Active filters are frequency selective circuits that incorporate RC networks and amplifiers with feedback to produce a low-pass, high-pass, bandpass, and bandstop performance. These filters can replace standard passive LC filters in many applications. They offer the following advantages over standard passive LC filters.
Fig. 2-42 shows two types of low-pass active filter and two types of high-pass active filters. Note that these active filters use op-amps to provide the gain. The voltage divider, made up of R1 and R2, sets the circuit gain in the circuits of Fig. 2-42(a) and (c) as in any noninverting op-amp. The gain is set by R3 and/or R1 in Fig. 2-42(b) and by C3 and/ or C1 in Fig. 2-42(d). All circuits have what is called a second-order response, which means that they provide the same filtering action as a two-pole LC filter. The roll-off rate is 12 dB per octave or 40 dB per decade. Multiple filters can be cascaded to provide faster roll-off rates.
Two active bandpass filters and a notch filter are shown in Fig. 2-43. In Fig. 2-43(a), both RC low-pass and high-pass sections are combined with feedback to give a bandpass result. In Fig. 2-43(b), a twin-T RC notch filter is used with negative feedback to provide a bandpass result. A notch filter using a twin-T is illustrated in Fig. 2-43(c).
The feedback makes the response sharper than that with a standard passive twin-T. Active filters are made with integrated-circuit (IC) op-amps and discrete RC networks. They can be designed to have any of the responses discussed earlier, such as Butterworth and Chebyshev, and they are easily cascaded to provide even greater selectivity. Active filters are also available as complete packaged components. The primary disadvantage of active filters is that their upper frequency of operation is limited by the frequency response of the op-amps and the practical sizes of resistors and capacitors. Most active filters are used at frequencies below 1 MHz, and most active circuits operate in the audio range and slightly above. However, today op-amps with a frequency range up to one microwave (.1 GHz) mated with chip resistors and capacitors have made RC active filters practical for applications up to the RF range.
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