1
PBL 385 73
June 1999
PBL 385 73
Speech Circuit for constant current
feeding systems
Description.
PBL 38573 is a monolithic integrated speech transmission circuit for use in
electronic telephones. It is intended specially for telephone lines with constant current
feed. Maximum line current less than 130 mA in DIL package (100 mA in SO package).
It is designed to accomodate either a low impedance dynamic or an electret microphone.
A signal summing point at the transmitter input is available for DTMF dialler- and
possible monitor or handsfree signals. An available internally preset line length
compensation can be adjusted or shut off in low gain mode with external resistors.
Application dependent parameters such as line balance, side tone level, transmitter and
receiver gains and frequency responces are set independently by external components
which means an easy adaption to various market needs. The setting of the parameters
if carried out in certain order will counteract the interaction between the settings. A DC-
supply is provided to feed microphones and diallers.
+
REC
7
5
8
12
4
3
2
14
13
11
1
6
AR
AT
DC-supply
PBL 385 73
+
9
10
AM
+
Mute
(active low)
DC supply for
external devices
1
3
2
5
4
Telephone
line
DTMF - input
MIC.
1. Impedance to the line and radio interference suppression
2. Transmitter gain and frequency responce network
3. Receiver gain and frequency responce network
4. Sidetone balance network
5. DC supply components
Key features.
Minimum number of external
components for function,with one filte-
red DC-supply. 6 capacitors and 10
resistors.
Easy adaption to various market
needs.
Mute control input for operation with
DTMF - generator.
Transmitter and receiver gain
regulation for automatic loop loss
compensation. Disconnectable.
Extended current and voltage range
5 - 130mA (DIP), down to 2 V.
Differential microphone input for good
balance to ground.
Balanced receiver output stage.
Stabilized DC - supply for low current
CMOS diallers and electret
microphones.
Short start up time.
Excellent RFI performance.
In 14- pin DIP and SO packages.
Figure 1. Functional diagram.
PBL 385 73
PBL 385 73
14-pin plastic SO
14-pin plastic DIP
2
PBL 385 73
Figure 4. Circuit with external compon-
ents for test set up.
DIP package pinning.
Maximum Ratings
Parameter
Symbol
Min
Max
Unit
Line voltage, t
p
= 2 s
V
L
0
18
V
Line current, continuous DIP package
I
L
0
130
mA
Line current, continuous SO package (depending on mounting)
I
L
0
100
mA
Operating temperature range
T
Amb
-40
+70
C
Storage temperature range
T
Stg
-55
+125
C
No input should be set on higher level than pin 4 (+C).
Figure 2. Test set up without rectifier
bridge.
Figure 3. Test set up with rectifier
bridge.
+
= 350
+ LINE
- LINE
ARTIFICIAL
LINE
I
L
V
2
V
1
V
L
R = 0-4k
L
0 ohm when artificial
line is used
MUTE
PBL 38573
with external
components
See fig. 4
Z
Mic = 350
Z
Rec
MIC
REC
R
feed = 400
+400
600
C
E = 48.5V
V
3
V
4
C = 1
F when artificial line is used
470
F when no artificial line
V
M
DC
I
V
DC
5H+5H
+
I
M
+
+ LINE
- LINE
I L
V
2
V1
V
L
R = 0 - 4k
L
MUTE
PBL 38573
with external
components
See fig. 4
Z
Mic
Z
Rec
MIC
REC
R
feed = 400
+400
600
+
E = 50.0V
V 3
V4
1
F
V
M
Uz= 15-16V
5H+5H
DC
I
V
DC
= 350
= 350
I
M
+
REC.
350
7
12
4
3
2
14
13
11
1
6
AR
AT
DC-supply
PBL 385 73
+
9
10
AM
+
Mute
DC supply
Mic.
350
47
F
18k
22k
100nF
75
910
6.2k
560
11k
100nF
62k
47nF
47
F
910
15nF
310
+ Line
-Line
2.7k
R4
R5
C11
C3
R6
C2
C1
R3
R10
C6
R11
R7
R8
R9
C5
R12
11k
R
G
8
5
3
PBL 385 73
Electrical Characterisics
At T
Amb
= + 25
C. No cable and line rectifier unless otherwise specified.
Ref.
Parameter
fig.
Conditions
Min
Typ
Max
Unit
Line voltage, V
L
2
I
L
= 15 mA
3.3
3.7
4.1
V
2
I
L
= 100 mA
11
13
15
V
Transmitting gain, note 1
2
20
10
log (V
2
/ V
3
); 1 kHz
No gain regulation
R
g
= 20k
41
43
45
dB
Transmitting gain, note 1
2
20
10
log (V
2
/ V
3
); 1 kHz
With gain regulation
R
L
= 0, R
G
=
41
43
45
dB
R
L
= 400
43.5
45.5
47.5
dB
R
L
= 900
- 2.2 k
46
48
50
dB
Transmitting range of
2
1 kHz, R
L
= 0 to 900
3
5
7
dB
regulation
Transmitting frequency
2
200 Hz to 3.4 kHz
-1
1
dB
response
Microphone input impedance
2 1.7//(2.7) note 3 k
Transmitter input impedance
2
1 kHz
13.5
17
20.5
k
pin 3
Transmitter dynamic output 2 200 Hz - 3.4 kHz
1.5 V
p
2% distortion, I
L
= 20 - 100 mA
Transmitter max output 2 200 Hz - 3.4 kHz
3 V
p
I
L
= 0 - 100 mA, V
3
= 0 - 1 V
Transmitter output noise 2
Psoph-weighting, Rel 1 V
rms
, R
L
= 0
-75
dB
Psoph
Receiving gain, note 1
2
20
10
log (V
4
/ V
1
); 1 kHz
No gain regulation
R
g
= 20k
-18.5 -16.5 -14.5 dB
Receiving gain, note 1
2
20
10
log (V
4
/ V
1
); 1 kHz
With gain regulation
R
L
= 0, R
G
=
-18.5 -16.5 -14.5 dB
R
L
= 400
-16 -14 -12
dB
R
L
= 900
- 2.2 k
-13.5 -11.5 -9.5
dB
Receiving range of regulation
2
1 kHz, R
L
= 0 to 900
3
5
7
dB
Receiving frequency response
2
200 Hz to 3.4 kHz
-1
1
dB
Receiver input impedance
2
1 kHz,
38
k
Receiver output impedance
2
1 kHz,
3(+310)note 3
Receiver dynamic output
2
200 Hz - 3.4 kHz
0.5
V
p
note 2
2% distortion, I
L
= 20 - 100 mA
Receiver max. output
3
Measured with line rectifier
0.9
V
p
200 Hz - 3.4 kHz, I
L
= 0 - 100 mA,
V
1
= 0 - 50 V
Receiver output noise
2
A-weighting, Rel 1V
rms
, with cable
-85
dB
A
0 - 3 km, = 0.4 mm
0 - 5 km, = 0.5 mm,
Mute input voltage 2
0.3 V
at mute (active low)
DC-supply voltage, note 4
2
I
L
= 20 - 100 mA
Pin 7
I
DC
= 0 mA
2.1
2.35
2.6
V
I
DC
= 2 mA
1.95
2.2
2.6
V
DC-supply voltage, pin 7
4
V
DC
= 2.35 V
0.1
A
leakage current (no supply)
Notes
1. Adjustable to both higher and lower values with external components.
2. The dynamic output can be doubled, see applications information.
3. External resistor in the test set up.
4. The DC output voltage is reduced at low line voltage (see fig. 8).
4
PBL 385 73
+L
TO
TI
+C
Mute
GR
DCO
RI
-L
MI 2
MI 1
MO
1
2
3
4
5
6
7
12
11
10
9
8
RE 2
RE 1
14
13
1
2
3
4
5
6
7
13
12
17
16
9
8
+L
TO
TI
+C
Mute
GR
DCO
RE 2
RE 1
RI
-L
MI 2
MI 1
MO
4
5
11
10
14
14-pin DIP
14-pin SO
Figure 5. Pin configurations.
Pin Descriptions:
Refer to figure 5.
Pin
Name
Function
1
+L
Output of the DC-regulator and transmit amplifier. This pin is connected to the line through a polarity
guard diode bridge.
2
TO
Output of the transmit amplifier. This pin is connected through a resistor of 47 to 100 ohm to -L,
sets the DC-resistance of the circuit. The output has a low AC output impedance and the
signal is used to drive a side tone balancing network.
3
TI
Input of transmit amplifier. Input impedance 17 kohm
20 %.
4
+C
Positive power supply terminal for most of the circuitry inside the PBL 385 73 ( current consumption
about 1 mA). The +C pin shall be connected to a decoupling capacitor of 47
F to 150
F.
5
Mute
Maximum voltage (to mute) is 0.3 V, current sink requirement of external driver is 100
A.
6
GR
Input of the gain regulation with line length.
7
DCO
Output of the auxiliary DC-supply.
8
MO
Output of the microphone amplifier or DTMF-amplifier.
9
MI 1
Microphone amplifier Inputs. Input impedance 1.7 kohm
20 %.
10
MI 2
11
-L
The negative power terminal, connected to the line through a polarity guard diode bridge.
12
RI
Input of receiver amplifier. Input impedance 38 kohm
20 %.
13
RE 1
Receiver amplifier outputs. Output impedance is approximately 3 ohm.
14
RE 2
}
}
5
PBL 385 73
Figure 7. Block connections.
Figure 6. AC-impedance.
Functional description
Design procedure; ref. to fig.4.
The design is made easier through that all
settable parameters are returned to gro-
und (-line), this feature differs it from bridge
type solutions.To set the parameters in the
following order will result in that the
interaction between the same is minimized.
1. Set the circuit impedance to the line,
either resistive (600
) or complex. (R3
and C1). C1 should be big enough to give
low impedance compared with R3 in the
telephone speech frequency band.Too
large C1 will make the start-up slow. See
fig. 6.
2. Set the DC-characteristic that is
required in the PTT specification or in case
of a system telephone,in the PBX
specification (R6).There are also internal
circuit dependent requirements like supply
voltages etc.
3. Set the attac point where the line
length regulation is supposed to cut in
(R15 and R16). Note that in some countries
the line length regulation is not allowed. In
most cases the end result is better and
more readily achieved by using the line
length regulation (line loss compensation)
than without. See fig. 13.
4. Set the transmitter gain and
frequency response.
5. Set the receiver gain and frequency
response. See text how to limit the max.
swing to the earphone.
6. Adjust the side tone balancing
network.
7. Set the RFI suppression
components in case necessary. In two
piece telephones the often "helically"
wound cord acts as an aerial. The
microphone input with its high gain is
especially sensitive.
8. Circuit protection. Apart from any
other protection devices used in the de-
sign a good practice is to connect a 15V
1W zener diode across the circuit , from
pin 1 to -Line.
Impedance to the line
The AC- impedance to the line is
set by R3, C1 and C2. Fig.4. The circuits
relatively high parallel impedance will not
influence it to any noticeable extent. At low
frequencies the influence of C1 can not be
neglected. Series resistance of C1 that is
dependent on the temperature and the
quality of the component will cause some
of the line signal to enter pin 4. This
generates a closed loop in the transmitter
amplifier that in its turn will create an
active impedance thus lowering the
impedance to the line. The impedance at
high frequencies is set by C2 that also
acts as a RFI suppressor.
In many specifications the
impedance towards the line is specified as
a complex network. See fig. 6. In case a).
the error signal entering pin 4 is set by the
ratio
Rs/R3 (910
), where in case b). the
ratio at high frequencies will be Rs/220
because the 820
resistor is bypassed by
a capacitor. To help up this situation the
complex network capacitor is connected
directly to ground, case c). making the ratio
Rs/220
+820
and thus lessening the
error signal. Conclusion: Connect like in
case c) when complex impedance is
specified.
DC - characteristic
The DC - characteristic that a
telephone set has to fulfill is mainly given
by the network administrator. Following
parameters are useful to know when the
DC behaviour of the telephone is to be set:
The voltage of the feeding system
The line feeding resistance 2 x.......
ohms.
The maximum current from the line at
zero line length.
The min. current at which the telephone
has to work (basic function).
The lowest and highest voltage
permissible across the telephone set.
The highest voltage that the
telephone may have at different line
currents. Normally set by the
network owners specification.The
lowest voltage for the telephone is
normally set by the voltages that are
needed for the different parts of the
telephone to function. For ex. for
transmitter output amplifier, recei-
ver output amplifier, dialler, speech
switching.
1
2
+Line
R3
R6
PBL 38 573
+
3
C1
C2
-Line
Rs
1
How to connect a
complex network.
220
+820
//Cx
Example:
a)
b)
c)
4
220
820
Cx
+
1
AM
AT
AR
2
Transmitter summing
input
Mute
+ Line
- Line
3
4
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