PBL 3852
microphone amplifier a similar RC
combination ought to be connected from
the other input, pin 13, to ground.
An electret microphone with a built in
FET amplifier is to be seen from outside
as a high impedance constant current
the circuits reference and this way to all
functions, resulting among other things in positive output swing is then limited by the
a bad mute. Hence it is better regarding peak output current of the microphone
noise perfomance and mute to rather use amplifier. The negative swing is limited by
transmitted signal (See fig. 9). The
the gain of the microphone amplifier than
the gain of the microphone itself (in case
the saturation voltage of the output
amplifier. The output of the amplifier is
DC-vice at internal reference level
(1.16V). The lowest negative level for the
generator and is normally specified with a of electret) flat out. A more suitable level
load resistance of ≈ 2k. This is to be
considered as max. value and by using it
will render the max. gain from the micro-
phone. This level of input signal that is
unnecessary high will result in clipping in
the microphone amplifier and in mute
condition permeate through the input to
of gain from the microphone is achieved
by using a load resistance of 200 - 470Ω. signal is reference minus one diode and
Gain setting to the line is done at the
input of the transmitter.
It is possible to use the microphone
sat. transistor drop (1.16-0.6-0.1 = 0.46V).
The correct clipping level is found by
determining the composite AC- and DC-
amplifier as a limiter ( added to the limiter load that gives a maximum symmetrical
in the transmitter output stage ) of the
unclipped signal at the 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 transmitter gain when an
electret microphone is used can then be
adjusted with the load resistor of the
electret microphones buffer amplifier.
DC ( ref. ≈ 1.16V )
Strong
cc gen.
ref. minus a diode ≈ 0.5V
11
DC-load = R4+R5
DC-
load
AC-
load
AC-load = R4+R5// (R6+ZDTMF) //ZTI
Z
DTMF = DTMF generator impedance
Figure 9. Microphone amplifier output clipping.
PBL 3852
(b)
PBL 3852
(c)
PBL 3852
4
(d)
(a)
PBL 3852
11
13
11
13
11
13
11
13
12
M
M
M
M
12
12
2
12
+
+
+
+
Dynamic
microphone
Magnetic
microphone
For dynamic mic.with
reduced gain
regulation
Unbalanced electret
mic. with balanced
signal, DC-supply from
pin 4.
DC1
DC1
R
(f)
4
(g)
PBL 3852
(h)
(e)
PBL 3852
PBL 3852
PBL 3852
11
13
11
11
13
11
13
Im
13
12
M
M
M
M
12
12
12
+
+
+
+
C
Unbalanced electret
microphone
+
Balanced electret
microphone with reduced
gain regulation
Ry
Balanced electret
microphone
Rx
Balanced electret microphone.
An additional RC filterlink is
recommended if pin 4 is used
as a supply.
DC1
Mic. ampl. supplies the mic.
current Im, set by Rx and Ry.
Figure 10. Microphone solutions.
7
ERICSSON [ ERICSSON ]
