LM4876

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SNAS054E – FEBRUARY 2000 – REVISED MAY 2013

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 Characteristics 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 account for the amplifier's dropout

voltage, two additional voltages, 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)

(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 DISSIPATION section.

After satisfying the LM4876's power dissipation requirements, 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

impedance set at 20k

Ω, the feedback resistor is found using Equation 11.

where

•

(11)

The last step in this design example is setting the amplifier's -3dB low frequency bandwidth. To achieve the

desired ±0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-

fifth the lower bandwidth limit and the high frequency response must extend to at least five times the upper

bandwidth limit. The results is an

and an

amplifier's lower bandpass frequency limit. Find the coupling capacitor's value using Equation 12.

Ci

(12)

The result is

1/(2

π*20kΩ*20Hz) = 0.398µF.

Use a 0.39µF capacitor, the closest standard value.

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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