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LT1228 - 100MHz Current Feedback Amplifier with DC Gain Control

TheLT1228 makes it easy to electronically control the gain of signals from DC to video frequencies. The LT1228 implements gain control with a transconductance amplifier (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 impedance differential input and a current source output with wide output voltage compliance. The transconductance, gm, is set by the current that flows into Pin5, 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 feedback 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 impedance loads, such as cables, with excellent linearity at high frequencies.

Features Very Fast Transconductance Amplifier Bandwidth: 75MHz Outputs Swing Within 0.8V of Supplies gm = 10 x 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: 25M Ohm, 6pF Wide Supply Range: ±2V to ±15V Inputs Common Mode to Within 1.5V of Supplies Supply Current: 7mA Available in 8-Lead PDIP and SOIC Packages Typical Application

Applications Video DC Restore (Clamp) Circuits Video Differential Input Amplifiers Video Keyer/Fader Amplifiers AGC Amplifiers Tunable Filters Oscillators Packaging

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