LT1228CJ8PBF(Rev_C) データシートの表示（PDF） - Linear Technology

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LT1228CJ8PBF
(Rev.:Rev_C)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

TYPICAL APPLICATIO S

Voltage Controlled State Variable Filter

10k

VC

180Ω

1k

2N3906

LT1006

100pF

51k

3.3k

VIN

100Ω

–5V

5V

3k

7

3+

5

gm

2–

4

18pF

–5V

3.3k

100Ω

3.3k

5V

100Ω

7

3+

5

gm

2–

4

18pF

–5V

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

3k

1+

CFA

8–

1k

6 BANDPASS

OUTPUT

1+

6

CFA

8–

1k

LOWPASS

OUTPUT

LT1228 • TA18

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 50µA 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.

1228fc

18

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