LM4917 datasheet(13/18 Pages) NSC

13 / 18 page

Application Information (Continued)

The value of R

f is 30k

Ω.

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

L = 100Hz/5= 20Hz

(8)

and

H = 20kHzx5= 100kHz

(9)

As stated in the External Components section, both R

i in

conjunction with C

i, and RL, create first order highpass fil-

ters. Thus to obtain the desired low frequency response of

100Hz within ±0.5dB, both poles must be taken into consid-

eration. 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.

≥ 1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF (10)

The high frequency pole is determined by the product of the

desired high frequency pole, f

H, and the closed-loop gain,

V. With a closed-loop gain of 1.5 and fH = 100kHz, the

resulting GBWP = 150kHz which is much smaller than the

LM4917’s GBWP of 3MHz. This figure displays that if a

designer has a need to design an amplifier with a higher

gain, the LM4917 can still be used without running into

bandwidth limitations.

LM4917 Datasheet(PDF) 13 Page - National Semiconductor (TI)