8

LT1797

APPLICATIO S I FOR ATIO

Supply Voltage

The positive supply pin of the LT1797 should be bypassed

with a small capacitor (about 0.1

µF) within an inch of the

pin. When driving heavy loads an additional 4.7

µF 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

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.

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

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.