external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4871 is unity-gain stable which gives a designer

maximum system flexibility. The LM4871 should be used in

low gain configurations to minimize THD+N values, and

maximize the signal to noise ratio. Low gain configurations

require large input signals to obtain a given output power. In-

put signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the sec-

tion, Audio Power Amplifier Design, for a more complete

explanation of proper gain selection.

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

width is dictated by the choice of external components

shown in

Figure 1. The input coupling capacitor, C

first order high pass filter which limits low frequency re-

sponse. This value should be chosen based on needed fre-

quency response for a few distinct reasons.

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenua-

tion. But in many cases the speakers used in portable sys-

tems, whether internal or external, have little ability to repro-

duce signals below 100 Hz to 150 Hz. Thus, using a large

input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

C

reach its quiescent DC voltage (nominally 1/2 V

charge comes from the output via the feedback and is apt to

create pops upon device enable. Thus, by minimizing the ca-

pacitor size based on necessary low frequency response,

turn-on pops can be minimized.

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the LM4871 turns

on. The slower the LM4871’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

(in the range of 0.1 µF to 0.39 µF), should produce a virtually

clickless and popless shutdown function. While the device

will function properly, (no oscillations or motorboating), with

C

to turn-on clicks and pops. Thus, a value of C

1.0 µF is recommended in all but the most cost sensitive de-

signs.

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Audio Amplifier

Given:

Power Output

1 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

and add the output voltage. Using this method, the minimum

supply voltage would be (V

V

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(3)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 4.6V. But since 5V is a stan-

dard voltage in most applications, it is chosen for the supply

rail. Extra supply voltage creates headroom that allows the

LM4871 to reproduce peaks in excess of 1W without produc-

ing audible distortion. At this time, the designer must make

sure that the power supply choice along with the output im-

pedance does not violate the conditions explained in the

Power Dissipation section.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 4.

(4)

R

(5)

From Equation 4, the minimum A

Since the desired input impedance was 20 k

Ω, and with a

A

allocation of R

is to address the bandwidth requirements which must be

stated as a pair of −3 dB frequency points. Five times away

from a −3 dB point is 0.17 dB down from passband response

f

f

As stated in the External Components section, R

junction with C

C

The high frequency pole is determined by the product of the

desired frequency pole, f

With a A

150 kHz which is much smaller than the LM4871 GBWP of

4 MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4871 can still be used without running into bandwidth limi-

tations.

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