OP481GSZ datasheet(16/20 Pages) AD

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OP281/OP481

Rev. C | Page 16 of 20

OP281-B

20kΩ

600Ω

SPEAKER

50kΩ

10kΩ

1µF

10kΩ

POT.

1MΩ

1µF

INPUT

1MΩ

OP281-A

11kΩ

300kΩ

0.1µF

1µF

1MΩ

1µF

MIC OUT

2.2kΩ

ELECTRET

MIC

audio bandwidth. A 2.2 kΩ resistor is used to bias the electret

microphone. This resistor value may vary depending on the

specifications of the microphone. The output of the microphone is

ac-coupled to the noninverting terminal of the op amp. Two 1 MΩ

resistors are used to provide the dc offset for single-supply use.

The OP281-B amplifier (see Figure 45) can provide up to 15 dB of

gain for the headset speaker. Incoming audio signals are ac-coupled

to a 10 kΩ potentiometer that is used to adjust the volume. Again,

two 1 MΩ resistors provide the dc offset with a 1 μF capacitor

establishing an ac ground for the volume-control potentiometer.

Because the OP281 is a rail-to-rail output amplifier, it would have

difficulty driving a 600 Ω speaker directly. Here, a Class AB buffer

is used to isolate the load from the amplifier and to provide the

necessary current to drive the speaker. By placing the buffer in

the feedback loop of the op amp, crossover distortion can be

minimized. Q1 and Q2 should have minimum betas of 100. The

600 Ω speaker is ac-coupled to the emitters to prevent quiescent

current from flowing into the speaker. The 1 μF coupling capacitor

makes an equivalent high-pass filter cutoff at 265 Hz with a 600 Ω

load attached. Again, this does not pose a problem because it is

outside the frequency range for telephone audio signals.

Figure 45. Two-Way Amplifier in a Battery-Powered Telephone Headset

The OP281-A op amp provides about 29 dB of gain for audio

signals coming from the microphone. The gain is set by the

300 kΩ and 11 kΩ resistors. The gain bandwidth product of the

amplifier is 95 kHz, which yields a −3 dB rolloff at 3.4 kHz for

the set gain of 28. This is acceptable because telephone audio is

band limited for 300 kHz to 3 kHz signals. If higher gain is

required for the microphone, an additional gain stage should be

used, because adding more gain to the OP281 would limit the

The circuit in Figure 45 draws around 250 μA of current. The

Class AB buffer has a quiescent current of 140 μA, and roughly

100 μA is drawn by the microphone itself. A CR2032 3 V

lithium battery has a life expectancy of 160 mA hours, which

means this circuit can run continuously for 640 hours on a

single battery.

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