LM4890 datasheet(20/31 Pages) NSC

20 / 31 page

Application Information (Continued)

As stated in the External Components section, R

IN in con-

junction with C

IN create a highpass filter.

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

The high frequency pole is determined by the product of the

desired frequency pole, f

H, and the differential gain, AVD.

With a A

VD = 3 and fH = 100kHz, the resulting GBWP =

300kHz which is much smaller than the LM4890 GBWP of

2.5MHz. This calculation shows that if a designer has a need

to design an amplifier with a higher differential gain, the

LM4890 can still be used without running into bandwidth

limitations.

The LM4890 is unity-gain stable and requires no external

components besides gain-setting resistors, an input coupling

capacitor, and proper supply bypassing in the typical appli-

cation. However, if a closed-loop differential gain of greater

than 10 is required, a feedback capacitor (C4) may be

needed as shown in Figure 2 to bandwidth limit the amplifier.

This feedback capacitor creates a low pass filter that elimi-

nates possible high frequency oscillations. Care should be

taken when calculating the -3dB frequency in that an incor-

rect combination of R

and C

will cause rolloff before

20kHz. A typical combination of feedback resistor and ca-

pacitor that will not produce audio band high frequency rolloff

is R

3 = 20k

Ω and C

4 = 25pf. These components result in a

-3dB point of approximately 320 kHz.

HIGHER GAIN AUDIO AMPLIFIER

20019224

FIGURE 2.

LM4890 Datasheet(PDF) 20 Page - National Semiconductor (TI)