PBL 385 70
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 sup-
pression. Gain setting to the line is done at
the input of the transmitter. The microphone
amplifier has its own 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. transistor 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 out-
put. This signal is then fed into the trans-
mitter 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.
11
(a)
R
A
3
11
(b)
C
A
R
A
C
A
3
R
A
11
(c)
C
A
3
a
(a),(c), (d)
f
(a and b)
attn. = R
TI
//(R
TI
+R
A
)
no attn. = R
A
= 0
C
B
R
B
attenuation
a
(b),(e)
f
11
C
C
R
A
(d)
3
11
(e)
R
A
C
A
3
C
C
R
A
11
(f)
3
C
A
R
B
C
A
C
B
R
B
C
B
a
big C
A
R
B
attn.without dc.
small C
A
(f)
f
attenuation
attn.without dc.
Figure 10. Network and frequency plots between microphone amplifier and transmitter.
Transmitter amplifier
The transmitter amplifier in PBL38570
consists of three stages. The first stage is
an amplitude limiter for the input signal at
TI, in order to prevent the transmitted sig-
nal to exceed a certain set level and cause
distortion. The second stage amplifies
further the signal from the first and adds it
to a DC level from an internal DC-regulation
loop in order to give the required DC
characteristic to the telephone set. The
output of this stage is TO. The third stage
is a current generator that presents a high
impedance towards the line and has its
gain from TO to +L. The gain of this
amplifier is ZL/R6 where ZL is the
impedance across the telephone line.
Hence, the absolute maximum signal
How to calculate the gains in the transmitter channel.
See fig. 2 and 4.
Microphone amplifiers first stage 19 dB.
Microphone amplifiers regulated second stage 10.5 dB - 15.5 dB
Regulation interval 10.5 - 15.5 dB
low gain 19.0 + 10.5 dB = 29.5 dB
high gain 19.0 + 15.5 dB = 34.5 dB
V
2
R
M
R
5
R
load
=
⋅
G
M
⋅
⋅
G
TX
⋅
V
3
Z
mic
+
R
M
R
4
+
R
5
R
6
RM = Microphone amplifier input resistance
Rload = Rline // Rtelephone
PBL
385 70
DC
( ref.
≈
1.2V)
constant
current
generator
ex. calculate the gain of the transmitter stage GTX at 0 - line length:
ref. minus
a diode
≈
0.5V
43
=
20log(
11
DC-
load
AC-
load
(1.7 / /2.7)k
(17 / /22)k
)
+
29.5
+
20log(
)
350Ω
+
(1.7 / /2.7)k
18k
+
(17 / /22)k
600Ω / /910Ω
)
+
G
TX
+
20log(
75Ω
DC-load = R4+R5
AC-load = R4+R5//Z
TI
43
= −2.51+
29.5
−
9.17
+
G
TX
+
13.66
G
TX
=
11.52
dB
Figure 9. Micophone amplifier output
clipping.
7
ERICSSON [ ERICSSON ]
