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Патент USA US3033950

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May s, 1962
A 3,033,940
E. coxr‘ _
Filed Sept. 9, 1960
3 Sheets-Sheet l
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May s, 1962
May 8, 1962
J. E. cox
Filed sept. 9. 1960
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Vhybrid vnetwork 17 via monitor relay contacts M1, M2 and
» the output or collector electrodes c are inductiv'ely coupled
bientenvironmental changes such as fluctuations in supply
voltage, temperature,V etc. In fact, it has been found
to the operator headset 15 via arepeat coil 25.
The base biasing Circuits for the various transistors in
clude a voltage divider having resistors RYlS--RYZG
connected between battery 35 and ground 36 whenY con
facts TR1 close; The capacitor RQ4 vis connected to
istics occurring as components age.V
point 37 on the voltage divider and functions as a de
pedance of any’connecting circuits may beV matched.
,coupling capacitor by .charging when 'the potential at v
Contrastthis with the situation where specially designed '
components are used and a high cost, low production
item is required each time that it is necessary to change
that this feedback control is adequate to stabilize the
circuit against changes caused by the drifting character
Since commercialgra-de components aroused, a wide
range Vof component values are available and the im
point 37 goes up and discharging when the potential
goes down to prevent:V the amplifiers from responding to
iluctuations of» source voltage. The base electrode b
an impedance.
' of the transistor 18 is biased bythe potential appearingat
In addition to the above described use of negative
point 38 on the voltage divider, the biasing circuitex
tending through a parallel circuitV including the left
' feedback to control the frequency response of the am
pliñer, a tone control circuit is provided in the receive '
hand winding of relay TR and the diode 34a. The base
electrodes b of transistors 19 and 2@ are biased to the
More specifically, the resistor RY17 and ca
_ pacitor 49 are connected in Yseries across the left-hand
point 3S through resistors RY1'6_ and RYIS, respectively.
winding of repeat coil 25.
The capacitor 40 offers
The base to emitter bias for each of the transistors is
a high impedance to low frequencies and a low impedance
completed to groundpoint 26, the emiter circuit to tran 20 to high frequencies with the value of the capacitor chosen
sistor 18 beingvia resistor RY11, to transistor ‘19 being
via resistors RY13, -RY14, and to transistor
20 beingì via
resistors RYIZ, RYM.
- to pass frequencies which are higher than the upper
» limits of thefnormal voice band. Thus, the higher order
The electrical values of these ~ l »harmonics resulting from the use of unmatched tran
resistors are chosen to make the emitters of the various
transistors more positive than their respective bases. The
sistors or other commercialV grade components are at
tenuated by the resistor RY17 and the ñdelity of the
presence vof these resistors in the emitter. biasing circuits
greatly increases the input impedance of the amplifiers.
speech signal remains unimpared.
"Therefore, a resistor RY9 is connected across the input '
complete understanding of how the hybrid circuit 17
Next, reference is made to FIGS. 2,-4 >>for a more
terminals of the ampliiiers .19, 2G to match the input
functions. A hybrid circuitfby deñnition, is a combina
`irnpedance'of thefampliners to the impedance of the 30 tion of elements affording paths betweenv three branches
hybrid network 17.
marked A, B and C in the drawings: First (Branch C)
The various collectors areV supplied from the negative
is a two-way circuit including both transmit and receive
potential point 37, thecircuit to the collector of tran
signals, second (Branch A) Vis a one-way transmitV chan
sistor 18 being completed viathe right-hand winding of
nel and third (Branch B) is acne-Way receive channel.
transformer 2.1. The 'collector of transistor L19 is sup 35 These three branches are arranged so that A can send
plied from point 37 via the lower halfand the collector
to C and B can receive from C, but A and B are elîec
-oftransistor 20 is ysupplied via the upper half of the lett
tively isolated -?rom each other; therefore, A cannot send
hand Winding of transformer 25.
v . K
to B.
With this biasing arrangement there is a negative feed
i back which ‘stabilizes the D.C. supply and the ampliíier 40
More specifically, if the potential at
point 37 becomes .more negative, the .bias on the various .
base l.electrodes causes a'grea'ter collector-emitter current
FIG. 2 shows the hybrid circuit 17 redrawn-from FiG.
1 as an analogous Wheatstone bridge. In this redrawn
version, Branch A (the transmit-channellis that portion `
of the hybridV circuit which is _connected to repeat coil
21. Branch B (the receive channel) is that portion of
v - ñow.. With this increased current, the 1R drop'across the
the hybrid circuit Kwhich is connected to the receiver
.Y :emitter biasing »resistors RY11-RY14 increases, .the bias
on; the ‘emitters drops, and the'collector-emitter current
reduces. Thus, the circuit is self-stabilizing and cor
“REC” of the operator’sheadset. And, Branch C (the
V`'tvvo-way'channel) is that Vportion of the'hybrid circuit
which is connected over the tip and ring leads T and R
rects for ñuctuations of power supply. Moreover, ifv the
to the cord circuit 1li-(FIG. l).
'gain of th'e ampliñer tends to go up, more emitter current
By Way of analysis, reference is made to the super
is drawn, the IRV drop increases, and the VVcollector-emitter 50 position theory Which states that currents flowing vin a
¿current reduces to restore the ampliñer gain Vto normal.
linear network as- a result of the Vsimultaneous applica
»It should be V‘understood that a gain control may be pro
V'tion of a number of voltages distributed in any manner
yidcd to allow either a continuously variable or an'in
through the network, is the sum of the component cur
cremental adjustment in the volume of the sound heardA
rents l'at any point that would be caused by the individual
by the operator. Thus, the negative Vvfeedback provided
`by the voltage drop across resistors RY11--¢RY14 'auto
'matic-ally controlsV the gain of the amplifier.
`Still further, the frequency response of the ampli-tiers
¿is greatly improved by the feedback, since the resistors
voltages if acting separately. Thus, `the hybrid ’circuit
17 may be analyzed byexamining the relation between
`currents in branches A and B separately, currents in
branches A and C separately, and currents in branches
B and ÍC separately.
-RYll--RY14 function as emitter degeneration resist 60
Pursuing this superposition principle, the Wheatstone
bridge of FIG. 2 is redrawn in FIG. 3 as an >equivalent
Van'ces which reduce harmonic distortion, and especially
circuit. In this figure, the legs of the Wheatstone bridge
y :third harmonic distortion, This is because the distor
tions produced in the amplifiers is fed back «by fluctua
are arranged as parallel resistance circuits, the receiver
tions of the emitter bias potential into the input of the
is replaced by an equivalent resistance RR, and the
primary winding of the repeat coil 21 is replaced by an
`amplifier to cancel the original distortion. In'addition,
equivalent generator E and resistance RG. Branch C is
there is no »reactance in the,V resistors RY1rV1-RY14;
replaced by equivalent resistance RL. Each of the Wheat
therefore, the 'response over the entire audio frequency
Vbandis uniform, thus retaining the full fidelity of the
stone bridge resistors RYl-RYS have the same resist
'original signal.y
ance. Also the balancing resistor RYÃG, the transmitter
»Another advantage'ot the negative feedback provided
resistance RG, the receiver resistance RR and RY9 in
'by 'the 'IVR drop across the emitter biasing resistors-is that
vthel transistors 18--20 and other components may be in-V
sistance. The currents ilowing through :the resistors RY7
expensive, commercial >grade devices because the negative Y
and RYB are designated I1 and I2 respectively. The cur- '
‘feedback signal causes the amplifiers to make-up anyV
parallel and branch C resistance RL have the same re
renali/any, flowing in the matching resistor RY9 and
hybrid loss andi-entiers lthe circuit insensitive to am 75 the receiver RR is designated I3.
Also, by the superposition theory, if the receiver re
ohms, and resistors RY9 and RYld have a resistance of
sistance RR and the balancing resistor RY9 are not con
900 ohms.
nected into the bridge the currents Il and I2 are by
When the circuit is used in a 900 ohm oiiice, the
Ohm’s law equal to the BMF. of the generator E di
biasing potentials applied to the various electrodes of
vided by the total resistance of each leg, i.e.
the transistors would n-ot be correct; therefore, the re
sister` RYZS is connected between ground point 26 and
the left-hand end of the upper winding of transformer
21. Thisrresistor both soaks-up any additional power
resulting from changes in resistance values of hybrid net
work i7 and provides a correct input impedance for
Branch A of the hybrid network. It is not necessary to
make any further changes in the ampliñer Acircuits when
Since resistances RYl-RYS are equal to each other and
modified for use in a 900 ohm office.
resistance RYiQ equals resistance RL, current I1 is equal
It is thought that the nature of the invention may be
to current I2. Thus, the potential diiîerence between 15.Y `understood best from the following detailed description
points a and e is zero because the IR drops across re
of the manner in which the circuit of FIG. l operates.
sistors RY7 and RY6 are equal to the 1R drops across
resistors RYS and RYE., and the IR drops across re
sistors RY4 and RYS are‘equal to the IR drops across- re
sistors RYB and RYZ.
If it is now assumed that matching resistor RY9 and
the receiver resistance RR are connected, in parallel, across
When the operator takes position, the key K1 is thrown
to the open position shown in the drawing, thereby dis
connecting the position circuit 11 from the preceding
operator positions 29a.
Also, the operator inserts a
plug attached to the headset 1.6 into jacks Sti, 3l thereby
completing a transmit channel from the headset 16 trans
points a and e no current will flow through either re
mitter “TRNS” to the hybrid network 17 via the transmit
ampliíier 18 and completing a receive channel from the
tial dii‘ierence across points ¿i and e is equal to zero. 25 hybrid network »17 to the headset 16 receiver “REC” via
Therefore, the electromotive force oi' the operator’s trans
the receive amplifiers including transistors i9, 2i).
mitter has no effect on her receiver.
Means are provided for telephonically coupling the
in FIG. 4 the Wheatstone bridgeof FlG. 2 is redrawn
transmitter oi the operator headset to the transmit channel
as an equivalent circuit to show the relation between cur- f
of the hybrid circuit via the inductive coupling between
rents in branches A and C. Again, the repeat coil windin g 30 the windings of a double wound relay. More particularly,
is shown as a generator E and a resistor RG. The resist
the talking battery is supplied over a circuit which may
sistor RY9 or the receiver resistance RR since the poten
ance legs or" the Wheatstone bridge are shown as two
be traced from ground through the right-hand winding of
the transmit relay TR, the tip of the jack 3.1. and associated
separate resistance circuits, and the loop to the subscriber
station is shown as resistor RL. The resistance of the
load across the tip and ring conductors (RL) is equal to
the balancing resistor RYîtl and is equal to the resistance
of the transmitter RG. Again the currents I1 and I2 ñow
through the legs of the Wheatstone bridge while current
plug, the transmitter “TRNS,” the -tip of jack 3%, and Y
associated plug, capacitor 32 and parallel resistor RYZd
to battery 33. In this circuit resistor-RY24 limits current
flow and capacitor 32 acts as a filter to quiet the battery
33. Diodes 34a, 34h are connected across the windings
I3 ñows through balancing resistor RYltl, and current
I4 flows through the subscriber loop.
of relay TR to prevent voltage surges from damaging
transistor i8 when the operator’s headset‘is plugge -in
If it is assumed that balancing resistor~ R‘Yi@ and the
or removed.
loop resistance EL are not connected to the bridge the
Responsive to the operation of transmit relay TR, con
currents Il and i2 are equal because the resistance of
tacts TR1 close thereby energizing the voltage divider
the two legs is equal. This equality of the currents l1 and
RYlS~-RY20 which completes the transistor collector
i2 continues after the resistances RYltl and RL are con 45 and bias supplies, and contacts TR2 close to complete an
nected to the bridge because these resistances are equal.
obvious circuit for energizing relay 34 in position cir
The 1R drop across resistor RY@ is less than the IR
cuit 11.
drops across the resistors RY7-§-RY§+RY5 Thus,
Responsive to the operation of position relay' 34, con- i
there is a potential difference between points h and d,
tacts 4l open to break a supervisory circuit extending to and current L_. ñows through the load resistance RL (ie. 50 the succeeding operator positions 29b. Contacts 42, d3 "
the subscriber loop). This current I4 is equal to the po«
close to extend the heavily inked talking conductors from
tential difference between the points Iz and d divided' by
cord circuit l@ to the operator circuit l2 via capacitors
the resistance RL. For similar easons, a potential dif
CL'CZ. Contacts 44 close to complete a busy test circuit
ference also exists across points b and f. Therefore, a
and contacts 45 and d6 close to extend a monitoring cir
current I3 liows through the balancing resistor RYlt), 55 :uit from operator circuit 12 to cord circuit 10.
_ The voice communication path between the oper
the current value being equal to the potential ditîerence
ator’s headset 16 and the cord circuit l@ is now completed
between points b and f divided by the resistance RYlti.
and nothing further happens until voice signals appear on
In view or" the equality of the various resistances, current
the lines connected to jacks 13b and Mb, or until the oper
I3 equals current I4. Thus, when the operator is speaking
into her transmitter, a voice current isigenerated (the 60 ator’s Voice acts upon the transmitter “TRNS” of head
set l16 to modulate the talking battery 33. In either
Eli/LF. of generator E) and applied across the resistance
event, the voice »signals pass .through the resistance hybrid
circuits of the hybrid network to vary the current Ig in
network Á1.7' in a direction which completes a path for i
the subscriber loop. In this manner, the voice currents
carrying a conversation from a subscriber to the operator
in Branch A appear in Branch C.
or from the operator to a subscriber, as the case may be.
The relation between the currents in Branches B and
However, there is no transmission of signals between the
C is not shown; however, it should be understood that
transmit and receive channels A and B extending between
it is similar to the relation between the currents in
hybrid circuit `1.7 and the headset` 1_6 providing the line im
Branches A and C.
pedance at jack 13b or ll’db matches RYltì.
In this exemplary hybrid network construction, when 70 Incoming voice signals which appear in Branch B are
the operator circuit 12 is used in a 60G ohm oñice each
applied to the base or control electrode b of the transistors
of the resistors RYll-RY8 has a resistance of 425 ohms,
i9, 2t) via resting contacts Ml and M2. Responsive there.
and resistors RY‘E and RYllB have a resistance of 600
' ohms. If the operator circuit is used in a 90() ohm otiice,
to, current flowing through the emitter-collectorcircuits
e-c of transistors 19, 20 and the primary winding of re- `
each of the resistors RYE-RYB has a resistance of 635 75 peat coil 25 are amplilied. By inductive coupling across
verator headset, the circuit may also provide an intercon
nection betweenv a four-wire headset, and a four-wire
repeat coil 25, these ampliñed signals are applied to the re
ceiver “REC” of headset 16. `A vari'stor 52 is connected-in
the receive Vcircuit to shunt excessive voltages to ground,
Vthereby preventing Vacoustical shock to the operator.
» Outgoing voiceV signals originating the transmitter'
‘.‘TRNS” are inductively coupled> across the windings of
-relay TR to the 'base of transistor 181. Thereafter, the
switchboard. Briefly, a four-wire switchboard is one hav
ing segregated transmit and receive channels, each com
sa' 1 pleted over a two-wire metallic path.
That is, a first sub
Vamplified currents flowing in the emitter-collector circuit
scriber transmits to a second subscriber over aríirst two
wire, one-way voice channel providing east-west lcom
munication while the second subscriber transmits to the
age of the transistor’llâ appear in Branch A of the hybrid
tirst subscriber over. a second two-wire, one-way channel
network 17.
providing west-east communication.
To converse with the subscribers, the operator must be
able both to talk and to listen over either channel. For
Means are provided for monitoring conversations eX
l tended, over paths through the cord circuit without de-t>
example, fthe operator talks to the second subscriberV over
grading the voice signals carried over such paths while _. - vthe east-west‘channel and listens over the west-east chan
nel. Conversely, the operator talks tothe ñrst subscriber
allowing the operator to hear at normal levels. Basically, '
a monitor circuit >is a connection extending from the
over the west-east channel and listens over the cast-west
channel. Y To prevent circuit oscillation there mus-t be' no
operator headset to the talking conductors that are then`
carrying subscriber’s voice signals. lf the monitor circuit
is merely connected in'parallel with the subscriber’s sta
tions, the voice currents are divided by the parallel circuits.
feedback between the two one-way channels. Heretofore,
There is a sharp» reduction of signal strength and consc
In carrying out this invention,` a single resistance hybrid '
network is used to interconnect the operator and the four
quential degrading Vof the voice'signals.
the operator circuit Vhas required-a pair of hybrid coils
to prevent such feedback.
To avoid these problems, previous monitor circuits have
included specially designed, high cost hybrid coils bridged
‘ -
wire switchboard circuit. . To illustrate the manner in
making it difficult to'monitor on connections Where the
which this is accomplished, FIG. 5 Vhas been prepared as
a simplified circuit diagram illustrating only those corn
ponents necessary forj an understanding of the invention.
The remaining components (not shown in FIG. 5) are the
signal strength'is weak.
`same as those shown in FIG. 1. Similar reference num
across the line, all of` which caused -a loss of about l() db
of «signal strength and of listening qualities to theroperator
erals are used in lFIGS. 1 and 5 to designate similar parts
with a prime mark (’) added to identify those shown in
In accordance with an aspect of this invention, the
monitor circuit is connectedrto lthepsubscriber lines via
resistors RYZî and RYZZV which sharply limit the Vcur
cuit impedance accordingly. Since Vthe resistors RYil
-REF-§14» in the'emitter circuits increase the input impedance
of the transistors `19, 20, to match the impedance of the
nected to jacks 6i) while' the four-wire line leading to the
called subscriber station is connected to jacks 61.V Thus,
' ingresistor, suchV as RY9 when contacts M1, M2 are
the `east-west channel is connected across points h' and
closed. The advantage of this arrangement is that the
monitor circuit presents a negligible input impedanceV
d', while the west-east channel is connected across points
40 b’ and f’ of the hybrid network 17’. VBecause the hybrid
y during monitoring conditions.
The busy indicating circuit RYZS is disconnected in
network is balanced,V the-energy in "one channel of the
Vfour-wire circuit does _not reach the other channel. On
the other hand, the connections to the transmit channel
>the position circuit whenvthe plugs 13a and 14a ar
A' and to the receive channel B’ are displaced from the
Thus, to monitor a calL-the operator closes a circuit .
(not shown). which operates monitor relay M. Respon
~ sive thereto, contacts M1 and kM2 operate to switch theYV
input kcircuit of> the transistors-amplifiers 19, Ztl from its
With >this Varrange
ment, the amplifiers provide the Ygain which permits-re
connections to the four-wire Vswitchboard circuit by one
resistor in the hybrid network; therefore, there is an un
balanced condition providing the transfer of energy that
allows communication between the operator and the
normal connection with hybrid network 17 to a connec
ftion `with the Vmonitor conductors.
Y .
duction in current drawnL from the line during monitoring
with no reduction in'signal received by the operator.V In
While the principles of the invention have been de
scribed in connection with specific apparatus, it is to be
vclearly understood that this description is made only by
addition, the value of the resistors'R-Yzl and 'RYZZ is so j
way of example and not as a limitation to _the scope of
high that subscribers engaged in the conversation being
the claims.
monitored hear no _clicks or other indication that the
Vvcircuit is being monitored.
_ Busy Test
" wire line leading to the calling subscriber station is con
`monitor circuit, there is no need for an impedance match
y.plugged into the jacks Y13b and 14h. l .
nro. 5.
When used in aV four-wire system, the resistor RY10
is omitted and the second-ofV the Ytwo one-way channels is
Vconnected in" lieu thereof. More specifically, the four
rent that is drawn from the line and raise the monitor cir
To make a busy test, the operator touches the tip of a .
'cord circuit such as 14a to the sleeve oían associated
jack such as 1411. If busy tone is‘ïon the sleevegthe signal'
l. In an operator telephone circuit, the combination
comprising a two-way voice channel including a plu
rality ‘ot talking conductors terminated by a resistance
hybrid network, said hybridnetwork having a character
istic impedance which matches the impedance of said
two-way ‘voice channel, an ¿operator headset, a pair of
one-way voice channels including a transmit channel and
receiver “RECÍ’ >A contact (not shown) of a relay in the
a receive channel for interconnecting said hybrid network
position .circuit 11V disconnects this Ypath when the plug lilar
and said headset, means in each of said one-way chan
nels for: @amplifying voice currents, and means including
is fun-y inserted Eto jack 14h.
. If the vbusy signal -is a battery marking applied to the
a negative feedback circuit for stabilizing said amplifiers.
v sleeve of jack Í-14b, the capacitor 51 charges when the
2. In an operator circuit, the combination comprising
operator touches the *tip of the plug to the associated jack. , _ aY ,two-way voice channel including a pair of conductors,
~Thus, the `operator hears a "click when capacitor< 51 70 means including a resistance hybrid network for terminat
charges. The varistor S2 shunts excessive voltages to
ing said two-way channel in its characteristic impedance,
_ ground, thus precluding acoustical shock to the operator.
an operator headset, a `pair of one-way voice channels
including a transmit and a receive channel for inter
An Operator Circuit for Use in a Four-WirefSwitchboard
connecting said hybrid network and said headset, means
In addition tothe above described'use as a converter`V
f between a two-wire telephone system and a four-wire op 75 in each of Vsaid one-way channels for amplifying voice
current is transmitted over thetalking conductors, con
tacts 44, and through current limiting resistor RSI-23»V to the `
currents, and means including a negative feedback cir
cuit for stabilizing said ampliiier.
3. in an operator circuit, the combination comprising
a pair of one-way voice channels including four wires
for providing a two-wayvoice channel, means including
a resistance hybrid network for terminating said four
Wires in their characteristic impedance, an operator head
seat, a pair of one-way voice channels including a trans
necting a two-way voice channel and a pair of one-wayv
voice channels, the resistance of said hybrid network be
ing selected to match the impedance of said two-Way
channel whereby the characteristic impedance of the cir
cuit into which said plug-in unit is connected may be
changed by switching the plug-in units, means on said
plug-in unit for amplifying voice currents ingveachV of said
one-way channels, and means including :a negative feed
back circuit for stabilizing the output of said amplifying
said hybrid network and said headset, means in each of 10 means against changes caused by drifting characteristics
of component in said amplifying means whereby inexpen
said channels that interconnect said hybrid network and
sive commercial grade components may be used in said
said headset for amplifying voice currents, and means
mit channel »and a receive channel for interconnecting
including a negative feedback circuit for stabilizing said
component circuit.
9. In a telephone switchboard, the combination com
4. A telephone switchboard comprising a plurality of 15 prising `a plurality of talking conductors for interconnect
ing an operator circuit :and cord circuits at said switch
cord circuits, an operator circuit, and a resistance hybrid
board, a resistance hybrid network having three branches,
network, means including a plurality of talking con
means for coupling *one of said branches to said pair of
ductors for extending a two-way voice channel from
talking conductors, the characteristic impedance of said
said cord circuits to said resistance hybrid network, an
operator headset in said operator circuit, a transmit chan 20 hybrid network being matched to the characteristic im
pedance of said talking conductors, an operator headset
nel and a receive channel in said operator circuit for
including a transmitter and a receiver, means for con
interconnecting said hybrid network and said headset,
necting a second brauch of said hybrid network to a
means in said transmit channel for amplifying voice cur
transmitter in said headset,_ means for connecting the
rents, means in said receive channel for amplifying voice
currents, each of said amplifying means comprising a 25 third branch of said hybrid network to the receiver in
said operator headset, means interposed between said
transistor connected in a common emitter configuration,
transmitter and said hybrid network for amplifying Voice
and means for biasing the emitters of said transistors
signals originating at said transmitter, means interposed
between said hybrid network and said receiver for ampli
30 fying voice signals received over said talking conductors,
current for stabilizing said amplifiers.
each of said amplifying means comprising a transistor
5. A telephone switchboard comprising a plurality `of
through at least one voltage dropping resistor to provide
a negative feedback which varies :as a function of emitter
cord circuits, an operator circuit, a headset, means in
connected in a common emitter configuration, and means
comprising at least one resistor for applying biasing p-o
cluding a plurality of talking conductors for extending a
tential to the emitter of each of said transistors whereby
two-way voice channel from said cord circuits to said
operator circuit, a plug-in unit for attachment into said 35 increased current through an emitter causes a larger IR
dropV across said resistor thereby reducing the biasin
operator circuit, said plug-in unit comprising a hybrid
potential applied to said emitter.
network, a transmit channel, and a receive channel, said
l0. An operator telephone circuit comprising la head
hybrid network interconnecting said two-way channel
set having a transmitter and a receiver, a transmit chan
and said transmit and receive channels, means in said
transmit channels for amplifying voice currents originat 40 nel coupled to said transmitter and a receive channel
coupled to said receiver, a two-way communication chan
ing in said headset, means in said receive channel for
amplifying voice currents received over said two-way
nel extending to said operator circuit, means including a
`hybrid resistance network for interconnecting said two
voice channel, each of said amplifying means comprising
a transistor connected in -a common emitter configura
way channel and said transmit and receive channels,
tion, and means for biasing the emitters of said tran 45 means comprising a transistorized amplifier in said trans
mit channel for amplifying voice signals origin-ating in
sistors through at least one voltage dropping resistor to
provide a negative feedback controlled by emitter current
said transmitter, means comprising a transistorized ampli
Iier in said receive channel for amplifying voice signals
for stabilizing said amplifiers.
received over said two-way channel, and means for sta
6. A telephone switchboard comprising a plurality of
cord circuits, an operator circuit, and an operator head 50 bilizing the output of said transistors comprising a nega
tive feedback circuit for controlling the gain of said
set, means for extending a two-way voice channel from
transistors, said gain being selected to compensate for
said cord circuits to said loperator circuit and a unit for
loss of signal strength in said hybrid network and said
plugging into said operator circuit, said plug-in unit com
negative feedback to compensate for changes in ambient
prising a resistance hybrid network, a transmit channel
and a receive channel, means including said hybrid net 55 environmental conditions, and for drifting component
work for interconnecting said two-way channel and said
1l. The operator telephone circuit of claim l0 and
headset Via said transmit and receive channels, and ampli
means comprising a double wound relay, said coupling
lier means in both said transmit channel and said receive
between said transmit channel and said transmitter being
channel for amplifying voice-currents, the impedance of
said resistance hybrid network being matched to the 60 completed by the inductive coupling across said relay
impedance of said two-way channel whereby the char
12. The operator telephone circuit of claim 1l and
acteristic impedance of said operator circuit may be
means responsive to operation of said relay for energiz
changed to match the characteristic impedance of a
telephone office by switching said plug-in units.
ing said amplifiers.
13. In a telephone switchboard, at least one cord cir
7. The telephone circuit of claim 6 wherein the re 65
cuit for telephonically interconnecting subscriber lines,
sistance of the hybrid networks on some of said plug-in
an operator headset having at least a receiver, -an operator
units is selected to match the impedance of a 600 ohm
circuit for interconnecting said cord circuit and said head
telephone oñìce and the resistance of the hybrid net- v set, means in said operator circuit for monitoring con
works on other of said plug-in units is selected to match 70 versations extended over paths through said cord circuit,
the impedance of a 900 ohm telephone office.
at least one amplifier having input and output circuits,
y8. A component circuit for use in a telephone system
said monitoring means including a pair of talking con
comprising a plug-in unit for interconnecting two- and
ductors coupled between said subscriber lines and the in
put circuit of said amplifier via current limiting resistors,
unit including a resistance hybrid network for intercon 75 and means for coupling the output circuit of said ampli
four-wire transmission channels, means on said plug-in
1 i
fier to Vsaid receiver ofsaid operator headset thereby
limiting resistor for transmitting busy signals from said
allowing the voperator Vto hear at normal ievels without
cord circuit toV said headset. Y »
degrading voice signals on said subscriber lines.
14. In an operator telephone circuit, the combination
16. An operator'circuit for use -in a teiephone system
comprising a two-way voice channel >for ‘interconnecting
comprising a headset having a transmitter and a receiver, 5 Tsaid operator circuit and a telephone line, an operator
a transmit channel coupled to said transmitter and a re
ceive Vchannel coupled to said receiver, a two-way com
headset, means comprising La'rresistance hybrid network
v>in said operator circuit for> interconnecting said two-way
inunication `channel extending to said operator'circuit,
means 4including va resistance hybrid network for inter
connecting said two-way channel and said transmit and
receive channels, means for amplifying voice signals ap
Y channel and said headset,`A at least one ampliñerphaving
:input and output circuits interposed between said hybrid
network and said operator headset, kthe input circuit of
said ampiiiier being normally coupled to .y said hybrid
'peering in said hybrid network comprising an ampliñer
coupled between said hybrid network and said receiver, Y
"meanscomprising la negative-gfeedbackV circuit for stabiiiz
'ing >the output of said ampliñena monitor circuit includ
ring current limiting resistors, 4and means for Switching
said amplifier from said coupling with said hybrid cir
cuit to a coupling with said monitor circuit.
V»network and the output circuit of said amplifier being`
Y coupled to said headset, a monitor circuit including cur
`rent limiting resistors for extending voice signals between
V`said ktelephone line and said operator circuit, means for
switchingithe input circuit of said amplifier from said
15 A telephone switchboard comprising `a pair of tallt- Y
‘ing conductors for interconnecting'cord circuits and an
A`operator circuit, a hybrid network terminating said con
normal coupling with said hybrid network to a coupling
with said monitor circuit, Vmeans for extending busy sig
nals from said telephone line to said headset independ
20 ently
` ductors in said operator circuit, an operator headset, a Y
of said amplifier, >and means associated with said
last named means for shunting excessive voltages thereby
"transmit channel and a receive channel for interconnect Y ’preventing acoustical shock to the operator.
ing Ysaid'hybridl network and said headset, means kin each Y,
'17, Ina telephone system, a transmitter, a voice chan
of said channels for amplifying voice currents, each‘of 25,:nel, a two-winding relay, means for supplying talking
said :amplifying means comprising a transistor Yconnected
in a common emitter configuration, means lfor appiying
'a biasing potential to -the emitters of said transistors
battery yto said transmitter throughone of said windings,
and means for coupling said voice channel to the other
.of said windings, whereby saidtransmitter and saidtvoice
_through voltage dropping resistors to provide a negative
channel are telephonically coupled via the inductive cou
_feedback circuit for stabilizing said amplifying means, -aV 30 f_pling between said windings.
¿monitor ' circuit `intercormecting said cord -circuit and saidV
1Voperator circuit via current Vlimiting resistors, means >for
switchingsaid 'amplifying means in said -receive channel
' ’from connection lwith’said hybridnetwork to connection .
iwith saidjm'onitor circuit, and means including a vcurrent 35
‘ 18. The'telephone system of claim 17 and a pair of
diodes, one of said diodes being connected across Veach
of said windings.
No ‘references cited.
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