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TEA1098ATV

Speech and handsfree IC

Philips Electronics 

Philips Semiconductors

Speech and handsfree IC

Product specification

TEA1098A

The earpiece amplifier has a rail-to-rail output EARO,

biased at two diodes voltage. It is designed for use with low

ohmic (real) loads (150 Ω) or capacitive loads (100 nF in

series with 100 Ω).

AGC (pin AGC)

The TEA1098A performs automatic line loss

compensation, which fits well with the true line attenuation.

The automatic gain control varies the gain of some

transmit and receive amplifiers in accordance with the DC

line current. The control range is 6.45 dB for Gv(MIC-LN) and

Gv(IR-RECO) and 6.8 dB for Gv(HFTX-LN), which corresponds

approximately to a line length of 5.5 km for a 0.5 mm

twisted-pair copper cable.

To enable this gain control, pin AGC must be shorted to

pin LN. The start current for compensation corresponds to

a line current of typically 23 mA and the stop current to

57 mA. The start current can be increased by connecting

an external resistor between pins AGC and LN. It can be

increased up to 40 mA (using a resistor typically 80 kΩ).

The start and stop current will be maintained in a ratio

equal to 2.5. By leaving the AGC pin open-circuit, the gain

control is disabled and no line loss compensation is

performed.

Handsfree application

As can be seen from Fig.6, a loop is formed via the

sidetone network in the line interface part and the acoustic

coupling between loudspeaker and microphone of the

handsfree part.

When this loop gain is greater than 1, howling occurs. In a

full duplex application this would be the case. The

loop-gain has to be much lower than 1 and therefore has

to be decreased to avoid howling.

This is achieved by the duplex controller. The duplex

controller of the TEA1098A detects which channel has the

‘largest’ signal and then controls the gains of the

microphone and loudspeaker amplifiers so that the sum of

the gains remains constant.

As a result, in handsfree application, the circuit can be in

three stable modes:

1. Transmit mode (TX mode).

The gain of the microphone amplifier is at its maximum

and the gain of the loudspeaker amplifier is at its

minimum.

2. Receive mode (RX mode).

The gain of the loudspeaker amplifier is at its

maximum and the gain of the microphone amplifier is

at its minimum.

3. Idle mode.

The gain of the amplifiers is halfway between their

maximum and minimum value.

The difference between the maximum gain and minimum

gain is called the switching range.

handbook, full pagewidth

telephone

line

sidetone

HYBRID

DUPLEX

CONTROL

Fig.6 Handsfree telephone set principles.

acoustic

coupling

MGM299

2000 Aug 18

11

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