OP193/OP293

Rev. C | Page 13 of 20

FUNCTIONAL DESCRIPTION

The OP193/OP293 operational amplifiers are single-supply,

micropower, precision amplifiers whose input and output ranges

maximum, while the output delivers ±5 mA to a load. Supply

current is only 15 μA.

A simplified schematic of the input stage is shown in Figure 26.

The input transistors, Q1 and Q2, are PNP devices, which permit

the inputs to operate down to ground potential. The input transis-

tors have resistors in series with the base terminals to protect

the junctions from overvoltage conditions. The second stage is

an NPN cascode that is buffered by an emitter follower before

driving the final PNP gain stage.

The OP193 includes connections to taps on the input load resis-

The OP293 have two additional transistors, Q7 and Q8. The

behavior of these transistors is discussed in the Output Phase

Reversal—OP193 and Output Phase Reversal—OP293 sections.

The output stage, shown in Figure 25, is a noninverting NPN

totem-pole configuration. Current is sourced to the load by

Emitter Follower Q1, while Q2 provides current sink capability.

When Q2 saturates, the output is pulled to within 5 mV of

ground without an external pull-down resistor. The totem-pole

output stage supplies a minimum of 5 mA to an external load,

even when operating from a single 3.0 V power supply.

Q4

Q1

Q5

Q3

Q2

OUTPUT

I1

I2

I3

V+

FROM

INPUT

STAGE

V–

Figure 25. OP193/OP293 Equivalent Output Circuit

By operating as an emitter follower, Q1 offers a high impedance

load to the final PNP collector of the input stage. Base drive to

Q2 is derived by monitoring Q1’s collector current. Transistor

Q5 tracks the collector current of Q1. When Q1 is on, Q5 keeps

Q4 off, and Current Source I1 keeps Q2 turned off. When Q1 is

driven to cutoff (that is, the output must move toward V−), Q5

allows Q4 to turn on. Q4’s collector current then provides the

base drive for Q3 and Q2, and the output low voltage swing is

DRIVING CAPACITIVE LOADS

The OP193/OP293 amplifiers are unconditionally stable with

capacitive loads less than 200 pF. However, the small signal,

unity-gain overshoot improves if a resistive load is added. For

example, transient overshoot is 20% when driving a 1000 pF,

10 kΩ load. When driving large capacitive loads in unity-gain

configurations, an in-the-loop compensation technique is

recommended, as illustrated in Figure 30.

2kΩ

2kΩ

+INPUT

Q1

Q7

Q8

Q3

Q4

D1

I2

I3

I4

Q5

Q6

I5

I6

V–

V+

I1

TO

OUTPUT

STAGE

Q2

NULLING

TERMINALS

(OP193 ONLY)

–INPUT

OP293

ONLY

Figure 26. OP193/OP293 Equivalent Input Circuit