Abstract:
A circuit and a method for correcting an offset is provided that includes a current amplifier and an adjusting circuit for correcting an offset of an output current of the current amplifier. Wherein the adjusting circuit has a controlled current source, an output of the controlled current source is connected to the current amplifier for impressing an output current of the controlled current source in the current amplifier, an input of the controlled current source to form a regulation element of a control loop is connected by a first switching device of the adjusting circuit to an output of the current amplifier and to form a holding element is disconnected from the output of the current amplifier by the first switching device. The controlled current source, acting as a regulation element in the control loop, is set up to regulate the offset to a minimum by setting of a current value of the output current, and the controlled current source, acting as a holding element, is set up to hold the current value, associated with the minimum, of the output current.

Description:
[0001]    This nonprovisional application claims priority to German Patent Application No. DE 10 2009 060 504.4, which was filed in Germany on Dec. 23, 2009, and to U.S. Provisional Application No. 61/289,846, which was filed on Dec. 23, 2009, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a circuit and method for setting an offset output current for an input current amplifier. 
         [0004]    2. Description of the Background Art 
         [0005]    A current amplifier (CC-OPV) is known, for example, from “Halbleiterschaltungstechnik” (Semiconductor Technology), Tietze, Schenk, 12 th  edition, 2002, pages 563-565. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to improve a circuit with a current amplifier as much as possible. Accordingly, a circuit is provided which can be monolithically integrated on a semiconductor chip. 
         [0007]    The circuit can have a current amplifier and an adjusting circuit. The current amplifier has a current input and at a current output outputs the amplified input current as an output current at a current output of the current amplifier. A current amplifier for amplifying small input currents can also be called an input current amplifier. The adjusting circuit is set up to correct an offset of the output current of the current amplifier. 
         [0008]    The adjusting circuit can have a controlled current source. The controlled current source provides an output current, which depends on a control variable, particularly a control voltage. The output current of the controlled current source is also constant with a constant control variable. 
         [0009]    The output of the controlled current source can be connected to the current amplifier for impressing the output current of the controlled current source in the current amplifier. Preferably, the controlled current source is connected to an output of the current amplifier. Alternatively, the controlled current source can also be connected to an input of the current amplifier. 
         [0010]    An input of the controlled current source, to form a regulation element of a control loop, can be connected by a first switching device of the adjusting circuit to the output of the current amplifier. The first switching device is, for example, a semiconductor switch, particularly a transmission gate or a field-effect transistor. 
         [0011]    The input of the controlled current source, to form a holding element, moreover, is disconnected from the output of the current amplifier by the first switching device. The controlled current source in the closed switch position of the first switching device as a first function therefore has a regulation function as a regulation element of the control loop and in synergy in the open switch position of the first switching device as a second function has a holding function as a holding element. The control loop in this regard can be closed by the first switching device. The control loop is disconnected by opening of the first switching device. In the disconnected state, the output current of the controlled current source acting as a holding element for the amplification of a temporally succeeding input signal is substantially constant. 
         [0012]    The controlled current source, acting as a regulation element in the control loop, is set up to regulate the offset to a minimum by setting a current value of the output current. The minimum offset is achieved when the output current from a current amplifier has reached a steady state; therefore it is substantially constant, ideally zero. 
         [0013]    The regulation function is ended when the steady state is attained. The controlled current source, now acting as a holding element, is set up to hold the output current value associated with the offset minimum. The controlled current source acting as a holding element holds the output current substantially constant in this regard at least for the duration of an amplification of input signals of the current amplifier. 
         [0014]    The object of the invention further is to provide a method for correcting an offset of a current amplifier. Accordingly, a method is provided for correcting an offset of an output current of a current amplifier of a circuit. In this regard, the method can be carried out by a control device. 
         [0015]    In the method, a controlled current source, to form a regulation element of a control loop, is connected by a first switching device to an output of the current amplifier. The control loop in this case is formed to regulate to a steady state. 
         [0016]    The offset is regulated to a minimum by setting a current value of the output current of the controlled current source, acting as a regulation element. The current value in a regulated state belongs to the minimum offset. The regulation occurs when an input signal of the current amplifier has a constant value. Therefore, only a direct current value but not an alternating current is present at the input of the current amplifier during the regulation. Ideally, the direct current value, present at the input of the current amplifier, of the input signal is zero. 
         [0017]    The controlled current source, to form a holding element for holding the output current value, associated with the minimum, of the controlled current source, is disconnected by the first switching device from the output of the current amplifier. In this regard, the current value is held by the controlled current source until amplification, following the regulation, of a time-variant input signal has occurred at the input of the current amplifier. 
         [0018]    The embodiments described hereinafter relate to the circuit and to the adjusting method. The functional features of the circuit in this regard emerge from the method features. Method features can be derived from the functions of the circuit. 
         [0019]    In an embodiment, the controlled current source of the circuit has a capacitor. The capacitor in this case can be formed by an integrated capacitor, for example, a MIM capacitor, or by a capacitor of an active component, such as the gate-source capacitor of a field-effect transistor. Preferably, the first switching device is connected to the capacitor. Preferably, a current can be connected for charging the capacitor by the first switching device. 
         [0020]    An embodiment provides that it is possible to control the controlled current source by a control voltage. In this regard, the control voltage can be generated by an element of the controlled current source itself. 
         [0021]    According to an refinement, the controlled current source has a transistor. The transistor is preferably a field-effect transistor. The transistor controls the output current of the controlled current source by a control voltage at the control input of the transistor. 
         [0022]    In another embodiment, it is provided that the controlled current source has a storage device, such as a capacitor, for storing the control voltage. The output current of the controlled current source can be kept constant in its function as a holding element by means of the stored control voltage. Alternatively, in a more elaborate embodiment, a digital value as well for controlling the controlled current source could be stored. 
         [0023]    According to an embodiment, the capacitor of the controlled current source, acting as a regulation element, can be connected to the output of the input current amplifier. The connection can occur by means of the first switching device for charging the capacitor until a steady state is attained for the minimum offset. In the steady state, the charging current is reduced to a minimum by the capacitor. 
         [0024]    According to another embodiment, the adjusting circuit can have a constant current source, which is connected to the current amplifier for impressing a constant current. The constant current source can be connected to an output of the current amplifier. Alternatively, the constant current source can also be connected to an input of the current amplifier. In this case, the constant current of the constant current source is also amplified by the current amplifier. Preferably, the current flow, produced by the constant current at the output of the current amplifier, is greater than the maximum offset of the current amplifier. The maximum offset can be determined, for example, by means of a simulation. 
         [0025]    It is provided in an embodiment that an output current of the controlled current source, said current which is impressed in the current amplifier, at the output of the current amplifier causes a current flow that is directed opposite to a current flow of the constant current. In this case, the constant current together with the offset can be compensated predominantly by the current flow caused by the controlled current source. The output current of the controlled current source is impressed in the output of the current amplifier. Both the constant current of the constant current source and the output current of the controlled current source are impressed in the output of the current amplifier and have an opposite current direction. Alternatively, one of the two or both currents of the constant current source and the controlled current source can be impressed in an input in the amplification path of the current amplifier and act according to the amplification in an opposite current direction at the output of the current amplifier. 
         [0026]    In an embodiment, it is provided that the current amplifier has a first current mirror and a second current mirror for current amplification. The outputs of the current mirrors are connected to the current output of the current amplifier and therefore to the output of the circuit. The first current mirror of an amplification can be assigned a positive signal current at the current input of the current amplifier and the second current mirror of an amplification a negative signal current at the current input of the current amplifier. Preferably, the constant current source and/or the controlled current source are connected to the first and/or second current mirror. 
         [0027]    The current amplifier can have a current summing node connected to the output of the current amplifier. Preferably, a first current and a second current are summed in the current summing node. The second current is the constant current of the constant current source or is based on the constant current of the constant current source. The second current is the output current of the controlled current source or is based on the output current of the controlled current source. 
         [0028]    The constant current source can be connected to the output of the current amplifier directly or via a component, such as a field-effect transistor. Preferably, the controlled current source is connected to the output of the current amplifier directly or via a component, such as a field-effect transistor. The first current or the second current enters the summation with a negative sign. If the constant current source and the controlled current source are connected to the output of the current amplifier, the constant current of the constant current source or the output current of the controlled current source enters the summation with a negative sign. 
         [0029]    In an embodiment, the adjusting circuit has a second switching device. The second switching device is connected via an input of the adjusting circuit to the output of the current amplifier and to the circuit output. The output of the circuit can be disconnected from the output of the current amplifier and can be connected to the output of the current amplifier by means of the second switching device. The second switching device is, for example, a semiconductor switch, particularly a transmission gate or a field-effect transistor. 
         [0030]    According to an embodiment, it is provided that the adjusting circuit has a third switching device. The third switching device is connected to the capacitor of the controlled current source and is formed to discharge the capacitor in the closed state. 
         [0031]    In an embodiment, the circuit has a control circuit which is connected to the adjusting circuit. 
         [0032]    The control circuit, to control the first switching device, can be connected to a first control terminal of the first switching device. The control circuit, to control the second switching device, is preferably connected to a second control terminal of the second switching device. The control circuit, to control the third switching device, is preferably connected to a third control terminal of the third switching device. The control circuit preferably has a number of delay elements for a time-dependent control. 
         [0033]    The control circuit can be set up in a first step to disconnect the output of the current amplifier from the circuit output by opening the second switching device. Preferably, the control circuit is set up in a second step to connect the capacitor of the controlled current source to the output of the current amplifier by closing the first switching device, whereby after the second step the capacitor is charged by a charging current and by the charging of the capacitor an output current of the controlled current source is increased until a minimum is attained at a current value of the steady state of the charging current. Preferably, the control circuit is set up in a third step to disconnect the charged capacitor of the controlled current source from the output of the current amplifier by opening the first switching device. 
         [0034]    The control circuit can be set up in a fourth step to connect the output of the current amplifier to the circuit output by closing the second switching device. 
         [0035]    According to an embodiment, the method has several process steps, which are carried out, for example, by a state machine or a program sequence in an arithmetic unit. First, the third switching device can be temporarily closed, so that the capacitor is discharged via the third switching device. Then, the third switching device is opened again. 
         [0036]    Next, in a process step the capacitor of the controlled current source is connected to the output of the current amplifier by closing the first switching device. Moreover, an output of the current amplifier is disconnected from a circuit output by opening of the second switching device, so that the regulation process produces no desirable output signal. After this process step, the capacitor is charged by the charging current. An output current of the controlled current source is increased by the charging of the capacitor until the charging current attains a minimum. 
         [0037]    In a subsequent process step, the charged capacitor of the controlled current source is disconnected from the output of the current amplifier by opening of the first switching device. In a subsequent process step, the output of the current amplifier is connected to the circuit output by closing of the second switching device in order to output a signal, amplified by the current amplifier, as an output signal. 
         [0038]    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 
         [0039]    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: 
           [0040]      FIG. 1   a  shows a schematic illustration of an input current amplifier; 
           [0041]      FIG. 1   b  shows a schematic illustration of an input current amplifier with an adjusting circuit for adjusting the offset output current; 
           [0042]      FIG. 2   a  shows a circuit diagram of a first exemplary embodiment; 
           [0043]      FIG. 2   b  shows a schematic diagram; and 
           [0044]      FIG. 3  shows a circuit diagram of another exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    A current amplifier  100  with a low input impedance is shown schematically in  FIG. 1 , which is also called an input current amplifier below. Current amplifier  100  has a current input and a current output. The input current through the current input in this case is output at the output amplified by the current amplification of the current amplifier. In this case, a signal output current Io at the circuit output is superposed by an undesirable offset Ioff at the output. The offset Ioff is caused by process variations during the production of amplifier transistors of input current amplifier  100  and is shown schematically in  FIG. 1   a  as current source Ioff. The amplifier-inherent offset Ioff in this case can be positive or negative in regard to the current direction at the circuit output. 
         [0046]    To correct the offset Ioff at the circuit output, an adjusting circuit  200 , which compensates at least predominantly the Offset Ioff at the output of the circuit and in the ideal case subtracts completely the amplifier-intrinsic offset Ioff, is provided in  FIG. 1   b.    
         [0047]    An example for an input current amplifier  100  with a low-ohmic input impedance at the current input of current amplifier  100  is shown in  FIG. 2   a  as a circuit diagram. Further, an exemplary embodiment of an adjusting circuit  200  for adjusting the offset Ioff of input current amplifier  100  is shown in  FIG. 2   a . PMOS transistors  123  and  124  form a first current mirror of input current amplifier  100  with a first transformation ratio. The first current mirror  123 ,  124  is connected to the supply voltage V+. NMOS transistors  125  and  126  form a second current mirror of input current amplifier  100  with a second transformation ratio. The second current mirror  125 ,  126  is connected to ground. In the ideal case, the first transformation ratio and the second transformation ratio would be precisely the same. Because of process deviations during production, the first transformation ratio and the second transformation ratio, however, do not turn out precisely the same and therefore cause the offset Ioff at the current output of current amplifier  100 . PMOS transistor  112  and NMOS transistor  111 , connected to input  101  of input current amplifier  100 , are used to adjust the voltage at input  101  by means of the gate voltages Vn and Vp. For example, the voltage at input  101  is adjusted to half the operating voltage V+/2 by means of gate voltages Vn and Vp. 
         [0048]    Output transistor  124  of the first current mirror is connected via PMOS transistor  131  to a current summing node  105  and an output  102  of input current amplifier  100 . Output transistor  126  of the second current mirror is connected via NMOS transistor  132  to current summing node  105  and output  102  of input current amplifier  100 . Transistors  131  and  132  are controlled by the gate voltages Vcp and Vcn and cause an increase in the output resistance of input current amplifier  100  (cascode current mirror). 
         [0049]    Further, adjusting circuit  200 , which is connected to input current amplifier  100  for adjusting and therefore for correcting the offset Ioff, is shown in  FIG. 2   a . Preferably, adjusting circuit  200  is formed to adjust the offset Ioff to a minimum, preferably to the value of zero. Adjusting circuit  200  has two current sources, a controlled current source  210  and a constant current source  220 , which in the exemplary embodiment of  FIG. 2   a  are connected to output  102  of input current amplifier  100 . 
         [0050]    Constant current source  220  generates a constant current I 2 . Constant current I 2  is greater in terms of value than the maximum expected offset Ioff. The maximum expected offset Ioff can be determined, for example, by simulating process deviations. Constant current source  220  in the exemplary embodiment of  FIG. 2   a  is connected to PMOS output transistor  124  of the first current mirror via terminal  203  of adjusting circuit  200  and terminal  103  of input current amplifier  100 . The output current of output transistor  124  of the first current mirror and the constant current I 2  are summed in the terminal node. Constant current source  220  is therefore connected via PMOS transistor  131  to output  102  of input current amplifier  100 . It would also be possible to connect constant current source  220  directly to output  102  of input amplifier  100 . In the exemplary embodiment of  FIG. 2   a , constant current source  220  has a current source  224  and a current mirror comprising PMOS transistors  225 ,  226  to generate constant current I 2 . 
         [0051]    Controlled current source  210  generates a controlled current I 1  as the output current. The controlled current source  210  in the exemplary embodiment of  FIG. 2   a  is connected to NMOS output transistor  126  of the second current mirror via terminal  204  of adjusting circuit  200  and via terminal  104  of input current amplifier  100 . The output current of NMOS output transistor  126  of the second current mirror and output current I 1  of controlled current source  210  are summed in the terminal node. Controlled current source  210  is therefore connected via NMOS transistor  132  to output  102  of input current amplifier  100 . It would also be possible to connect controlled current source  210  directly to output  102  of input amplifier  100 . 
         [0052]    Controlled current source  210  has a capacitor  212 . A voltage Uc dropping across capacitor  212  controls output current I 1  of controlled current source  210 . In the exemplary embodiment of  FIG. 2   a , an NMOS transistor  213  is provided as an element for voltage-current conversion. The voltage Uc dropping across capacitor  212  in this case is present as gate-source voltage at NMOS transistor  213 . If the voltage Uc dropping across capacitor  212  is zero, NMOS transistor  213  blocks. With an increasing voltage Uc, the gate-source voltage increases and turns on NMOS transistor  213 , so that output current I 1  also increases. Output current I 1  increases until the sum of the amplifier-intrinsic offset Ioff, constant current I 2 , and output current I 1  of controlled current source  210  reaches a minimum. Capacitor  212  is no longer charged and the voltage Uc is constant. An especially rapid adjustment of the steady state is achieved in this way, so that the time during which the input current amplifier is not available for current amplification of the input signal Isig is minimized. 
         [0053]    Constant current source  220  and controlled current source  210  in this regard are connected to output  102  of the input current amplifier  100  in such a way that the constant current I 2  and output current I 1  of controlled current source  210  are summed, whereby one of the two currents enters the summation with a negative sign. The current direction, acting in node  105  and therefore at output  102 , of the constant current I 2  and the current direction, acting in node  105  and therefore at output  102 , of the output current I 1  of the controlled current source  210  are therefore opposite. If the technical current direction in  FIG. 2   a  is considered, constant current I 2  flows into summing node  105 . In contrast, output current I 1  of controlled current source  210  flows out of summing node  105 , therefore enters the summation as negative. 
         [0054]    As an alternative to the exemplary embodiment of  FIG. 2   a , constant current source  220  and/or controlled current source  210  can be connected to current input  101  of current amplifier  100 . If constant current source  220  is connected to input  101 , constant current I 2  is amplified by current amplifier  100 . If controlled current source  210  is connected to input  101 , the output current I 1  thereof is amplified by current amplifier  100 . If both constant current source  220  and controlled current source  210  are connected to current input  101 , a difference current (I 1 -I 2 ) between constant current I 2  and output current I 1  of controlled current source  210  is amplified accordingly by current amplifier  100 . In these three embodiment variants as well, a regulation of the offset Ioff to a minimum is possible, so that in the case of amplification of an input current signal Isig no or only a negligible offset Ioff interferes with the output signal Io of the circuit. 
         [0055]    Constant current I 2 , which is greater than the offset Ioff in value, is impressed on output  102  of input current amplifier  100  by adjusting circuit  200 , shown in  FIG. 2   a , for adjusting the offset (Ioff. Likewise at output  102  of current amplifier  100 , output current I 1  of controlled current source  210  is impressed with the current direction opposite to I 2 . Adjusting circuit  200 , moreover, has a first switching device S 1  and a second switching device S 2 . First switching device S 1  in this regard is connected to output  102  of adjusting circuit  200  and to an input  219  of controlled current source  210 . In the closed state, first switching device S 1  connects output  102  of adjusting circuit  200  to input  219  of controlled current source  210  and forms a control loop, whereby controlled current source  210  acts as a regulation element of this control loop. In said control loop, the actual value is the current Ic through terminal  102 , which also charges capacitor  212 . Current Ic is the same as the current through current output  102  of current amplifier  100  and therefore the same as the resulting offset Ioff, which is minimized by the regulation. The actual value is compared with the target value zero, the generation of which requires no component. The control variable of the control loop is output current I 1  of controlled current  210 . 
         [0056]    For regulation, second switching device S 2  is open and disconnects output  202  of the circuit from output  102  of adjusting circuit  200 . The input signal current Isig is zero in this case. As a result, the resulting current, which results from the summation of the output current of first current mirror  123 ,  124 , of the output current of second current mirror  125 ,  126 , and of constant current I 2 , flows out at output  102  of input current amplifier  100 . Output current I 1  of controlled current source  210  is equal to zero because of the initially still discharged capacitor  212 . 
         [0057]    By charging capacitor  212  by charging current Ic, the gate of NMOS transistor  213  is controlled so that the controlled current source  210  as a regulation element sets a current value of output current I 1  of controlled current source  210 , so that the current through output  102  is regulated to a steady state, whereby output current I 1  of controlled current source  210  again draws off specifically the sum of constant current I 2  and the amplifier-intrinsic offset Ioff. In this case, the offset Ioff active at output  102  is regulated to a minimum and thereby to a constant value, ideally zero. In the steady state, the current value of output current I 1  of controlled current source  210  is constant. 
         [0058]    The amplifier-intrinsic offset Ioff can be positive or negative. Capacitor  212  and NMOS transistor  213  form the regulation element of the control loop. In the steady state, output current I 1  is equal to the (signed) sum of the constant current I 2  and amplifier-intrinsic offset Ioff. In the steady state case, therefore, a constant current no longer flows out of output  102  of input current amplifier  100 , so that charging current Ic as well is zero. 
         [0059]    A diagram for the control signals of switching devices S 1 , S 2 , and S 3  of adjusting circuit  200  is shown schematically in  FIG. 2   b . Between time points t 1  and t 4 , second switching device S 2  is opened and disconnects circuit output  202  from output  102  of input current amplifier  100 . Before, during, or after the opening of second switching device S 2 , a third switching device S 3  is closed, which in the closed state short-circuits capacitor  212 , so that capacitor  212  discharges via third switching device S 3  between time points t 2  and t 3 . 
         [0060]    At time point t 5 , both second switching device S 2  and third switching device S 3  are in the switch position open “0.” In contrast, first switching device S 1  between time points t 5  and t 6  is controlled into the switch position closed “1.” Between time points t 5  and t 6 , capacitor  212  is connected via first switching device S 1  to output  102  of input current amplifier  100 . Between the time points t 5  and t 6 , therefore, as previously described, capacitor  212  is charged until the steady state is attained. 
         [0061]    At time t 6 , first switching device S 1  is opened and again disconnects capacitor  212  from output  102  of input current amplifier  100 . Only a very low leakage current thereby flows through capacitor  212 , the gate of transistor  213  and first and third switching device S 1 , S 3 , so that the charging of capacitor  212  is substantially retained for a longer time. The charge is stored in capacitor  212  as storage device, so that the current value of output current I 1  of controlled current source  210  remains substantially constant. Timewise after time point t 6 , at time point t 7 , second switching device S 2  is closed and the output of input current amplifier  100  is connected to circuit output  202 . A time difference is therefore provided between time points t 6  and t 7 . Switching devices S 1  and S 2  are preferably not closed simultaneously. Switching devices S 1 , S 2 , S 3  are preferably semiconductor switches, for example, in the form of field-effect transistors or transmission gates. 
         [0062]    Between time points t 8  and t 9 , a voltage signal Vsig is sent to a capacitor Cm of a touch screen. If the screen is touched, the capacitor Cm is changed and moreover a signal current Isig is produced, which flows as an input current via input  101  into/out of input current amplifier  100  and is amplified by input current amplifier  100 . A readjustment of output current I 1  of controlled current source  210  can occur, for example, before each signal or before a group of signals with the signal voltage Vsig. Preferably, the adjustment of output current I 1  of controlled current source  210  occurs within a time interval of, for example, 500 us. For example, the adjustment of output current I 1  of controlled current source  210  occurs periodically. Advantageously, the time interval or the periods can be adjusted. 
         [0063]    Another exemplary embodiment for the use of a touch screen is shown schematically in  FIG. 3  as circuit diagram. The exemplary embodiment of  FIG. 3  also has an input current amplifier  100  with a low-ohmic input impedance. Input current amplifier  100  has two current mirrors  121  and  122  and four transistors  111 ,  112 ,  131 ,  132  analogous to  FIG. 2   a . Constant current source  220 , with current source  223  and NMOS transistors  221  and  222 , to output constant current I 2  is formed accordingly complementary to constant current source  220  of  FIG. 2   a . Therefore, constant current I 2  in keeping with the technical current direction flows into constant current source  220 . A controlled current source  210  with a capacitor  212  and a PMOS transistor  211  is also formed complementary to controlled current source  210  of  FIG. 2   a . Switching devices S 1  and S 3  are accordingly closed. The operation of adjusting circuit  200  corresponds here substantially to the operation of the adjusting circuit of  FIG. 2   a . If the control loop with the regulation element of controlled current source  210  with PMOS transistor  211  and capacitor  212  is activated by the closing of first switching device S 1 , current Ic flows to charge capacitor  212  into output  102  of input current amplifier  100  until in the steady state output current I 1  of controlled current source  210  is the same as the (signed) sum of constant current I 2  and offset Ioff. 
         [0064]    Furthermore, a control circuit  300 , which has an interface  310  to an arithmetic unit  400 , such as, for example, a microprocessor, is shown in  FIG. 3 . Control circuit  300  is formed to control the described time course. Control circuit  300  is set up in a first step to disconnect output  102  of input current amplifier  100  from circuit output  202  by opening the second switching device S 2 . To this end, control circuit  300  via output  301  sends a control signal, for example, according to  FIG. 2   b , to second switching device S 2 . 
         [0065]    Control circuit  300  is set up in the first step to close a third switching device S 3 , so that capacitor  212  is discharged via third switching device S 3 . To this end, control circuit  300  via output  303  sends a control signal, for example, according to  FIG. 2   b , to third switching device S 3 . This step is optional, and thus the regulation can be started also with a partially charged capacitor  212 . 
         [0066]    Control circuit  300  is set up in a second step to connect capacitor  212  of controlled current source  210  to output  102  of input current amplifier  100  by closing first switching device S 1 . To this end, control circuit  300  via output  302  sends a control signal, for example, according to  FIG. 2   b , to first switching device S 1 . After the second step, capacitor  212  is charged by a charging current Ic. An output current I 1  of controlled current source  210  is increased by the charging of capacitor  212  until the charging current Ic attains a minimum. 
         [0067]    Control circuit  300  is set up in a third step to disconnect charged capacitor  212  of controlled current source  210  from output  102  of input current amplifier  100  by opening first switching device S 1 . Furthermore, control circuit  300  is set up in a fourth step to connect output  102  of input current amplifier  100  to circuit output  202  by closing second switching device S 2 . 
         [0068]    Control circuit  300  for generating the signals and their time sequence has a logic and a number of delay elements, for example, at least two delay elements (not shown in  FIG. 3 ). The delay elements are triggered by arithmetic unit  400  via interface  310  to generate the signals Vsig. 
         [0069]    The invention is not limited to the shown embodiment variants in  FIGS. 1 through 3 . For example, it is possible to provide a different input current amplifier. It is also possible to provide a different voltage-current conversion of the controlled current source instead of transistors  213 ,  211 . The functionality of the circuit according to  FIG. 2   a  can be used especially advantageously for a touch screen. 
         [0070]    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.