of Microprocessors

BASIC ANALOG MODULES 211
INPUT~OUTPUT~ -~t------
..L C z -10 ~!,
I ~-20 : ~
-= -30 ..
FC = 2"I RC
LOG FREOUENCY
(A)
R2
RI
INPUT o-~VV'v--+-i
C
>--+--0 OUTPUT
FC= 2"~2C
PASSBAND GAIN = ~
lBI
R2
C
RI
INPUT o-J\M~"""-I
(C)
'----------~OUTPUT
~ c
TUNE +
~A -
INPUTO>---- t OUTPUT
(D)
Fig. 6-21. Steps toward a voltage-controlled low-pass filter. (A) Single-pole
R-C low-pass filter. (8) Active version of A. (C) Tunable version of 8
using a VCA. (D) Improved version of C with constant gain.
VCA in Fig. 6-21C has a constant passband gain because the previous
expression is multiplied by G, which then cancels the G in the denominator,
leaving Ao = R2/Rl. It would seem that the problem is solved, but closer
analysis reveals that at low cutoff frequencies the input signal level to the
VCA may become extremely large to offset its corresponding low gain and
therefore will be subject to severe distortion.
In Fig. 6-21D, the same elements have been rearranged to overcome
the signal level problem. Essentially, both the input signal and the feedback
signal go through the VCA. The tendency for the op-amp output amplitude