LM4870 datasheet(7/8 Pages) NSC

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Application Information (Continued)

are required for 1.0W of output power. Since 5V rails are

common, it is chosen to be the supply voltage. Supply volt-

age (up to a maximum of 5.5V) above the required 4.6V will

allow the LM4870 to reproduce transient signals at greater

than 1.0W of output power. The extra voltage will also in-

crease the device power dissipation, so the designer must

make sure that the conditions explained in the Power Dissi-

pationsection are not violated.

Once the power dissipation issues have been addressed,

the required differential gain can be determined from Equa-

tion 4:

From equation 4, the minimum A

vd is A vd = 2.83; use Avd=

Since the amplifier output is bridged, giving a gain of 2, and

the desired system gain is 3, the ratio of feedback (R

f)toin-

put (R

i) resistor is 1.5:1. If Ri is set to the required input im-

pedance of 20 k

Ω, then R

f = 30 kΩ. 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 better than the required ±0.25 dB specified.

L = 100 Hz/5 = 20 Hz

H = 20 kHz5 = 100 HzAs stated in the External Compo-

nents section, R

i in conjunction with Ci create a highpass

filter.

i ≥ 1/(2π * 20 kΩ * 20 Hz) = 0.397 µF;

use 0.39 µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the differential gain, A

. With a A

vd = 3 and fH = 100 kHz, the resulting GBWP =

150 kHz which is much smaller than the LM4870 GBWP of

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

design an amplifier with a higher differential gain, the

LM4870 can still be used without running into bandwidth

problems.

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