Versatile telephone transmission circuit with
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
between pin MUTE and pin VCC (pins 20 and 21).
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 kΩ 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.
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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