Datasheet

LT1228
18
1228fd
TYPICAL APPLICATIONS
Voltage Controlled State Variable Filter
LT1228 • TA18
–
+
–
+
g
m
CFA
LOWPASS
OUTPUT
8
3
2
1k
4
1
6
100Ω
18pF
–5V
3.3k
7
5
–
+
–
+
g
m
CFA
BANDPASS
OUTPUT
8
3
2
1k
4
1
6
100Ω
18pF
–5V
3.3k
7
5
100Ω
V
IN
5V
5V
3k
3k
–
+
100pF
LT1006
1k
180Ω
10k
V
C
51k
–5V
2N3906
3.3k
3.3k
f
O
= 100kHz AT V
C
= 0V
f
O
= 200kHz AT V
C
= 1V
f
O
= 400kHz AT V
C
= 2V
f
O
= 800kHz AT V
C
= 3V
f
O
= 1.6MHz AT V
C
= 4V
The state variable filter has both lowpass and bandpass
outputs. Each LT1228 is configured as a variable integrator
whose frequency is set by the attenuators, the capacitors
and the set current. Because the integrators have both
positive and negative inputs, the additional op amp nor-
mally required is not needed. The input attenuators set
the circuit up to handle 3V
P–P
signals.
The set current is
generated with a simple circuit that
gives logarithmic voltage to current control. The two PNP
transistors should be a matched pair in the same package
for best accuracy. If discrete transistors are used, the
51k resistor should be trimmed to give proper frequency
response with V
C
equal zero. The circuit generates 100µA
for V
C
equal zero volts and doubles the current for every
additional volt. The
two 3k resistors divide the current
between the two LT1228s. Therefore the set current of
each amplifier goes from 50µA to 800µA for a control
voltage of 0V to 4V. The resulting filter is at 100kHz for V
C
equal zero, and changes it one octave/V of control input.