Abstract:
A driver circuit is provided, which in an operating mode drives a component that only supplies output power when a driving input signal exceeds a first threshold value. The driver circuit includes a differential amplifier whose output signal controls the driving input signal, a reference signal generator that supplies a reference input of the differential amplifier, and an external feedback that applies a signal, which is dependent on the output signal, to a feedback input of the differential amplifier. The driver circuit also has an adapter circuit and an internal feedback that can be activated in a compensation mode as an alternative to the external feedback, which internal feedback provides a signal to both the feedback input and the adapter circuit even for input signals that do not exceed the first threshold. Further, the adaptor circuit generates from the signal, and stores, a compensation signal that compensates an offset signal acting alone at the reference input when the reference signal generator is switched off. The adaptor circuit feeds the stored compensation signal, together with a reference signal provided by the reference signal generator, to the reference input or the feedback input when the external feedback is activated.

Description:
[0001]     This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 10 2004 058 595.4 filed in Germany on Nov. 26, 2005, which is herein incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a driver circuit, which in an operating mode drives a component that supplies output power when a driving input signal exceeds a first threshold value, having a differential amplifier whose output signal controls the driving input signal, having a reference signal generator that supplies a reference input of the differential amplifier, and having an external feedback that applies a signal, which is dependent on the output signal, to a feedback input of the differential amplifier.  
         [0004]     The invention further relates to a method for compensation of offset currents in such a driver circuit.  
         [0005]     2. Description of the Background Art  
         [0006]     A typical example of such a component is a laser diode in which a laser effect occurs only above a laser threshold. For a laser diode, the external feedback takes place by the radiated optical power of the laser diode and a photodiode that is connected to the feedback input and receives a portion of the radiated optical power. When the laser diode radiates a comparatively high optical power, the photodiode supplies a high photocurrent to the feedback input of the differential amplifier. This reduces the difference at the input of the differential amplifier, which reduces the output signal of the differential amplifier, and thereby reduces the optical power of the laser diode. Similarly, a relatively low radiated optical power leads to an increase in the difference and thereby to an increase in the optical power. The feedback thus closes a control loop, by which a stable optical power is established at a stable input signal difference in the steady state.  
         [0007]     In this context, a signal difference corresponding to the quotient of the output signal and the gain of the differential amplifier is established between the reference input and the feedback input.  
         [0008]     Ideal differential amplifiers deliver reproducibly identical output signals for specific reference signal values and thus possess a reproducibly stable characteristic curve. In real differential amplifiers, however, shifts in the characteristic curves arise through offset currents of the differential amplifiers. The offset currents can be represented in an equivalent schematic as an additive offset of the reference signal.  
         [0009]     In the case of a driver circuit with a differential amplifier that has such an offset, therefore, signal distortion occurs at the reference input. In the absence of countermeasures, such a signal distortion is stabilized by the external feedback. When the reference signal is switched off, the offset current alone acts as a reference signal in the equivalent schematic. Under certain circumstances, namely when the laser threshold is exceeded, the external feedback then establishes a final output power even though the switched off reference signal generator should likewise reduce the output power to zero.  
         [0010]     For this reason, such behavior is always problematic when small output power levels are to be established, as is the case for a laser diode in a CD or DVD unit in read operation, for example.  
       SUMMARY OF THE INVENTION  
       [0011]     It is therefore an object of the invention to provide an improved driver circuit in which offset currents of a differential amplifier are compensated.  
         [0012]     This object is attained in a method of the aforementioned type in that the driver circuit has an adapter circuit and an internal feedback that can be activated in a compensation mode as an alternative to the external feedback, the internal feedback providing a signal to both the feedback input and an adapter circuit even for input signals that do not exceed the first threshold, in that the adaptor circuit generates from the signal, and stores, a compensation signal that compensates an offset signal acting alone at the reference input when the reference signal generator is switched off, and in that the adaptor circuit feeds the stored compensation signal, together with a reference signal provided by the reference signal generator, to the reference input or the feedback input when the external feedback is activated.  
         [0013]     This object is further attained in a method of the aforementioned type by the following steps: activation of an internal feedback that can be activated in a compensation mode as an alternative to the external feedback and that provides a signal to both the feedback input and the adapter circuit even for input signals which do not exceed the first threshold, storage of a compensation signal that is generated from the signal and that compensates an offset signal acting alone at the reference input when the reference signal generator is switched off, and, when the external feedback is activated, feeding of the stored compensation signal to the reference input or the feedback input, in addition to the feeding of a reference signal provided by the reference signal generator.  
         [0014]     In compensation mode with the reference signal generator switched off and the external feedback deactivated, a detected feedback signal can be unambiguously associated with an undesired offset current of the differential amplifier. The generation and storage of a compensation signal in the compensation mode, together with the additional feeding of the compensation signal in the operating mode, leads to the desired compensation of problematic offset currents.  
         [0015]     With regard to embodiments of the driver circuit, it is preferred for the internal feedback to have a threshold filter that only allows feedback signals to pass which exceed a second threshold.  
         [0016]     Due to this embodiment, the adaptation process takes place for output signals of the differential amplifier, which result in feedback signals of a minimum amplitude determined by the second threshold. As a result, the compensation values determined in the compensation mode that are above the second threshold value but below the first threshold value can be transmitted to the later operating mode better than would be the case in adaptation with arbitrarily small output signals of the differential amplifier.  
         [0017]     Another embodiment includes a threshold filter as a current source that is connected to the internal feedback and that receives or emits a current up to a predetermined maximum current amplitude corresponding to the second threshold.  
         [0018]     This embodiment represents a particularly simple and continuous form of threshold filtering. The current source can be implemented, for example, as a source of negative currents, hence as a current sink, that is connected to the internal feedback and receives small currents, and only allows currents in excess of its maximum current to pass by into the feedback.  
         [0019]     It is also preferred for the internal feedback to have an output coupling circuit that couples a feedback signal out of the internal feedback and supplies it to the adapter circuit.  
         [0020]     As a result of such a coupling taking place in the internal feedback in parallel with the continuation of the feedback signal, the adaptation can take place continuously in the compensation mode and with no disturbing influence on the loop including the differential amplifier and inner feedback, in contrast to a switchover that supplies the feedback signal to the adapter circuit at certain times and to the feedback input of the differential amplifier at certain times.  
         [0021]     Moreover, the output coupling circuit can have a current mirror that reflects a current fed out of the differential amplifier into the internal feedback, reflecting the current into a current branch leading to the feedback input as well as into a measuring branch of the adapter circuit.  
         [0022]     The coupling with the current mirror has the advantage that any desired transmission ratios between the currents in different branches of the current mirror can be established by the number and dimensioning of the elements used. It is thus possible to set the attenuation of the signal feedback, for example.  
         [0023]     The adapter circuit can have a detector that detects a signal amplitude of the coupled-out feedback signal and transmits it to a control unit.  
         [0024]     The signal amplitude of the feedback signal is a measure of the amplitude of the offset current. This embodiment thus permits indirect measurement of the offset current.  
         [0025]     Moreover, the detector can periodically sample the signal amplitude.  
         [0026]     By means of the periodic sampling, a sequence of discrete measurements is produced, which can easily be processed by the subsequent control unit into stepwise changes in a compensation signal.  
         [0027]     The detector can also be embodied as a clocked comparator, as this has a very high sensitivity. This measure permits a minimization of the interaction between the measurement intervention and the internal feedback.  
         [0028]     Another embodiment provides that the control unit can store the signal amplitude and, by controlling a compensating current source, creates a compensating current at the reference input which at least partially compensates the offset current.  
         [0029]     By this embodiment, the appropriate value of the compensating current is determined successively, because an initial partially completed compensation, which still does not lead to a change in feedback signal in the compensation mode, is stored and can thus serve as a basis for further change in the compensating current.  
         [0030]     With regard to embodiments of the method, it is thus preferred for generation of the compensation signal to be performed in a stepwise manner, wherein the signal supplied to the adapter circuit is compared to a third threshold and wherein the compensation signal is changed in a stepwise fashion from a starting value until the signal supplied to the adapter circuit crosses the third threshold.  
         [0031]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:  
         [0033]      FIG. 1  illustrates a conventional arrangement of a driver circuit with external feedback;  
         [0034]      FIG. 2  is a characteristic curve of a laser diode;  
         [0035]      FIG. 3  illustrates characteristic curves of a differential amplifier with and without offset currents of different polarities;  
         [0036]      FIG. 4  is a block diagram of an example embodiment of a driver circuit according to the present invention;  
         [0037]      FIG. 5  is a circuit diagram illustrating an example of possible circuit implementations of various blocks from  FIG. 4 ; and  
         [0038]      FIG. 6  shows timing diagrams of signals such as those arising during the course of an example embodiment of the inventive method in the block diagram in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0039]      FIG. 1  shows a conventional driver circuit  10 , which drives a laser diode  12 . The driver circuit  10  has a differential amplifier  14  that has a reference input  16  and a feedback input  18 , as well as a reference signal generator  20 , a control unit  22 , and a photodiode  24 . The reference input  16  is fed by the reference signal generator  20 , which is controlled by the control unit  22 . Connected to the feedback input  18  is the photodiode  24 , which, during operation of the driver circuit  10  and the laser diode  12 , receives a portion of the optical power radiated by the laser diode  12  through an optical coupling  25  and converts it into a photocurrent. The photocurrent serves as the feedback signal Ifb. The differential amplifier  14  provides a current I as a control signal for the laser diode  12 , whereby the current depends on the gain G and the difference between the signals at the reference input  16  and the feedback input  18 . In this way, the power radiated by the laser diode  12  and the signal shape of the optical output signal are determined by the reference signal generator  20  and the control unit  22 , and are regulated in an external feedback that is connected by the optical coupling  25  between the laser diode  12  and the photodiode  24 .  
         [0040]      FIG. 2  shows a characteristic curve  26  of the laser diode  12 . In this context, the optical power P of the laser diode is plotted as a function of the driving input signal I. As is evident from the course of the characteristic curve  26 , an optical power P does not appear until the driving input signal I exceeds a threshold SW 1 . This threshold SW 1  corresponds in the case of the laser diode  12  to the laser threshold, which must be exceeded for the laser effect to occur. The laser diode  12  thus represents an example of a component that only provides an output power P when a driving input signal I exceeds a first threshold SW 1 .  
         [0041]      FIG. 3  shows typical characteristic curves of a differential amplifier, for example the differential amplifier  14  from  FIG. 1 . In this context, the output signal I of the differential amplifier  14  is plotted as a function of the reference signal Iref, wherein the characteristic curves have been recorded in an open loop condition, which is to say with a constant signal at the feedback input  18 . The output signal I of the differential amplifier  14 , which in  FIG. 3  is plotted at the ordinate, represents for example the driving input signal I for the laser diode  12 , which in  FIG. 2  is plotted along the abscissa.  FIG. 3  shows a total of three characteristic curves  28 ,  30  and  32 , wherein the number  28  designates an ideal characteristic curve. The ideal characteristic curve  28  is characterized in that it passes through the coordinate origin with no offset current, so that even a small change in the reference signal Iref from zero results in a finite change in the output signal I.  
         [0042]     As already mentioned above, however, real differential amplifiers have offset currents which can be represented in an equivalent schematic as additive effects on the signal at the reference input, where the additive effects can be positive as well as negative. The characteristic curve  30  shown in dashed lines results from shifting the ideal characteristic curve  28  to the right, which corresponds to a negative offset current: If the ideal characteristic curve  28  is considered as a function of Iref, then the characteristic curve  30  can be generated as the identical function with the argument (Iref−Ioff), where Ioff represents the offset current. Analogously, the characteristic curve  32 , produced by shifting the ideal characteristic curve  28  to the left, represents a positive offset current that could be represented in a functional representation as a positive offset in an argument Iref+Ioff.  
         [0043]      FIG. 4  shows a block diagram of an example embodiment of a driver circuit  34 , with which both the positive and negative offsets of the characteristic curves  32  and  30  relative to the ideal characteristic curve  28  can be adapted in a special compensation mode, by which a compensation of the shifts can occur even in a normal operating mode. The driver circuit  34  includes, among other items, a laser diode  12 , a differential amplifier  14  with reference input  16  and feedback input  18 , a reference signal generator  20 , a control unit  22 , and a photodiode  24  that is connected to the feedback input  18  of the differential amplifier  14 .  
         [0044]     In addition to these elements, the driver circuit  34  also has an output stage  36 , a switch  40 , a threshold filter  42 , a threshold combining element  44 , an attenuator  46 , an output coupling circuit  48 , a detector  50 , a compensating current source  52 , and a combining element  54  and/or a combining element  55 . In this regard, the output stage  36  serves only to further amplify the output signal I of the differential amplifier  14  into an input signal I′ of the laser diode  12 . The output coupling  38  serves to couple out a feedback signal that is fed to the feedback input  18  of the differential amplifier  14  through an internal feedback in a compensation mode. The internal feedback is activated by closing the switch  40  and includes the threshold filter  42 , the threshold combining element  44 , the attenuator  46 , and the output coupling circuit  48 .  
         [0045]     In this context, the threshold filter  42  defines a second threshold value SW 2  for regulation by the internal feedback; said second threshold value is smaller than the first threshold value SW 1  acting in the external feedback, corresponding, for example, to the laser threshold of the laser diode  12 . The control unit  22  switches the driver circuit  34  into a compensation mode by closing the switch  40 . The output signal I of the differential amplifier  14  that is coupled into the internal feedback through the output coupling  38  is combined in the combining element  44  with the comparatively low second threshold value SW 2  of the threshold filter  42 . In this regard, this combination can take place, for example, in such a manner that the threshold filter  42  can draw a current from the combining element  44  up to a predefined maximum value, so that the combining element  44  only transmits a signal to the attenuator  46  when the maximum value defined by the threshold filter  42  is exceeded by the output signal I coupled out of the differential amplifier  14 . The portion of the output signal I of the differential amplifier  14  that exceeds the second threshold value SW 2  is attenuated by the attenuator  46  to such a degree that a stable internal feedback is ensured. The attenuated signal is applied to the feedback input  18  of the differential amplifier  14  through the output coupling circuit  48  as feedback signal Ifb_i of the internal feedback.  
         [0046]     Since the second threshold value SW 2  which acts in the internal feedback is lower than the first threshold value SW 1  which acts in the external feedback, a relatively large feedback signal Ifb_i initially appears at the feedback input  18  of the differential amplifier  14  when switch  40  is closed, which is to say in compensation mode. As a result, the input signal difference at the differential amplifier  14  drops, and consequently so does the amplitude of the output signal I that is combined with the input signal difference by the gain G. With appropriate dimensioning of the second threshold value SW 2 , in comparison to the first threshold value SW 1 , the input signal I′ of the laser diode  12  then drops below the laser threshold so that the optical power radiated by the laser diode  12  ceases. As a result, the optical coupling between the laser diode  12  and the photodiode  24  also ceases so that the external feedback, which is closed through this optical coupling in the operating mode, is deactivated.  
         [0047]     In addition to the deactivation of the external feedback through the closing of the switch  40 , the reference signal generator  20  is also switched off in the compensation mode so that it no longer provides a signal to the reference input  16  of the differential amplifier  14 . If the characteristic curve of the differential amplifier  14  corresponds to the ideal characteristic curve  28  from  FIG. 3 , the output signal I of the differential amplifier  14  will then also drop to zero and the input signal difference between the inputs  16  and  18  of the differential amplifier  14  will vanish.  
         [0048]     In contrast, if the differential amplifier  14  has a characteristic curve  32  from  FIG. 3  that is shifted to the left by a positive offset, then even when the reference signal is switched off an output signal I will appear, which is fed back to the feedback input  18  of the differential amplifier  14  through the internal feedback as an attenuated signal Ifb_i. By an adapter circuit formed of the detector  50 , the control unit  22 , the compensating current source  52 , and the combining element  54  and/or the combining element  55 , this undesirable offset can be learned in the compensation mode and can be compensated in the subsequent operating mode. The output coupling circuit  48  couples a signal out of the internal feedback in which the feedback signal Ifb_i is reflected. The reflection can be identical, for example, so that a signal Ifb_i is fed into the detector  50  of the adapter circuit.  
         [0049]     The detector  50  compares the fed-in feedback signal Ifb_i to a predefined third threshold value SW 3 , and if the signal exceeds or drops below the third threshold value SW 3 , the detector supplies an appropriate signal to the control unit  22 . The control unit  22  controls the detector  50  by means of the dashed connection between the blocks  22  and  50  in such a manner that, for example, the detector  50  samples its input signal at predetermined time intervals specified by the control unit  22  and compares it to the third threshold value SW 3 . The third threshold value SW 3  can, e.g., be dimensioned such that it corresponds to the value f(SW 3 ) in the graph in  FIG. 3 . In this regard, the value f(SW 3 ) in  FIG. 3  is drawn relatively high on the I-axis for reasons of clarity, and is brought still closer to the coordinate origin in implementations of the invention.  
         [0050]     If the third threshold value SW 3  is immediately exceeded at the beginning, as is the case in the characteristic curve  32  from  FIG. 3 , the control unit  22  triggers a stepwise change in a compensating current by the compensating current source  52 , which is applied through the combining element  54  to the reference input  16  and/or through the combining element  55  to the feedback input  18  of the differential amplifier  14 , and is intended to compensate the offset current acting there. In order to achieve the effect of a positive (negative) compensating current at the reference input  16 , feed-in to the feedback input  18  must take place with a negative (positive) polarity. As has already been mentioned, the characteristic curve  32  corresponds to a positive offset current so that in this case the control unit  22  establishes a negative compensating current of the compensating current source  52  if the compensating current is fed in through the reference input. This is reflected in a change in the feedback signal Ifb_i by means of the internal feedback.  
         [0051]     In the case of the characteristic curve  32  from  FIG. 3 , the application of a negative compensating current into the combining element  54  results in a shift to the right of the characteristic curve  32 . As a result, the point of intersection of the characteristic curve  32  with the I-axis drops. With successive increases in the negative compensating current by the adapter circuit, the characteristic curve  32  shifts successively further downward until the value crosses below f(SW 3 ). This downward crossing is detected by the detector  50  and is registered by the control unit  22 . The control unit  22  then commands the compensating current source  52  to maintain the last compensating current value used, and to use it with activated external feedback in the subsequent operating mode.  
         [0052]     In similar fashion, a characteristic curve  30  in  FIG. 3  that is initially shifted to the right by a negative offset current, is shifted to the coordinate origin by successive determination of a compensating positive compensation current.  
         [0053]      FIG. 5  shows a circuit diagram as an example of possible circuit implementations of various blocks from  FIG. 4 . Thus, the coupling  38  can be accomplished by a transistor  56 , which is connected through an emitter resistor  58  to a supply voltage  60 , which is controlled by the output signal of the differential amplifier  14 , and whose collector is connected to the threshold combining element  44 . In the circuit  34  shown in  FIG. 5 , the output signal of the differential amplifier can be a voltage or a current. The output stage  36  can likewise be implemented through a transistor  62  which is connected through an emitter resistor  64  to a supply voltage  60 , which likewise is controlled by the output signal I of the differential amplifier  14 , and whose collector current serves as the input signal I′ of the laser diode  12 . The threshold filter  42  can include, for example, a variable current source  66  that draws from the threshold combining element  44  a current of variable amplitude, but predefined maximum amplitude, and which is connected between the threshold combining element  44  and a ground connection  68 .  
         [0054]     A current mirror  70 , which has three branches  72 ,  74  and  76 , mirrors a current flowing in the first branch  72  as feedback current Ifb_i from the threshold combining element  44  through a transistor  78  and a resistor  80  to the ground potential  68 , to the other two branches  74  and  76 , each of which likewise has its own transistor  82 ,  86  and emitter resistor  84 ,  88 . The collector of the transistor  82  of the second branch  74  is connected to the feedback input  18  of the differential amplifier  14 , thus closing the internal feedback. As in the case in  FIG. 4 , the switch  40  serves to deactivate the external feedback. But unlike  FIG. 4 , it is arranged in  FIG. 5  such that it deactivates the external feedback in the closed state.  
         [0055]     In the implementation in  FIG. 5 , the attenuator shown as block  46  in  FIG. 4  results from the transmission ratio of the currents in the branches  72  and  74 . The current mirrored in the third branch  76  constitutes a current coupled out of the internal feedback, so the third branch  76  in combination with the other two branches  72 ,  74  of the current mirror  70  represents the function of the output coupling circuit  48  from  FIG. 4 . In this context, the transistors and resistors can be dimensioned such that a different current is fed into the measuring branch  76  than into the feedback branch  74 . For example, by means of a higher current in the measuring branch  76 , the voltage drop can be increased through a measurement resistor  92 , which increases the sensitivity of the measurement. The detector  50  from  FIG. 4  is implemented, for example, by a comparator  90  in conjunction with the measurement resistor  92 . The comparator  90  can take the form of a “latched comparator” or a “clocked comparator.” A clocked comparator of this nature has, in addition to differential inputs  90 . 1  and  90 . 2 , a clock input  94 . When the clock signal is switched on, a positive feedback is activated within the comparator  90 , which latches the state at the output  90 . 3  of the comparator  90 . This state is then, which is to say until the next pulse of the clock signal, independent of the input signal. As a result of the positive feedback, the gain of the comparator  90  is very large at the time of switchover, so even the smallest changes between the differential inputs  90 . 1  and  90 . 2  result in unambiguous signals at the output  90 . 3 .  
         [0056]      FIG. 6  shows time behaviors of signals such as those that can be obtained at various points in the circuits in  FIGS. 4 and 5  when the differential amplifier  14  has the characteristic curve  30  shifted to the right by a negative offset current as shown in  FIG. 3 . The high level of the signal  93  in  FIG. 6   a  represents an active compensation mode, while the low signal level there represents an active operating mode. As is evident from  FIG. 3 , in the case of the characteristic curve  30  as output signal I of the differential amplifier  14 , a zero signal, or a signal that is smaller than the value f(SW 3 ) in  FIG. 3 , is established. Consequently, no current flows initially in the internal feedback, so that the feedback signal Ifb_i is also zero at first. This is represented by the initially low signal level in  FIG. 6   b,  which represents the time behavior  95  of the signal Ifb_i.  
         [0057]      FIG. 6   c  illustrates sampling of the feedback signal Ifb_i by the detector  50 . In this context, the pulses  96 ,  98  and  100  each correspond to periods of time in which the control unit  22  applies a clock signal to the clock input  94  of the comparator  90  in  FIG. 5 , thus recording a measurement.  FIG. 6   d  illustrates a possible curve  102  of a compensating current Icomp. Initially, i.e. at the first measurement by the pulse  96 , the compensation value is still zero. Since the detector  50  determines at the first measurement that the feedback signal Ifb_i is below the third threshold value SW 3 , the control unit  22  sets a first base value of a positive compensating current by means of the compensating current source  52 , wherein the setting in  FIG. 6  takes place in each case with a delay dt. This base value corresponds to the first stage  104  in the signal  102 . As a result, the characteristic curve  30  from  FIG. 3  is shifted somewhat to the right toward the coordinate origin.  
         [0058]     If the shift is not yet enough to place the characteristic curve  30  above the threshold value f(SW 3 ), the base value of the set compensating current was apparently too small, and at the next measurement pulse  98  the detector  50  again determines that the feedback signal Ifb_i lies below the third threshold value SW 3 . In consequence, the control unit  22 , with the aid of the compensating current source  52 , increases the compensating current by a predetermined step size corresponding to the level height in  FIG. 6   d  at the transition from the level  104  to the level  106 . For the following discussion, it is assumed that the compensating current is now so large that it overcompensates the negative offset current. In the illustration in  FIG. 3 , this means that the characteristic curve  30  has been shifted far enough to the left that the threshold value f(SW 3 ) now lies below the shifted characteristic curve  30 .  
         [0059]     Then, with the reference signal generator  20  still switched off, an output signal I appears at the output of the differential amplifier  14 , and this signal is reflected by an increase in the feedback signal Ifb_i. In  FIG. 6   b,  this is represented by the level  108  in the signal curve  95 . In keeping with the aforementioned assumption, Ifb_i will be so large that the detector  50  will determine at its next sampling pulse  100  that the third threshold SW 3  has been exceeded. The detector signals this by a switchover to a high signal level  110  in the signal  112  that is fed into the control unit  22  through the closed switch  94 . The control unit  22  thus registers the crossing above the third threshold SW 3  and terminates the compensation mode, which is represented in  FIG. 6   a  by the falling edge in the signal  93 . At the same time, the compensating current source  52  latches the compensating current value that was determined. Consequently, in a subsequent operating mode, operation always approaches the ideal characteristic curve  28  from  FIG. 3 .  
         [0060]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.