PBL 385 41
V
16
14
12
10
8
6
4
2
V telephone line
V line
V pin 4
V pin 2
V pin 9
V pin 8
(DC supply)
I
20
Figure 8. DC-Characteristics. (R6=75
Ω
)
R6 will set the slope of the DC-char. and
the rest of the level is set by some constants
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 thus the gain of both transmitter
and receiver. See the table under gain
regulation. R6 also acts as power
protection for the circuit, this must be kept
in mind when low values of R6 are con-
sidered.
microphone amplifier can be used for
dynamic or electret transducers. See fig.
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
resistance of
≈
2k. This is to be considered
as max. value and by using it will render
the max. gain from the microphone. This
level of input signal that is unnecessary
high will result in clipping in the microphone
amplifier and could in mute condition
permeate through the input to the circuits
reference and this way to all functions,
resulting among other things in a bad mute.
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
L
40
60
80
100
120
mA
V
Line
≈
2
+
1.5
⋅
R
6
⋅
I
line
V
telephoneline
≈
1.5
V
+
V
line
Microphone amplifier
The microphone amplifier in PBL385 41 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
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
to the limiter in the transmitter output stage
) 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. The output of the amplifier is DC-
vice at internal reference level (1.2V). The
lowest negative level for the signal is
reference minus one diode and sat. trans-
istor 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 output.
This signal is then fed into the transmitter
amplifier at a level that renders a
symmetrical signal clipping on the line.
(adjust with ratio R4,R5) The total trans-
mitter gain when an electret microphone is
used can then be adjusted with the load
resistor of the electret microphones buffer
amplifier.
ERICSSON [ ERICSSON ]
