LT2178 datasheet(13/28 Pages) LINER

13 / 28 page

LTC6258/LTC6259/LTC6260

6258960fa

For more information www.linear.com/LTC6258

OPERATION

The LTC6258 family input signal range extends beyond the

negative and positive power supplies. Figure 1 depicts a

Simplified Schematic of the amplifier. The input stage is

comprised of two differential amplifiers, a PNP stage Q1/

Q2 and NPN stage Q3/Q4 that are active over different

ranges of common mode input voltage. The PNP stage

is active between the negative power supply to approxi-

mately 1V below the positive supply. As the input voltage

approaches the positive supply, transistor Q5 will steer the

tail current I1 to the current mirror Q6/Q7, activating the

NPN differential pair and the PNP pair becomes inactive

for the remaining input common mode range. Also for the

input stage, devices Q17, Q18 and Q19 act to cancel the

bias current of the PNP input pair. When Q1/Q2 is active,

the current in Q16 is controlled to be the same as the

current Q1/Q2. Thus, the base current of Q16 is normally

equal to the base current of the input devices of Q1/Q2.

Similar circuitry (not shown) is used to cancel the base

current of Q3/Q4. The buffer and output bias stage uses

a special compensation technique to take full advantage

of the process technology to drive high capacitive loads.

The common emitter topology of Q14/Q15 enables the

output to swing from rail to rail.

LT2178 Datasheet(PDF) 13 Page - Linear Technology