LM4849 datasheet(18/19 Pages) NSC

18 / 19 page

Application Information (Continued)

supply voltage must also not create a situation that violates

of maximum power dissipation as explained above in the

Power Dissipation section.

After satisfying the LM4849’s power dissipation require-

ments, the minimum differential gain needed to achieve 1W

dissipation in an 8

Ω load is found using Equation (13).

(11)

Thus, a minimum overall gain of 2.83 allows the LM4849’s to

reach full output swing and maintain low noise and THD+N

performance.

The last step in this design example is setting the amplifier’s

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

pass band magnitude variation limit, the low frequency re-

sponse 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 gain variation for

both response limits is 0.17dB, well within the ±0.25dB

desired limit. The results are an

L = 100Hz/5 = 20Hz

(12)

and an

H = 20kHzx5= 100kHz

(13)

As mentioned in the Selecting Proper External Compo-

nents section, R

i (Right & Left) and Ci (Right & Left) create

a highpass filter that sets the amplifier’s lower bandpass

frequency limit. Find the coupling capacitor’s value using

Equation (17).

≥ 1/(2πR

ifL)

(14)

The result is

1/(2

π*20kΩ*20Hz) = 0.397µF

(15)

Use a 0.39µF capacitor, the closest standard value.

The product of the desired high frequency cutoff (100kHz in

this example) and the differential gain A

VD, determines the

upper passband response limit. With A

VD = 3 and fH =

100kHz, the closed-loop gain bandwidth product (GBWP) is

300kHz. This is less than the LM4849’s 3.5MHz GBWP. With

this margin, the amplifier can be used in designs that require

more differential gain while avoiding performance,restricting

bandwidth limitations.

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