LT1797
8
1797fb
APPLICATIONS INFORMATION
Supply Voltage
The positive supply pin of the LT1797 should be bypassed
with a small capacitor (about 0.1µF) within an inch of the
pin. When driving heavy loads an additional 4.7µF electro-
lytic capacitor should be used. When using split supplies
the same is true for the negative supply pin.
Inputs
The LT1797 is fully functional for an input signal range
from the negative supply to the positive supply. Figure 1
shows a simplified schematic of the amplifier. The input
stage consists of two differential amplifiers, a PNP stage
Q3/Q4 and an NPN stage Q1/Q2 that are active over dif-
ferent ranges of input common mode voltage. The PNP
differential pair is active for input common mode voltages
V
CM
between the negative supply to approximately 1.3V
below the positive supply. As V
CM
moves closer toward
the positive supply, the transistor QB1 will steer the tail
current I1 to the current mirror Q5/Q6, activating the
NPN differential pair and the PNP pair becomes inactive
for the rest of the input common mode range up to the
positive supply.
The input offset voltage and the input bias current are
dependent on which input stage is active. The input offset
voltage is trimmed on a single 5V supply with the common
mode at 1/2 supply and is typically 1mV with the PNP stage
active. The input offset of the NPN stage is untrimmed and
is typically 1.5mV. The input bias current polarity depends
on the input common mode voltage. When the PNP dif-
ferential pair is active, the input bias currents flow out of
the input pins. They flow in the opposite direction when
the NPN input stage is active. The offset error due to the
input bias currents can be minimized by equalizing the
noninverting and inverting source impedance.
The input stage of the LT1797 incorporates phase reversal
protection to prevent false outputs from occurring when
the inputs are driven up to 5V beyond the rails. Protective
resistors are included in the input leads so that current
does not become excessive when the inputs are forced
beyond the supplies or when a large differential signal is
applied.
Output
The output is configured with a pair of complementary
common emitter stages Q19/Q20, which enable the output
to swing from rail-to-rail. The output voltage swing of the
LT1797 is affected by input overdrive as shown in the Typi-
cal Performance Characteristics. When monitoring input
voltages within 50mV of V
+
or within 8mV of V
–
, some
gain should be taken to keep the output from clipping. The
output of the LT1797 can deliver large load currents; the
short-circuit current limit is typically 50mA at ±5V. Take
care to keep the junction temperature of the IC below the
absolute maximum rating of 150°C. The output of the
amplifier has reverse biased diodes to each supply. If the
output is forced beyond either supply, unlimited current
will flow through these diodes.
The LT1797 can drive capacitive loads up to 200pF on
a single 5V supply in a unity gain configuration. When
there is a need to drive larger capacitive loads, a resistor
of a couple hundred ohms should be connected between
the output and the capacitive load. The feedback should
still be taken from the output so that the resistor isolates
the capacitive load to ensure stability. The low input bias
current of the LT1797 makes it possible to use high value
feedback resistors to set the gain. However, care must
be taken to insure that the pole formed by the feedback
resistors and the total capacitance at the inverting input
does not degrade stability.
