HIGHER GAIN AUDIO AMPLIFIER

The LM4860 is unity-gain stable and requires no external

components besides gain-setting resistors, an input coupling

capacitor, and proper supply bypassing in the typical appli-

cation. However if a closed-loop differential gain of greater

than 10 is required, then a feedback capacitor is needed, as

shown in

Figure 2, to bandwidth limit the amplifier. The feed-

back capacitor creates a low pass filter that eliminates un-

wanted high frequency oscillations. Care should be taken

when calculating the −3 dB 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

100 k

Ω and C

point of approximately 320 kHz. 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

requires 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 LM4860 is a bridged

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

single-endedly in applications, such as PC cards, which re-

quire that one side of the load is tied to ground.

Figure 3

shows a common single-ended application, where V

used to drive the 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 ca-

pacitor, designated C

forms a highpass filter. The −3 dB point of this highpass 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

470 µF. V

the output that is not used, is connected

through a 0.1 µF capacitor to a 2 k

Ω load to prevent instabil-

ity. While such an instability will not affect the waveform of

V

AUDIO POWER AMPLIFIER DESIGN

Design a 500 mW/8

Ω Audio Amplifier

Given:

Power Output:

500 mWrms

Load Impedance:

8

Ω

Input Level:

1 Vrms(max)

Input Impedance:

20 k

Ω

Bandwidth:

A designer must first determine the needed supply rail to ob-

tain the specified output power. Calculating the required sup-

ply rail involves knowing two parameters, V

dropout voltage. The latter is typically 0.7V. V

determined from equation 3.

For 500 mW of output power into an 8

Ω load, the required

V

adding V

that exists in many applications and for this reason, a supply

rail of 5V is designated. Extra supply voltage creates dy-

namic headroom that allows the LM4860 to reproduce peaks

in excess of 500 mW without 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 condi-

tions 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.

From equation 4, the minimum A

A

Since the desired input impedance was 20 k

Ω, and with an

A

R

capacitor is not needed. The final design step 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 which is bet-

a low and high frequency pole of 4 Hz and 100 kHz respec-

tively. As stated in the External Components section, R

conjunction with C

C

use 2.2 µF.

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 LM4860 GBWP of

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

design an amplifier with a higher differential gain, the

LM4860 can still be used without running into bandwidth

problems.

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