LM4919 datasheet(14/15 Pages) NSC

14 / 15 page

Application Information (Continued)

for the supply rail. Extra supply voltage creates headroom

that allows the LM4919 to reproduce peak in excess of

10mW without producing audible distortion. At this time, the

designer must make sure that the power supply choice along

with the output impedance does not violate the conditions

explained in the Power Dissipation section. Once the power

dissipation equations have been addressed, the required

gain can be determined from Equation 2.

(4)

From Equation 4, the minimum AV is 1; use A

V = 1. Since the

desired input impedance is 20k, and with a A

V gain of 1, a

ratio of 1:1 results from Equation 1 for R

f to R. The values

are chosen with R

i = 20k and Rf = 20k. The final design step

is to address the bandwidth requirements which must be

stated as a pair of -3dB frequency points. Five times away

from a -3dB point is 0.17dB down from passband response

which is better than the required ± 0.25dB specified.

L = 100Hz/5 = 20Hz

H = 20kHz*5= 100kHz

As stated in the External Components section, R

i in con-

junction with C

i creates a

≥ 1/(2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.

The high frequency pole is determined by the product of the

desired frequency pole, fH, and the differential gain, A

V. With

an AV

V = 1 and fH = 100kHz, the resulting GBWP = 100kHz

which is much smaller than the LM4919 GBWP of 3MHz.

This example displays that if a designer has a need to design

an amplifier with higher differential gain, the LM4919 can still

be used without running into bandwidth limitations.

LM4919 Datasheet(PDF) 14 Page - National Semiconductor (TI)