REV. B

–12–

OP281/OP481

Low-Side Current Monitor

In the design of power supply control circuits, a great deal of

design effort is focused on ensuring a pass transistor’s long-term

reliability over a wide range of load current conditions. As a

result, monitoring and limiting device power dissipation is of

prime importance in these designs. Figure 7 shows an example

of a 5 V, single-supply current monitor that can be incorporated

into the design of a voltage regulator with fold-back current

limiting or a high current power supply with crowbar protection.

The design capitalizes on the OPx81’s common-mode range

that extends to ground. Current is monitored in the power sup-

ply return path where a 0.1

very small voltage drop. The voltage at the inverting terminal

becomes equal to the voltage at the noninverting terminal through

the feedback of Q1, which is a 2N2222 or equivalent NPN tran-

sistor. This makes the voltage drop across R1 equal to the voltage

voltage is given by:

R2

R1 ¥

Ê

ËÁ

ˆ

¯˜

decreases with higher supply current being sensed. For the

5V

RETURN TO

GROUND

5V

R2

2.49k

R1

100

0.1

Q1

SINGLE

CHANNEL

OPx81

Figure 7. Low-Side Load Current Monitor

Low Voltage Half-Wave and Full-Wave Rectifiers

Because of its quick overdrive recovery time, an OP281 can be

configured as a full-wave rectifier for low frequency (<500 Hz)

applications. Figure 8 shows the schematic.

3V

OP281-A

2k

A1

3V

OP281-B

A2

R1 = 100k

R2 = 100k

FULL-WAVE

RECTIFIED

OUTPUT

HALF-WAVE

RECTIFIED

OUTPUT

Figure 8. Single-Supply Full-Wave and Half-Wave

Rectifiers Using an OP281

10

0%

100

90

SCALE 0.1V/DIV

SCALE 0.1ms/DIV

Figure 9. Full-Wave Rectified Signal

Amplifier A1 is used as a voltage follower that will track the input

voltage only when it is greater than 0 V. This provides a half-wave

rectification of the input signal to the noninverting terminal of

amplifier A2. When A1’s output is following the input, the invert-

ing terminal of A2 will also follow the input from the virtual

ground between the inverting and noninverting terminals of A2.

With no potential difference across R1, no current flows through

either R1 or R2, therefore the output of A2 will also follow the input.

Now, when the input voltage goes below 0 V, the noninverting

terminal of A2 becomes 0 V. This makes A2 work as an inverting

amplifier with a gain of 1 and provides a full-wave rectified version

of the input signal. A 2 k

W resistor in series with A1’s noninverting

input protects the device when the input signal becomes less

than ground.

Battery-Powered Telephone Headset Amplifier

Figure 10 shows how the OP281 can be used as a two-way

amplifier in a telephone headset. One side of the OP281 can be

used as an amplifier for the microphone, while the other side can

be used to drive the speaker. A typical telephone headset uses a

600

W speaker and an electret microphone that requires a supply

voltage and a biasing resistor.

3V

OP281-B

20k

20k

Q1

Q2

3V

1 F

600

SPEAKER

50k

10k

1 F

10k

POT.

3V

1M

1M

1 F

1 F

INPUT

1M

3V

OP281-A

11k

300k

0.1 F

1 F

3V

1M

1 F

MIC OUT

2.2k

3V

ELECTRET

MIC

Figure 10. Two-Way Amplifier in a Battery-Powered

Telephone Headset