LT1228I(Rev_C) 查看數據表（PDF） - Linear Technology

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LT1228I
(Rev.:Rev_C)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

FEATURES

■ Very Fast Transconductance Amplifier

Bandwidth: 75MHz

gm = 10 × ISET

Low THD: 0.2% at 30mVRMS Input

Wide ISET Range: 1µA to 1mA

■ Very Fast Current Feedback Amplifier

Bandwidth: 100MHz

Slew Rate: 1000V/µs

Output Drive Current: 30mA

Differential Gain: 0.04%

Differential Phase: 0.1°

High Input Impedance: 25MΩ, 6pF

■ Wide Supply Range: ±2V to ±15V

■ Inputs Common Mode to Within 1.5V of Supplies

■ Outputs Swing Within 0.8V of Supplies

■ Supply Current: 7mA

■ Available in 8-LeaUd PDIP and SOIC Packages

APPLICATIO S

■ Video DC Restore (Clamp) Circuits

■ Video Differential Input Amplifiers

■ Video Keyer/Fader Amplifiers

■ AGC Amplifiers

■ Tunable Filters

■ Oscillators

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

LT1228

100MHz Current Feedback

Amplifier with DC Gain Control

DESCRIPTIO

The LT®1228 makes it easy to electronically control the

gain of signals from DC to video frequencies. The LT1228

implements gain control with a transconductance ampli-

fier (voltage to current) whose gain is proportional to an

externally controlled current. A resistor is typically used to

convert the output current to a voltage, which is then

amplified with a current feedback amplifier. The LT1228

combines both amplifiers into an 8-pin package, and

operates on any supply voltage from 4V (±2V) to 30V

(±15V). A complete differential input, gain controlled

amplifier can be implemented with the LT1228 and just a

few resistors.

The LT1228 transconductance amplifier has a high imped-

ance differential input and a current source output with wide

output voltage compliance. The transconductance, gm, is

set by the current that flows into Pin 5, ISET. The small signal

gm is equal to ten times the value of ISET and this relationship

holds over several decades of set current. The voltage at Pin

5 is two diode drops above the negative supply, Pin 4.

The LT1228 current feedback amplifier has very high input

impedance and therefore it is an excellent buffer for the

output of the transconductance amplifier. The current feed-

back amplifier maintains its wide bandwidth over a wide

range of voltage gains making it easy to interface the

transconductance amplifier output to other circuitry. The

current feedback amplifier is designed to drive low imped-

ance loads, such as cables, with excellent linearity at high

frequencies.

TYPICAL APPLICATIO

R3A

10k

+

VIN R2A

10k

–

R3

100Ω

Differential Input Variable Gain Amp

15V

4.7µF

3+ 7

gm

2–

5

4

–15V

R2

100Ω

+

4.7µF

R4

1.24k

R6

6.19k

1+

6

CFA

VOUT

ISET

8–

RF

R5

470Ω

10k

R1

RG HIGH INPUT RESISTANCE

270Ω 10Ω EVEN WHEN POWER IS OFF

–18dB < GAIN < 2dB

VIN ≤ 3VRMS

LT1228 • TA01

Frequency Response

6

3

0

ISET = 1mA

VS = ±15V

RL = 100Ω

–3

–6

–9

–12

ISET = 300µA

–15

–18

ISET = 100µA

–21

–24

100k

1M

10M

100M

FREQUENCY (Hz)

LT1228 • TA02

1228fc

1

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