10
LT1077
1077fa
APPLICATIO S I FOR ATIO
WUUU
resistors.) When the output is high the pull-down resistor
draws 80µA which dominates the micropower current
budget. This situation is much worse in the gain of one
circuit with V
–
=0V. At 100V common mode, the output has
to sink 2µA. At a minimum output voltage of 20mV
competing devices require a 10k pull-down resistor. As
the output now swings to 10V, this resistor draws 1mA of
current.
Since the output of the LT1077 cannot go exactly to
ground, but can only approach ground to within a few
millivolts, care should be exercised to ensure that the
output is not saturated. For example, a 1mV input signal
will cause the amplifier to set up in its linear region in the
gain 100 configuration shown below; however, it is not
enough to make the amplifier function properly in the
voltage follower mode.
Voltage Follower with Input Exceeding the Negative Common Mode Range (V
S
= 5V, 0V)
Single supply operation can also create difficulties at the
input. The driving signal can fall below 0V—inadvertently
or on a transient basis. If the input is more than a few
hundred millivolts below ground, two distinct problems
can occur on previous single supply designs, such as the
LM124, LM158, OP-20, OP-21, OP-220, OP-221, OP-420
(a and b), OP-90/290/490 (b only):
a) When the input is more than a diode drop below
ground, unlimited current will flow from the substrate
(V
–
terminal) to the input (this can destroy the unit). On the
LT1077, resistors in series with the input protect the
device even when the input is 5V below ground.
b) When the input is more than 400mV below ground
(at 25°C), the input stage saturates and phase reversal
occurs at the output (this can cause lock-up in servo
systems). Due to a unique phase reversal protection
circuitry, the LT1077’s output does not reverse, as
illustrated below, even when the input is at –1.0V.
100mV
1mV
99R
5V
R
+
–
OUTPUT
SATURATED
≈ 3.5mV
1mV
5V
+
–
LT1077 • AI01
Gain 100 Amplifier Voltage Follower
1ms/DIV
2V
1ms/DIV
2V
2V
4V
4V
4V
0
0
0
LT1077
NO PHASE REVERSAL
OP-90
EXHIBITS OUTPUT
PHASE REVERSAL
1ms/DIV6V
P-P
INPUT
–1.0 TO 5.0V
