vides automatic gain compensation to reduce the gain as

loop length decreases The AGC range can be adjusted by

from 0 to 6 dB

RECEIVE AMPLIFIER

This buffer amplifier provides the necessary gain or loss for

the receive signal RCV IN should be AC coupled to SIDE-

TONE (pin 4) Automatic gain control is built into the amplifi-

er to reduce the gain as loop length decreases The AGC

range is adjusted in common with the transmit AGC range

with a range of adjustment for maximum loss from 0 to 6 dB

Push-pull complementary outputs provide balanced direct

drive to a dynamic transducer which may have an imped-

ance as low as 100X The effective receive gain can be

reduced by adding a resistor in series with the transducer

The receive gain is automatically reduced by 20 dB when

the MUTE input is pulled high

SIDETONE CIRCUIT

The level of Sidetone cancellation may be adjusted by con-

necting an external balance impedance to SIDETONE (pin

lation the balance impedance should be approximately 10

times the subscriber line input impedance Some typical

component values to match a precise 600X termination for

test purposes are shown in

Figure 2 Use the component

values shown in the Applications Section for better results

over a wide range of telephone line impedances

DTMF AMPLIFIER

An additional transmit amplifier is included to enable the

open-emitter output of a conventional DTMF generator to

be connected to the line via the transmit output stage This

path includes the transmit AGC section When the MUTE

input is pulled high the DTMF input is enabled and the MIC

input disabled When MUTE IN is open-circuit or pulled to

enabled

VOLTAGE REGULATOR OUTPUTS

A precision band-gap voltage reference controls a regulator

to provide bias for internal circuits Two auxiliary outputs are

low voltage pulse dialer or DTMF generator In order to pro-

tect this output in low voltage situations where the instanta-

neous voltage across the Speech Circuit may swing below

maximum available output current In speech mode MUTE

the Tip-

Ring voltage and the available output current is limited to

200 mA This is adequate to power a DTMF generator in

standby mode When MUTE IN is pulled high to switch the

Speech Circuit to the DTMF dialing mode

switched to a 3V regulated output and up to 2 mA may be

drawn from it to power the active DTMF generator

a low voltage 2-wire electret microphone such as the Primo

TLH5201 – 4

See Figure 3

Note

applications

FIGURE 2 TP5700A Telephone Speech Circuits

4