Audio power amplifier apa operation and measurement
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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!


Measuring Class-D and Class-AB Outputs


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

1kHz3kHz10kHz30kHz100kHz300kHz1MHz


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 !


QUESTIONS ?


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