemented as a class A, B, or AB amplifier. In another example, the output stage might be a single-ended configuration rather than push-pull. In yet another example, the output stage might be powered with one power supply rail (positive or negative) referenced to ground with an AC-coupled output. The scope of this disclosure should therefore not be limited to the particular amplifier configuration depicted.

In the example depicted in <FIG>, the modulation scheme employed by switching amplifiers <NUM> and <NUM> results in rail voltages that track the input signal in a continuous manner similar to, for example, class H amplifier topologies. Examples of class H topologies are described in <CIT>, No. <CIT>, No. <CIT>, and No. <CIT>. However, it should be noted that implementations are contemplated in which other modulation schemes are employed. For example, the switching amplifiers might drive the power supply rails of the linear amplifier to a number of discrete voltage levels using some form of stepped or quantized voltage rails. These discrete levels may be coarsely quantized as with, for example, class G topologies, an early example of which is the Dynaharmony HMA <NUM> high-power audio amplifier by Hitachi (<NUM>). Alternatively, the discrete levels may be more finely quantized. The scope of this disclosure should therefore not be limited to a particular rail modulation scheme.

As mentioned above, linear amplifier <NUM> may be a class A amplifier, in which case bias circuitry <NUM> is configured such that that both output transistors conduct <NUM>% of the time. In some implementations, the standing or bias current of the output stage is fixed by bias circuitry <NUM> at about half of the expected peak current. Alternatively, and according to a particular class of implementations, the standing current of the output stage set by bias circuitry <NUM> may also be modulated in response to the input signal as represented by bias control blocks <NUM> and <NUM> which receive the modulated rail voltages V+ and V- as inputs, respectively. This allows for an additional increase in power efficiency on top of that provided by modulation of the power supply rails. An example of an implementation of such bias control circuitry is described with reference to <FIG> and <FIG>.

<FIG> is a block diagram of an example of a dual-mode switching amplifier <NUM> that may be employed with some embodiments. The depicted amplifier topology has features in common with some conventional class D amplifiers. The digital input signal (e.g., an N-bit audio stream) is converted to the analog domain by digital-to-analog (D/A) converter <NUM>, the output of which is integrated by integrator <NUM>. A synchronization (sync) signal is injected into the output of integrator <NUM> for comparison by comparator <NUM>. The output of comparator <NUM> drives output stage <NUM> which drives a load (not shown) via an output LC filter. Feedback is provided via one or more feedback elements depicted in this example as feedback elements <NUM> and <NUM> having respective transfer functions Hi and H<NUM>.

Amplifier <NUM> also includes loop configuration logic <NUM> that is configured to set or control various loop parameters and aspects of the operation of amplifier <NUM>. Logic <NUM> may be programmable and may be implemented with any of a variety of programmable logic devices (e.g., a field programmable gate array or FPGA), firmware controlled devices, discrete circuitry, and software. Logic <NUM> monitors various aspects of loop operation (as represented by dashed input lines) that may include, for example, the digital input signal, the one-bit output of comparator <NUM>, the gate drives of the power transistors of output stage <NUM>, the output signal after the LC filter, load characteristics, etc. Logic <NUM> uses these inputs to control various aspects of loop operation (as represented by dashed output lines) including, for example, synchronization control (via sync signal), duty cycle monitoring and level control, dead-band timing, dynamic loop delay control (e.g., using programmable delay lines (not shown)), controlling the transfer characteristic of integrator <NUM>, manipulation of the transfer functions of feedback elements <NUM> and <NUM>, etc..

Logic <NUM> may be configurable (e.g., via config/comm link) to cause amplifier <NUM> to operate differently in distinct modes of operation such as for example, the rail modulation and signal amplification modes described herein. The way in which logic <NUM> uses its various inputs to control the various aspects of loop operation can be optimized for the desired efficiency and fidelity associated with each mode. For example, at least some of the stringent requirements associated with the high fidelity amplification of an audio signal in signal amplification mode can be relaxed in rail modulation mode to achieve further gains in efficiency beyond the gains realized by rail modulation.

For example, class D amplifiers are characterized by switching losses which correlate with switching frequency. Fidelity also correlates with switching frequency meaning that there is a tradeoff between efficiency and fidelity. For rail modulation mode, logic <NUM> might optimize the switching characteristics of switching amplifier <NUM> for maximum power conversion rather than high fidelity. This might include implementation of so-called "soft switching;" the selection of the optimal times to switch the power transistors of the output stage to minimize their switching losses. That is, in the rail modulation mode, the power transistors could be switched such that they are transitioned at or near "ideal" moments in terms of the instantaneous voltage and current across each transistor as determined by monitoring of the LC output filter. Other examples of loop operation parameters that may be optimized via logic <NUM> for a given operational mode include switching frequency (a lower frequency improves efficiency), attack time, decay time, and soft switching (e.g., Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS)), etc..

In another example, logic <NUM> could enforce a nonzero DC bias on the modulator output in rail modulation mode, e.g., by appropriate signal processing within the switching amplifier signal chain such as, for example, enforcing a minimum duty cycle that yields a net DC output potential, or injecting a DC offset into the front-end of the closed-loop servo.

For example, the switching amplifier may be configured to have a lower switching frequency in rail modulation mode than in signal amplification mode. In another example, the switching amplifier alternatively or additionally may be configured to have a shorter decay time in rail modulation mode than in signal amplification mode. In another example, the switching amplifier alternatively or additionally may be configured to apply soft switching of its output stage in a rail modulation mode, but not in signal amplification mode. In another example, the switching amplifier alternatively or additionally may be configured to enforce a nonzero DC bias at its output stage in rail modulation mode, but not in signal amplification mode.

Implementations enabled by the present disclosure include systems in which only a single channel of amplification is provided as well as systems in which multiple channels are provided and in which at least some level of synchronization among the channels is maintained. Examples of such multi-channel systems include home and professional cinema systems.

Techniques for creating content for cinema involve mixing digital audio signals to generate a digital audio soundtrack for presentation in combination with the visual component(s) of the overall cinematic presentation. Portions of the mixed audio signals are assigned to and played back over a specific number of predefined channels, e.g., <NUM> in the case of Dolby Digital <NUM>, <NUM> in the case of Dolby Surround <NUM>, and as many as <NUM> in the case of Dolby Atmos, all industry standards.

<FIG> shows an example of a cinema environment <NUM> (viewed from overhead) in which a particular implementation may be practiced. A projector <NUM>, a sound processor <NUM>, and a bank of audio power amplifiers <NUM> operate cooperatively to provide the visual and audio components of the cinematic presentation, with power amplifiers <NUM> driving speakers and sub-woofers deployed around the environment (connections not shown for clarity). Sound processor <NUM> may be any of a variety of computing devices or sound processors including, for example, one or more personal computers or one or more servers, or one or more cinema processors such as, for example, the Dolby Atmos Cinema Processor CP850 from Dolby Laboratories, Inc. Interaction with sound processor <NUM> by a sound engineer might be done through a laptop <NUM>, a tablet, a smart phone, etc., via, for example, a browser-based html connection. Audio measurements and sound processing will typically be done with the sound processor which includes analog or digital inputs to receive microphone feeds, as well as outputs to the power amplifiers that drive the speakers.

The depicted environment can be configured via sound processor <NUM> and amplifier configuration interface <NUM> to playback soundtracks having different numbers of audio channels (e.g., <NUM>, <NUM>, <NUM>, <NUM>, etc.), with different subsets of the amplifiers and the speakers corresponding to the different channels. Configuration interface <NUM> (with input, for example, from laptop <NUM>) and appropriate interconnect cabling (not shown for clarity) may configure a subset of power amplifiers <NUM> to drive each subset or array of speakers with the audio for the corresponding channel in accordance with any of a variety of digital audio formats (e.g., Dolby <NUM>, <NUM>, or Atmos).

Power amplifiers <NUM> include switching amplifiers <NUM> and linear amplifiers <NUM>. Switching amplifiers <NUM> might be implemented in individual chassis and enclosures (e.g., for mounting in racks), or included in a single chassis and enclosure (as represented by dashed box <NUM>). Each of linear amplifiers <NUM> may have its own chassis and enclosure (as represented by dashed box <NUM>). Such configurations may allow, for example, existing amplifier products to be repurposed to provide rail modulation to an external linear amplifier. In another alternative, each of linear amplifiers <NUM> may be integrated with a corresponding one of the loudspeakers in cinema environment <NUM> which are represented by the shaded blocks distributed throughout the environment. As yet another alternative, each of linear amplifiers <NUM> may be integrated with one or more of switching amplifiers <NUM>. Other suitable variations on this theme will be apparent to those of skill in the art.

With appropriate interconnect cabling and configuration via interface <NUM>, one or more of switching amplifiers <NUM> may be configured to modulate the supply rail(s) of one of linear amplifiers <NUM>, that can itself be configured to drive one of the speakers in cinema environment <NUM> such as, for example, one of screen monitors <NUM>. As these monitors tend to dominate the sound experience in such an environment, the selection of a linear amplifier might be warranted for higher-fidelity sound reproduction at the expense of power efficiency.

Such a configuration might include one of the switching amplifiers (operating in rail modulation mode) modulating the positive power supply rail of the linear amplifier and another switching amplifier (also operating in rail modulation mode) modulating the negative rail as described, for example, with reference to <FIG>. The configuration of each of the switching amplifiers to operate in an optimized rail modulation mode may be accomplished, for example, as described with reference to <FIG> via configuration interface <NUM> and the config/comm link associated with each amplifier.

In contrast with the screen monitor channels, surround channels, e.g., as represented by overhead (<NUM>), left (<NUM>), right (<NUM>), and rear (<NUM>) monitors, or subwoofers (<NUM>) might warrant a different balance between power efficiency and fidelity, resulting in the use of switching amplifiers <NUM> to directly drive corresponding monitors. The configuration of each of switching amplifiers <NUM> to operate in an optimized signal amplification mode may be accomplished as described, for example, with reference to <FIG> via configuration interface <NUM> and the config/comm link associated with each amplifier. As will be appreciated, implementations in which the operation of each switching amplifier can be optimized for each of its different modes of operation provides considerable flexibility in configuring a multi-channel amplifier system such as that depicted in <FIG>.

According to various implementations, the system depicted in <FIG> may be configured such that one or multiple speakers correspond to a particular audio channel. Moreover, amplifiers <NUM> may be combined (via appropriate interconnect cabling and configuration interface <NUM>) in various ways to support a particular audio channel. For example, when operating in signal amplification mode, switching amplifiers <NUM> may each be configured to amplify the audio signal for a corresponding channel. Alternatively, switching amplifiers <NUM> may be configured to operate in parallel, half-bridge, or full-bridge configurations for a given audio channel. Similarly, linear amplifiers <NUM> (with power supply rail(s) modulated by one or more of switching amplifiers <NUM>) may be configured to operate in parallel, half-bridge, or full-bridge configurations for a given audio channel. In addition, switching amplifiers <NUM> may be configured to operate in parallel, half-bridge, or full-bridge configurations in rail modulation mode to modulate one of the rails of one of linear amplifiers <NUM>. Such configurations may provide, for example, for greater power handling flexibility or high efficiency operation.

Various implementations enabled by the present disclosure allow designers and/or system installers to strike the appropriate balance between amplifier distortion and power efficiency for particular applications. In the case of audio amplification, the use of a linear output stage strikes that balance in favor of higher-fidelity sound reproduction. However, despite the fidelity of even the best linear amplifiers, other factors in the system can still be the source of undesirable artifacts. For example, the transducers in the monitors driven by the audio amplifiers are mechanical systems constructed with paper, rubber, plastics, kapton, aluminum, and a variety of other materials, each having its own resonance and potential to produce undesirable artifacts. Moreover, the production of such artifacts is typically more pronounced at higher power levels.

According to a particular implementation, a linear amplifier (e.g., amplifier <NUM> of <FIG> or <NUM> of <FIG>) may have associated feedback terminals for receiving transducer feedback so that undesirable artifacts from such transducers can be reduced as part of the linearization of the overall system loop in which the amplifier and speaker are included. As will be appreciated by those of skill in the art, there are a variety of ways in which such transducer feedback may be introduced. For example, according to some implementations, the feedback may be introduced using a nested loop that is local to the output stage. Such transducer feedback could be acquired via any of a variety of mechanical, electrical, acoustic, or other mechanisms. Examples include a separate winding adjacent the voice coil, capacitive or other transducers to track the motion of the speaker cone, pressure sensors on the baffle adjacent the cone, etc..

Implementations in which the switching amplifiers and the linear amplifiers are integrated with different chassis present opportunities for placing the linear amplifier in close proximity to the speaker it is driving. That is, if the linear amp enclosure is separate from the switching amplifier(s) modulating its power supply rail(s), the linear amp enclosure can be placed closer, and even immediately adjacent, the speaker it is driving. Thus, its transducer feedback terminal(s) on the exterior of its enclosure may be connected through very short cable(s) to the transducer feedback mechanism(s) associated with the speaker. This may be advantageous in applications such as the cinema environment of <FIG> in that there might be dozens, if not hundreds, of feet of cable between the equipment racks and some speakers. Close proximity means a shorter loop to linearize and therefore better performance. It is also unlikely that the speakers will include active electronics to power any transducer feedback sensors or to transmit the transducer feedback over any significant distance.

According to some implementations, the linear amplifier can be implemented in the same enclosure with the speaker; that is, a powered speaker in which the power is being supplied by external switching amplifiers. This would eliminate the need to bring the transducer feedback of the speaker and the transducer feedback terminals of the linear amplifier outside the enclosure, allowing a wider variety of methods to employ feedback, and potentially improving the linearization of the transducer artifacts.

<FIG> illustrates an example of an implementation of an amplifier <NUM> in which both the supply rail voltages and the bias current of the output stage are modulated. As described above, modulating the rail voltages results in supply voltages that are just large enough to support the swing of the amplified signal. Modulation of the standing current of the output stage further reduces power dissipation.

In the example depicted in <FIG>, the output stage of amplifier <NUM> is implemented with output transistors <NUM> and <NUM> in a push-pull configuration between positive and negative power supply rails V+ and V- and is configured for class A operation in the driving of load <NUM>. However, it should be noted that implementations are contemplated that employ other configurations and biasing schemes. For example, the output stage of amplifier <NUM> might be configured for class A, B, or AB operation. In another example, the output stage might be a single-ended configuration rather than push-pull. In yet another example, the output stage might be powered with one power supply rail (positive or negative) referenced to ground. The scope of this disclosure should therefore not be limited to the particular amplifier configuration depicted.

Amplifier <NUM> includes rail modulation circuitry <NUM> and <NUM> that may be configured to modulate their respective power supply rails according to any of a variety of modulation schemes. For example, a suitable rail modulation scheme might result in rail voltages that track the input signal in a continuous manner similar to, for example, class H amplifier topologies. In another example, the rail modulation circuitry might drive the power supply rails of the linear output stage to a number of discrete voltage levels using some form of quantization circuitry. These discrete levels, or stepped rails, may be coarsely quantized as with, for example, class G topologies, or more finely quantized. The scope of this disclosure should therefore not be limited to a particular rail modulation scheme.

According to a particular subset of implementations, rail modulation circuitry <NUM> and <NUM> may be implemented with switching amplifiers, e.g., class D amplifiers. These amplifiers might be configurable as described elsewhere herein, but might also be purpose-built for rail modulation. Alternatively, rail modulation circuitry may be any of a wide variety of conventional circuits used to modulate the rails of a linear amplifier. Amplifier <NUM> may include (or be preceded by) delay circuitry <NUM> which delays the input signal before amplification by the linear output stage to ensure proper synchronization with the modulation of the supply rails.

According to a particular subset of implementations, the output stage bias current includes a static component and a dynamic component. The static component is set based on the minimum load amplifier <NUM> is expected to drive. For example, if amplifier <NUM> is an audio amplifier, it might be expected to drive loudspeaker loads of <NUM> to <NUM> ohms. If it is known that amplifier <NUM> will not see a load that is lower than <NUM> ohms, the static component of the bias current can be set to support a maximum bias current based on <NUM> ohms rather than a much higher maximum that would be required to drive <NUM> ohms. The dynamic component of the output stage bias current may be based on the modulated power supply rails, modulating the standing current around the level corresponding to the static component. Alternatively, the dynamic component of the output stage bias current may be based on a feed forward signal that corresponds to or is based on the input signal (as indicated by the dashed lines). In addition, both the static and dynamic components of the control of the output stage bias current may be configurable such that one or both may be turned on or off at set up time.

A particular implementation of bias circuitry <NUM> is shown in <FIG>. In the depicted implementation, R4 represents the speaker load (<NUM> ohms in this example), and Q1, Q2, and Q3 form a triple Darlington (e.g., in place of output transistor <NUM> of <FIG>) of which Q3 is the output device that sources current to the load on positive cycles. Q4, Q5, and Q6 form another triple Darlington (e.g., in place of output transistor <NUM> of <FIG>) of which Q4 is the output device that sinks current from the load on negative cycles. V1 and V2 are the positive and negative power supply rails for the output stage which may be, for example, modulated rails from a class D stage in modulator mode as described above (e.g., V+ and V-of <FIG>) R1 and R2, in conjunction with the bias-setting network, determine the bias current in the output stage. The voltage across R3 modulates the voltage across resistors R1 and R2. This, in turn, modulates the bias current in the output stage. Diodes D1-D6 are intended to cancel out the VBE voltage drops of output devices Q1-Q6.

The active components in the dashed-line box include current sources that allow R3 to float with respect to circuit ground. As the positive and negative rails of the output stage are modulated, the current through the branch formed by Q8, Q10, and R5 determines the current that is reflected through the current mirrors (to Q7, Q9 and Q11, Q12). This, in turn, is converted into a voltage across R3. Because R3 is in parallel with R1 and R2, the same voltage modulates the output stage bias current. It should be noted that the circuit depicted in <FIG> may be designed to operate in class A, class B, or class A/B.

Static control of the bias current is set by R5 in shunt with R7 when switch S1 is closed. Multiple switches and resistors may be used if multiple loads impedances are to be supported (e.g., <NUM>, <NUM>, <NUM> ohms, etc.). Knowing maximum output power of the amplifier and the load impedance connected to it, the output bias could be selected to avoid wasting excess energy. Dynamic control of the bias current is set by the gain through the current mirrors which is a factor of R5 (and R7) as well as the design of the current sources.

<FIG> shows a modified version of the circuit of <FIG> in which (optional) operational amplifiers U1 and U2 are used as buffers to ensure that the output stage doesn't load the modulated bias voltage. These may not be necessary for some implementations. In some implementations, U1 and U2 might be replaced with simpler circuits such as, for example, emitter followers.

<FIG> is a circuit diagram that is the basis for amplifier output stage efficiency calculations described below. V2 and V3 are positive and negative voltage rails, respectively, and are generally equal in magnitude but opposite in sign. Q1 and Q2 are output stage devices for conveying power from the power supply (V2, V3) to the load R3 (also to be referred to as RL). In the following examples: V2 = -V3 = 30V; and RL = <NUM> Ohms. The combination of V1, R1, and R2 determine the bias current for the output stage. V1 determines the class of operation of the output stage. V4 is the AC input signal source. For the following calculations, efficiency is determined by the ratio of the RMS power consumed by the load to the average power delivered by the power supply (which includes both rails for a bipolar design).

The calculations below are for traditional static (non-modulated) rail cases (class A and class B) using push-pull output stages as shown in <FIG> for sine waves. These calculations provide the basis for the modulated rail case, and will show that the maximum efficiency that can be achieved with static rails only at maximum output voltages, will be the case for all signal levels for the modulated case.

Class B push-pull efficiency calculations for VOUT = VCC (assume the following variables are the same as the Class A example calculation from above).

With regard to power supply current during Class A operation, current drawn from the power supply is constant, and is determined by the biasing network which is designed with the intended worst-case load in mind. The output devices steer current demanded by the load according to the input signal. However, the power supply sees a constant current. In Class B operation, the power supply sees primarily the load current which flows through the power supply filter capacitors.

The above analyses of the static voltage rail cases for class A and B output stages show two important things. First, efficiency is a function of output signal level and is maximum when the output level is equal to the rail voltage (Vcc, Vee). Second, efficiency is proportional to the RMS-to-peak ratio (inverse crest factor) of the signals being amplified (e.g., Sine wave = <MAT>; <MAT>; Square wave = <NUM>; Pink Noise ~ <NUM> (or higher depending on distribution).

In theory, the modulated rail case then strives to achieve the maximum efficiency case all the time as allowed by these linear push-pull topologies regardless of input signal level. Further losses then depend on the signal crest factors (e.g., see "k" factors shown in calculations above).

For example, to appreciate this improvement, if the input signal RMS level is <NUM>% of the rail voltage <NUM>% all of the time, then the overall efficiency is not much better than <NUM>%. However if modulating the voltage rail, the peak level of the signal can be nearly equal to the rail <NUM>% of the time. The resulting efficiency then will depend on the crest factor of the input signal. Since audio has typical crest factors of 3dB or more, this means an efficiency of nearly <NUM>% for all signal levels when biased for Class A operation.

Moreover, and as stated elsewhere herein, further efficiency improvements can be made by modifying the bias current in one or both of the following manners: (<NUM>) Making a static bias current adjustment based on known load impedance; and/or (<NUM>) Modulated bias current based on rail modulation. Depending on the precision of the bias current modulation and the lower limit allowed for the modulated voltage rails to keep the output devices turned on, even higher efficiencies may be attained.

Claim 1:
An amplifier system (<NUM>, <NUM>), comprising:
a linear amplifier (<NUM>, <NUM>) having a power supply rail; and
a first switching amplifier (<NUM>, <NUM>, <NUM>), the first switching amplifier being configurable into a first mode in which an audio input signal of the linear amplifier is amplified by the linear amplifier and the first switching amplifier operates to modulate the power supply rail of the linear amplifier based on the audio input signal of the linear amplifier, the first switching amplifier being configurable into a second mode in which an audio input signal of the first switching amplifier is amplified by the first switching amplifier,
wherein the first switching amplifier is configured to modulate the power supply rail of the linear amplifier only in the first mode, and
wherein the linear amplifier is not used when the first switching amplifier operates in the second mode.