Datasheet

LM4809, LM4809LQBD
www.ti.com SNAS126F –FEBRUARY 2001–REVISED APRIL 2013
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 an
f
L
= 100Hz/5 = 20Hz (9)
and a
f
H
= 20kHz
∗
5 = 100kHz (10)
As stated in the Selecting Proper External Components section, both R
i
in conjunction with C
i
, and C
o
with 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)
C
o
≥ 1 / (2π * 32Ω * 20Hz) = 249µF; use 330µF. (12)
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 LM4809's GBWP of 900kHz. This figure displays that if a designer has a need to design an amplifier
with a higher gain, the LM4809 can still be used without running into bandwidth limitations.
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