March 1994

10

Philips Semiconductors

Product specification

Versatile telephone transmission circuit with

dialler interface

TEA1065

MUTE input

When MUTE = HIGH the DTMF input is enabled and the

microphone and receiving amplifier inputs are inhibited.

When MUTE = LOW or open-circuit the DTMF input is

inhibited and the microphone and receiving amplifier

inputs are enabled. Switching the MUTE input will cause

negligible clicks at the earpiece outputs and on the line. An

electrostatic discharge protection diode is connected

Dual-tone multifrequency input DTMF

When the DTMF input is enabled, dialling tones may be

sent onto the line. The voltage gain from DTMF to LN is

typ. 12.5 dB less than the gain of the microphone amplifier

and varies with R7 in the same way as the gain of the

microphone amplifier. This means that the tone level at the

DTMF input has to be adjusted after setting the gain of the

microphone amplifier. When R7 = 68 k

Ω the gain is

typically 25.5 dB. The signalling tones can be heard in the

earpiece at a low level (confidence tone).

Receiving amplifiers: IR, QR

+, QR− and GAR

The receiving amplifier has one input IR and two

complementary outputs, QR

+ (non-inverting) and

QR

− (inverting). These outputs may be used for

single-ended or differential drive, depending on the type

and sensitivity of the earpiece used (see Fig.10). Gain

from IR to QR

+ is typically 31 dB with R4 = 100 kΩ, which

is sufficient for low-impedance magnetic or dynamic

earpieces which are suitable for single-ended drive. By

using both outputs (differential drive) the gain is increased

by 6 dB. Differential drive can be used when earpiece

impedance exceeds 450

Ω as with high impedance

dynamic, magnetic or piezoelectric earpieces.

The output voltage of the receiving amplifier is specified for

continuous-wave drive. The maximum output voltage will

be higher under speech conditions where the ratio of peak

and RMS value is higher.

The gain of the receiving amplifier can be adjusted over a

range of

−11 dB to +8 dB to suit the sensitivity of the

transducer that is used. The gain is proportional to external

resistor R4 connected between GAR and QR

+.

Two external capacitors, C4 = 100 pF and C7 = 1 nF, are

necessary to ensure stability. A larger value of C4 may be

chosen to obtain a first-order low-pass filter. The “cut-off”

frequency corresponds with the time constant R4

× C4.

Fig.10 Alternative receiver arrangements:

(a) dynamic earpiece with an impedance less than 450

Ω;

(b) dynamic earpiece with an impedance more than 450

Ω;

(c) magnetic earpiece with an impedance more than 450

Ω, resistor (1) may be connected to prevent

distortion (inductive load);

(d) piezoelectric earpiece, resistor (2) is required to increase the phase margin (stability with capacitive

load).

handbook, full pagewidth

MBA554

(1)

QR

−

QR

+

5

4

VEE

16

QR

−

QR

+

5

4

QR

−

QR

+

5

4

(2)

QR

−

QR

+

5

4

(a)

(b)

(c)

(d)