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.
