Active Filters using Op-amp

Active Filters

A filter is a circuit that is designed to pass a specified band of frequencies while attenuating all the signals outside that band. It is a frequency selective circuit.

The filters are basically classified as active filters and passive filters. The passive filter networks use only passive elements such as resistors, inductors and capacitors. On the other hand, active filter circuits use the active elements such as op-amps, transistors alongwith the resistors, inductors and capacitors. Modem active filters do not use inductors as the inductors are bulky, heavy and nonlinear. The inductors generate the stray magnetic fields. The inductors dissipate considerable amount of power.

1. Advantages of Active Filters

The active filters have the following advantges over the passive filters,

1. All the elements alongwith op-amp can be used in the integrated form. Hence there is reduction is size and weight.

2. In large quantities, the cost of the integrated circuit can be much lower than its equivalent passive network.

3. Due to availability of modem ICs, variety of cheaper op-amps are available.

4. The op-amp gain can be easily controlled in the closed loop fashion hence active filter input signals is not attenuated.

5. Due to flexibility in gain and frequency adjustments, the active filters can be easily timed.

6. The op-amp has high input impedance and low output impedance hence the active filters using op-amp do not cause loading of the source or load.

7. The inductors are absent in the active filters hence the modem active filters are more economical.

8. Active filters can be realized under number of class of functions such as Butterworth, Thomson, Chebyshev, Cauer etc.

9. The response is improved as compared to passive filters due to ready availability of high quality components.

10. The design procedure is simpler than that for the passive filters.

11. Active filters can provide voltage gains, incontrast the passive filters often show a significant voltage loss.

Inspite of the above advantages, the active filters also have certain limitations. The finite bandwidth of the active devices places a limit on the highest frequency of operation. The active elements are much more sensitive to the temperature and the  environmental changes than the passive elements. Hence due to changes in the parameters due to the environmental changes, the active filter performance deviates from its ideal response. The requirement of d.c. power supply is another disadvantage of the active filters. The passive filters do not require the d.c. supply.

2. Frequency Response Characteristics of Filters

The most commonly used filters are,

1. Low Pass (LP) filter 2. High Pass (HP) filter 3. Band Pass (BP) filter 4. Band Reject (BR) filter. This is also called Band Stop or Band Ellimination filter. 5. All Pass filter.

a. Low Pass Filter

The Fig. 3.4.1 shows the frequency response of low pass filter. A low pass filter has a constant gain from 0 Hz to a high cut-off frequency, fH. Hence, the bandwidth of this filter is also fH. The ideal characteristics is shown in Fig. 3.4.1 (a).

The circuit allows the range of frequencies from 0 to fH. This range is known as the pass band. The range of frequencies beyond f_H is completely attenuated and hence called as stop band.

Practically, the gain of the filter decreases as the frequency increases and at f - f_H, the gain is down by 3 dB and after f_H' it decreases at a higher rate. After the end of transition band, the gain becomes zero.

Using proper design techniques, precision component values and high speed op-amps, the practical response can be obtained very close to the ideal response.

b. High Pass Filter

The Fig. 3.4.2 shows the frequency response of high pass filter. For a high pass filter, fL is the low cut off frequency. The range of frequency 0 < f < f_L is the stop band where f is the operating frequency. While the range of frequency f > f_L is the passband. The Fig. 3.4.2(a) shows the ideal high pass filter characteristics while Fig. 3.4.2(b) shows the practical high pass filter characteristics. 

The transition band is practically not shown in the characteristics as it is very small. Hence, practically, also range upto f_L is called as stop band and f > f_L as pass band. The range upto f_L is completely attenuated by high pass filter.

c. Band Pass Filter.

The Fig. 3.4.3 shows the characteristics of band pass filter. The band pass filter has two stop bands. The range of frequency 0 < f < f_L and range of frequency f_H < f < oo are two stop bands while the range f_L < f < f_H is the pass band. The bandwidth is thus f_H - f_L. The Fig. 3.4.3 (a) shows the ideal characteristics while the Fig. 3.4.3 (b) shows the practical characteristics.

The low pass filter passes only lower frequency range, the high pass filter passes only higher frequency range while the band pass filter only passes the band of frequency between low and high frequency limits defined by fL and fH.

d. Band Elimination Filter

The band elimination filter is also called band rejection filter or band stop filter. Its characteristics is exactly opposite to that of band pass filter. There are two pass bands while one stop band. The stop band is between the two frequencies fL and f_H. The two ranges 0 < f < f_L and f_H < f < ∞ are the two pass bands. The frequency response characteristics of band elimination filter is shown in Fig. 3.4.4. The Fig. 3.4.4 (a) shows the ideal response while the Fig. 3.4.4 (b) shows the practical response. 

At frequency f - f_C, the practical characteristics shows a notch and hence, this filter is also called Notch filter.

The frequency fc in both band pass and band elimination filters is called centre frequency as it is approximately at the centre of the pass band and stop band respectively for the two filters.

e. All Pass Filter

The all pass filter passes all the frequencies but it produces the phase shift between the input and output The output and input voltages are equal in amplitude for all the frequencies but with the phase shift between the two. The frequency upto which the input-output amplitudes remains same is decided by the unity gain bandwidth ( UGB ) of the op-amp used. The characteristics showing phase shift between input and output is shown in Fig. 3.4.5.

Review Questions

1. State the advantages of active filters.

2. Draw and explain the frequency characteristics of various types of active filters.