OP281 Datasheet, PDF (13/16 Pages) Analog Devices – Ultralow Power, Rail-to-Rail Output Operational Amplifiers

OP281/OP481
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 kW and 11 kW 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 band-
width. A 2.2 kW 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 MW
resistors are used to provide the dc offset for single-supply use.
The OP281-B amplifier can provide up to 15 dB of gain for the
headset speaker. Incoming audio signals are ac-coupled to a
10 kW potentiometer that is used to adjust the volume. Again,
two 1 MW resistors provide the dc offset with a 1 mF 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 W speaker directly. Here, a class AB buffer
is used to isolate the load from the amplifier and also provide
the necessary current drive to 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 W speaker is ac-coupled to the emitters to prevent any qui-
escent current from flowing in the speaker. The 1 mF coupling
capacitor makes an equivalent high-pass filter cutoff at 265 Hz
with a 600 W load attached. Again, this does not pose a problem,
as it is outside the frequency range for telephone audio signals.
The circuit in Figure 10 draws around 250 mA of current. The
class AB buffer has a quiescent current of 140 mA while roughly
100 mA 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
Single OPx81 Channel SPICE Macro-model
* 9/96, Ver. 1
*
* Copyright 1996 by Analog Devices
*
* Refer to “README.DOC” file for License Statement. Use of this
* model indicates your acceptance of the terms and provisions in the
* License Statement.
*
* Node Assignments
*
noninverting input
*
| inverting input
*
| | positive supply
*
| | | negative supply
*
| | | | output
*
| | | ||
*
| | | ||
.SUBCKT OPx81 1 2 99 50 45
*
* INPUT STAGE
*
Q1 4 1 3 PIX
Q2 6 7 5 PIX
I1 99 8 1.28E-6
EOS 7 2 POLY(1) (12, 98) 80E-6 1
IOS 1 2 1E-10
RC1 4 50 500E3
RC2 6 50 500E3
RE1 3 8 108
RE2 5 8 108
V1 99 13 DC .9
V2 99 14 DC .9
D1 3 13 DX
D2 5 14 DX
*
* CMRR 76dB, ZERO AT 1kHz
*
ECM1 11 98 POLY(2) (1, 98) (2, 98) 0 .5 .5
R1 11 12 1.59E6
C1 11 12 100E-12
R2 12 98 283
*
* POLE AT 900kHz
*
EREF 98 0 (90, 0) 1
G1 98 20 (4, 6) 1E-6
R3 20 98 1E6
C2 20 98 177E-15
*
* POLE AT 500kHz
*
E2 21 98 (20, 98) 1
R4 21 22 1E6
C3 22 98 320E-15
*
* GAIN STAGE
*
CF 45 40 8. 5E-12
R5 40 98 65. 65E6
G3 98 40 (22, 98) 4.08E-7
D3 40 41 DX
D4 42 40 DX
V3 99 41 DC 0.5
V4 42 50 DC 0.5
*
* OUTPUT STAGE
*
ISY 99 50 1.375E-6
RS1 99 90 10E6
RS2 90 50 10E6
M1 45 46 99 99 POX L=1.5u W=300u
M2 45 47 50 50 NOX L=1.5u W=300u
EG1 99 46 POLY(1) (98, 40) 0.77 1
EG2 47 50 POLY(1) (40, 98) 0.77 1
*
* MODELS
*
.MODEL POX PMOS (LEVEL=2, KP=25E-6, VTO=-0.75,
LAMBDA=0.01)
.MODEL NOX NMOS (LEVEL=2, KP=25E-6, VTO=0.75,
LAMBDA=0.01)
.MODEL PIX PNP (BF=200)
.MODEL DX D(IS=1E-14)
.ENDS
REV. B
–13–