OP777

–10–

REV. 0

BASIC OPERATION

The OP777 amplifier uses a precision Bipolar PNP input stage

coupled with a high-voltage CMOS output stage. This enables

this amplifier to feature an input voltage range which includes the

negative supply voltage (often ground-in single-supply applications)

and also swing to within 1 mV of the output rails. Additionally, the

input voltage range extends to within 1 V of the positive supply rail.

The epitaxial PNP input structure provides high breakdown voltage,

high gain, and input bias current figure comparable to that obtained

with “Darlington” input stage amplifier but without the drawbacks

(i.e., severe penalties for input voltage range, offset, drift and noise).

PNP input structure also greatly lowers the noise and reduces the dc

input error terms.

Supply Voltage

The amplifiers are fully specified with a single 5 V supply and, due

to design and process innovations, can also operate with a supply

voltage from 2.7 V up to 30 V. This allows operation from most

split supplies used in current industry practice, with the advantage

of substantially increased input and output voltage ranges over

conventional split-supply amplifiers. The OP777 series is specified

for single supply application. With PSRR of 130 dB (0.3

µV/V) and

CMRR of 110 dB (3

µV/V) offset is minimally affected by power

supply or common-mode voltages. Dual supply,

±15 V operation

is also fully specified.

Input Common-Mode Voltage Range

The OP777 is rated with an input common-mode voltage which

extends from minus supply to 1 V of the Positive supply. However,

the amplifier can still operate with input voltages slightly below

with a single supply of 2.7 V and negative dc common-mode volt-

ages applied at the inputs terminals. A 400 mV p-p input is then

applied to the noninverting input. It can be seen from the graph

below that the output does not show any distortion. Micropower

operation is maintained by using large input and feedback resistors.

TIME – 0.2ms/DIV

0V

+3V

OP777

100k

100k

100k

100k

0.1V

0.27V

Figure 43. OP777 Configured as a Difference Amplifier

Input Over Voltage Protection

When the input of an amplifier is more than a diode drop below

input pins which can destroy the device. In the case of OP777,

differential voltage equal to the supply voltage will not cause any

problem (see Figure 44). OP777 has built in 500

Ω internal current

limiting resistors, in series with the inputs, to minimize the chances

of damage. It is a good practice to keep the current flowing into the

inputs below 5 mA. In this context it should also be noted that the

high breakdown of the input transistors removes the necessity for

clamp diodes between the inputs of the amplifier; a feature that is

mandatory on many precision op amps. Unfortunately, such

clamp diodes greatly interfere with many application circuits such

as precision rectifiers and comparators. The OP777 series is free

from such limitations.

30V

V p-p = 32 V

OP777

Figure 44a. Unity Gain Follower

TIME – 400 s/DIV

Figure 44b. Input Voltage Can Exceed the Supply Voltage

Without Damage