Low voltage versatile telephone
transmission circuit with dialler interface
Rp = 16.2 kΩ
Leq = C3 × R9 × Rp
Fig.4 Equivalent impedance circuit.
Microphone inputs (MIC+ and MIC−) and gain
adjustment pins (GAS1 and GAS2)
The TEA1067 has symmetrical microphone inputs. Its
input impedance is 64 kΩ (2 × 32 kΩ) and its voltage gain
is typically 52 dB (when R7 = 68 kΩ, see Fig.14). Dynamic,
magnetic, piezoelectric or electret (with built-in FET source
followers) microphones can be used. Microphone
arrangements are shown in Fig.11.
The gain of the microphone amplifier can be adjusted
between 44 dB and 52 dB to suit the sensitivity of the
transducer in use. The gain is proportional to the value of
R7 which is connected between GAS1 and GAS2. Stability
is ensured by the external capacitor C6 which is connected
between GAS1 and SLPE. The value of C6 is 100 pF but
this may be increased to obtain a first-order low-pass filter.
The cut-off frequency corresponds to the time constant
R7 × C6.
Mute input (MUTE)
When MUTE is HIGH the DTMF input is enabled and the
microphone and receiving amplifier inputs are inhibited.
The reverse is true when MUTE is LOW or open-circuit.
MUTE switching causes only negligible clicking on the
earpiece outputs and line. If the number of parallel sets in
use causes a drop in line current to below 6 mA the speech
amplifiers remain active independent to the DC level
applied to the MUTE input.
Dual-tone multi-frequency input (DTMF)
When the DTMF input is enabled dialling tones may be
sent onto the line. The voltage gain from DTMF to LN is
typically 25.5 dB (when R7 = 68 kΩ) and varies with R7 in
the same way as the microphone gain. The signalling
tones can be heard in the earpiece at a low level
Receiving Ampliﬁer (IR, QR+, QR− and GAR)
The receiving amplifier has one input (IR), one
non-inverting complementary output (QR+) and an
inverting complementary output (QR−). These outputs
may be used for single-ended or differential drive
depending on the sensitivity and type of earpiece used
(see Fig.12). IR to QR + gain is typically 31 dB (when
R4 = 100 kΩ), this is sufficient for low-impedance
magnetic or dynamic microphones which are suited for
single-ended drive. Using both outputs for differential drive
gives an additional gain of 6 dB. This feature can be used
when the earpiece impedance exceeds 450 Ω
(high-impedance dynamic or piezoelectric types).
The receiving amplifier gain can be adjusted between 20
and 39 dB with single-ended drive and between 26 and
45 dB with differential drive, to match the sensitivity of the
transducer in use. The gain is set with the value of R4
which is connected between GAR and QR+. Overall
receive gain between LN and QR+ is calculated by
substracting the anti-sidetone network attenuation (32 dB)
from the amplifier gain. Two external capacitors C4 and
C7, ensure stability. C4 is normally 100 pF and C7 is
10 × the value of C4. The value of C4 may be increased to
obtain a first-order low-pass filter. The cut-off frequency
will depend on the time constant R4 × C4.
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 peak to RMS
ratio is higher.