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.

Continuous output, amplitude proportional to input

– 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.

Switched output, duty cycle and short-term

average proportional to input

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.

Input referred to APA ground S

– 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.

Input referred to the source, not APA ground

S

Single-Ended Inputs: Disadvantages • Input DC blocking cap are practically always

required in single-supply systems.• No rejection of input noise or interference.

APA input = sum of these signals

S

Noisy power currents in the ground between source &

APA produce voltages that add to the APA input signal.

S

APA input = sum of these signals

Radiation from other circuits near APA induces voltages that add to the input signal.

Differential Inputs: Advantages• Input blocking caps may not be required.• High rejection of input noise and interference.

net input = only intended signal

With ground reference at source, voltages induced by

noisy ground currents are the same at both inputs &

are rejected by APA CMRR.

S

net input = only intended signal

If input leads are closely spaced, voltages induced

by radiation are essentially the same at both inputs and are rejected by APA CMRR.

S

Differential Inputs Cont’d.• Differential inputs may be connected to either

differential or single-ended sources.

Differential Source

- If the audio source DC is within the APA common-mode range, DC blocking caps are optional.

- However, be sure source DC offset is not a problem!

- Input caps may still be used if high-pass filtering is needed.

caps may be optional

Single-ended Source

- Input DC blocking caps are required with a single-supply single-ended source.

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.

Psuedo-Differential Source

caps may be optional

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.

Output referred to APA ground Z

loa

d

– 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.

Output independent of APA ground

Zlo

ad

Single-Ended Outputs: Disadvantages• A large output DC blocking cap is required in

single-supply systems.

The blocking capacitor is required to prevent high DC current through the load.

Zlo

ad

The cap must be very big for good low-frequency response:

- 8Ω, 50Hz: C ~= 390μF. - 4Ω, 20Hz: C ~= 2200μF! Z

load

Differential Outputs: Advantages• Output power is nearly 4 times S/E output

power.

• DC blocking capacitor is not required.

Outputs sum to 2 x S/E outputs

The 2 outputs are opposite in phase, so their voltages sum across the load to provide twice the voltage and 4 times the power of single-ended.

Zlo

ad +

2 x -

DC load current is negligible

NO output cap is required – when input is zero, output is a small DC offset, so DC load current is negligible.

Zlo

ad

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.

S

Zlo

ad

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.

VDD/2

-

+

-

+

Input Signal

RF

RIN

R

R

CBYPASS

A

B

2

2

Gain amp

Inverting amp

Noise on input

Noise on output

Noise on output

1Noise coupled into inputs is amplified to the outputs

1

2 RF coupled into inputs or outputs can cause RF Rectification –

BAD!

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.

Vin PWM LOGIC

Vcc

Vcc

- +

PGA

feedback OUT+

OUT- feedback

H-Bridge - +

- +

+

-

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.

- +

OUT+

OUT-

Error Voltage

Triangle

Wave

Comparator Outputs

+

-

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.

Vcc ON

ON

off

off

- + - +

Vcc off

off

ON

ON

+ + - -

Positive Output Polarity

Negative Output Polarity

Duty cycle determines the short-term average, the

amplitude of the audio output

Q1 Q2

Q1 Q2

Q4 Q3

Q4 Q3

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.

With no LC filter, ripple current is limited only by loudspeaker

inductance, usually 20 to 60 uH.

With no input signal, switching at 250kHz,

approximate peak ripple current would be

Vcc * 1uS / L.

For Vcc = 12V and L = 30uH, peak ripple current would be

~ 0.4A.

With an 8Ω load, including extra power burned in the

APA, this would waste nearly 1/2 watt.

BD (Filter-Free) Modulation• A newer technique, BD modulation, permits

operation without an output LC filter.

Vcc ON

off

ON

off

+ -

Vcc off

ON

off

ON

+ -

Q1 Q2

Q1 Q2

Q4 Q3

Q4 Q3

OUTP

OUTN

Differential Load

Voltage

Ripple Current

OUTP OUTN

OUTP OUTN

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

0V

-5V

+5V

Load Current

OUTP

OUTN

Differential Load

Voltage

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.

• 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!

A Note About Output Filtering

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.

Positive Output Polarity

Negative Output Polarity

Duty cycle determines the short-term average, the audio output amplitude

The audio output may be an ordinary

1kHz signal.

However, it’s very difficult to see any audio output in the

switching waveforms!

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.

Class-D

Rflt Cflt

Rflt Cflt

Audio unclear in switching output

RC filter shows audio with small ripple

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.

Class-D

Rflt Cflt

Rflt Cflt

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.

Measure single-ended outputs to ground.

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.

The audio output may be an ordinary 1kHz signal.

But the switching output may

rise and fall in 10nS.

With a typical 12V supply, this means an equivalent slew rate of 1200V/uS!

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.

Frequency

1.0KHz 3.0KHz 10KHz 30KHz 100KHz 300KHz 1.0MHzDB(V(_V1.1)) DB(V(_V2.2))

-40

-30

-20

-10

-0

1kHz 3kHz 10kHz 30kHz 100kHz 300kHz 1MHz

0

-10

-20

-30

-40

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 ?