LT1228 Linear Technology, LT1228 Datasheet - Page 18

• Manufacturer Part Number: LT1228
• Description: 100MHz Current Feedback Amplifier with DC Gain Control
• Manufacturer: Linear Technology

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
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
LT1228
Voltage Controlled State Variable Filter
18
TYPICAL
A
PPLICATI
P–P
signals.
V
O
C
V
IN
f
f
f
f
f
O
O
O
O
O
100
U
180
= 100kHz AT V
= 200kHz AT V
= 400kHz AT V
= 800kHz AT V
= 1.6MHz AT V
10k
100
3.3k
S
100
–5V
3
2
C
C
C
C
C
= 0V
= 1V
= 2V
= 3V
= 4V
+
–
51k
3
2
2N3906
5V
+
–
g
7
m
–5V
5V
g
4
7
m
–5V
18pF
4
5
3k
18pF
3.3k
3.3k
3.3k
5
1k
best accuracy. If discrete transistors are used, the 51k
resistor should be trimmed to give proper frequency
response with V
for V
additional volt. The two 3k resistors divide the current
between the two LT1228s. Therefore the set current of
each amplifier goes from 50 A to 800 A for a control
voltage of 0V to 4V. The resulting filter is at 100kHz for V
equal zero, and changes it one octave/V of control input.
1
8
8
1
100pF
+
–
+
–
C
CFA
CFA
equal zero volts and doubles the current for every
3k
1k
LT1006
1k
6
+
–
6
C
equal zero. The circuit generates 100 A
BANDPASS
OUTPUT
LT1228 • TA18
LOWPASS
OUTPUT
C