Figure 2-27 illustrates the response curve typical of a low-pass active filter using a general-purpose operational amplifier. Outside the passband, the attenuation is computed at 12 dB per octave. However, at high frequencies the attenuation of the filter is less than predicted. In simple theory, the operational amplifier is considered to be perfect, and, for a typical general-purpose operational amplifier, this perfection proves to be acceptable up to 100 kHz. However, above 100kHz the output impedance and other characteristics of the amplifier can no longer be ignored. The combined effect of these factors causes a loss of attenuation at high frequencies. General-purpose operational amplifiers are most effective in the audio frequency range. For higher frequency applications, a broad band amplifier such as the LM318 or TL291 should be used.
When the frequency spectrum of the input signal is especially wide, the high-frequency rejection characteristic must be considered. This is true when the input to the filter
is a rectangular signal. Figure 2-28 shows the response of a low-pass active filter to a I-MHz square-wave signal.
The high-frequency-cutoff problem is resolved by using a simple RC filter ahead of the active filter. The combination of an RC filter and an active filter having superior low-frequency performance will significantly improve high-frequency cutoff. In addition, an impedance adapter should be inserted between the two filters shown in Figure 2-29.