Datasheet

18
LT1228
1228fc
Voltage Controlled State Variable Filter
TYPICAL APPLICATIO S
U
The state variable filter has both lowpass and bandpass
outputs. Each LT1228 is configured as a variable integra-
tor whose frequency is set by the attenuators, the capaci-
tors 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 tran-
sistors 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.
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