REV. B

–10–

OP281/OP481

APPLICATIONS

Theory of Operation

The OPx81 family of op amps is comprised of extremely low

powered, rail-to-rail output amplifiers, requiring less than 4

mA

of quiescent current per amplifier. Many other competitors’

devices may be advertised as low supply current amplifiers but

draw significantly more current as the outputs of these devices

are driven to a supply rail. The OPx81’s supply current remains

under 4

mA even with the output driven to either supply rail.

Supply currents should meet the specification as long as the

inputs and outputs remain within the range of the power supplies.

Figure 1 shows a simplified schematic of a single channel for the

OPx81. A bipolar differential pair is used in the input stage.

PNP transistors are used to allow the input stage to remain

linear with the common-mode range extending to ground. This

is an important consideration for single-supply applications. The

bipolar front end also contributes less noise than a MOS front

end with only nano-amps of bias currents. The output of the op

amp consists of a pair of CMOS transistors in a common source

configuration. This setup allows the output of the amplifier to

swing to within millivolts of either supply rail. The headroom

required by the output stage is limited by the amount of current

being driven into the load. The lower the output current, the

closer the output can go to either supply rail. TPCs 7, 8, and 9

show the output voltage headroom versus load current. This

behavior is typical of rail-to-rail output amplifiers.

+IN

–IN

OUT

Figure 1. Simplified Schematic of a Single OPx81 Channel

Input Overvoltage Protection

The input stage to the OPx81 family of op amps consists of a

PNP differential pair. If the base voltage of either of these input

transistors drops to more than 0.6 V below the negative supply,

the input ESD protection diodes will become forward biased,

and large currents will begin to flow. In addition to possibly

damaging the device, this will create a phase reversal effect at

the output. To prevent these effects from happening, the input

current should be limited to less than 0.5 mA.

This can be done quite easily by placing a resistor in series with

the input to the device. The size of the resistor should be pro-

portional to the lowest possible input signal excursion and can

be found using the following formula:

R

=

0.5

where:

For example, a single channel of the OPx81 is to be used with a

single-supply voltage of +5 V where the input signal could possibly

go as low as –1 V. Because the amplifier is powered from a single

be 2 k

W.

Input Offset Voltage

The OPx81 family of op amps was designed for low offset

voltages less than 1 mV.

OP281

+3V

100k

100k

100k

100k

400mV p-p

–0.1V

–0.27V

Figure 2. Single OPx81 Channel Configured as

Input Common-Mode Voltage Range

The OPx81 is rated with an input common-mode voltage range

Figure 2 shows a single OPx81 channel configured as a difference

amplifier with a single-supply voltage of 3 V. Negative dc voltages

are applied at both input terminals creating a common-mode volt-

age that is less than ground. A 400 mV p-p input signal is then

applied to the noninverting input. Figure 3 shows the input and

output waves. Notice how the output of the amplifier also drops

slightly negative without distortion.

10

0%

100

90

0.2ms

0.1V

0V