LM4898LD datasheet(14/17 Pages) NSC

14 / 17 page

Application Information (Continued)

AUDIO POWER AMPLIFIER DESIGN

Design a 1W/8

Ω Audio Amplifier

Given:

•

Power Output

•

Load Impedance

Ω

•

Input Level

1Vrms

•

Input Impedance

20k

Ω

•

Bandwidth

100Hz–20kHz ± 0.25dB

A designer must first determine the minimum supply rail to

obtain the specified output power. The supply rail can easily

be found by extrapolating from the Output Power vs. Supply

Voltage graphs in the Typical Performance Characteristics

section. A second way to determine the minimum supply rail

is to calculate the required Vopeak using Equation 7 and add

the dropout voltages. Using this method, the minimum sup-

ply voltage is (Vopeak +(V

DO TOP+(VDO BOT )),where VDO

BOT

and V

DO TOP are extrapolated from the Dropout Voltage

vs. Supply Voltage curve in the Typical Performance Char-

acteristics section.

(7)

Using the Output Power vs. Supply Voltage graph for an 8

Ω

load, the minimum supply rail just about 5V. Extra supply

voltage creates headroom that allows the LM4898 to repro-

duce peaks in excess of 1W without producing audible dis-

tortion. At this time, the designer must make sure that the

power supply choice along with the output impedance 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 Equation 8.

(8)

f /Ri =AVD

From Equation 8, the minimum A

VD is 2.83. Since the de-

sired input impedance was 20k

Ω, a ratio of 2.83:1 of R

f to Ri

results in an allocation of R

i = 20k

Ω for both input resistors

and R

f= 60k

Ω for both feedback resistors. The final design

step is to address the bandwidth requirement which must be

stated as a single -3dB frequency point. Five times away

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

which is better than the required ±0.25dB specified.

H = 20kHz * 5 =100kHz

The high frequency pole is determined by the product of the

desired frequency pole, f

H , and the differential gain, AVD

.With a A

VD = 2.83 and fH = 100kHz, the resulting GBWP =

150kHz which is much smaller than the LM4898 GBWP of

10MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4898 can still be used without running into bandwidth

limitations.

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