PBL 385 71
V
16
14
12
10
8
V telephone line
V line
V pin 4
V pin 2
6
4
2
I
L
mA
20
40
60
80
100
120
Figure 8. DC - Characteristics. (R6 =75Ω)
R6 will set the slope of the DC-char. and
microphone amplifier can be used for
therestofthelevelissetbysomeconstants dynamic or electret transducers. See fig.
temperature stable reference to prevent
overhearing to other parts and functions
on the chip.It is possible to use the
microphone amplifier as a limiter ( added)
tothelimiterinthetransmitteroutputstage
of the transmitted signal. See fig.9. The
positive output swing is then limited by the
peak output current of the microphone
amplifier. The negative swing is limited by
the saturation voltage of the output
amplifier.TheoutputoftheamplifierisDC-
vice at internal reference level (1.2V). The
lowest negative level for the signal is
reference minus one diode and saturated
tranistor drop. (1.2-0.6-0.1 = 0.5V) The
correct clipping level is found by
determining the composite AC- and DC-
load that gives a maximum symmetrical
unclipped signal at the output. This signal
is then fed into the transmitter amplifier at
a level that renders a symmetrical signal
clippingontheline.(adjustwithratioR4,R5)
The total transmitter gain when an electret
microphone is used can then be adjusted
with the load resistor of the electret
microphones buffer amplifier.
in the circuit as shown in the equation
below. The slope of the DC-char. will also
influence the line length regulation (when
used)and thusthegainofbothtransmitter
and receiver. See the table under gain
regulation.R6alsoactsaspowerprotection
for the circuit, this must be kept in mind
when low values of R6 are considered.
10. An electret microphone with a built in
FET amplifier is to be seen from outside as
a high impedance constant current gene-
rator and is normally specified with a load
resistanceof≈ 2k.This istobeconsidered
as max. value and by using it will render
the max. gain from the microphone. This
level of input signal that is unnecessary
highwillresultinclippinginthemicrophone
amplifier and could in mute condition
permeate through the input to the circuits
reference and this way to all functions,
resultingamongotherthingsinabadmute.
Hence it is better regarding noise
perfomance and mute to rather use the
gain of the microphone amplifier than the
gain of the microphone itself (in case of
electret) flat out. A more suitable level of
gain from the microphone is achieved by
using a load resistance of 330 - 820Ω. A
low microphone impedance will also
improve RFI suppression. Gain setting to
the line is done at the input of the transmit-
ter. The microphone amplifier has its own
V
V
Line ≈ 2 +1. 5 R 6 I line
telephoneline ≈ 1. 5 V + V line
Microphone amplifier
The microphone amplifier in PBL38571 is
divided into two stages. The first stage is a
true differential amplifier providing high
CMRR (-55 to -65 dB typical) with voltage
gain of 19 dB. This stage is followed by a
gain regulated amplifier with a regulation
range of 5 ± 2 dB. The input of the
6
ERICSSON [ ERICSSON ]
