Page 31 - Analog and Digital Filter Design

28 Analog and Digital Filter Design

filter with the appropriate frequency response. Thus they avoid the use of induc-
tors. Because there are gain and bandwidth limitations for all op-amps, the per-
formance of the filter can be restricted. Active filter designs were once restricted
to frequencies below 100 kHz, but wide bandwidth op-amps (particularly cur-
rent feedback types) are now allowing filter designs up to a few megahertz
(MHz). This makes them suitable for video signal filtering.

Figure 1.1 0
An Operational Amplifier
(op-amp)

Active filters have the advantage of being smaller than passive types, and
integrated circuit designs allow them to be miniaturized further. Unfortunately
active filters do have disadvantages: op-amps add noise to the signals; the
signal’s amplitude is limited by the op-amp’s output slew rate and the power
supply voltage; and harmonic distortion can also be introduced, particularly at
the output stage.
Active filters are more suited to designs that are not very demanding, where
rapid changes in amplitude occur as the frequency of the signal is changed. Even
in a nondemanding filter design the signals within a filter circuit can be many
times the applied voltage. For example, a signal may have an amplitude of, say,
one volt, and this may be multiplied typically to perhaps ten volts within the
filter. Devices within the filter must therefore be able to handle signals with large
amplitudes at frequencies well beyond the passband required.

Integrated circuit (IC) filters are now quite common because they can be much
smaller than active filters using op-amps and very much smaller than passive
filters. Their small size supports the general trend to miniaturize equipment. The
IC filters fall into two categories: continuous time and switched capacitor.

Continuous time filters use a number of op-amp circuits within the IC, and often
integrating resistors and capacitors too. The filter response is selected by the
addition of further resistors or capacitors around the IC. Continuous time filters