OP196/OP296/OP496

REV.

–13–

A Micropower False-Ground Generator

Some single supply circuits work best when inputs are biased

above ground, typically at 1/2 of the supply voltage. In these

cases, a false-ground can be created by using a voltage divider

buffered by an amplifier. One such circuit is shown in Figure 5.

This circuit will generate a false-ground reference at 1/2 of the

supply voltage, while drawing only about 55

µA from a 5 V

supply. The circuit includes compensation to allow for a 1

µF

bypass capacitor at the false-ground output. The benefit of a

large capacitor is that not only does the false-ground present a

very low dc resistance to the load, but its ac impedance is low as well.

6

2

3

10k

OP196

100

4

5V OR 12V

0.022 F

1 F

240k

240k

1 F

2.5V OR 6V

7

Figure 5. A Micropower False-Ground Generator

Single-Supply Half-Wave and Full-Wave Rectifiers

An OP296, configured as a voltage follower operating from a

single supply, can be used as a simple half-wave rectifier in low

frequency (<400 Hz) applications. A full-wave rectifier can be

configured with a pair of OP296s as illustrated in Figure 6.

A1

8

1

3

4

5V

1/2

OP296

2k

2

A2

5

6

2Vp-p

<500Hz

7

1/2

OP296

R1

100k

R2

100k

VOUTA

FULL-WAVE

RECTIFIED

OUTPUT

VOUTB

HALF-WAVE

RECTIFIED

OUTPUT

10

0%

100

90

500mV

1V

500µs

500mV

f = 500Hz

INPUT

VOUTB

(HALF-WAVE

OUTPUT)

VOUTA

(FULL-WAVE

OUTPUT)

Figure 6. Single-Supply Half-Wave and Full-Wave

Rectifiers Using an OP296

The circuit works as follows: When the input signal is above

0 V, the output of amplifier A1 follows the input signal. Since

the noninverting input of amplifier A2 is connected to A1’s

output, op amp loop control forces A2’s inverting input to the

same potential. The result is that both terminals of R1 are at the

same potential and no current flows in R1. Since there is no

current flow in R1, the same condition must exist in R2; thus,

the output of the circuit tracks the input signal. When the input

signal is below 0 V, the output voltage of A1 is forced to 0 V.

This condition now forces A2 to operate as an inverting voltage

follower because the noninverting terminal of A2 is also at 0 V.

The output voltage of VOUTA is then a full-wave rectified

version of the input signal. A resistor in series with A1’s

noninverting input protects the ESD diodes when the input

signal goes below ground.

Square Wave Oscillator

The oscillator circuit in Figure 7 demonstrates how a rail-to-rail

output swing can reduce the effects of power supply variations

on the oscillator’s frequency. This feature is especially valuable

in battery powered applications, where voltage regulation may

not be available. The output frequency remains stable as the

supply voltage changes because the RC charging current, which

is derived from the rail-to-rail output, is proportional to the

supply voltage. Since the Schmitt trigger threshold level is also

proportional to supply voltage, the frequency remains relatively

independent of supply voltage. For a supply voltage change

from 9 V to 5 V, the output frequency only changes about 4 Hz.

The slew rate of the amplifier limits the oscillation frequency to

a maximum of about 200 Hz at a supply voltage of 5 V.

59k

1/2

OP296/

OP496

100k

100k

FREQ OUT

fOSC =

< 200Hz @ V+ = 5V

1

RC

C

V+

R

2

3

4

8

1

Figure 7. Square Wave Oscillator Has Stable Frequency

Regardless of Supply Voltage Changes

A 3 V Low Dropout, Linear Voltage Regulator

Figure 8 shows a simple 3 V voltage regulator design. The regu-

lator can deliver 50 mA load current while allowing a 0.2 V

dropout voltage. The OP296’s rail-to-rail output swing easily

drives the MJE350 pass transistor without requiring special

drive circuitry. With no load, its output can swing to less than

the pass transistor’s base-emitter voltage, turning the device

nearly off. At full load, and at low emitter-collector voltages, the

transistor beta tends to decrease. The additional base current is

easily handled by the OP296 output.

The AD589 provides a 1.235 V reference voltage for the regula-

tor. The OP296, operating with a noninverting gain of 2.43,

drives the base of the MJE350 to produce an output voltage of

3.0 V. Since the MJE350 operates in an inverting (common-

emitter) mode, the output feedback is applied to the OP296’s

noninverting input.

E