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Audio Power Amplifier (APA) Operation and Measurement. Stephen Crump http://e2e.ti.com Audio Power Amplifier Applications Audio and Imaging Products 18 August 2010. Contents. Audio Power Amplifier Operation Class-D APA Operation Measuring Class-D and Class-AB Outputs.
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Audio Power Amplifier (APA) Operation and Measurement Stephen Crumphttp://e2e.ti.comAudio Power Amplifier ApplicationsAudio and Imaging Products18 August 2010
Contents • Audio Power Amplifier Operation • Class-D APA Operation • Measuring Class-D and Class-AB Outputs
Audio Power Amplifier Operation APA ClassesInput ConfigurationsOutput ConfigurationsFully Differential APAs
Audio Power Amplifier Classes • There are two classes of audio power amplifiers in common use. • Class-AB – continuous output. The traditional configuration. • Class-D – switching output. We will examine Class-D in detail later. • Class-D output is the short-term average of the switching waveform.
Advantages and Disadvantages • Advantages and disadvantages of Class-AB. • Simple application. • Inexpensive (but not necessarily in SYSTEM cost). • Low efficiency, high power drain and heat generation. • Advantages and disadvantages of Class-D. • High efficiency, low power drain and heat generation. • Somewhat more expensive (but not in SYSTEM cost). • More complicated application. • Class-D advantages usually are compelling.
APA Input Configurations • There are two common input configurations. • Single-ended – single input line referred to ground. Traditional configuration. • Differential – a pair of input lines. A superior configuration. • May be connected to a differential source OR a single-ended source.
Single-Ended Inputs: Disadvantages • Input DC blocking cap are practically always required in single-supply systems. • No rejection of input noise or interference.
Differential Inputs: Advantages • Input blocking caps may not be required. • High rejection of input noise and interference.
Differential Inputs Cont’d. • Differential inputs may be connected to either differential or single-ended sources.
Differential Inputs Cont’d. • Psuedo-differential sources use a single output with a midrail bias. • Treat these like differential sources for wiring to the differential inputs of APAs.
Differential Input Connections • Keep the 2 input leads close together. • With single-ended sources connect the APA input ground lead at source ground, NOT at APA ground. • This lets the CMRR of the APA reject any common-mode radiation or any ground noise between the APA and the source.
APA Output Configurations • There are two common output configurations. • Single-ended – single output line. Traditional configuration. • Differential – a pair of output lines. Also called BTL (for Bridge-Tied Load). • Must be connected to a floating load.
Single-Ended Outputs: Disadvantages • A large output DC blocking cap is required in single-supply systems.
Differential Outputs: Advantages • Output power is nearly 4 times S/E output power. • DC blocking capacitor is not required.
Fully Differential APAs • Fully differential APAs use differential circuits at inputs, outputs and all intermediate stages. • They have all the advantages of differential inputs and outputs, with increased CMRR, PSRR and RF immunity from balanced differential operation throughout the IC. • All recent differential APAs from TI use fully differential architecture.
Fully Differential vs. Traditional • APAs with differential inputs and outputs, like master-slave IC’s, may not be fully differential. • These cannot match the performance of fully differential APAs. Noise on input Gain amp Noise coupled into inputs is amplified to the outputs 1 1 Noise on output 2 RF coupled into inputs or outputs can cause RF Rectification – BAD! 2 2 Noise on output Inverting amp
Class-D Audio Power Amplifier Operation BenefitsBlock Diagram and Circuit Description of OperationOutput WaveformsAD and BD Modulation
Class-D APA Benefits • Class-D audio power amplifiers offer greater efficiency than amplifiers like Class-AB. • They therefore reduce power consumption of products in which they’re used. • Product power budgets are reduced. • Battery life is extended in portable products. • Heat generation is reduced. • These benefits reduce product cost and improve product performance.
Class-D APA Block Diagram • Below is a block diagram for a fully differential Class-D audio power amplifier. • Most TI Class-D amplifiers are fully differential. • Single-ended implementations are possible.
Class-D Differential APA Circuits • A programmable-gain differential amplifier feeds a differential integrator and comparator. • The integrator takes feedback from the output pulse train, subtracts it from the input signal and low-pass filters the result. • The comparator compares integrator output to a triangle wave to set output pulse width. • PWM (pulse width modulation) interface logic drives output FET gates. • A MOSFET bridge supplies switching pulses to a loudspeaker, which low-pass filters them to produce an audio output.
Class-D Analog/PWM Conversion • The integrator produces an error voltage at its output that reflects the input after feedback. • The comparator switches when the error voltage crosses the output of the triangle wave oscillator. • PWM logic converts the comparator outputs to gate drive signals for the H-bridge.
Class-D Output Waveforms • The PWM output switches at a frequency well above the audio frequency range. • Its short-term average is the audio-band output.
AD Modulation • AD modulation, the simplest technique, puts the full differential output voltage across the load at all times, varying the duty cycle to control output. • (Differential or BTL AD modulation is shown on the preceding page. In differential AD modulation the outputs are always switched in opposite phase.) • AD modulation is a powerful technique, but it can generate high ripple current in the load at the switching frequency. • So AD modulation generally requires an LC filter before the load to eliminate the ripple current.
AD Modulation Ripple Current • Without the LC filter, AD modulation ripple current wastes power and may increase the power handling requirement of the speaker.
BD (Filter-Free) Modulation • A newer technique, BD modulation, permits operation without an output LC filter.
BD Modulation Characteristics • BD modulation requires a differential output. • When there is no input, BD modulation switches the opposing outputs nearly in parallel. • So the differential voltage across the load is limited to very low duty cycle and ripple current is reduced dramatically.
BD Modulation Waveforms OUTP OUTN Differential Load Voltage +5V 0V -5V Load Current Current Increasing Current Decaying Filter free modulation output voltage and current waveforms, example signal • As input increases, output duty cycles are modulated in opposite phase to produce a net load voltage at twice the switching frequency.
A Note About Output Filtering • BD modulation eliminates the problem of ripple current without an output LC filter. • However, a output filter may be required for EMC even with BD modulation. • This will depend on the system or product configuration!
Viewing Class-D Outputs • Look again at an earlier graph of Class-D output. • The switching waveform doesn’t look much like the audio output.
RC Filter for Viewing Class-D Output • To view the audio content of a Class-D output use an RC low-pass filter at each output. • Filter frequency should be 30 to 40 kHz. • Recent work shows that 330Ω+15nF works best.
Measuring Differential Outputs • Single-ended outputs are measured between output and ground. • HOWEVER ! – measure differential outputs BETWEEN the 2 output lines to be accurate. • Do not connect probe ground to a differential output – that will short it to ground. • Measure single-ended outputs to ground. • Measuring a differential output vs. ground is NOT accurate, and it overlooks half the output voltage. • Connect a scope probe to each side and use a math difference function.
Class-D Output Rise and Fall • A Class-D switching waveform has very fast rise and fall, or equivalent slew rate. • Very few other devices can match this.
Filters for Measuring Class-D APAs • Many audio analyzers require filtering because extreme slew rates of Class-D waveforms cause slew-induced distortion in their input stages. • A first-order RC filter with time constant around 4.7μS eliminates this problem in most cases. • At high gains such analyzers may require second-order filters. These may be cascaded RCs, with time constants around 2.7μS. • Be aware that there is some frequency response rolloff in the audio band! It is generally not large enough to cause significant loss in results.
1st and 2nd Order Filter Responses • Schematics and frequency responses for suggested 1st and 2nd order filters appear at right. 0 -10 -20 -30 -40 1kHz 3kHz 10kHz 30kHz 100kHz 300kHz 1MHz
Other Filter Possibilities • It’s possible active filters could be used for measuring outputs of Class-D amplifiers. • HOWEVER, active filters can have the same slewing problems as analyzers. • It’s possible transformers could be used for measuring outputs of Class-D amplifiers. • HOWEVER, transformers often have problems like saturation and overshoot. • MAKE SURE YOUR FILTER DOES NOT ADD TROUBLE !