11

LT1498/LT1499

Rail-to-Rail Input and Output

The LT1498/LT1499 are fully functional for an input and

output signal range from the negative supply to the posi-

tive supply. Figure 1 shows a simplified schematic of the

amplifier. The input stage consists of two differential

amplifiers, a PNP stage (Q1/Q2) and an NPN stage (Q3/

Q4) which are active over different ranges of input com-

mon mode voltage. A complementary common emitter

output stage (Q14/Q15) is employed allowing the output

to swing from rail-to-rail. The devices are fabricated on

Linear Technology’s proprietary complementary bipolar

process to ensure very similar DC and AC characteristics

for the output devices (Q14/Q15).

The PNP differential input pair is active for input common

moves further toward the positive supply, the transistor

Q7 activating the NPN differential pair, and the PNP

differential pair becomes inactive for the rest of the input

common mode range up to the positive supply.

The output is configured with a pair of complementary

common emitter stages that enables the output to swing

from rail to rail. Capacitors C1 and C2 form local feedback

loops that lower the output impedance at high frequencies.

5V Small-Signal Response

200ns/DIV

1498/99 G27

±15V Small-Signal Response

200ns/DIV

1498/99 G29

5V Large-Signal Response

2

µs/DIV

1498/99 G28

±15V Large-Signal Response

2

µs/DIV

1498/99 G30