Datasheet LT1228 (Analog Devices) - 9

LT1228
APPLICATIONS INFORMATION
The LT1228 contains two amplifiers, a transconductance
Resistance Controlled Gain
amplifier (voltage-to-current) and a current feedback ampli- If the set current is to be set or varied with a resistor or fier (voltage-to-voltage). The gain of the transconductance potentiometer it is possible to use the negative temperature amplifier is proportional to the current that is externally coefficient at Pin 5 (with respect to Pin 4) to compensate programmed into Pin 5. Both amplifiers are designed to for the negative temperature coefficient of the transcon- operate on almost any available supply voltage from 4V ductance. The easiest way is to use an LT1004-2.5, a 2.5V (±2V) to 30V (±15V). The output of the transconductance reference diode, as shown below: amplifier is connected to the noninverting input of the current feedback amplifier so that both fit into an eight
Temperature Compensation of gm with a 2.5V Reference
pin package. R
TRANSCONDUCTANCE AMPLIFIER
ISET gm V The LT1228 transconductance amplifier has a high imped- be 4 2.5V ance differential input (Pins 2 and 3) and a current source 2E 5 g output (Pin 1) with wide output voltage compliance. The R ISET Vbe voltage to current gain or transconductance (gm) is set by the current that flows into Pin 5, I LT1004-2.5 SET. The voltage at Pin 5 is two forward biased diode drops above the nega- V– LT1228 • TA04 tive supply, Pin 4. Therefore the voltage at Pin 5 (with The current flowing into Pin 5 has a positive temperature respect to V–) is about 1.2V and changes with the log of coefficient that cancels the negative coefficient of the the set current (120mV/decade), see the characteristic transconductance. The following derivation shows why a curves. The temperature coefficient of this voltage is about 2.5V reference results in zero gain change with temperature: –4mV/°C (–3300ppm/°C) and the temperature coefficient of the logging characteristic is 3300ppm/°C. It is important q ISET that the current into Pin 5 be limited to less than 15mA. Sincegm = × = 10 •I kT 3.87 SET THE LT1228 WILL BE DESTROYED IF PIN 5 IS SHORTED   TO GROUND OR TO THE POSITIVE SUPPLY. A limiting akT cTn and Vbe = Eg – where a = In  resistor (2k or so) should be used to prevent more than q  Ic  15mA from flowing into Pin 5. ≈ 19.4 at 27°C(c = 0.001, n = 3, Ic = 100µA) The small-signal transconductance (gm) is given as g E m = 10 • ISET, with gm in (A/V) and ISET in (A).This rela- g is about 1.25V so the 2.5V reference is 2Eg. Solving tionship holds over many decades of set current (see the the loop for the set current gives: characteristic curves). The transconductance is inversely  akT proportional to absolute temperature (–3300ppm/°C). The 2Eg –2 Eg – input stage of the transconductance amplifier has been  q  2akT ISET = or ISET = designed to operate with much larger signals than is pos- R Rq sible with an ordinary diff-amp. The transconductance of the input stage varies much less than 1% for differential input signals over a ±30 mV range (see the characteristic curve Small-Signal Transconductance vs DC Input Voltage). 1228fd 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Simplified Schematic Applications Information Typical Applications Package Description Revision History Typical Applications Related Parts