REV. B

OP281/OP481

–13–

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 k

W and 11 kW resistors. The gain bandwidth product of the

amplifier is 95 kHz, which, for the set gain of 28, yields a –3dB

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 k

W 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 M

W

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 k

W potentiometer that is used to adjust the volume. Again,

two 1 M

W 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