OP281/OP481
Rev. D | Page 16 of 20
00291-045
3V
OP281-B
20kΩ
20kΩ
Q1
Q2
3V
600Ω
SPEAKER
50kΩ10kΩ
1µF
10kΩ
POT.
3V
1MΩ 1MΩ
1µF
1µF
INPUT
1MΩ
3V
OP281-A
11kΩ 300kΩ
0.1µF
1µF
1µF
3V
1MΩ
1µF
MIC OUT
2.2kΩ
3V
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.
