REV. B

OP183/OP283

Low Noise Microphone Amplifier for Multimedia

The OP183 family is ideally suited as a low noise microphone

preamp for low voltage audio applications. Figure 40 shows a gain

of 100 stereo preamp for the AD1849 16-bit SoundPort Stereo

Codec chip. The common-mode output buffer serves as a “phan-

tom power” driver for the microphones.

bandwidth and is not sensitive to false-ground perturbations. The

simple false-ground circuit shown achieves good rejection of low

frequency interference using standard off-the-shelf components.

Amplifier A3 biases A1 and A2 to the middle of their input

common-mode range. When operating on a +3 V supply, the

center of the OP283’s common-mode range is 0.75 V. This notch

filter effectively squelches 60 Hz pickup at a filter Q of 0.75. To

reject 50 Hz interference, simply change the resistors in the twin-T

section (R1 through R5) from 2.67 k

Ω to 3.16 kΩ.

The filter section uses an OP283 dual op amp in a twin-T configu-

ration whose frequency selectivity is very sensitive to the relative

matching of the capacitors and resistors in the twin-T section.

Mylar is the material of choice for the capacitors, and the relative

matching of the capacitors and resistors determines the filter’s pass

band symmetry. Using 1% resistors and 5% capacitors produces

satisfactory results.

A Low Voltage Frequency Synthesizer for Wireless

Transceiver

The OP183’s low noise and the low voltage operation capability

serves well for the loop filter of a frequency synthesizer. Figure 42

shows a typical application in a radio transceiver. The phase noise

performance of the synthesizer depends on low noise contribution

from each component in the loop as the noise is amplified by the

frequency division factor of the prescaler.

CRYSTAL

+3V

VARACTER

DIODE

900MHz

REFERENCE

OSCILLATOR

PHASE

DETECTOR

÷

PRESCALER

RF

OUT

V

CONTROL

VCO

OP183

Figure 42. A Low Voltage Frequency Synthesizer for a

Wireless Transceiver

The resistors used in the low-pass filter should be of low to

moderate values to reduce noise contribution due to the input bias

current as well as the resistors themselves. The filter cutoff

frequency should be chosen to optimize the loop constant.

+5V

1/2 OP283

10µF

+5V

1/2 OP213

10µF

1/2 OP283

15

17 MINL

MINR

19 CMOUT

AD1849

LEFT

ELECTRET

CONDENSER

MIC

INPUT

RIGHT

ELECTRET

CONDENSER

MIC

INPUT

Figure 40. Low Noise Stereo Microphone Amplifier for

Multimedia Sound CODEC

A +3 Volt 50 Hz/60 Hz Active Notch Filter with False Ground

To process ac signals, it may be easier to use a false-ground bias

rather than the negative supply as a reference ground. This would

reject the power-line frequency interference which oftentimes can

obscure low frequency physiological signals, such as heart rates,

blood pressures, EEGs, ECGs, et cetera.

25k

Ω

1k

Ω

8

1

4

3

2

A1

+3V

C1

1µF

2.67k

Ω

2.67k

Ω

C2

1µF

2.67k

Ω

1/2 OP283

10k

Ω

V

IN

Ω

R5

1.33k

Ω

(2.67k

Ω ÷ 2)

2µF

(1µF

× 2)

7

6

5

A2

V

O

1/2 OP283

1k

Ω

1

3

2

A3

OP183

C5

0.015µF

10k

Ω

70

Ω

0.75V

C6

1µF

+3V

C4

1µF

A1, A2, AND A3 = 1/2 OP283

Ω

Q = 0.75

NOTE: FOR 50Hz APPLICATIONS

CHANGE R1–R4 TO 3.1k

Ω

AND R5 TO 1.58k

Ω (3.16kΩ ÷ 2).

Ω

R2

2.67k

Ω

75k

Ω

Figure 41. +3 Volt Supply 50 Hz/60 Hz Notch Filter with

Pseudo Ground

Figure 41 shows a 50 Hz/60 Hz active notch filter for eliminating

line noise in patient monitoring equipment. It has several kilohertz

–10–