Active and Passive filters – A Comparison:
The simplest approach to building a filter
is with passive components (resistors, capacitors, and inductors). In
the R-F range it works quite well but with the lower frequencies,
inductors create problems. AF inductors are physically larger and
heavier, and therefore expensive. For lower frequencies the inductance
is to be increased which needs more turns of wire. It adds to the series
resistance which degrades the inductor’s performance.
Input and output impedances of passive filters are both a problem,
especially below RF. The input impedance is low, that loads the source,
and it varies with the frequency. The output impedance is usually
relatively high, which restricts the load impedance that the passive
filter can drive. There is no isolation between the load impedance and
the passive filter. Thus the load will have to be considered as a
component of the filter and will have to be taken into consideration
while determining filter response or design. Any change in load
impedance may significantly alter one or more of the filter response
characteristics.
