LT1228(Rev_A) Просмотр технического описания (PDF) - Linear Technology

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LT1228
(Rev.:Rev_A)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

TYPICAL APPLICATI S

Filters

Single Pole Low/High/Allpass Filter

VIN

LOWPASS

INPUT

R3A

1k

R3

120Ω

VIN

HIGHPASS

INPUT

3+

gm

2–

C

5 330pF

ISET RG

1k

1+

CFA

8–

RF

1k

R2A

1k

6

VOUT

R2

120Ω

fC =

10

2π ×

ISET

C

×

RF +

RG

1

×

R2

R2 + R2A

fC = 109 ISET FOR THE VALUES SHOWN

LT1228 • TA16

LT1228

Allpass Filter Phase Response

0

1mA SET CURRENT

–45

–90

–135

100µA SET CURRENT

–180

10k

100k

1M

10M

FREQUENCY (Hz)

LT1228 • TA17

Using the variable transconductance of the LT1228 to

make variable filters is easy and predictable. The most

straight forward way is to make an integrator by putting a

capacitor at the output of the transconductance amp and

buffering it with the current feedback amplifier. Because

the input bias current of the current feedback amplifier

must be supplied by the transconductance amplifier, the

set current should not be operated below 10µA. This limits

the filters to about a 100:1 tuning range.

The Single Pole circuit realizes a single pole filter with a

corner frequency (fC) proportional to the set current. The

values shown give a 100kHz corner frequency for 100µA

set current. The circuit has two inputs, a lowpass filter

input and a highpass filter input. To make a lowpass filter,

ground the highpass input and drive the lowpass input.

Conversely for a highpass filter, ground the lowpass input

and drive the highpass input. If both inputs are driven, the

result is an allpass filter or phase shifter. The allpass has

flat amplitude response and 0° phase shift at low frequen-

cies, going to –180° at high frequencies. The allpass filter

has –90° phase shift at the corner frequency.

17

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