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LT1228

Voltage Controlled State Variable Filter

LT1228 • TA18

–

CFA

LOWPASS

OUTPUT

100

Ω

18pF

–5V

3.3k

–

CFA

BANDPASS

OUTPUT

100

Ω

18pF

–5V

3.3k

100

Ω

VIN

–

100pF

LT1006

180

Ω

10k

51k

–5V

2N3906

3.3k

fO = 100kHz AT VC = 0V

fO = 200kHz AT VC = 1V

fO = 400kHz AT VC = 2V

fO = 800kHz AT VC = 3V

fO = 1.6MHz AT VC = 4V

APPLICATI

TYPICAL

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 3VP–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 VC equal zero. The circuit generates 100µA

for VC 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 50

µA to 800µA for a control

voltage of 0V to 4V. The resulting filter is at 100kHz for VC

equal zero, and changes it one octave/V of control input.

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