LT1797 Linear Technology, LT1797 Datasheet - Page 8

• Manufacturer Part Number: LT1797
• Description: 10MHz/ Rail-to-Rail Input and Output Op Amp in SOT-23
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1797
Supply Voltage
The positive supply pin of the LT1797 should be bypassed
with a small capacitor (about 0.1 F) within an inch of the
pin. When driving heavy loads an additional 4.7 F 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
different ranges of input common mode voltage. The PNP
differential pair is active for input common mode voltages
V
below the positive supply. As V
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
differential 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.
8
CM
between the negative supply to approximately 1.2V
U
U
CM
W
moves closer toward
U
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 out-
put to swing from rail-to-rail. The output voltage swing of
the LT1797 is affected by input overdrive as shown in the
Typical Performance Characteristics. When monitoring
input voltages within 50mV 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
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.
5V. Take care to keep the junction temperature of the IC
+
or within 8mV of V
–
,