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TEA1111AT

Speech circuit with dialler interface,regulated supply and earpiece volume control

Philips Electronics 

Philips Semiconductors

Speech circuit with dialler interface, regulated

supply and earpiece volume control

Product specification

TEA1111A

Transmit stage (pins MIC+, MIC− and DTMF)

MICROPHONE AMPLIFIER (PINS MIC+ AND MIC−)

The TEA1111A has symmetrical microphone inputs.

The input impedance between pins MIC+ and MIC− is

68 kΩ (2 × 34 kΩ). The voltage gain from pins MIC+/MIC−

to pin LN is set at 44.2 dB (typical) at 600 Ω line load.

Automatic gain control is provided on this amplifier for line

loss compensation.

DTMF AMPLIFIER (PIN DTMF)

When the DTMF amplifier is enabled, dialling tones may

be sent on line. These tones are also sent to the receive

output QR at a low level (confidence tone), the level is

controlled by pin VCI.

The TEA1111A has an asymmetrical DTMF input.

The input impedance between DTMF and VEE is 20 kΩ

and it is biased at VEE. The voltage gain from pin DTMF to

pin LN is set at 25.9 dB.

Automatic gain control has no effect on the DTMF

amplifier.

Receiving stage (pins IR, GAR, QR and VCI)

The receive part consists of an earpiece amplifier and a

volume control block.

EARPIECE AMPLIFIER

The earpiece amplifier has one input (IR) and one output

(QR). The input impedance between pin IR and pin VEE is

22 kΩ. When pin VCI is tied to VEE, the voltage gain from

pin IR to pin QR is set at 27.4 dB (typical) which reduces

the attenuation of the receive signal by the anti-sidetone

network from 32 dB to 4.6 dB. The gain can be decreased

by connecting an external resistor RGARext between

pins GAR and QR; the adjustment range is 6 dB.

Two external capacitors CGAR (connected between

pins GAR and QR) and CGARS (connected between

pins GAR and VEE) ensure stability. Capacitor CGAR

provides a first-order low-pass filter. The cut-off frequency

corresponds to the time constant CGAR × RGARint. Where

RGARint is the internal resistor (123 kΩ typical) which sets

the gain. The relationship CGARS = 10 × CGAR must be

complied with to ensure stability.

The output voltage of the earpiece amplifier is specified for

continuous wave drive. The maximum output swing

depends on the DC line voltage, the RCC resistor, the ICC

current consumption of the circuit, the IP current

consumption of the peripheral circuits and the load

impedance.

Automatic gain control is provided on this amplifier for line

loss compensation.

VOLUME CONTROL (PIN VCI)

A positive DC voltage applied to pin VCI allows the gain of

the earpiece amplifier to be increased in steps of 4.85 dB.

The volume control range is 27.4 to 41.9 dB (14.5 dB

typical). A proportional voltage decoder at pin VCI defines

a gain of 27.4 dB when VVCI equals VEE and a gain of

41.9 dB when VVCI equals VDD.

The intermediate steps correspond to: VVCI = 13-- VDD

and VVCI = 23-- VDD .

Automatic gain control (pin AGC)

The TEA1111A performs automatic line loss

compensation. The automatic gain control varies the gain

of the microphone amplifier and the gain of the receive

amplifier in accordance with the DC line current.

The control range is 6.0 dB (which corresponds

approximately to a line length of 5 km for a 0.5 mm

diameter twisted-pair copper cable with a DC resistance of

176 Ω/km and an average attenuation of 1.2 dB/km).

The IC can be used with different configurations of feeding

bridge (supply voltage and bridge resistance) by

connecting an external resistor RAGC between

pins AGC and VEE. This resistor enables the Istart and Istop

line currents to be increased (the ratio between Istart and

Istop is not affected by the resistor). The AGC function is

disabled when pin AGC is left open circuit.

Mute function (pin MUTE)

The mute function performs the switching between the

speech mode and the dialling mode.

When MUTE is LOW, the DTMF input is enabled and the

microphone and receive amplifier inputs are disabled.

In this mode, the DTMF tones are sent to the receive

output at a low level (confidence tone).

When MUTE is HIGH, the microphone and receiving

amplifiers inputs are enabled while the DTMF input is

disabled. The MUTE input is provided with an internal

pull-up current source to VDD.

1999 Nov 22

9

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