MS-2212

Technical Article

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©2011 Analog Devices, Inc. All rights reserved.

November 2011 | Page 4 of 5

Table 1.

Competitor

Type

Spec (mA)

Follower (mA)

Vol (mA)

Voh (mA)

LM358

Bipolar

30V

2

0.707

0.506

0.671

LM393

Bipolar

36V

2.5

0.548

0.565

0.567

OP184

Bipolar

30V

2

1.239

1.188

6.683

A

Bipolar

24V

0.45

0.361

3.442

0.708

B

Bipolar

30V

3.4

2.785

2.051

3.998

C

Bipolar

30V

4.5

4.063

5.336

3.786

AD8605

CMOS

5V

1.2

0.998

0.544

0.625

A

CMOS

5V

0.9

0.511

0.361

10.152

B

CMOS

5V

2.4

1.916

2.759

2.475

C

CMOS

5V

1.4

1.039

0.822

0.667

Red highlighted values indicate exceeds data sheet limit.

Figure 6.

CMOS Rail-to-Rail Op Amps

The CMOS op amps have an interesting behavior. In some

cases, the supply current actually goes down when driven to

a rail. The output stage of a CMOS op amp consists of

common source PMOS and NMOS transistors, and gain is

reasonable value of transconductance, the drive circuit is

designed to set the quiescent current to a certain value. As

the output is driven into the rail, the drive circuit will

decrease the drive on the complementary transistor.

Depending on the transfer characteristics from the top

transistor to the bottom transistor, the current will actually

decrease. Note the wide variation in behavior among the

four CMOS op amps selected.

Finally, in the desire to reduce die size and, therefore, cost,

some circuits, such as bias circuits and the associated startup

circuit, may be shared by both op amps. As mentioned

previously (8), if one op amp operates outside of its normal

range and causes the bias circuit to malfunction, then the

other op amp will malfunction also.

In battery operated systems or when using low current series

regulators, the additional supply current should be

considered. Battery life may be less than calculated, or the

regulator may not start up under all conditions, especially

over temperature.

Tips

For new designs, the easiest solution is “Don’t use op amps

as comparators!” If you must, or have used one by accident

as a comparator:

•

Check the data sheet to see if the manufacturer

has any information on operation as a

comparator. Some manufacturers are adding

this information (9,10).

•

If the information is not there, ask the

manufacturer if it is available.

•

If they cannot provide it, measure several date

codes yourself using the circuits shown previously,

and add 50% for a safety factor.

V+

I2

I1

Q1

Q3

Q4

Q2

V–

Q5

Q6

R6

R3

R2

R1

R4

R5

D1

INPUT FROM

SECOND GAIN

STAGE