possible high frequency oscillations. Care should be taken

when calculating the −3dB frequency in that an incorrect

combination of R

typical combination of feedback resistor and capacitor that

will not produce audio band high frequency rolloff is R

100k

Ω and C

point of approximately 320kHz. Once the differential gain of

the amplifier has been calculated, a choice of R

and C

External Components Description section.

VOICE-BAND AUDIO AMPLIFIER

Many applications, such as telephony, only require a voice-

band frequency response. Such an application usually re-

quires a flat frequency response from 300Hz to 3.5kHz. By

adjusting the component values of Figure 2, this common

application requirement can be implemented. The combina-

tion of R

lowpass filter. Using the typical voice-band frequency range,

with a passband differential gain of approximately 100, the

following values of R

tions stated in the External Components Description sec-

tion.

R

Ω,R

Five times away from a −3dB point is 0.17dB down from the

flatband response. With this selection of components, the

resulting −3dB points, f

spectively, resulting in a flatband frequency response of

the passband. If a steeper rolloff is required, other common

bandpass filtering techniques can be used to achieve higher

order filters.

SINGLE-ENDED AUDIO AMPLIFIER

Although the typical application for the LM4861 is a bridged

monoaural amp, it can also be used to drive a load single-

endedly in applications, such as PC cards, which require that

one side of the load is tied to ground. Figure 3 shows a

common single-ended application, where V

drive the speaker. This output is coupled through a 470µF

capacitor, which blocks the half-supply DC bias that exists in

all single-supply amplifier configurations. This capacitor,

designated C

highpass filter. The −3dB point of this high pass filter is

1/(2

πR

product of R

cies to the load. When driving an 8

Ω load, and if a full audio

spectrum reproduction is required, C

470µF. V

a 0.1 µF capacitor to a 2k

Ω load to prevent instability. While

such an instability will not affect the waveform of V

good design practice to load the second output.

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 needed supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graph 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 dropout voltage. Using this method, the mini-

mum supply voltage would be (V

typically 0.6V.

(3)

For 1W of output power into an 8

Ω load, the required V

opeak

is 4.0V. A minumum supply rail of 4.6V results from adding

V

in many applications and for this reason, a supply rail of 5V

is designated. Extra supply voltage creates dynamic head-

room that allows the LM4861 to reproduce peaks in excess

of 1Wwithout clipping the signal. 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 Equa-

tion 4.

(4)

R

(5)

From equation 4, the minimum A

Since the desired input impedance was 20k

Ω, and with a A

vd

of 3, a ratio of 1:1.5 of R

20k

Ω,R

Ω. The final design step is to address the

bandwidth requirements which must be stated as a pair of

−3dB frequency points. Five times away from a −3db point is

0.17dB down from passband response which is better than

high frequency pole of 20Hz and 100kHz respectively. As

stated in the External Components section, R

tion with C

C

i

≥ 1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

With a A

100kHz which is much smaller than the LM4861 GBWP of

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

design an amplifier with a higher differential gain, the

LM4861 can still be used without running into bandwidth

problems.

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