Datasheet

LM4917
SNAS238G –AUGUST 2004–REVISED MAY 2013 www.ti.com
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
f
L
= 100Hz / 5 = 20Hz (9)
and
f
H
= 20kHz x 5 = 100kHz (10)
As stated in the External Components Description section, both R
i
in conjunction with C
i
, and R
L
, create first
order highpass filters. Thus to obtain the desired low frequency response of 100Hz within ±0.5dB, both poles
must be taken 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 response is
better than 0.5dB down at 100Hz.
C
i
≥ 1 / (2π*20kΩ*20Hz) = 0.397µF; use 0.39µF (11)
The high frequency pole is determined by the product of the desired high frequency pole, f
H
, and the closed-loop
gain, A
V
. With a closed-loop gain of 1.5 and f
H
= 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.
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