MS-2212

Technical Article

www.analog.com

©2011 Analog Devices, Inc. All rights reserved.

November 2011 | Page 2 of 5

input protection diodes, phase reversal, and several other op

amp characteristics but argues that careful attention to these

details can pay off. He does briefly mention RRO op amps,

but not supply current.

As supply voltages decreased, one of the methods used to try

to maintain a large voltage swing, was to convert the classic

output stage to a “rail to rail” output stage. A classic output

stage is shown in Figure 1. Referring to the non-rail-to-rail

output, the output can only get within about 1 V of the

positive supply.

To get closer to the rails, the output stage transistors were

changed to a common emitter configuration as shown in

Figure 2.

Figure 2. Bipolar rail-to-rail output

The “rail-to-rail” output is not really rail-to-rail, but can get

within 50 mV to 100 mV of the supply depending on the

size of the output transistor and the load current.

Comparing these two output stages, there are three

important things to note: First, the classic output stage has

current gain, but a voltage gain less than one, and very low

output impedance. Second, the rail-to-rail output stage is a

common emitter stage and, thus, has voltage gain,

output operating more than several hundred millivolts away

but not if the output is close to the rail. Third, the output can

be considered as a classic two transistor ratioed current

mirror. This is the crux of the problem.

In normal operation, the middle stage will pull the base-

collector node down, driving more current into the load and

raising the voltage. With negative feedback, as the output

voltage rises, the input stage and middle stage will reduce the

drive until the closed loop is balanced.

When used as a comparator, the middle stage will pull the

base-collector node down, trying to close the loop, but with

no feedback, it continues to pull harder and harder. This

additional current finds a path from the positive supply pin

to the negative supply pin and appears as additional supply

current. There are several different ways of driving the

output stage, and combined with the difference in mobility

between holes and electrons, the increase in supply current

is usually not symmetrical.

To quantify this effect, a bipolar op amp and a CMOS op

amp were obtained from Analog Devices and three of its

major analog competitors. For comparison purposes, the

venerable LM358 dual op amp (non-RRO) and LM393 dual

comparator were also included. The supply current was

measured as a function of supply voltage using three circuits.

Figure 3 shows the classic method for measuring supply

current. The ammeters are connected as shown so that the

supply current of the resistive divider is not included.

Figure 3.

Two ammeters are used to verify that the supply current is

accurate and does not include any undesired current path

through the input pins. The resistor values are noncritical,

and are selected to ensure that the input to the op amp is