Active Filters Using Op Amp

Active filters are frequency-selective circuits that incorporate RC networks and amplii ers with feedback to produce low-pass, high-pass, bandpass, and bandstop performance. These i lters can replace standard passive LC i lters in many applications. They offer the following advantages over standard passive LC filters.

Figure-1.1 Block diagram or schematic symbols for filters.

• Gain. Because active filters use amplifiers, they can be designed to amplify as well as filter, thus offsetting any insertion loss.
• No inductors. Inductors are usually larger, heavier, and more expensive than capacitors and have greater losses. Active filters use only resistors and capacitors.
• Easy to tune. Because selected resistors can be made variable, the filter cutoff frequency, center frequency, gain, Q, and bandwidth are adjustable
• Isolation. The amplifiers provide very high isolation between cascaded circuits because of the amplifier circuitry, thereby decreasing interaction between filter sections.
• Easier impedance matching. Impedance matching is not as critical as with LC filters. 

Fig. shows two types of low-pass active filters 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.1.2(a) and (c) as in any noninverting op amp. The gain is set by R3 and/or R1 in Fig.1.2(b) and by C3 and/ or C1 in Fig. 1.2(d). All circuits have what is called a second-order response, which means that they provide the same i ltering 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. 

Figure 1.2 Types of active filters. (a) Low-pass. (b) Low-pass. (c) High-pass. (d ) High-pass.

Figure 1.3 Active bandpass and notch filters. (a) Bandpass. (b) Bandpass. (c) High-Q notch. 

Two active bandpass filters and a notch filter are shown in Fig.1.3. In Fig.1.3(a), both RC low-pass and high-pass sections are combined with feedback to give a bandpass result. In Fig.1.3(b), a twin-T RC notch i lter is used with negative feedback to provide a bandpass result. A notch i lter using a twin-T is illustrated in Fig. 1.3(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 i lters 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 frequency ranges up to one microwave (.1 GHz) mated with chip resistors and capacitors have made RC active i lters practical for applications up to the RF range. 

 

🔗Switched capacitor filter (SCF) or analog Sampled data or commutating filter