Патент USA US3033950код для вставки
May s, 1962 A 3,033,940 OPERATOR J.TELEPHONE'MRCUIT E. coxr‘ _ Filed Sept. 9, 1960 3 Sheets-Sheet l „SNN .Y l@ n MNR( É§§àls» lIi _ \\ \\ . INVENToR. Jl. ral B V.. „Sà@ , #Mix/fr Il; May s, 1962 J. E. CÓX OPERATOR TELEPHONE CIRCUIT 3,033,940 May 8, 1962 J. E. cox 3,033,940 OPERATOR TELEPHONE CIRCUIT Filed sept. 9. 1960 s sheets-sheet :s LA; A; vvvvvvv -à iRvSm@ `Rm È. `ï -bëm»SS S.«Ènä x„SQës ìwg INVENTOR. .zi mr BY - Ä/ A//ß/f//ff É? 3,033,940 Y Vhybrid vnetwork 17 via monitor relay contacts M1, M2 and » the output or collector electrodes c are inductiv'ely coupled d 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 channel. 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 . l 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 characteristics. 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 Y i ' 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 55 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. 3,033,940 5 . Also, by the superposition theory, if the receiver re d „ 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 and 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. _ 40 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. ' ' Y 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. 65 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 . - 3,033,940 S 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. - ' _ _ Monitoring 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 l Y 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 ' l 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 l ~ 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 45 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 50 subscribers. , 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 ' Y» " 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' i claim: ' A s . 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. Y . 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 ` 3,033,940 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 amplifiers. 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 characteristics. 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 windings. 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 3,633,946 . Y Y Y 1 i 12 _ 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. t t Y No ‘references cited.