OP181_15 Datasheet, PDF (13/16 Pages) Analog Devices – Ultralow Power, Rail-to-Rail Output Operational Amplifiers

OP181/OP281/OP481
Amplifier A1 is used as a voltage follower that will only track the ^{The OP281-B amplifier can provide up to 15 dB of gain for the}
input voltage when it is greater than 0 V. This provides a half-
headset speaker. Incoming audio signals are ac coupled to a
wave rectification of the input signal to the noninverting
10 kΩ potentiometer that is used to adjust the volume. Again,
terminal of amplifier A2. When A1’s output is following the
two 1 MΩ resistors provide the dc offset with a 1 µF capacitor
input, the inverting terminal of A2 will also follow the input
establishing an ac ground for the volume control potentiometer.
from the virtual ground between the inverting and noninverting ^{Because the OP281 is a rail-to-rail output amplifier, it would}
terminals of A2. With no potential difference across R1, no
have difficulty driving a 600 Ω speaker directly. Here, a class AB
current flows through either R1 or R2, therefore the output of
buffer is used to isolate the load from the amplifier and also
A2 will also follow the input. Now, when the input voltage goes ^{provide the necessary current drive to the speaker. By placing}
below 0 V, the noninverting terminal of A2 becomes 0 V. This
the buffer in the feedback loop of the op amp, crossover
makes A2 work as an inverting amplifier with a gain of 1 and
distortion can be minimized. Q1 and Q2 should have minimum
provides a full-wave rectified version of the input signal. A 2 kΩ ^{betas of 100. The 600 Ω speaker is ac coupled to the emitters to}
resistor in series with A1’s noninverting input protects the
prevent any quiescent current from flowing in the speaker. The
OBSOLETE device when the input signal becomes less than ground.
A Battery Powered Telephone Headset Amplifier
Figure 43 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 Ω speaker and an electret microphone that requires a
supply voltage and a biasing resistor.
0.1µF _11kΩ
+3V
+3V
2.2kΩ
1µF ^1MΩ
ELECTRET _1MΩ
MIC
300kΩ
+3V
1µF
OP281-A
MIC OUT
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, as it is outside the frequency range for
telephone audio signals.
The circuit in Figure 43 draws around 250 µA of current. The
class AB buffer has a quiescent current of 140 µA while 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 could run continuously for 640 hours on a
single battery.
SPICE Macro-Model
* OP181 SPICE Macro-model
* 9/96, Ver. 1
*
* Copyright 1996 by Analog Devices
*
* Refer to “README.DOC” file for License Statement. Use of this
1µF _10kΩ
50kΩ
* model indicates your acceptance of the terms and provisions in
* the License Statement.
+3V
*
* Node Assignments
20kΩ
*
*
+3V
INPUT ^1µF
Q1
*
1µF
*
noninverting input
| inverting input
| | positive supply
| | | negative supply
10kΩ
+3V _POT.
1MΩ
1µF
1MΩ
OP281-B
20kΩ
Q2
600Ω
SPEAKER
*
| | | | output
*
| | | ||
*
| | | ||
.SUBCKT OP181 1 2 99 50 45
*
* INPUT STAGE
Figure 43. A Battery Powered Telephone Headset
Two-Way Amplifier
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, for the set gain of 28, yields a –3 dB
rolloff at 3.4 kHz. This is acceptable since 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, as adding any more gain to the OP281 would limit the
audio bandwidth. A 2.2 kΩ resistor is used to bias the electret
microphone. This resistor value may vary depending on the
specifications on the microphone being used. 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.
*
Q1 4 1
Q2 6 7
I1 99 8
EOS 7 2
IOS 1 2
RC1 4 50
RC2 6 50
RE1 3 8
RE2 5 8
V1 99 13
V2 99 14
D1 3 13
D2 5 14
*
* CMRR 76dB,
*
3 PIX
5 PIX
1.28E-6
POLY(1)
1E-10
500E3
500E3
108
108
DC .9
DC .9
DX
DX
(12, 98) 80E-6 1
ZERO AT 1kHz
REV. 0
–13–