AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1W into an 8

Ω Load

The following are the desired operational parameters:

Power Output

1W

RMS

Load Impedance

8

Ω

Input Level

1V

RMS

Input Impedance

20k

Ω

Bandwidth

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

find the minimum supply voltage is to use the Output Power

vs Supply Voltage curve in the Typical Performance Char-

acteristics section. Another way, using Equation (8), is to

calculate the peak output voltage necessary to achieve the

desired output power for a given load impedance. To ac-

count for the amplifier’s dropout voltage, two additional volt-

ages, based on the Dropout Voltage vs Supply Voltage in the

Typical Performance Characteristics curves, must be

added to the result obtained by Equation (8). This results in

Equation (9).

(8)

V

CC

≥ (V

(9)

The Output Power vs Supply Voltage graph for an 8

Ω load

indicates a minimum supply voltage of 4.6V. This is easily

met by the commonly used 5V supply voltage. The additional

voltage creates the benefit of headroom, allowing the

LM4876 to produce peak output power in excess of 1W

without clipping or other audible distortion. The choice of

supply voltage must also not create a violation of maximum

power dissipation as explained above in the Power Dissi-

pation section.

After satisfying the LM4876’s power dissipation require-

ments, the minimum differential gain is found using Equation

(10).

(10)

Thus, a minimum gain of 2.83 allows the LM4876’s to reach

full output swing and maintain low noise and THD+N perfor-

mance. For this example, let A

The amplifier’s overall gain is set using the input (R

feedback (R

set at 20k

Ω, the feedback resistor is found using Equation

(11).

R

(11)

The value of R

Ω.

The last step in this design example is setting the amplifier’s

-3dB low frequency bandwidth. To achieve the desired

quency response must extend to at least one-fifth the lower

bandwidth limit and the high frequency response must ex-

tend to at least five times the upper bandwidth limit. The

results is an

f

and an

F

As mentioned in the External Components section, R

create a highpass filter that sets the amplifier’s lower band-

pass frequency limit. Find the coupling capacitor’s value

using Equation (12).

Ci

≥ 1/(2πRif

(12)

The result is

1/(2

π*20kΩ*20Hz) = 0.398µF.

Use a 0.39µF capacitor, the closest standard value.

The product of the desired high frequency cutoff (100kHz in

this example) and the differential gain, A

upper passband response limit. With A

100kHz, the closed-loop gain bandwidth product (GBWP) is

150kHz. This is less than the LM4876’s 4MHz GBWP. With

this margin, the amplifier can be used in designs that require

more differential gain and avoid performance-restricting

bandwidth limitations.

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