Audio Amplifier Research Articles

A 4Ω 2.65W Class-D Audio Amplifier With Embedded DC-DC Boost Converter, Current Sensing ADC and DSP for Adaptive Speaker Protection

In this paper a class-D smart speaker driver is presented that can deliver 2.65 W at 1% THD into a 4Ω load. Maximal output power is maintained at low battery voltage by supplying the class-D amplifier from a DC-DC boost converter. Speaker damage is avoided by a speaker protection algorithm that runs on an embedded DSP. The protection algorithm estimates the membrane excursion and voice coil temperature using a speaker model that tracks the speaker impedance which is determined by measuring the speaker current with less than 2% relative error. A sample & hold technique is presented that rejects the load current ripple by sampling at the moments where the instantaneous load current equals the average current. At full output power the combined efficiency of the class-D amplifier and DC-DC boost converter is higher than 80%. The complete system is implemented on a single chip that measures 6.6 mm2 and is fabricated in a 0.14 μm CMOS technology with a 5 V gate-oxide option.

A THD-reduction high-efficiency audio amplifier using inverter-based OTAs with filter-output feedback

This paper presents an integrated low-voltage THD-reduction high-efficiency class-D audio amplifier using inverter-based operational transconductance amplifiers (OTAs). We propose a negative feedback loop which can compensate for external perturbations and improving output precision. The compensator increases the audio-frequency loop gain, and leads to a better rejection of audio-frequency disturbances. The use of inverter-based OTA and comparator provides low-voltage operation and low-power dissipation. The audio amplifier operates with a 1.5V supply voltage with maximum power efficiency of 90%. The proposed class-D amplifier was implemented using a TSMC 0.18-μm 1P6M CMOS process, and the active chip area is 1.87mm2.

The Stingray embedded acoustic instrument

Many traditional acoustic musical instruments are convenient to use: a performer picks one up, provides an energetic excitation, and the resulting sound radiates immediately. No wires, protocols, or software updates are ever required. The same cannot be said for the vast majority of prior digital musical instruments. The present work addresses how to endow a digital musical instrument with the convenience, look, feel, and personality of a traditional acoustic musical instrument via enclosure prototyping techniques, audio amplification, and embedded computation. For example, the Stingray is an embedded acoustic instrument. Although its battery needs to be occasionally charged, it otherwise can give the impression of a traditional acoustic musical instrument. The control inputs for the Stingray include a piano keyboard and force-feedback motorized faders. The faders allow the performer to interact expressively with the sound, while the piano keyboard enables the precise selection of notes. If desired, the Stingray can be programmed using physical models, including models of hypothetical acoustic instruments that would be infeasible to build physically. In this configuration, the Stingray expressively transforms the performer's gestures into radiated sound in an energy-conserving manner.

A digital input class-D audio amplifier with sixth-order PWM

A digital input class-D audio amplifier with a sixth-order pulse-width modulation (PWM) modulator is presented. This modulator moves the PWM generator into the closed sigma—delta modulator loop. The noise and distortions generated at the PWM generator module are suppressed by the high gain of the forward loop of the sigma—delta modulator. Therefore, at the output of the modulator, a very clean PWM signal is acquired for driving the power stage of the class-D amplifier. A sixth-order modulator is designed to balance the performance and the system clock speed. Fabricated in standard 0.18 μm CMOS technology, this class-D amplifier achieves 110 dB dynamic range, 100 dB signal-to-noise rate, and 0.0056% total harmonic distortion plus noise.

A design methodology for high-order class-D audio amplifiers

This paper presents a design methodology for high-order class-D amplifiers, based on their similarity with sigma---delta ( $$\Upsigma\Updelta$$ Σ Δ ) modulators, for which established theory and toolboxes are available. The proposed methodology, which covers the entire design flow, from specifications to component sizing, is validated with three design examples, namely a second-order, a third-order, and a fourth-order class-D amplifier. Moreover, the third-order class-D amplifier has been integrated on silicon and characterized, further confirming the validity of the whole design flow. The achieved results demonstrate that high-order class-D amplifiers can achieve total-harmonic-distortion (THD) performance compatible with the specifications of high-end audio applications (THD ? 90 dB), which would be unfeasible with conventional first-order class-D amplifiers.

A Class-D Amplifier for a Digital Hearing Aid with 0.015% Total Harmonic Distortion Plus Noise

A class-D audio amplifier for a digital hearing aid is described. The class-D amplifier operates with a pulsecode modulated (PCM) digital input and consists of an interpolation filter, a digital sigma-delta modulator (SDM), and an analog SDM, along with an H-bridge power switch. The noise of the power switch is suppressed by feeding it back to the input of the analog SDM. The interpolation filter removes the unwanted image tones of the PCM input, improving the linearity and power efficiency. The class-D amplifier is implemented in a 0.13-μm CMOS process. The maximum output power delivered to the receiver (speaker) is 1.19 mW. The measured total harmonic distortion plus noise is 0.015%, and the dynamic range is 86.0 dB. The class-D amplifier consumes 304 μW from a 1.2-V power supply.

Wavelet-Based Approach to Evaluation of Signal Integrity

In this paper, we present a new approach to evaluation of signal integrity that is based on signal energy density as a function of time and frequency, represented by its wavelet scalogram. Using signal integrity ratio and cumulative energy ratio, we illustrate signal integrity analysis with simulated examples, followed by the demonstration of their usefulness through analysis of experimental data of a real audio amplifier. These figures of merit represent the extent to which the integrity of a signal is diminished by the electromagnetic interference effects and/or nonlinear processes.

Design of Power Supply for Intelligent Power Amplifier Based on DSP

This paper designs a new type of the control system of switching power supply for audio power amplifier, which is used in DSP-controlled power amplifier. The basic frame and the software and hardware design of the system are described. The system uses TMS320F2812 as the main controller, to reduce the hardware circuit structures, and realize the audio power amplifier digital control. The experimental results show that the design has good regulator function, which not only satisfies certain control accuracy, but also meets the real-time requirements, and has great future in high audio power amplifier.

A Novel 3D Filter-less Class-D Audio Amplifier with Stereo Enhancement

This paper presents a novel 3D filter-less class-D audio amplifier with the stereo enhancement structure. The principle of this stereo enhancement scheme has been analyzed. It shows that the amplifier with this proposed circuit has a better live effect than the traditional class-D audio amplifiers. In this amplifier, the input audio signal from one channel can be amplified in two channels at the same time by way of cross-coupling. The ratio of gains in the two channels also can be adjusted by changing the external resistance, which improves the stereo effect. A practical implementation in the SMIC CMOS 0.18-μm process has been done to validate the theoretical results. When a sine signal with 1 kHz frequency and 100 mV amplitude is applied in one channel and a DC applied signal in the other, the amplifier demonstrates a gain difference of 9.4 dB. A double-sided natural sampling PWM is used in the proposed class-D amplifier to eliminate low-pass filter. The experimental result shows that the amplifier can deliver 2.5 W × 2 into 2 Ω loads at 3.3 V power supply and the THD + N is less than 0.1% without the external filters.

Multiplierless Implementation of an Aliasing-Free Digital Pulsewidth Modulator

Digital pulsewidth modulators are used to transform nonconstant amplitude signals into pulsed signals, such that the information lying in the signal amplitude is encoded in the widths of pulses. Because of the inherent aliasing distortion in digital pulsewidth-modulated signals, additional signal processing steps are required to make pulsewidth modulation (PWM) suitable for applications like digital audio amplification or burst-mode radio-frequency transmitters. These processing steps, however, entail an undesirable increase in computational effort. This brief presents a multiplierless implementation of a digital aliasing-free pulsewidth modulator using lookup tables, adders, and arithmetic shifts only. Mathematical equations of asymmetric double-edge PWM are given, as well as a modified aliasing-free version of this PWM technique that directly integrates the distortion-avoiding signal processing steps into the pulsewidth modulator. Based on these equations, a multiplierless implementation of the aliasing-free PWM (AF-PWM) is developed. Simulation results obtained with a Simulink fixed-point model show that the proposed modulator implementation provides a feasible solution for realizing AF-PWM with low computational effort.

Note: A portable magnetic field for powering nanomotors, microswimmers, and sensors

Time-varying magnetic fields are the basis of many modern devices and are used to remotely power and steer nanomotors and microswimmers. However, the required magnetic field setups are often prohibitively bulky laboratory setups that require technical expertise to build, modify, or relocate. Here we introduce a programmable magnetic field setup based on consumer electronics that is both portable and easy to use. The complete setup consists of a laptop computer, an audio amplifier, and audio inductors and was used to create complex magnetic fields in 0.5-2000 Hz frequency range with up to 4.7 mT amplitude. The setup was also validated using an example application, namely a rotating magnetic field with a constant amplitude and fixed frequency, which has applications in powering nanosensors and microswimmers.

A Low-EMI 3-W Audio Class-D Amplifier Compatible With AM/FM Radio

A 3-W audio class-D amplifier compatible with AM/FM radio while requiring minimal external filtering is presented. The amplifier uses driver slew rate control to reduce electromagnetic interference (EMI) in the FM band and digital modulation to spread the PWM common-mode signal and place a null at the tuned AM channel. Two original feedback techniques are employed to linearize the H-bridge. One cancels signal-dependent propagation delays in the H-bridge to improve harmonic distortion and eliminate high-frequency noise folding into the audio band. The second technique boosts the power supply rejection ratio to simplify and reduce the cost of supply filters. The amplifier system, implemented in a standard 110-nm CMOS process, operates with a supply voltage ranging from 2.7 to 6.6 V and achieves an SNR of 95 dB and 85% efficiency.

Class-D Amplifier Power Stage With PWM Feedback Loop

This paper presents a second-order pulsewidth modulation (PWM) feedback loop to improve power supply rejection (PSR) of any open-loop PWM class-D amplifiers (CDAs). PSR of the audio amplifier has always been a key parameter in mobile phone applications. In contrast to class-AB amplifiers, the poor PSR performance has always been the major drawback for CDAs with a half-bridge connected power stage. The proposed PWM feedback loop is fabricated using GLOBALFOUNDRIES' 0.18-μm CMOS process technology. The measured PSR is more than 80 dB and the measured total harmonic distortion is less than 0.04% with a 1-kHz input sinusoidal test tone.

Performance comparison of integrated fully-differential filterless class-D amplifiers with different feedback techniques

Two integrated stereo fully differential filterless class-D amplifiers are presented in this paper. The object is to develop a modulation of a class-D audio amplifier with high power efficiency in this paper. The traditional H-bridge class-D audio amplifier has a shortcoming of large signal distortion which is worse than realized. However, the proposed circuit improves the drawback and provides high power efficiency at the same time. The circuit implements a modified scheme of pulse-width modulation. In this paper, we presented two class-D amplifiers, compared their differences and explained why the efficiency and distortion performance can be modified. The increase in total harmonic distortion (THD) is due to non-linearity in the triangle wave. To overcome this problem, a negative feedback from the output of the switching power stage is adopted to reduce the THD. When a 0.7-VPP and 1 kHz sine wave is used as an input signal, the minimum THD is 0.029 % and the maximum power efficiency is 83 %. The fully differential class-D audio amplifier is implemented with a TSMC 0.35-μm 2P4M CMOS process, and the chip area is 2.57 × 2.57 mm2 (with PADs).

Modelling of Non–Linear Distortion in Vacuum Triodes Using Trans–Characteristics Inverse and Newton’s Method

The increased interest in vacuum tube audio amplifiers led to an increased interest in mathematical modelling of such kind of amplifiers. The main purpose of this paper is to develop a novel global numerical approach in calculation of the harmonic distortion (HD) and intermodulation distortion (IM) of vacuum-triode audio amplifiers, suitable for applications using brute-force of modern computers. Since the 3/2 power law gives only the transcharacteristic inverse of a vacuum triode amplifier, unknown plate currents are determined in this paper iteratively using Newton’s method. Using the resulting input/output pairs, harmonic distortions and intermodulations are calculated using discrete Fourier transform and three different analytical methods.

Dynamic voltage scaling for series hybrid amplifiers

We present an optimization of the voltage scaling algorithm in low power audio class-G amplifier for headphones application to allow longer playback time. The optimization approach minimizes the voltage difference between the internal audio amplifier power supply and its output signal over a large range of operating conditions. The modeling is based on a behavioral model enabling accurate and rapid evaluation of efficiency and audio quality with realistic input stimuli. The model validated in practice is used to optimize the voltage scaling using only few power supply levels. Thanks to a global search algorithm followed by a local one, the optimization gives the better parameters for voltage scaling algorithm while keeping a good audio quality. The proposed configuration increases the efficiency up to 48% at nominal operation.

Single-Inductor Bipolar-Outputs Converter for the Supply of Audio Amplifiers in Mobile Platforms

Listening music with a headset is now a classical feature of mobile phones. A linear amplifier is still employed to drive the headset as the audio quality must challenge dedicated audio players with somewhat 100 dB of signal-to-noise ratio. This feature must also have the lowest impact on the platform autonomy, size, and cost. A symmetrical power supply is required by such audio amplifiers due to the standard jack connector of headsets. Using an inductive switch mode power supply (SMPS) is the most energy efficient solution to supply audio amplifiers from a battery. Classically, two dc/dc converters are employed to generate the required two symmetrical supply rails from a standard Lithium-Ion battery. It is at the cost of numerous bulky and expensive external components. A solution to reduce the number of external passive components is to consider a single-inductor, bipolar-outputs, (SIBO) dc/dc converter. An early prototype of such a converter has been realized on a 130-nm silicon process and offers primary results, nonoptimal but promising. The feedback control structure is studied based on a small signal model that serves to define the pairing of the loops for designing a decentralized controller. A set of controllers is designed by a pole placement technique and transient performances are demonstrated on audio patterns thanks to a piecewise linear model of the converter. Eighty percent peak efficiency is measured with the primary demonstrator but latter simulation results yield an expected improvement to 90%. The audio specifications appear very constraintful and the proposed SMPS does not meet entirely the latter figures. Limitations are detailed. However, many other applications can benefit from the proposed SIBO dc/dc converter.

Integrated Pop-Click Noise Suppression, EMI Reduction, and Short-Circuit Detection for Class-D Audio Amplifiers

Circuit techniques that overcome practical noise, reliability, and EMI limitations are reported. An auxiliary loop with ramping circuits suppresses pop-and-click noise to 1 mV for an amplifier with 4 V-achievable output voltage. Switching edge rate control enables the system to meet the EN55022 Class-B standard with a 15 dB margin. An enhanced scheme detects short-circuit conditions without relying on overlimit current events.

MOTIONAL FEEDBACK SYSTEM

An audio amplifier is operated in an audio system with optimized source impedance to minimize distortion in a loudspeaker that is paired with the audio amplifier. The audio amplifier provides an amplified audio signal to drive the loudspeaker. A variable output impedance of the audio amplifier is controlled using a feedback control loop to allow negative output impedance at low frequencies that changes to positive output impedance at higher frequencies. The change from negative output impedance to positive output impedance occurs at a determined threshold frequency or determined transitional frequency band.

Analytical Modeling of the Effect of Nonlinear Switching Transition Curves on Harmonic Distortion in Class D Audio Amplifiers

Practical switching transition times typically occupy less than one percent of the switching period in class D audio amplifiers. These transitions, however, are a very nonlinear function of current, which give rise to significant levels of distortion. This paper presents a method for determining the effect of nonlinear switching transition curves on harmonic distortion within half-bridge and full-bridge topologies, taking into account the interaction between the various dominant pulse-timing errors. An accurate analytical model, based on the well-established double Fourier series method of analysis, for determining the effect on harmonic distortion is introduced. The analytical model is accompanied by an equivalent simulation model using an existing strategy, and the results are verified experimentally.