TEA1098 Philips Semiconductors, TEA1098 Datasheet - Page 11

• Manufacturer Part Number: TEA1098
• Description: Speech and handsfree IC
• Manufacturer: Philips Semiconductors

Philips Semiconductors
H
The TEA1098 has an asymmetrical HFTX input, which is
mainly intended for use in combination with the TXOUT
output. The input impedance between HFTX and GND is
typically 20 k . The voltage gain between
pins HFTX and LN is set to 34.7 dB. Without output
limitation, the input stage can accept signals of up to
95 mV (RMS) at 2% THD (room temperature). The HFTX
input is biased at a voltage of two diodes.
Automatic gain control is provided for line loss
compensation.
Receive channels (pins IR, RECO, GARX and QR)
RX
The receive amplifier has one input (IR) which is
referenced to the line. The input impedance between pins
IR and LN is typically 20 k and the DC bias between
these pins is equal to the voltage of one diode. The gain
between pins IR (referenced to LN) and RECO is typically
29.7 dB. Without output limitation, the input stage can
accept signals of up to 50 mV (RMS) at 2% THD (room
temperature).
The receive amplifier has a rail-to-rail output (RECO),
which is designed for use with high ohmic (real) loads of
more than 5 k . This output is biased at a voltage of two
diodes.
Automatic gain control is provided for line loss
compensation.
E
The earpiece amplifier is an operational amplifier which
has an output (QR) and an inverting input (GARX).
Its input signal is fed by a decoupling capacitor from the
receive amplifier output (RECO) to two resistors which set
the required gain or attenuation from 3 to +15 dB
compared to the receive gain.
Two external capacitors C
and QR) and C
ensure stability. The C
low-pass filter. The cut-off frequency corresponds to the
time constant C
C
The earpiece amplifier has a rail-to-rail output (QR) biased
at a voltage of two diodes. It is designed for use with low
ohmic (real) loads of 150 , or capacitive loads of 100 nF
in series with 100 .
1999 Oct 14
ARPIECE AMPLIFIER
ANDSFREE TRANSMIT AMPLIFIER
GARS
Speech and handsfree IC
AMPLIFIER
10
(
C
PINS
GARS
GAR
GAR
IR
must be satisfied.
(
(connected between GAR and GND)
PINS
R
AND
GAR
e2
. The relationship
GARX
GAR
capacitor provides a first-order
RECO)
(connected between GAR
(
PINS
AND
QR)
HFTX
AND
LN)
11
AGC (pin AGC)
The TEA1098 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 G
and 6.8 dB for G
approximately to a line length of 5.5 km for a 0.5 mm
twisted-pair copper cable.
To enable this gain control, the pin AGC must be shorted
to pin LN. The start current for compensation corresponds
to a line current of typically 23 mA and a stop current of
57 mA. The start current can be increased by connecting
an external resistor between pins AGC and LN. It can be
increased by up to 40 mA (using a resistor of typically
80 k ). The start and stop current will be maintained at a
ratio of 2.5. By leaving the AGC pin open, the gain control
is disabled and no line loss compensation occurs.
Handsfree application
Figure 7 shows a loop is formed by the sidetone network
in the line interface section, and by the acoustic coupling
between loudspeaker and microphone in the handsfree
section. A loop-gain of greater than 1 causes howl.
To prevent howl in full duplex applications, the loop-gain
must be set much lower than 1. This is achieved by the
duplex controller which detects the channel with the
‘largest’ signal and controls the gains of the microphone
and the loudspeaker amplifiers so that the sum of their
gains remains constant.
Therefore in the handsfree application, the circuit can have
three stable modes:
1. Transmit mode (Tx mode).
2. Receive mode (Rx mode).
3. Idle mode.
The difference between the maximum and minimum gain
is called the switching range.
The microphone amplifier is at maximum gain, and the
loudspeaker amplifier is at minimum gain.
The microphone amplifier is at minimum gain, and the
loudspeaker amplifier is at maximum gain.
The microphone amplifier and the loudspeaker
amplifier are both midway between maximum and
minimum gain.
v(HFTX-LN)
, which corresponds
v(MIC-LN)
Product specification
TEA1098
and G
v(IR-RECO),