Datasheet

LM4811

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SNAS119D –DECEMBER 2000–REVISED APRIL 2013

Using Figure 28 for a 32Ω load, the minimum supply rail is 4.8V. Since 5V is a standard supply voltage in most

applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4811 to

reproduce peaks in excess of 70mW without clipping the signal. 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 POWER

DISSIPATION. Remember that the maximum power dissipation point from Equation 1 must be multiplied by two

since there are two independent amplifiers inside the package.

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 assuming a single

pole roll-off. As stated in External Components Description, C

and C

create first order highpass filters. Thus to

obtain the desired frequency low 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.

≥ 1 / (2π * 33 kΩ * 20 Hz) = 0.241µF; use 0.39µF. (4)

≥ 1 / (2π * 32Ω * 20 Hz) = 249µF; use 330µF. (5)

The high frequency pole is determined by the product of the desired high frequency pole, f

, and the closed-loop

gain, A

. With a closed-loop gain of 3.98 or +12dB and f

= 100kHz, the resulting GBWP = 398kHz which is

much smaller than the LM4811 GBWP of 1MHz. This figure displays that at the maximum gain setting of 3.98 or

+12dB, the LM4811 can be used without running into bandwidth limitations.

Product Folder Links: LM4811