APPLICATION INFORMATION

SINGLE-SUPPLY OPERATION

(b) Output Phase Reversal

Exhibited by LM358

(c) No Phase Reversal

Exhibited by LT1014

5

4

3

2

1

0

−1

−2

5

4

3

2

1

0

−1

5

4

3

2

1

0

−1

LT1014D-EP

includes ground, and the output swings within a few millivolts of ground.

Furthermore, the LT1014D has specific circuitry that addresses the difficulties of single-supply operation, both at

the input and at the output. At the input, the driving signal can fall below 0 V, either inadvertently or on a

transient basis. If the input is more than a few hundred millivolts below ground, the LT1014D is designed to deal

with the following two problems that can occur:

1. On many other operational amplifiers, when the input is more than a diode drop below ground, unlimited

400-

Ω resistors in series with the input (see schematic) protect the device even when the input is 5 V below

ground.

amplifiers saturates, and phase reversal occurs at the output. This can cause lockup in servo systems.

Because of unique phase-reversal protection circuitry (Q21, Q22, Q27, and Q28), the LT1014D outputs do

not reverse, even when the inputs are at -1.5 V (see Figure 25).

However, this phase-reversal protection circuitry does not function when the other operational amplifier on the

LT1014D is driven hard into negative saturation at the output. Phase-reversal protection does not work on an

amplifier:

• When 4's output is in negative saturation (the outputs of 2 and 3 have no effect)

• When 3's output is in negative saturation (the outputs of 1 and 4 have no effect)

• When 2's output is in negative saturation (the outputs of 1 and 4 have no effect)

• When 1's output is in negative saturation (the outputs of 2 and 3 have no effect)

At the output, other single-supply designs either cannot swing to within 600 mV of ground or cannot sink more

than a few microproamperes while swinging to ground. The all-npn output stage of the LT1014D maintains its low

output resistance and high gain characteristics until the output is saturated. In dual-supply operations, the output

stage is free of crossover distortion.

Figure 25. Voltage-Follower Response

With Input Exceeding the Negative Common-Mode Input Voltage Range

Copyright © 2008, Texas Instruments Incorporated

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