tion of R

combination of feedback resistor and capacitor that will not

produce audio band high frequency rolloff is R

C

proximately 320 kHz. Once the differential gain of the ampli-

fier has been calculated, a choice of R

then be calculated from the formula stated in the 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 300 Hz to 3.5 kHz. 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

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

flatband response. With this selection of components, the re-

sulting −3 dB points, f

spectively, resulting in a flatband frequency response of bet-

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 com-

mon single-ended application, where V

speaker. This output is coupled through a 470 µF capacitor,

which blocks the half-supply DC bias that exists in all single-

supply amplifier configurations. This capacitor, designated

C

The −3 dB point of this high pass filter is 1/(2

πR

should be taken to make sure that the product of R

is large enough to pass low frequencies to the load. When

driving an 8

Ω load, and if a full audio spectrum reproduction

is required, C

that is not used, is connected through a 0.1 µF capacitor to

a2k

Ω load to prevent instability. While such an instability will

not affect the waveform of V

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 ob-

tain 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 20 k

Ω, and with a

A

R

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 which is bet-

a low and high frequency pole of 20 Hz and 100 kHz respec-

tively. As stated in the External Components section, R

conjunction with C

C

The high frequency pole is determined by the product of the

desired high frequency pole, f

With a A

100 kHz which is much smaller than the LM4861 GBWP of

4 MHz. 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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