LM4808

www.ti.com

SNAS051D – FEBRUARY 2000 – REVISED MAY 2013

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 70mW/32

Ω Audio Amplifier

Given:

Power Output

70mW

Load Impedance

32

Ω

Input Level

1Vrms (max)

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.50dB

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 5, 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 5. For a single-

ended application, the result is Equation 6.

(5)

(6)

The Output Power vs Supply Voltage graph for a 32

Ω load indicates a minimum supply voltage of 4.8V. This is

easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom,

allowing the LM4808 to produce peak output power in excess of 70mW without clipping or other audible

distortion. The choice of supply voltage must also not create a situation that violates maximum power dissipation

as explained above in the POWER DISSIPATION section. Remember that the maximum power dissipation point

from Equation 1 must be multiplied by two since there are two independent amplifiers inside the package. Once

the power dissipation equations have been addressed, the required gain can be determined from Equation 7.

(7)

Thus, a minimum gain of 1.497 allows the LM4808 to reach full output swing and maintain low noise and THD+N

impedance set at 20k

Ω, the feedback resistor is found using Equation 8.

(8)

The last step in this design is setting the amplifier's

−3db frequency bandwidth. To achieve the desired ±0.25dB

pass band magnitude variation limit, the low frequency response must extend to at lease one

−fifth the lower

bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth limit. The

gain variation for both response limits is 0.17dB, well within the ±0.25dB desired limit. The results are an

(9)

and a

(10)

both poles must be taken into consideration. The combination of two single order filters at the same frequency

forms a second order response. This results in a signal which is down 0.34dB at five times away from the single

order filter

−3dB point. Thus, a frequency of 20Hz is used in the following equations to ensure that the response

is better than 0.5dB down at 100Hz.

Copyright © 2000–2013, Texas Instruments Incorporated

Submit Documentation Feedback

15

Product Folder Links: LM4808