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NJW1124

Voice Switched Speakerphone circuit

Japan Radio Corporation  

NJW1124

5.Line Amplifier

Line Amplifier transmits the signal from Tx attenuator to satellite unit. Line Amplifier consists of two operational

Amplifiers.

First Amplifier non-inverting input keeps reference voltage inside. First Amplifier is used as inverting amplifier.

Second Amplifier includes –1 fixed Gain.

These two amplifiers enable to differential output from single-ended signal.

C9 R6

100n 5.1k

TXO

LII

R7

51k

C10

LiO-

LiO+

Line Out

-1

VREF

Line Amplifier

Fig.5 Line Amplifier Block(26dB Application)

Outside parts

Function

C9

DC decoupling

R6

R7

Gain Setting

C10

oscillation prevention

recommend value

100nF～10µF

3kΩ～300kΩ

10p～100pF

Detail

-

Gv=R7/R6

Input impedance＝R6

Memo

Shape HPF : fc=1/(2π×C9×R6)

Recommend gain less than :26dB

Large resistance value may cause oscillating.

Shape LPF : fc=1/(2π×C10×R7)

Line Amplifier may oscillates, long transmission path becoming large capacitive load. In this case, add ceramic

capacitor (47p to 100p) between LII and LIO-.

Add it as close as possible to the terminal. The frequency should be cut more than you need.

LIO+,LIO- should not be short to GND. LIO+,LIO- terminal are biased to V+/2).

C11 R8

R9

100n 5.1k

51k

LINE Cable

TXO

CfL

LII

LiO-

LiO+

VREF

-1

Line Amplifier

Fig.6 Forbidden Circuit.

– 17 –

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