L6726

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 13

www.unisonic.com.tw

QW-R108-015.D



FUNCTIONAL DESCRIPTION

The gain of the transmitting and receiving amplifiers are continuously and euqually changed with the line length.

The starting point for the gain regulation is externally set .The gain regulation can be cut-off externally, see table

under gain regulation below.

The outputs of the transmitting and receiving amplifiers have internals limitations of the output amplitudes.

The circuit includes a temperature independent voltage reference used for regulation of the DC line voltage and for

the transmitting and receiving gain regulation. The DC voltage is very fast settled to its final value with a minimum of

overshoot.

The circuit uses a minimum of external components. In a normal practical case only 5 external capacitors of which

only one is an electrolytic/tantalum filter capacitor .The other capacitor are needed as radio interference suppression

capacitor over the line, one capacitor in the side-tone balancing network and one capacitor for low frequency cut-off

in each of the transmitting and receiving amplifiers. The frequency response is except for that, flat for both amplifiers.

The circuit will with external components get a very good return loss behavior both against purely resistive lines as

for instance 600 Ω and against complex networks as 900 Ω in parallel with 30nF.

To get a good CMRR the microphone input is balanced.

It is possible to add a push-button controlled cut-off of the transmitting amplifier to the circuit without any

disturbance of the other circuit functions

A mute input is included in the circuit. It gives:

1. Cut-off the microphone amplifier.

2. Reduced gain in the receiving amplifier.

3. Connection of the DTMF input.

The receiver amplifier is equipped with a high impedance input stage. This gives a less expensive RC-network on

the input.

Only resistive elements are used to set the receiving gain.

A push-pull power stage in the receiving amplifier gives a high output swing.

To get the extended low current and voltage range two resistors have to be connected from microphone input to

ground.

The side-tone balanced can be set by a RC-network without influence on other parameters. An inexpensive

solution requires only one capacitor while more capacitors can give better performance.

In fig.4①②③ and ④ are RC-links with the following functions:

1. For radio interference suppression and to give the correct return loss behavior.

2. To set the gain and frequency response for transmitting.

3. To set the gain and frequency response for receiving.

4. To set the side-tone level

A stabilized DC output, with a pre-set output voltage of 2.35V, is provided to facilitate interfacing to external COMS

dialer ICs. The voltage is adjustable with two resistors, as indicated in fig.5. The output requires some filtering to

avoid distortion on the telephone line. Note that a high current consumption of the external device will alters the DC

characteristics of the speech circuit.