Active Filters

Filters are circuits whose
gain varies with frequency. Typically filters are used
not
to allow some, but all to
frequencies pass

contain only passive components (R,C,1)
gain: 1=
X
wnt
shunt series
Passive filters +




Active
filters contains passive components 3 amplifiers
like op-amps


Passive filter w/ gain
floor

- #=
we-ic'
X



in
(to ground
-
shunt pos.

makes it
low
pass.


asymptotes
1
at
B
low
Very wi
very high w:

capacitor open
circuit
capacitor:short circuit
-




R unattached.
Resistor circuitis pot. div wI gain 20
log, =-12 d
Allows low w to pass, attenuates high frequencies to
minimum
gain, due to presence
of resistor in series with C.


The order
of a filter is the highest power join the denominator. Almost
always
equals power of 1/C.




--it
Active IP
filter with gain floor
I
menting amplifier configuration: -




: is--te virtual
earth

*
-
QIgain
... + 9.SdB in to
previous.
comparison

Advantage of op-amp circuit:
can have
gain 1;
-




-Low outputimpedance -



loading does not

affectfilter frequency response
-
Resistive inputimpedance -
doesn't vary with

frequency