For 70 mW of output power into a 32

Ω load, the required V

O-

PEAK

is 2.12 volts. A minimum supply rail of 2.42V results

from adding V

voltage in most applications, it is chosen for the supply rail.

Extra supply voltage creates headroom that allows the

LM4808 to reproduce peaks in excess of 70 mW without clip-

ping the signal. At this time, the designer must make sure

that the power supply choice along with the output imped-

ance does not violate the conditions explained 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 pack-

age.

Once the power dissipation equations have been addressed,

the required gain can be determined from Equation 4.

(4)

A

(5)

From Equation 4, the minimum gain is: A

Since the desired input impedance was 20k

Ω, and with a

gain of 1.26, a value of 27k

Ω is designated for R

5% tolerance resistors. This combination results in a nominal

gain of 1.35. The final design step is to address the band-

width requirements which must be stated as a pair of −3 dB

frequency points. Five times away from a −3dB point is

0.17dB down from passband response assuming a single

pole roll-off. As stated in the External Components section,

both R

der highpass filters. Thus to obtain the desired frequency low

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 re-

sponse is better than 0.5dB down at 100Hz.

C

C

The high frequency pole is determined by the product of the

desired high frequency pole, f

V

. With a closed-loop gain of 1.35 and f

sulting GBWP = 135kHz which is much smaller than the

LM4808 GBWP of 900kHz. This figure displays that if a de-

signer has a need to design an amplifier with a higher gain,

the LM4808 can still be used without running into bandwidth

limitations.

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