Rf/Ri = AVD/2
From Equation 2, the minimum AVD is 2.83; use AVD = 3. Since the desired input impedance was 20kΩ, and with a AVD
impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an allocation of Ri = 20kΩ and Rf = 30kΩ. 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 pass band response which is better than the required ±0.25dB specified.
fL = 100Hz/5 = 20Hz
fH = 20kHz * 5 = 100kHz
As stated in the External Components section, Ri in conjunction with Ci create a high pass filter.
Ci ≥ 1/(2π*20kΩ*20Hz) = 0.397μF; use 0.39μF
The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AVD. With a
AVD = 3 and fH = 100kHz, the resulting GBWP = 300kHz which is much smaller than the CM8600 GBWP of 2.5MHz. This
figure displays that if a designer has a need to design an amplifier with a higher differential gain, the CM8600 can still be used
without running into bandwidth limitations.
FIGURE 2. HIGHER GAIN AUDIO AMPLIFIER
The CM8600 is unity-gain stable and requires no external components besides gain-setting resistors, an input coupling capacitor,
and proper supply bypassing in the typical application. 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 eliminates possible high frequency oscillations. Care should be taken when calculating the -3dB frequency
in that an incorrect combination of R3 and C4 will cause rolloff before 20kHz. A typical combination of feedback resistor and
capacitor that will not produce audio band high frequency rolloff is R3 = 20kΩ and C4 = 25pf. These components result in a
-3dB point of approximately 320kHz.
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