Abstract:
A telephone line interface circuit is disclosed having intelligent line current control that can adjust to the various requirements of worldwide telephone systems (DC masks), and/or can reduce the power consumed by the interface circuit. A controller reads a voltage value and based on the theoretical model of a telephone line network, and the expected current/voltage characteristic, as dictated by the appropriate DC mask. The controller sets the line current (via an output voltage) to an optimum value and causes the line voltage to adjust so as to comply with the DC mask and/or minimize power dissipation in the circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following U.S. Patent Applications: U.S. patent application Ser. No. 09/212,707, entitled TELEPHONE LINE INTERFACE CIRCUIT WITHOUT HOOKSWITCH, filed Dec. 16, 1998; U.S. patent application Ser. No. 09/312,136 entitled ELECTRONIC INDUCTOR WITH TRANSMIT SIGNAL TELEPHONE LINE DRIVER, filed May 14, 1999; U.S. patent application Ser. No. 09/312,218 entitled TELEPHONE LINE INTERFACE CIRCUIT WITH VIRTUAL IMPEDANCE, filed May 14, 1999; U.S. application Ser. No. 09/312,178 entitled METHOD AND APPARATUS FOR DIGITAL PABX DETECTION AND MODEM INTERFACE PROTECTION, filed May 14, 1999. All of these applications are commonly owned by the assignee of the present application. The disclosure of all those applications are explicitly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of modem circuits, and more particularly, to a telephone line interface circuit. 
     2. Description of Related Art 
     Existing telephone line interface circuits use a passive network in the electronic inductor (EI) which provides feedback from the Tip and Ring voltage to the transistor&#39;s bias and adjusts the line current so that the line voltage is maintained as low as possible. The circuit power dissipation is then calculated by using the worst-case voltage and current values. Components satisfying the maximum power requirements are then used. This power rating can be substantially higher than the rating that could be achieved by using an “intelligent” control of the current and voltage. 
     Additionally, problems arise when designing telephone line interface circuits for worldwide applications due to the specific V-I characteristics (i.e. requirements) of the telephone lines in different countries. These various V-I characteristics are commonly defined as “DC masks.” Specifically, various countries have different requirements as to the minimum and/or maximum levels of DC off-hook line voltage permissible for a given Tip and Ring circuit. For example, in the United States, the DC line current cannot exceed 6.6 volts at 20 mA. In France, the DC line current cannot exceed 60 mA while the voltage can be as high as 40 volts. Examples of various DC masks are shown in FIGS.  10 (A)- 10 (D). The solid line on each graph represents a particular V-I boundary constraint within which the interface circuit must operate. These DC mask requirements, however, cannot be met with a single passive network. 
     Thus, there is a need for an intelligent line current control which can reduce the power rating over conventional telephone line circuits, and which can comply with various worldwide DC masks using a single circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides intelligent line current and voltage control that can adjust to the various requirements of worldwide telephone systems, and/or can reduce the power consumed by the interface circuit. In a basic configuration (FIG.  1 ), a voltage divider is connected across the rectified Tip and Ring voltage (VTR) and provides feedback of the line voltage Vtr to an electronic inductor and to an analog-to-digital converter (ADC)  10 . The output of the ADC  10  is then provided to a controller  30 . The controller  30  may be implemented as a microcontroller, using either hardware or software control. Based on the theoretical model of a telephone line network and the expected voltage/current characteristic, as dictated by the appropriate DC mask, the controller  30  sets the line current to an optimum value (via an output voltage) VDAC and causes the line voltage to adjust so as to minimize power dissipation in the circuit. The controller  30  can set the line current precisely via a digital-to-analog converter (DAC)  20 , by changing the voltage at the base terminal of the electronic inductor transistor Q 1  and measuring the emitter voltage Ve. The line current will be equal to the emitter voltage Ve divided by the emitter resistor Re. 
     Furthermore, the controller  30  can calculate the power dissipation in the emitter resistor Re from the value of the emitter voltage Ve, and determine the power dissipation in the transistor Q 1 . While determining the optimum voltage/current line setting, the controller  30  also takes into account the specific requirements of a particular DC mask, depending upon the country of operation. A switch S 1  is enabled to increase the dynamic range of the ADC  10  with respect to VTR, by adding a resistor R 3  in parallel with R 2  so that a relatively large VTR can be measured within the limited voltage range of the ADC  10  (typically 0-4V). 
     In an alternative embodiment (FIG.  2 ), the ADC  10  reads the current between tip and ring (Itr) directly from the emitter of the transmitter Q 1  by measuring Ve and dividing by Re. The controller  30  can select either switch S 2  or S 3  to read either voltage Vtrdc or Ve. In other embodiments, various feedback lines, controlled by the controller  30 , are used to provide for greater current control. The electronic inductor transistor may also be configured as a Darlington pair. Various switches may be added to provide greater control over the operating range of the interface circuit, and are controlled by the controller  30 . 
     The control logic of the controller may be implemented in software, and the circuit adjusted using either a static, dynamic or static-dynamic combination control method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exact nature of this invention, as well as its objects and advantages, will become readily apparent from consideration of the following specification as illustrated in the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein: 
     FIG. 1 is a schematic diagram of a telephone line interface circuit with DC line voltage control according to the present invention; 
     FIG. 2 is a schematic diagram of a telephone line interface circuit according to a second embodiment of the present invention having DC line voltage control with direct current reading; 
     FIG. 3 is a schematic diagram of the equivalent DC circuit for the telephone line and modem line interface; 
     FIG.  4 (A) is a flow-chart of the static DC line voltage control logic according to the present invention; 
     FIG.  4 (B) is a flow-chart of the dynamic DC line voltage control logic according to the present invention; 
     FIG.  4 (C) is a flow-chart of an optimized DC line voltage control logic according to a preferred embodiment of the present invention; 
     FIG. 5 is a schematic diagram of a telephone line interface circuit with DC line voltage control and having multiplier switches; 
     FIG. 6 is a schematic diagram of a telephone line interface circuit with DC line voltage control using a Darlington pair configuration; 
     FIG. 7 is a schematic diagram of a telephone line interface circuit with DC line voltage control using a Darlington pair configuration and having direct current reading; 
     FIG. 8 is a schematic diagram of a telephone line interface circuit with DC line voltage control and having analog multiplier compensation; 
     FIG. 9 is a schematic diagram of a telephone line interface circuit with DC line voltage control having DAC calibration; 
     FIG.  10 (A) is graph of a first DC mask; 
     FIG.  10 (B) is graph of a second DC mask; 
     FIG.  10 (C) is graph of a third DC mask; 
     FIG.  10 (D) is graph of a fourth DC mask; and 
     FIG. 11 is a graph of the typical operating range of a telephone line interface circuit with DC line voltage control. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the basic principles of the present invention have been defined herein specifically to provide a telephone line interface circuit with intelligent line current control. 
     The basic principles of the present invention may be understood with reference to FIG.  3 . 
     FIG. 3 is a schematic diagram of the circuit of FIG.  1  and includes the DC equivalent circuit for a telephone company central office network (CO). The DC characteristic of the CO can be modeled as a voltage source Vbatt, in series with a loop resistance Rloop. Diode bridge D 1 , part of the modem telephone line interface, can be modeled as two diodes D 1 A and D 1 B, equivalent to a voltage drop of approximately 1.4V. 
     Resistors R 1  and R 2  form a resistor divider with respect to voltage VTR′, and their value is chosen to be very large to make the current through them negligible compared to the current through Q 1 . Therefore, the current through Q 1  is equal to the line current Itr. An operational amplifier U 1  converts the voltage Vtrdc to the current required to drive the base of the transistor Q 1 , and the negative input of U 1  is connected to Ve, the emitter voltage of Q 1 , effectively making the voltage at node Vtrdc equal to Ve (virtual ground property of operational amplifiers). Since the base current of Q 1  is negligible compared to the line current ITR, the value of ITR can be calculated as Ve (voltage across Re) divided by the value of Re. In this configuration, the transistor/operational amplifier configuration operates as a voltage-controlled current source (VCCS). 
     When the modem is on-hook, switch S 1  is enabled, placing resistor R 3  in parallel with resistor R 2 , U 1  is disabled, and transistor Q 1  is turned off (ITR=0). 
     When the modem goes off-hook, U 1  is enabled, and the voltage feedback from Tip and Ring at Vtrdc causes line current ITR to flow through Q 1 . 
     An analog-to-digital converter (ADC)  10  reads the voltage at Vtrdc, and based on this reading the controller  30  determines the voltage at Tip and Ring, VTR, according to the following equations: 
     
       
           VTR=VTR ′+1.4V 
       
     
     
       
         ( VTR′−Vtrdc )/ R   1 =( Vtrdc−VDAC )/ R   2 , which solved for  VTR ′ yields 
       
     
     
       
           VTR ′=( R   1 / R   2 )×( Vtrdc−VDAC )+ Vtrdc , and therefore 
       
     
     
       
           VTR =( R   1 / R   2 )×( Vtrdc−VDAC )+ Vtrdc+ 1.4V  [1] 
       
     
     The factor (R 1 /R 2 ) is herein referred to as the electronic inductor “multiplier.” If switch S 1  is enabled, the value of R 2  is substituted with the parallel combination of R 2  and R 3 , or R 2 //R 3 . 
     The controller  30  can also measure the line current at any point in time by using the ADC reading according to the following equation: 
     
       
           ITR=Ve/Re . Since 
       
     
     
       
           Vtrdc=Ve , then 
       
     
     
       
           Itr=Vtrdc/Re   [2] 
       
     
     Therefore, the controller  30  can monitor both the line voltage and line current at any point in time, by using the same ADC reading at node Vtrdc. 
     When the modem is on-hook, ITR is equal to zero and VTR represents the battery voltage at the CO (there is no voltage drop across the loop resistance). Hence, 
     
       
           VTR (on-hook)= Vbatt   [3] 
       
     
     When the modem is off-hook, the controller  30  calculates both VTR and ITR based on the steady-state ADC reading, and then estimates the loop resistance Rloop according to the following equations: 
     
       
         ( V batt− VTR )/ R loop= ITR , which solved for  R loop yields 
       
     
     
       
           R loop=( V batt− VTR )/ ITR   [4] 
       
     
     Equations [1], [2], [3], and [4] form the basis for two independent methods of controlling the line voltage as a function of the line current. Specifically, equations [1] and [2] can be used to implement “dynamic” DC voltage control, whereas equations [3] and [4] combined with [1] and [2] can be used to implement “static” DC voltage control. 
     In the dynamic method, the modem goes off-hook and the controller  30  sets an arbitrary initial DAC  20  setting, which results in a known VDAC voltage and switches the modem off-hook. The ADC  10  then measures voltage Vtrdc, and the controller  30  uses equation [1] to calculate VTR and equation [2] to calculate ITR. If the value of VTR is outside the range specified by the DC mask at the current ITR, the controller  30  increases VDAC to decrease VTR or decreases VDAC to increase VTR, respectively. 
     The DAC setting can be changed one bit at a time or in larger steps first and single bits later, depending on the difference between the VTR measured and the target value, and the process continues until VTR is within a specified range of the target value. On each iteration, the controller  30  recalculates ITR, as this value changes when the DAC  20  setting changes. 
     In the static method, the controller  30  sets an arbitrary initial DAC setting which results in a known VDAC voltage and uses equation [3] to measure Vbatt while the modem is on-hook, immediately before going off-hook. When the modem goes off-hook, the controller  30  measures voltage Vtrdc, and uses equations [1] and [2] to calculate VTR and ITR, respectively. Using equation [4], the controller then calculates Rloop. Since, 
     
       
           VTR=V batt− ITR×R loop, where  ITR=Vtrdc/Re , then 
       
     
     
       
           VTR=V batt−( Vtrdc/Re )× R loop, which solved for  Vtrdc  yields 
       
     
     
       
           Vtrdc =( V batt− VTR )×( Re/R loop)  [5] 
       
     
     Substituting the expression for Vtrdc from equation [5] into equation [2] yields 
     
       
           VDAC =( R   2 / R   1 )[ V batt( Re/R loop)(1 +R   1 / R   2 )− 
       
     
     
       
           VTR (1 +R   1 × Re/R   2 × R loop+ Re/R loop)+1.4]  [6] 
       
     
     Equation [6] relates the value of the DAC setting VDAC to line voltage VTR, assuming that Vbatt and Rloop are known. Using equation [6], therefore, the controller  30  can calculate the DAC setting as a function of the target value for VTR. The target value for VTR is based on the DC mask requirements and other considerations, such as power and distortion. The controller  30  has stored values for the DC masks shown in FIGS.  10 (A)- 10 (D), as well as the maximum power curve  13  of the transistor Q 1 , illustrated in FIG.  11 . 
     In both methods, the controller  30  always calculates the power dissipation in the modem line interface as 
       P modem= VTR×ITR   [7] 
     and adjusts the DAC  20  setting so that the condition Pmodem&lt;Pmax is satisfied, where Pmax if the maximum power rating of the modem line interface. 
     The details of the control logic used by the controller  30  in the static method are illustrated in the flow-chart of FIG.  4 (A). A final DAC  20  setting that satisfies both the DC mask and power requirements can be stored by the controller  30  in a memory register MemDAC, and used as the initial DAC value (best guess) the next time the modem goes off-hook (steps  40 - 46 ). If the modem is stationary at one location (which is the case most of the time in practical applications), using the stored previous, optimal DAC value will result in fewer iterations and a faster execution of the algorithm. 
     Immediately before going off-hook, the controller measures Vbatt using equations [1] and [3] (VDAC=0), and writes an initial value to the DAC, which results in a known voltage VDAC. This setting can be an arbitrary default value, or it can be the DAC  20  setting used by the modem in the previous off-hook session and stored in register MemDAC. 
     After going off-hook (step  48 ), the controller waits (step  50 ) for a steady-state line voltage condition, typically 100 ms, measures Vtrdc, and calculates VTR and ITR using equations [1] and [2], respectively (step  52 ). Using equation [4], the controller  30  then calculates Rloop (step  54 ). Knowing Vbatt and Rloop, it is possible to calculate an optimal operating point (VTR) within the DC mask, which minimizes distortion and power dissipation in the line interface, for example (step  56 ). 
     Using equation [6], an optimal value for VDAC can be calculated (step  58 ), and a corresponding value is written to the DAC (step  60 ). This setting is also stored in register MemDAC (step  62 ), so it can be used as initial value for the DAC the next time the modem goes off-hook. This prevents unnecessary transients when the modem goes off-hook on the same telephone line as the previous off-hook session. 
     Static voltage control has the benefit of using Vbatt and Rloop to calculate the optimal operating point in the V-I domain, because all the DC circuit parameters of the loop are known. This method, however, does not use feedback from the line voltage and current to correct the possible discrepancies between calculated values and actual values. 
     Such discrepancies may arise as a result of erroneous instantaneous voltage readings by the ADC  10 , or changing DC circuit parameters during the same off-hook session. It is not uncommon that the battery voltage at the CO changes over time during a power outage, for example, which would cause a considerable variation of Vbatt in the course of the same off-hook session. In this case, the controller  30  would not be able to detect a change in value of Vbatt. 
     Discrepancies between calculated and actual line voltage values can also be caused by relative error between the ADC  10  reading and DAC  20  output voltage, for example. Relative error can be minimized by calibrating the output voltage of the DAC  20  against the ADC  10  reading, as shown in FIG.  9 . While the modem is idle, the controller  30  enables switch S 9  and reads through the ADC  10  the output of the DAC  20  for each DAC setting. A look-up table representing calibrated values of the DAC  20  vs. the ADC  10  can be stored in a memory and used by the controller  30  when selecting an appropriate value for the DAC  20  to obtain a target VTR based on the ADC  10  reading. 
     The control logic for the dynamic control method will now be described with reference to FIG.  4 (B). Immediately before going off-hook, the controller  30  writes an initial value to the DAC  20 , which results in a known voltage VDAC (step  72 ). This setting can be an arbitrary default value, or it can be the DAC setting used by the modem in the previous communication session and stored in register MemDAC. 
     After going off-hook, the controller waits for a steady-state line voltage condition, typically 100 ms (step  76 ), measures Vtrdc (step  78 ), and calculates VTR and ITR using equations [1] and [2], respectively (step  80 ). If the value of VTR is outside the range specified by the DC mask at the current ITR (step  82 ), the controller  30  increases VDAC to decrease VTR or decreases VDAC to increase VTR, respectively (step  86 ). 
     The process continues until VTR is within a specified range of the target value. On each iteration, the controller  30  recalculates ITR as this value changes when the DAC  20  setting changes. The DAC setting can be changed one bit at a time, or starting with larger steps first and single bits later, depending on the difference between the VTR measured and the target value. 
     When VTR is within target, the controller  30  calculates the power dissipation in the modem line interface by multiplying the values of VTR and ITR, and compares this value with the maximum power rating Pmax (step  84 ). Since the voltages at Re and other points in the modem circuit are known, it is possible to calculate the power dissipation in each component at any one time and compare it with their individual power rating. If the power rating is within limits, the controller  30  stores the DAC setting in register MemDAC, so it can be used as initial value for the DAC  20  the next time the modem goes off-hook. This prevents unnecessary transients when the modem goes off-hook on the same telephone line as the previous off-hook session. 
     Dynamic voltage control has the benefit of using continuous feedback from the line voltage and current, whereby the controller  30  can correct the DAC  20  setting recursively until the target values are obtained. Furthermore, the algorithm can run periodically to maintain the voltage and power within the target, in case the DC loop parameters change during an off-hook session. In this method, however, the controller  30  does not use knowledge of Vbatt and Rloop and cannot optimize the values of VTR and ITR to minimize distortion and power dissipation, within the V-I domain specified by the DC mask. 
     An optimal voltage control solution combines the benefits of the static and dynamic methods, and thus overcomes their disadvantages. In the presently preferred embodiment of the present invention, this is the method used by the controller  30 . However, those skilled in the art can choose the best method for a given application. A flowchart of this optimal method is shown in FIG.  4 (C). 
     Immediately before going off-hook, the controller  30  measures Vbatt using equations [1] and [3] (VDAC=0), and writes an initial value to the DAC  20 , which results in a known voltage VDAC. This setting can be an arbitrary default value, or it can be the DAC setting used by the modem in the previous off-hook session and stored in a register MemDAC (steps  90 - 96 ). 
     After going off-hook (step  98 ), the controller waits for a steady-state line voltage condition, typically 100 ms (step  100 ), measures Vtrdc, and calculates VTR and ITR using equations [1] and [2], respectively (step  102 ). Using equation [4], the controller  30  then calculates Rloop (step  104 ). Knowing Vbatt and Rloop, it is possible to calculate an optimal operating point (VTR) within the DC mask, which minimizes distortion and power dissipation in the line interface, for example (step  106 ). Using equation [6], an optimal value for VDAC can be calculated, and a corresponding value is written to the DAC (step  110 ). 
     After a short delay, to allow for a steady-state operating condition (step  112 ), the controller  30  again measures Vtrdc (step  114 ) and calculates VTR and ITR using equations [1] and [2], respectively (step  116 ). If VTR and ITR are outside the optimal target range predicted by the expected values of Vbatt and Rloop (step  118 ), the controller  30  corrects the assumed values of Vbatt and Rloop and recalculates VDAC (step  120 ). The process continues recursively until VTR and ITR are within the optimal target in the V-I domain of the DC mask, and the power is within predicted limits (step  122 ). The controller  30  stores the DAC setting in register MemDAC (step  124 ), so it can be used as initial value for the DAC  20  the next time the modem goes off-hook. This prevents unnecessary transients when the modem goes off-hook on the same telephone line as the previous off-hook session. 
     The controller  30  can also determine the CO battery voltage Vbatt and the loop resistance Rs while off-hook, without necessarily reading the on-hook line voltage Vtr. Since equation [4] is true for any values of Vtr and Itr, the controller can take two independent readings of Vtr and Itr at state ( 1 ) and state ( 2 ). These two states can differ from each other by a different setting of the DAC, for example, or by having switch S 1  enabled or disabled, respectively. The following system of two equations in two unknowns, Vbatt and Rloop, can then be written: 
     
       
           V batt− R loop× Itr ( 1 )= Vtr ( 1 ) 
       
     
     
       
           V batt− R loop× Itr ( 2 )= Vtr ( 2 )  [8] 
       
     
     and the controller  30  can determine the values of Vbatt and Rloop. 
     The only error allowed by this method is due to the absolute inaccuracy of the ADC  10 , and the tolerance of components used in the determination of VTR and ITR. These quantities can be specified within reasonable limits and result in negligible offset. Some of the quantities that can be defined based on acceptable margins of error are the resolution of the ADC  10  and DAC  20  (typically 6 to 8 bits), the DC offset of the op-amp U 1 , and the tolerance of resistors R 1 , R 2 , R 3 , and Re, typically 1% to 5% for practical applications. 
     The control logic of the present invention may be implemented by various means known to those skilled in the art, including either a hardware or software implementation. 
     As stated above, switch S 1  is enabled to increase the dynamic range of the ADC  10  with respect to VTR, by adding resistor R 3  in parallel with R so that a relatively large VTR can be measured within the limited voltage range of the ADC  10  (typically 0-4V). For example, if VTR is expected to be as high as 60V while the modem is on-hook, R 3  is chosen so that the ratio (R 2 //R 3 )/(R 1 +R 2 //R 3 ) is approximately 15 (60V/4V). 
     Switch S 1  is also enabled while off-hook, typically to comply with TBR 21  specifications (Europe) whereby ITR is limited to 60 mA and VTR can be as high as 40V. The graph shown in FIG. 11 displays two operating regions in the V-I domain, for R 3 =340 K and R 2  750 K region, wherein each region can be selected by enabling and disabling S 1 , respectively. In FIG. 11, a power curve is also drawn. This represents an upper limit on the maximum power dissipation of the transistor Q 1  and represents an upper boundary on the other sets of curves. 
     An alternate embodiment of the present invention is shown schematically in FIG.  5 . In this embodiment, the controller  30  can control two switches, S 1  and a second switch S 4 , connected to the DAC  20 . This allows even greater control over the range of desired V-I characteristics as the values of R 3  and R 2  can be chosen to be independent of each other. Additionally, two switches S 5  and S 6  have been added to the feedback line of the op-amp U 1 . The controller  30  can select whichever switch provides the best “multiplier” factor for the resistor divider. Specifically, the values of R 2  and R 3  can be chosen to be as large as possible to minimize the multiplier factor, and the controller  30  can select either switch S 5  or S 6  to cover the operating range in the V-I domain as required. 
     Another embodiment of the present invention is illustrated schematically in FIG.  6 . The single transistor Q 1  described above has been replaced with a Darlington pair Q 2 , Q 3 . The Darlington pair configuration allows for larger line currents with less diode current from op-amp U 2 . Similarly, three switches have been added to the op-amp&#39;s U 1  feedback to provide an extra “multiplier” enhancement option, as described in reference to FIG.  5 . The features of circuits described in FIG. 5 and 6 can be combined with the direct current reading of the circuit of FIG. 2, as shown in FIG.  7 . Two switches S 1  and S 4  are used to select the appropriate resistor divider network. Switches S 2  and S 3  are used to select either a voltage or a current reading, respectively, and switches S 5 , S 6  and S 7  control the feedback to the op-amp U 1 . 
     Closing switches S 5 , S 6 , and S 7  causes the following equations to be true, respectively, regardless of the values of R 1 , R 2 , AND R 3 : 
     
       
           Vtrdc=Ve+Vbe ( Q   3 )+ Vbe ( Q   2 ) for  S   5 , closed, 
       
     
     which can be written as 
     
       
           Ve=Vtrdc −1.4V 
       
     
     assuming the base-emitter voltage to be 0.7V for each transistor 
     
       
           Vtrdc=Ve+Vbe ( Q   3 ) for  S   6  closed, 
       
     
     which can be written as 
     
       
           Ve=Vtrdc −0.7V 
       
     
     and 
     
       
           Vtrdc=Ve  for  S   7  closed. 
       
     
     Since the line current Itr is equal to Ve/Re, using the same multiplier factor (resistor divider ratio) and for the same Vtrdc, it is possible to increase Itr in steps of 0.7V/Re. This effectively allows additional degrees of freedom in loop current control, which can be used favorably to reduce the multiplier factor. 
     FIG. 8 shows a circuit configuration equivalent to FIG. 6, wherein switches S 6  and S 7  have been replaced by two analog feedback resistors R 4  and R 5 . Resistor R 4  is connected between the emitter of Q 3  and the negative feedback of U 1 , whereas resistor R 5  is connected between the negative feedback of U 1  and an arbitrary voltage reference VREF. 
     By a suitable selection of voltage VREF and resistor ratio R 4 /R 5 , it is possible to obtain equivalent results to the activation of switches s 6  and S 7  in the digital domain. For example, in FIG. 6 switch S 7  would normally be closed for large values of Itr and switch S 6  would normally be closed for medium values of Itr. In FIG. 8, if VREF is chosen to be equal to the maximum voltage Ve expected for the maximum current Itr, it can be easily seen that for large values of Itr the following is true: 
     
       
           Ve=VREF=Vtrdc  (this is equivalent to having switch S 7  closed) 
       
     
     For medium values of Itr, Ve is less than VREF and the ratio of R 4 /R 5  can be chosen so that Vtrdc is approximately equal to Ve+0.7V, which is equivalent to having switch S 6  closed. 
     The preferred embodiment is shown in FIG. 9 with the addition of a DAC calibration switch S 9 . The operation and scope of the circuitry of FIG. 9 has already been discussed. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.