PBL 388 14
switching a lot if the telephone is placed in
a noisy surrounding. It can not be required
that the other party has to know that he is
talking with somebody with a handsfree
telephone in a noisy environment and thus Loudhearing:
has to shout to get through.
samewayasonewoulddowhenspeaking
with somebody standing further away. On
thereceiversidewehaveavolumecontrol
to set the desired level.
power amplifier supply is set by resistor at
pin RE. The value of this resistor should
not be made so low that the speech circuit
will at any time ”current starve” as this
would cause high distortion on the line.
Becausethiskindofcurrentfeedsystemis
a co-operation between the speech circuit
and the power amplifier of the ”handsfree
circuit”, it will only function properly with
Ericsson speech circuits exept circuits
PBL3726/21 or PBL3853. (The two last
named circuits could feed the power
amplifier from the special supply they are
bothproviding).Thevoltageincreaseswith
increasing line current across the resistor
RE, which results in, that optimum current
is taken at all line currents. The current is
fed into a reservoir capacitor between -C
and+L. Thepoweramplifierisgroundedat
the positive rail, this to avoid that the gro-
und would have a small level shift in case
the-Lisusedforground.A leveldifference
in the ground between the circuits can
cause serious trouble in regard of RFI.
Everything is ground related to the two
possible points, those being the two
telephone wires. The reservoir capacitor
ischosenbetween470-2200µFdependent
on price contra efficiency. Because the
speech has a highly varying amlitude a big
capacitor will save energy to the real high
amplitude peaks. The power amplifier is a
simple output stage in order to render
maximum efficiency. A balanced output
stage would only lead to much increased
loudspeaker impedance, which is already
with a simple stage in the highest order.
The optimum loudspeaker impedance is
dependent on many factors like the
available voltage and current, if the
optimization is done agaist RMS value or
more towards speech like low RMS value
but with some high peaks. The optimum
loudspeaker impedance for RMS calculus
will be round 50 ohms, for speech ( music
power ) a 25 ohm loudspeaker is more
optimal and if it can be considered that it is
long time between the peaks, even a 16
ohm loudspeaker can be used.
By setting the CTR control input high
The circuit has no corresponding
function in the receiver channel in fear that
itwouldonly worsentheperformance.The
reason for this is that various tone signals
on the line are difficult to detect and to
separate because of the big level
differencies. A normal behaviour would be
that when one receives a high noise level
from the loudspeaker one automatically
risesonesownvoiceandcompensatesfor
the noise in the other end thus functioning
as a noise compensation for the receiver.
Thereisariskthattheloudspeakervolume
would be turned down but in that case it
would be difficult to hear the other party
from the noise.
Something that can be tried in a
”sophisticated” handsfree telephone is, to
let the volume control influence the gain
slightly also at the input of the receiver.
The circuit does not contain any
automatic volume controls ( type AGC).
These kind of functions can of course be
included externally to the inputs of the
receiverandtransmitterbutitisverydifficult
in this way to better the performance. The
speech switching is based to feel
differencies in signal levels where again
with a resistor to +supply the circuit will go
into half speech control mode. The
amplifiers in the other half of the signal
paths G3 and G4 will be set into maximum
gainconstantly.Thisdoesnotalteranything
in the speech control function because the
hysteresis function is set by the other two
controlled amplifiers. The purpose with
this is to lead the signal from the handset
microphone via the speech control trans-
mitter channel and deconnect the
”handsfree function”. If the loudhearing
mode is active with the loudspeaker on,
there will be no oscillation when the hand-
set is placed close to the loudspeaker
which would be the case in normal mode
when lifting and returning the handset.
Because the microphone in the handset
has lower sensitivity related to the
handsfree microphone, the 25 dB speech
controlthatisused,isenoughtocounteract
oscillation.Thereareothersolutionstothis
problem but none has the same speech
qualitythanthisone.Thisspeechcontrolis
needed so that the party in the other end of
the telephone line will not be disturbed by
the echo of his own voice, which can be
extremely disturbing.
theautomaticvolumecontrolsareworking The efficiency of the
to keep the levels constant. This results in loudspeaker power amplifier.
almost unsolveable problems with time
constants if these two systems are
combined. It is not even certain that
automaticvolumecontrolsaredesirable. If
one stands on the other side of the room,
where the telephone is placed, facing it,
one automatically rises ones voice the
The PBL 388 13 has an extremely
high efficiency when it comes to convert
the existing line current to loudspeaker
output power. It is possible to make a
telephone line fed ”handsfree” telephone
with just under 10 mA of line current.The
current that is taken for the loudspeaker
Information given in this data sheet is believed to be accurate and reliable. However no responsibility is assumed
for the consequences of its use nor for any infringement of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Ericsson
ComponentsAB.TheseproductsaresoldonlyaccordingtoEricssonComponentsAB's generalconditionsofsale,
unless otherwise confirmed in writing.
Specifications subject to change without
notice.
1522-PBL 388 14 Uen Rev.A
© Ericsson Components AB
May 1998
Ordering Information
Package
Temp. Range
Part No.
Plastic DIP -20 to 70°C
Plastic SO -20 to 70°C
Plastic SO -20 to 70°C
PBL 388 14/1N
PBL 388 14/1SO
PBL 388 14/1SO:T (Tape and Reel)
Ericsson Components AB
S-164 81 Kista-Stockholm, Sweden
Telephone: (08) 757 50 00
14
ERICSSON [ ERICSSON ]
