Abstract:
Resistive battery feed circuits are normally not used because of lack of power supply noise rejection and attendant crosstalk coupling. In this circuit the opamp and surrounding circuitry insure that power supply and ground noise do not couple into the tip and ring circuits as a metallic signal and improve crosstalk attenuation. In addition, the opamp is used to split the 48 volt supply.

Description:
BACKGROUND OF THE INVENTION 
     This invention is related to telephone line circuits and more particularly to the reduction of power dissipation and the elimination of inductors in such circuits. 
     A telephone line circuit is interposed between the PBX line and the associated telephone station. These circuits serve, among other things, to provide both AC and DC power to activate the telephone transmitter and receiver. These circuits presently contain inductors which serve to isolate the DC power supply and to allow for the circuit&#39;s AC impedance to differ from the DC resistance. One major aim of circuit designers has been the elimination of the costly, and bulky, inductors from the line circuits, thereby making the circuits fully resistive. 
     Resistive line feed circuits, however, have not been popular in the past for several reasons, namely because they do not reject power supply noise. Any noise on the battery supply shows up as an audible signal in the telephone. A second problem is that ground potential noise can be heard as an audible signal. Another problem with resistive-only line circuits is that one port could crosstalk to others through the power supply internal impedance. 
     SUMMARY OF THE INVENTION 
     I have designed a simple, but efficient, resistive battery feed line circuit that saves power dissipation and is physically smaller than previous designs operating from -48 VDC. It places two resistives battery feed circuits in series between -48 v and ground, each circuit operating on 24 volts. Ground noise is fed to the ring lead of one circuit by a Darlington emitter follower to that circuit&#39;s tip lead and the other circuit&#39;s ring lead through a source follower and directly to the tip lead of the other circuit. As a result, the noise signals on all of the leads move in unison thereby eliminating noise problems. Crosstalk is prevented in the same way as noise immunity is achieved by an opamp and a Darlington power filter. The circuit saves power over conventional line circuits by recycling the line feed current between two line circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features, together with the operation and utilization of the present invention, will be more apparent from the illustrative embodiment shown in conjunction with the drawing in which 
     FIG. 1 is a schematic drawing showing my resistive battery feed circuit, and 
     FIG. 2 shows a thermal shutdown circuit. 
    
    
     DETAILED DESCRIPTION 
     The circuit shown in FIG. 1 stacks two 24-volt resistive line feed circuits (R2, T2 and R1, T1) between ground and -48 volts. This results in two line circuits dissipating the same power as one of conventional design. Circuit T1, R1 operates between ground (lead T1) and -24 volts (lead R1) while circuit T2, R2 operates between -24 volts (T2 lead) and -48 volts (lead R2). Note that in both circuits the R- lead is the one most negative. 
     NOISE ELIMINATION 
     Ground noise is rejected in the following way. Any noise on ground, including audio voltages, is fed to ring 2 (R2) via capacitor C1, the Darlington emitter follower Q1, Q2 and resistor R11. These same ground noise signals are also fed to tip 2 (T2) via capacitor C2, the source follower amplifier A1 and resistor R13; to ring 1 (R1) via capacitor C2, source follower amplifier A1 and resistor R14; and to tip 1 (T1) via source follower Q10 and resistor R17. As a result, the ground noise signals appear as a common mode voltage on both tip/ring pairs, and accordingly, there is no differential voltage generated between the leads as a result of ground noise. 
     Power supply noise on the -48 volt lead is rejected by the Darlington filter circuit comprised of transistors Q1, Q2, and voltage divider resistors 1R1 and 1R2 and capacitor C1. Voltage divider resistors R1 and R2 establish a bias on the base of transistor Q1 while capacitor C1 filters the voltage to remove the AC noise component. The bias voltage should be selected so as to insure enough voltage drop across transistors Q1, Q2 to prevent their saturation by noise voltages riding on the -48 v power input. The resultant &#34;quiet&#34; bias voltage is delivered by the Darlington pair transistors Q1, Q2 to ring 2 (R2) via resistor R11. The voltage divider comprised of resistors R3, R4, R7 and R9 provides, at its midpoint, the -24 volt reference required for leads T2 and R1. 
     Power supply noise rejection for these leads is accomplished by the inherent power supply rejection of opamp A1. Since lead T1 is effectively grounded, power supply noise rejection is not a problem. This power supply noise rejection prevents other common mode or metallic noise signals from appearing on terminals T1-R1 or terminals T2-R2. 
     The Darlington transistors Q3, Q4, Q5 and Q6 feeding power to opamp A1 are used with voltage dividing resistors R3, R4, R7 and R9 so that only a fraction of the -48 volt source will appear across opamp A1. These transistors also function to reduce the power dissipation of opamp A1 because of the reduced voltage. This arrangement allows for the use of a low voltage opamp and could be eliminated if an opamp is used which can handle the -48 volt power directly. 
     CROSSTALK ELIMINATION 
     Digressing momentarily, it should be noted that transistor Q10 allows audio signal currents on tip 1 (T1) to flow via resistor R17 to the +5 volt supply lead while rejecting +5 v supply noise. Transistor Q10 could be eliminated and the emitter and collector leads connected directly to ground in which case the audio signal currents would flow on the ground lead. Such a situation, while possible in many situations, could cause crosstalk if circuits other than filtered battery feed circuits were to use the same ground. 
     Opamp A1 functions to provide a low impedance source of -24 v to the two circuits R1-T1 and R2-T2. Point 103, between terminals R1 and T2, is a low impedance point of -24 v that is used to provide a -24 v feed circuit to each of the communication stations via the associated battery feed resistors R14 and R13 in conjunction with battery feed resistors R17 and R11, respectively. Feedback for source follower opamp A1 to its negative input is taken from point 103 to insure that the wiring resistance from the output of opamp A1 to point 103 is within the feedback loop of opamp A1. This insures that point 103 has a low AC impedance and remains at -24 v. Low impedance at point 103 prevents the introduction of signals on the T2 terminal from signals on the R1 terminal (or the reverse). These undesirable signals, crosstalk, could also be coupled through the common opamp if care was not taken. However, opamp A1&#39;s negative power return, lead 101, is directly returned to -48 v via Darlington Q3, Q4 so as to prevent crosstalk between the circuits. Since opamp lead 101 carries audio signal current from the R1, T1 circuit, if this current were allowed to flow into the emitter of the Darlington power filter Q1, Q2, excessive crosstalk could result because of the emitter resistance of the Darlington pair. By returning the opamp power directly to -48 v any resultant signal voltage on the -48  v lead is filtered out by the Darlington power filter, as discussed above, before reaching the feed circuit, T2, R2. 
     LOOP CURRENT DETECTION 
     Loop current flowing between terminals T2 and R2 (or T1 and R1) is sensed by ground (low) appearing on leads A (or B) respectively. This occurs since current flowing between terminals T2 and R2, which signifies the off-hook condition of the communication station, generates a voltage across resistor R13 which, in turn, causes transistor Q7 to conduct. Resistors R10 and R12 determine the sensitivity of the circuit. The collector current of transistor Q7 drives transistor Q8 which provides the ground signal on lead A. Transistor Q8 is necessary because T2-R2 is at a potential below ground, i.e., -24 v to -48 v. Circuit R1, T1 uses only one transistor Q9 since that circuit operates between ground potential and -24 v. 
     The component values shown are for illustration purposes only and any other values may be used depending upon circuit sensitivity and requirements. Also, it should be noted that, while Darlington pairs are shown, other circuit configurations, such as superbeta pairs, or high gain transistors, or field effect transistors, may be substituted. 
     THERMAL PROTECTION 
     FIG. 2 shows the thermal shutdown circuit which may be used to inhibit chip power when the temperature of the chip reaches a first temperature threshold. The inhibit is maintained until the temperature of the chip decreases to a second predetermined temperature. In this manner, thermal hysteresis allows the circuit to cool sufficiently before restoring it to service. 
     Referring to FIG. 2, bias voltage 210 with resistors 2R1 and 2R2 establish a voltage at the base of transistor 2Q1 sufficiently below the base-to-emitter junction voltage of transistor 2Q1 to hold transistor 2Q1 in the nonconducting state. In turn, transistors 2Q1 and 2Q2 in the nonconducting state prevent transistor 2Q3 from conducting. It is well known in the art that the base-to-emitter junction voltage of a transistor decreases at a rate of approximately 2 millivolts per each degree centigrade rise in temperature. Thus, as the temperature of the illustrated circuit rises, the base-to-emitter junction voltage of transistor 2Q1 decreases. Resistors 2R1 and 2R2 are selected to establish a bias voltage across the base-to-emitter junction of transistor 2Q1 such that transistor 2Q1 starts to conduct when the temperature of the circuit is at approximately 150 degrees centigrade. 
     Transistor 2Q1, entering the conducting state, allows transistor 2Q2 to conduct which supplies additional forward bias current to the base of transistor 2Q1 via resistor 2R3 thereby driving transistor 2Q1 further into conduction. Transistor 2Q2 in the conducting state also permits transistor 2Q3 to conduct when applies an inhibit signal to the output which is applied between resistors R1 and R2 (FIG. 1) thereby removing power from the battery feed circuit. 
     The current supplied by transistor 2Q2 to the base of transistor 2Q1 via resistor 2R3 raises the bias voltage applied to the base-to-emitter junction of transistor 2Q1. Thus, transistors 2Q1, 2Q2 and 2Q3 remain in the conducting state when the temperature of the circuit starts to cool. In this manner, the values of resistors 2R1, 2R2 and 2R3 are selected so that the base-to-emitter junction voltage of transistor 2Q1 equals the bias voltage at a second or lower predetermined temperature at which transistors 2Q1, 2Q2 and 2Q3 start to turn off thereby disabling the inhibit applied to the battery feed circuit (FIG. 1) by transistor 2Q3.