Low voltage versatile telephone transmission circuit
with dialler interface and transmit level dynamic limiting
The output voltage of the receiving amplifier is specified for
continuous-wave drive. Fig.14 shows the maximum output
swing of the receiving amplifier as a function of the DC
voltage drop (VLN). The maximum output voltage will be
higher under speech conditions, where the ratio of the
peak to the RMS value is higher.
The gain of the receiving amplifier can be adjusted to suit
the sensitivity of the transducer used. The adjustment
range is between 20 dB and 39 dB with single-ended drive
and between 26 dB and 45 dB with differential drive. The
gain is proportional to the external resistor R4 connected
between GAR and QR+. The overall gain between LN and
QR+ can be found by subtracting the attenuation of the
anti-sidetone network (32 dB) from the amplifier gain.
Two external capacitors (C4 = 100 pF and C7 = 10 x C4 =
1 nF) ensure stability. A larger value may be chosen to
obtain a first-order low-pass filter. The cut-off frequency
corresponds with time constant R4 x C4. The relationship
C7 = 10 x C4 must be maintained.
5 VLN (V) 6
Valid for both options; THD = 2%, Iline = 15 mA.
Curve (1) is for a differential load of 47 nf (series
resistance = 100 Ω; f = 3400 Hz.
Curve (2) is for a differential load of 450 Ω;
f = 1 kHz.
Curve (3) is for a single-ended load of 150 Ω;
f = 1 kHz.
Fig.14 Typical output swing of the receiving
amplifier as a function of DC voltage drop
VLN with the load at the receiver output as