Logarithmic encoder with reduced settling time and method for automatic regulation of an effective feedback capacity of the logarithmic encoder

Abstract

Logarithm (100) comprehensive an input amplifier (101), a logarithmic component (102), and a feedback capacitor (103) which is designed to stabilize the logarithm (100), characterized by the fact that the logarithm (100) comprises a switching device (110), and the feedback capacitance (103) is divided into a plurality of n partial feedback capacitances (103a, 103b, 103c) connected in parallel, wherein a first partial feedback capacitor (103a) is a permanently stabilizing component of the logarithm converter (100), and wherein a number of n-1 of the n partial feedback capacitors (103a, 103b, 103c) are switchable partial feedback capacitors (103b, 103c), wherein the n-1 partial feedback capacitors (103b, 103c) can be switched on separately as stabilizing components in addition to the first partial feedback capacitor (103a), and where the switching device (110) is configured to individually switch the n-1 switchable partial feedback capacitors (103b, 103c) depending on an input current (I Ph ) of the logarithm (100) to be automatically switched on or off and thus depending on the input current (I Ph ) of the logarithm (100) automatically increases or decreases an effective feedback capacity (103) with which the logarithm (100) is stabilized, wherein the effective feedback capacitance (103) corresponds to a sum of the first partial feedback capacitance (103a) and all switched-on switchable partial feedback capacitances (103b, 103c), and wherein the switching device (110) comprises • at least one reference current source (111, 111a, 111b), • at least one reference transistor (112, 112a, 112b), • at least one switching amplifier (113a, 113b), and • at least one switch (114a, 114b).

Description

[0001] The invention relates to a logarithm comprising an input amplifier, a logarithmic component and a feedback capacitor, which is configured to stabilize the logarithm.

[0002] The invention further relates to a method for automatically adjusting the size of a feedback capacity of a logarithm.

[0003] As disclosed in IEEE JSSC, Vol. 30, No. 7, pp. 807-811 (July 1995), when using photodiodes as light sensors, a photocurrent is generated that spans a range of six decades and more, from near darkness (<1 mlux) to bright sunlight. Logarithmic converters are frequently used to process this dynamic range. In this process, the photodiode's photocurrent is converted into a voltage by the logarithmic converter, with the output voltage of the logarithmic converter being logarithmically proportional to the photocurrent corresponding to the logarithmic converter's input current.

[0004] Logarithmic components typically used are diodes or bipolar transistors.

[0005] In a standard logarithmic converter, an input current flows through a bipolar transistor, and the output voltage of the converter changes with the logarithm of the input current. This causes the impedance of the bipolar transistor to change over a large number of decades, which affects stability. A feedback capacitor C is typically used to stabilize the circuit. fb used.

[0006] As already revealed in Kauert, R.; Budde, W.; Kalz, A.: “A monolitic field segment photo sensor system”, IEEE JSSC, Vol.30, No.7, pp.807-811 (July 1995), a phase shift of the frequency response occurs, with the maximum phase shift depending on the ratio C fb / C d depends. Here, C corresponds to d the capacitance of the photodiode, where the capacitance is C d The photodiode represents the input capacitance of the logarithm.

[0007] To reduce this instability, the feedback capacitance C is used. fb increased.

[0008] In IEEE JSSC, Vol. 30, No. 7, pp. 807-811 (July 1995), this is achieved by adding another capacity to the feedback capacitor C. fb is connected in parallel.

[0009] The use of a large feedback capacitance C fb However, this leads to a longer settling time of the circuit at small input currents.

[0010] For small photocurrents, the settling time of a logarithmic converter can be up to one second or more. However, higher speeds are required, especially when using logarithmic converters to compress the measurement range of a photosensor.

[0011] DE 44 07 730 A1 discloses a variant for a switchable feedback capacitance, comprising a plurality of feedback impedances.

[0012] The object of the present invention is therefore to provide a solution for an improved logarithm that has an accelerated transient response without any loss of circuit stability, and wherein the solution should be cost-effective and not entail any major technical changes to a standard logarithm.

[0013] To solve the problem, a logarithm and a method according to the independent claims are proposed.

[0014] Further advantageous embodiments of the invention can be found in the dependent claims, the description, and the figures.

[0015] The proposed solution includes a logarithm converter comprising an input amplifier, a logarithmic component, and a feedback capacitor designed to stabilize the logarithm converter.

[0016] In particular, the logarithmic component is a bipolar transistor.

[0017] In particular, the logarithm includes a switching device.

[0018] In particular, the feedback capacitance is divided into a plurality of n partial feedback capacitances connected in parallel to each other.

[0019] In particular, a first partial feedback capacitor is a permanently stabilizing component of the logarithm.

[0020] In particular, a number of n-1 of the n partial feedback capacitors are designed as switchable partial feedback capacitors.

[0021] In particular, the n-1 switchable partial feedback capacitors can be switched on separately as stabilizing components in addition to the first partial feedback capacitor.

[0022] In particular, the switching device is designed to automatically switch the n-1 switchable partial feedback capacitors on or off individually depending on an input current of the logarithm.

[0023] In particular, the switching device is designed to automatically increase or decrease an effective feedback capacitance, with which the logarithm is stabilized, depending on the input current of the logarithm.

[0024] In particular, the effective feedback capacitance corresponds to the sum of the first partial feedback capacitance and all switched-on partial feedback capacitances. Specifically, the first partial feedback capacitance is not switchable. Specifically, the first partial feedback capacitance serves to provide basic stabilization for the logarithmic converter. Specifically, the effective feedback capacitance for small input currents of the logarithmic converter corresponds to the first partial feedback capacitance. Specifically, a small input current is typically a value of I. Ph <10nA.

[0025] In particular, additional partial feedback capacitors are added for larger input currents.

[0026] In particular, the additional partial feedback capacitors are switched on or off depending on a value for the input current.

[0027] In an embodiment according to the invention, the switching device of the logarithm is configured to switch the n-1 switchable partial feedback capacitors individually to a respective predetermined switching point.

[0028] In particular, a second partial feedback capacitor is switched to a first switching point. In particular, a further switchable partial feedback capacitor is switched to a further switching point, which differs from the first switching point.

[0029] In particular, a respective partial feedback capacitor is switched on at a respective switching point when the input current rises to or exceeds a corresponding value.

[0030] In particular, a respective partial feedback capacitor is switched off at a respective switching point when the input current drops to or falls below a corresponding value.

[0031] In a further embodiment according to the invention, the switching device comprises at least one reference current source. In particular, the switching device further comprises at least one reference transistor.

[0032] In particular, the switching device further comprises at least one switching amplifier.

[0033] In particular, the switching device further comprises at least one switch.

[0034] In a further embodiment according to the invention, the at least one reference current source is configured to set a respective value for a respective reference current.

[0035] In particular, a first reference current source is set up to establish a first value for a first reference current.

[0036] In particular, a further reference current source is set up to set a further value for a further reference current, wherein the further value for the further reference current differs from the first value for the first reference current.

[0037] In particular, at least one switching point is set by means of the respective value of the respective reference current, at which at least one of the switchable partial feedback capacitors is switched.

[0038] In a further embodiment according to the invention, each of the switching amplifiers is connected via one of the switches to each of the n-1 switchable partial feedback capacitors. In particular, a first switching amplifier is connected via a first switch to a switchable second partial feedback capacitor. In particular, the first switching amplifier is configured to switch the switchable second partial feedback capacitor on and off by means of the first switch in addition to the non-switchable first partial feedback capacitor. In particular, the first switching amplifier is configured to switch the switchable second partial feedback capacitor off again by means of the first switch.

[0039] In particular, a second switching amplifier is connected via a second switch to a switchable third partial feedback capacitor. Specifically, the second switching amplifier is configured to switch the switchable third partial feedback capacitor on and off using the second switch, in addition to the non-switchable first partial feedback capacitor and the switched-on second partial feedback capacitor. Specifically, the second switching amplifier is also configured to switch the switchable third partial feedback capacitor off again using the second switch.

[0040] In a further embodiment according to the invention, the at least one switching amplifier is configured to switch the respective switchable partial feedback capacitance of the n-1 switchable partial feedback capacitances depending on the input current by means of the respective switch.

[0041] In particular, with small input currents, especially with input currents of typically I Ph <10nA, all of the switchable n-1 partial feedback capacitors are switched off, so that the logarithm is stabilized exclusively by means of the non-switchable first partial feedback capacitor.

[0042] In particular, the at least one switching amplifier uses the respective switch to activate the respective switchable partial feedback capacitor assigned to it for an increasing input current.

[0043] In particular, the at least one switching amplifier switches off the respective switchable partial feedback capacitance assigned to it for a falling input current by means of the respective switch.

[0044] In a further embodiment according to the invention, the respective switch is configured to switch the switchable partial feedback capacitor of the n-1 switchable partial feedback capacitors connected to this switch as a function of a respective output voltage of the switching amplifier connected to this switch.

[0045] In particular, the respective switch becomes conductive when the respective output voltage of the switching amplifier connected to this switch exceeds a certain voltage value. Specifically, a respective switchable partial feedback capacitor is switched on when the respective switch connected to this switchable partial feedback capacitor is conductive. Specifically, the respective switch is blocked when the respective output voltage of the switching amplifier connected to this switch falls below a certain voltage value, specifically when the output voltage of the switching amplifier reaches zero. Specifically, a respective switchable partial feedback capacitor is switched off when the respective switch connected to this switchable partial feedback capacitor is blocked.

[0046] In a further embodiment according to the invention, the respective switching amplifier is configured to switch the respective switchable partial feedback capacitance to the respective predetermined switching point by means of the respective switch.

[0047] In particular, each predetermined switching point corresponds to a specific voltage value for the respective output voltage of the respective switching amplifier.

[0048] In a further embodiment according to the invention, the respective switching point is predetermined by the respective set value for the reference current.

[0049] In particular, the adjustable value for the reference current specifies a value for the input current for which the respective output voltage of the respective switching amplifier assumes a certain voltage value, at which the respective switch connected to this switching amplifier and thus the respective switchable partial feedback capacitance is switched.

[0050] In a further embodiment according to the invention, the respective switching point is predetermined by a respective amplification of the at least one switching amplifier.

[0051] In particular, the respective switching point results from a combination of the specified value for the reference current, the gain of the respective switching amplifier and the resulting respective output voltage, as well as a respective voltage value for which the respective switch connected to the respective switching amplifier becomes conductive or is blocked.

[0052] In a further embodiment according to the invention, the respective switching point is predetermined by a respective determinable offset of the at least one switching amplifier.

[0053] In particular, the respective switching point results from a combination of the specified value for the reference current, the respective determinable offset of the respective switching amplifier and the resulting respective output voltage, as well as a respective voltage value for which the respective switch connected to the respective switching amplifier becomes conductive or is blocked.

[0054] In a further embodiment according to the invention, a first switching point for a second partial feedback capacitor differs from a second switching point for a third partial feedback capacitor.

[0055] In particular, the switchable n-1 partial feedback capacitors are switched to n-1 different switching points.

[0056] The invention further relates to a method for automatically adjusting the size of an effective feedback capacity of the logarithm, wherein the feedback capacity is divided into a plurality of partial feedback capacities.

[0057] In particular, at least one switchable second partial feedback capacitor is automatically switched on or off depending on the input current of the logarithm converter, in addition to a fixed, non-switchable first partial feedback capacitor.

[0058] In particular, a plurality of n-1 switchable partial feedback capacitors are automatically switched on or off depending on the input current of the logarithm. Specifically, the magnitude of the effective feedback capacitance corresponds to the sum of the first partial feedback capacitance and all switched-on partial feedback capacitors of the n-1 switchable partial feedback capacitors.

[0059] In an embodiment of the method according to the invention, the switchable second partial feedback capacitor is switched on when the input current is equal to or greater than at least a predetermined value for a reference current.

[0060] In particular, the majority of n-1 switchable partial feedback capacitors are switched on when the input current is equal to or greater than at least a predetermined value for a reference current.

[0061] In a further embodiment of the method according to the invention, the at least one predetermined value for the reference current is set by means of a respective reference current source. In particular, a respective value for a respective reference current is predetermined by means of a respective reference current source.

[0062] In particular, a first value for a first reference current is set using a first reference current source.

[0063] In particular, n-1 different values ​​for n-1 reference currents are set using a plurality of n-1 reference current sources.

[0064] In a further embodiment of the method according to the invention, the logarithmic converter is stabilized only with the first partial feedback capacitor when the input current is smaller than a first value for a first reference current, which is set by means of a first reference current source. In particular, for small input currents for which the capacitance of the first partial feedback capacitor is sufficient and for which the settling time of the logarithmic converter does not exceed a certain length, the logarithmic converter is stabilized only with the first partial feedback capacitor. In particular, the settling time of the logarithmic converter should be as short as possible.

[0065] In particular, the logarithm is typically used with an input current of I Ph <10nA is stabilized only with the first partial feedback capacitor.

[0066] In a further embodiment of the method according to the invention, a respective k-th switching point is determined, with i ≥ 1 and k ≥ 1, according to a respective i-th value for an i-th reference current set by means of an i-th reference current source and according to a gain and / or an offset of a respective at least one k-th switching amplifier connected to the i-th reference current source and a resulting k-th output voltage of the k-th switching amplifier. Several switching amplifiers can be assigned to an i-th reference current source, wherein these several switching amplifiers have different gains and / or offsets from one another.

[0067] In particular, when the k-th switching point is reached, a (k+1)-th switchable partial feedback capacitor is switched by means of a k-th switch which is connected to the k-th switching amplifier.

[0068] In a further embodiment of the method according to the invention, the k-th switch adds a (k+1)-th switchable partial feedback capacitor when the k-th output voltage exceeds the k-th switching point.

[0069] In particular, the k-th switch switches off the (k+1)-th switchable partial feedback capacitor when the k-th output voltage falls below the k-th switching point.

[0070] In particular, the k-th switching point is determined by the value set by the i-th reference current source for the i-th reference current, the gain and / or offset of the k-th switching amplifier, and the switching behavior of the k-th switch connected to the k-th switching amplifier.

[0071] In particular, the k-th switch blocks when the k-th output voltage of the k-th switching amplifier is less than a certain k-th voltage value.

[0072] In particular, the k-th switch is conductive if the k-th output voltage of the k-th switching amplifier is equal to or greater than the specified k-th voltage value.

[0073] In a further embodiment of the method according to the invention, the size of the effective feedback capacitance is increased if at least one of the switchable partial feedback capacitances is added to the first partial feedback capacitance. In particular, the size of the effective feedback capacitance corresponds to the sum of the first partial feedback capacitance and all switched-on partial feedback capacitances.

[0074] In a further embodiment of the method according to the invention, the size of the effective feedback capacitance is reduced when at least one of the switched-on switchable partial feedback capacitances is switched off again.

[0075] In particular, the size of the effective feedback capacitance corresponds to the sum of the first partial feedback capacitance and all other switchable partial feedback capacitances that are still switched on.

[0076] The invention offers the advantage over a logarithm according to the prior art that, depending on an input current I Ph The logarithmic converter's effective feedback capacitance is automatically adjusted to compensate for its inertia. Simultaneously, the logarithmic converter's settling time is optimized by ensuring that the effective feedback capacitance is never larger than necessary for the current input current.

[0077] Thus, at any given time, according to the currently available input current I PhAdditional partial feedback capacitors are automatically switched on or off, ensuring circuit stability at all times and simultaneously guaranteeing that low input currents I Ph The settling time of the logarithm is significantly reduced compared to a logarithm according to the state of the art.

[0078] Further advantageous embodiments, features and functions of the invention are explained in connection with the examples shown in the figures.

[0079] This shows: Fig. 1. Schematic representation of a logarithm according to the state of the art; Fig. 2 phase reserve φ in for different input capacitances C d Dependence of the input current I Ph ; Fig. 3 Schematic representation of an embodiment of the logarithm according to the invention; Fig. 4 Settling time for small input currents of a logarithm converter according to the invention compared to a logarithm converter according to the prior art; Fig. 5 Schematic representation of a second embodiment of the logarithm according to the invention; and Fig. 6 Schematic representation of a further embodiment of the logarithm according to the invention.

[0080] Fig. Figure 1 shows a schematic representation of a logarithm 100 according to the state of the art. The in Fig. The logarithmic converter 100 shown comprises an input amplifier 101 and a logarithmic component 102 in the form of a bipolar transistor.

[0081] An input current I Ph The photocurrent from a photodiode D is converted using the bipolar transistor 102, resulting in an output voltage V. out of the logarithm 100 logarithmically proportional to the input current I Ph changes.

[0082] The impedance of the bipolar transistor 102 changes over a large number of decades, which affects its stability. To stabilize the circuit, the logarithmic amplifier 100 includes a feedback capacitor C. fb 103.

[0083] A phase shift in the frequency response depends on the ratio of the feedback capacitance C. fb 103 and a capacity C d the photodiode, which has an input capacitance C d of the logarithm, in the form C fb / C d dependent.

[0084] With a small feedback capacitance of 103 and a large input capacitance C d The phase shift can be almost 90°.

[0085] To reduce the phase shift, the feedback capacity 103 can be increased.

[0086] However, increasing the feedback capacity 103 has the disadvantage that the settling time of the logarithm 100 increases.

[0087] Fig. Figure 2 shows a typical frequency response of a logarithmic converter according to Fig. 1 for different input capacities C d with a feedback capacitance C fb of 50pF, where the phase margin φ depends on the input current I Ph was applied.

[0088] This shows that the larger the input capacitance C d The longer the phase shift, the greater the phase shift.

[0089] For small input currents I Ph , i.e., for input currents of I Ph For currents below 10 nA, the impedance of the logarithmic converter's bipolar transistor is very high. In this case, the circuit forms a capacitively feedback amplifier with a gain of C. fb / C dIt turns out that the phase shift for small input currents I Ph of less than 1nA, where a settling time of the logarithmic converter occurs for large feedback capacitances C fb takes a very long time, is small.

[0090] For large input currents (I Ph When the current is >1mA, the bipolar transistor has a low resistance. In this case, the circuit behaves like a differentiator.

[0091] In a transition region of a medium input current I Ph This results in a point of maximum phase shift, where the ratio C fb / C d from the feedback capacitance C fb and the input capacity C d The magnitude of the phase shift is defined. The maximum phase shift is observed for an input current in the range of approximately 1 µA to 10 µA.

[0092] Thus, only one input current I is considered. Phin the range of approximately 1µA to 10µA a large feedback capacitance C fb is needed to reduce the phase shift.

[0093] For small input currents I Ph However, due to the small phase shift, the feedback capacitance C fb can be chosen to be considerably smaller, thus reducing the settling time of the logarithm.

[0094] Through a variably adjustable feedback capacitance C fb , which corresponds to a value of the input current I Ph By adjusting the settings, the phase shift can be reduced and a short settling time of the logarithm can be achieved.

[0095] Fig. Figure 3 shows a first embodiment of the logarithm converter 100 according to the invention.

[0096] The in Fig. The logarithm 100 shown in Figure 3 according to the invention differs from the one shown in Figure 3. Fig. 1. The logarithm shown in the diagram is in accordance with the prior art in that the feedback capacity C fb of the in Fig. The logarithmic converter shown in Figure 1 is divided into a plurality of partial feedback capacitors 103a, 103b, 103c connected in parallel.

[0097] In the Fig. In the embodiment shown in section 3, a first partial feedback capacitance C ensures fb1 103a for basic circuit stability, while a second partial feedback capacitor C fb2 103b and a third partial feedback capacity C fb3 103c by means of a switching device 110 of the logarithm converter 100 according to the invention as a function of the input current I Ph can be added or switched off.

[0098] For small input currents I PhThe logarithm 100 therefore only has the first partial feedback capacity 103a, which is many times smaller than the feedback capacity of the logarithm according to Fig. 1. The minimum capacity of the first partial feedback capacitor 103a required for basic stability results from the necessary minimum value for a ratio C. fb1 / C d from the first partial feedback capacitance 103a and the input capacitance C d .

[0099] For larger input currents I Ph The phase shift in the logarithmic converter circuit 100 increases, such that the capacitance of the first partial feedback capacitor 103a is no longer sufficient for the stability of the circuit. For larger input currents I Ph The second partial feedback capacitance 103b is automatically activated by means of the switching device 110, and for even larger input currents I PhAdditionally, the third partial feedback capacitor 103c was also switched on.

[0100] For this purpose, the switching device 110 comprises according to the Fig. 3 in the illustrated embodiment a reference current source 111, a reference transistor 112, a plurality of switching amplifiers 113 and a plurality of switches 114.

[0101] A first switching amplifier 113a is configured to switch a first switch 114a and a second switching amplifier 113b is configured to switch a second switch 114b.

[0102] In the Fig. In the embodiment shown in Figure 3, the first switching amplifier 113a and the second switching amplifier 113b have different gains, such that the first gain of the first switching amplifier 113a differs from the second gain of the second switching amplifier 113b. The first output voltage of the first switching amplifier 113a depends on the input current I. Ph and the first amplification. Equivalently, a second output voltage of the second switching amplifier 113b depends on the input current I. Ph and the second reinforcement.

[0103] Using the reference current source 111, a value for a reference current I can be determined. lim be set. In particular, the value of the reference current I corresponds to lim a value for an input current I Ph , for which the first partial feedback capacitance 103a is no longer sufficient for the stability of the circuit.

[0104] Is the input current I Ph smaller than the value for the reference current l lim , so the magnitude of both the first and second output voltages corresponds to the value 0. Consequently, the first switch 114a and the second switch 114b are off, and thus the second partial feedback capacitor 103b and the third partial feedback capacitor 103c are switched off. For I Ph lim The logarithmizer 100 is therefore only compensated by the first partial feedback capacitance 103a.

[0105] Is the input current I Ph greater than the value for the reference current l lim , the first switching amplifier 113a and the second switching amplifier 113b are overdriven, wherein the first output voltage of the first switching amplifier 113a depends on the first gain and the second output voltage of the second switching amplifier 113b depends on the second gain.

[0106] ​If the first output voltage of the first switching amplifier 113a is greater than a certain first voltage value, the first switch 114a becomes conductive and the second partial feedback capacitor 103b is switched on.

[0107] If the second output voltage of the second switching amplifier 113b is greater than a certain second voltage value, the second switch 114b becomes conductive and the third partial feedback capacitor 103c is switched on.

[0108] In the case that the second gain of the second switching amplifier 113b is smaller than the first gain of the first switching amplifier 113a, a smaller input current I is required to reach the first voltage value at which the first switch 114a switches. Ph required to reach the second voltage value at which the second switch 114b is switched.

[0109] If the magnitude of the first output voltage is greater than or equal to the first voltage value, the first switch 114a becomes conductive. In this case, the logarithm converter 100 is compensated by the sum of the first partial feedback capacitance 103a and the second partial feedback capacitance 103b.

[0110] If the magnitude of the second output voltage is greater than or equal to the second voltage value, the second switch 114b also becomes conductive, so that the third partial feedback capacitor 103c is switched on in addition to the second partial feedback capacitor 103b already switched on by the first switching amplifier 113a. In this case, the logarithmic amplifier 100 is compensated with the full feedback capacitance, which results from the sum of all partial feedback capacitances.

[0111] If the input current I decreases Ph, so the first output voltage of the first switching amplifier 113a and the second output voltage of the second switching amplifier 113b also decrease accordingly.

[0112] In the event that the second gain of the second switching amplifier 113b is smaller than the first gain of the first switching amplifier 113a, the second output voltage is smaller than the first output voltage.

[0113] If both switches 114a, 114b are conducting, so that the second partial feedback capacitor 103b and the third partial feedback capacitor 103c are switched on, and the input current I decreases Phthen to the point that the second output voltage of the second switching amplifier 113b falls below the second voltage value, the second switching amplifier 113b blocks the second switch 114b and the third partial feedback capacitor 103c is switched off, so that the logarithm 100 is only compensated with the first partial feedback capacitor 103a and the second partial feedback capacitor 103b.

[0114] If the input current I decreases Ph If the voltage drops further, so that the first output voltage of the first switching amplifier 113a also falls below the first voltage value, the first switching amplifier 113a blocks the first switch 114a, thus also switching off the second feedback capacitor 103b. The logarithmic amplifier 100 is then only compensated by the first partial feedback capacitor 103a.

[0115] Thus, depending on the set value for the reference current l, the following results are obtained. limand the respective gain of the respective switching amplifiers 113a, 113b provide different switching points for the first switch 114a and the second switch 114b. The first switch 114a is thus switched by the first switching amplifier 113a at a first switching point, and the second switch 114b is switched by the second switching amplifier 113b at a second switching point, the first switching point being different from the second switching point.

[0116] In this way, the magnitude of the input current I is adjusted accordingly. Ph automatically, in addition to the first partial feedback capacity 103a, the second partial feedback capacity 103b and, if necessary, the third partial feedback capacity 103c are also switched on or off.

[0117] By switching on and off the second partial feedback capacitor 103b and the third feedback capacitor 103c depending on the input current IPh , and the one from the input current I Ph Based on the resulting output voltages of the switching amplifiers 113, where the first gain of the first switching amplifier 113a differs from the second gain of the second switching amplifier 113b, the size of the feedback capacitance with which the logarithm 100 is compensated is automatically adjusted.

[0118] Thus, at any given time, according to the currently available input current I Ph Additional partial feedback capacitors are switched on or off, ensuring circuit stability at all times and simultaneously realizing that at low input currents I Ph The settling time of the logarithm 100 is significantly reduced compared to a logarithm according to the state of the art.

[0119] In the Fig. In the embodiment shown in Figure 3, the feedback capacitance is divided into three partial feedback capacitances 103a, 103b, 103c, wherein the second partial feedback capacitance 103b and the third partial feedback capacitance 103c are controlled by means of a respective associated switching amplifier 113a, 113b depending on the input current I Ph can be added or removed. However, the number of partial feedback capacitors can be chosen arbitrarily, whereby the logarithmic amplifier 100 with n partial feedback capacitors has a number of n-1 switching amplifiers 113, which switch a number of n-1 switchable partial feedback capacitors.

[0120] In Fig. 4 is the respective settling time for a logarithm according to the invention. Fig. 3 and a state-of-the-art logarithm according to Fig. Figure 1 represents a jump in input current from 10pA to 100pA.

[0121] While the logarithm according to the state of the art Fig. 1 independent of the magnitude of the input current with the full feedback capacitance C fb When the inventive logarithm is compensated, it is used according to the invention. Fig. 3 for these small input currents is compensated only by the first partial feedback capacitance, which in this example is one third of the total feedback capacitance C. fb corresponds.

[0122] As in Fig. 4 can be seen to be the settling time for the logarithm according to the invention. Fig. 3 (Curve for 1 / 3 C fb ) significantly shorter than the settling time for the logarithm according to the state of the art (curve for C) fb ) according to Fig. 1.

[0123] By gradually switching partial feedback capacitors on and off depending on the magnitude of the input current, the settling time of the logarithmic converter can be significantly reduced.

[0124] Fig. Figure 5 shows a further embodiment of the logarithm converter 100 according to the invention.

[0125] Unlike the one in Fig. In the embodiment shown in 3, the switching amplifiers 113 in the Fig. In the embodiment shown in section 5, the same amplification is used, but different offsets are used.

[0126] Is the input current I Ph less than the value for the reference current I set by means of the reference current source 111 lim , so the logarithmizer 100 is only compensated by the first partial feedback capacity 103a.

[0127] If the input current I exceeds Ph the set value for the reference current I lim , so according to the input current I PhThe second partial feedback capacitor 103b is switched on at a first switching point, which depends on the first output voltage of the first switching amplifier 113a, by means of the first switch 114a, and the third partial feedback capacitor 103c is switched on at a second switching point, which depends on the second output voltage of the second switching amplifier 113b, by means of the second switch 114b.

[0128] If the input current I decreases Ph If the second output voltage of the second switching amplifier and / or the first output voltage of the first switching amplifier falls below the second voltage value and / or the first voltage value, then the second switching amplifier 114b blocks the second switch 114b and / or the first switching amplifier 114a blocks the first switch 114a. Correspondingly, the third partial feedback capacitor 103c and / or the second partial feedback capacitor 103b are switched off again.

[0129] To determine the first switching point and the second switching point, the following is used in the Fig. In the embodiment shown in Figure 5, a first value for a first offset is set for the first switching amplifier 113a and a second value for a second offset is set for the second switching amplifier 113b, wherein the first value for the first offset is different from the second value for the second offset.

[0130] The switching amplifiers 113a, 113b switch the second partial feedback capacitor 103b and the third partial feedback capacitor 103c on or off by means of the switches 114a, 114b when the input current I Ph the value for the reference current I set by means of the reference current source 111 limexceeds the threshold, whereby the first switch 114a is switched according to the first output voltage of the first switching amplifier 113a and the second switch 114b is switched according to the second output voltage of the second switching amplifier 113b. By selecting the respective offset of the switching amplifiers 113a and 113b, the respective output voltages of the first switching amplifier 113a and the second switching amplifier 113b are controlled such that the first switching amplifier 113a switches the first switch 114a at a first switching point and the second switching amplifier 113b switches the second switch 114b at a second switching point, the first switching point being different from the second switching point.

[0131] Fig. Figure 6 shows a further embodiment of the logarithm converter 100 according to the invention.

[0132] The switching amplifiers 113 have the same gain.

[0133] In the Fig. In the embodiment shown in Figure 6, the logarithmizer 100 has a plurality of reference current sources 111a, 111b, wherein a first value for a first reference current I is determined by means of a first reference current source 111a and a first reference transistor 112a. lim1 is set and a second value for a second reference current I is determined using a second reference current source 111b and a second reference transistor 112b. lim2 is being discontinued.

[0134] In this embodiment as well, the logarithm 100 is used for small input currents I Ph exclusively compensated with the first partial feedback capacitance 103a.

[0135] At a first switching point, the second partial feedback capacitor 103b is switched on or off by means of the first switch 114a according to the first output voltage of the first switching amplifier 113a.

[0136] At a second switching point, the third partial feedback capacitor 103c is switched on or off by means of the second switch 114b according to the second output voltage of the second switching amplifier 113a.

[0137] Here, the first switching point is determined by the set first value for the first reference current I. lim1 determined and the second switching point by the set second value for the second reference current I lim2 certainly.

[0138] When setting the first value for the first reference current I lim1 and the second value for the second reference current I lim2 for him I lim1 <l lim2 , the logarithm is used for an input current I Ph <l lim1 exclusively compensated with the first partial feedback capacitance 103a.

[0139] For an input current I ph with I lim1 ≤I Ph lim2 ​The logarithm 100 is compensated by the sum of the first partial feedback capacitance 103a and the second partial feedback capacitance 103b. For an input current I Ph ≤I lim2 The logarithmizer 100 is compensated with the total feedback capacity, i.e. the sum of the first partial feedback capacity 103a, the second partial feedback capacity 103b and the third partial feedback capacity 103c.

[0140] In this way, depending on the input current I Ph The size of the effective feedback capacity is automatically adjusted to compensate for the logarithmizer 100 while simultaneously optimizing the settling time of the logarithmizer.

[0141] In the Fig. In the embodiment shown in Figure 6, the partial feedback capacitors 103b, 103c and the switches 114a, 114b that control them are connected in reverse order than in the embodiment shown in Figure 6. Fig. 3 and Fig. 5 illustrated embodiments. The switches 114a, 114b and partial feedback capacitors 103b, 103c of the embodiment from Fig. 6 can be in the same order as in Fig. 3 and Fig. 5 can be switched. Likewise, switches 114a, 114b and partial feedback capacitors 103b, 103c of the embodiments from Fig. 3 and Fig. 5 in the order as in Fig. 6. The order of switching the partial feedback capacitors 103b, 103c and the associated switches 114a, 114b is irrelevant for the functionality.

[0142] In the Fig. 3, Fig. 5 and Fig. In the 6 illustrated embodiments, the feedback capacity is divided into three partial capacities.

[0143] However, the logarithmic mixer according to the invention can also have only two partial feedback capacitors or more than three partial feedback capacitors and a corresponding number of associated switches, which are switched by means of a corresponding number of associated switching amplifiers.

[0144] The ones in the Fig. 3, Fig. 5 and Fig. The 6 illustrated versions can be combined with each other.

[0145] In the exemplary embodiments, the input current I corresponds to Ph a photocurrent of a photodiode D, which photodiode D has a capacitance C d exhibits, with the capacity C d the photodiode D of the input capacitance C d This corresponds to the logarithm. However, any current source can also provide an input current I. Ph supply for the logarithm 100, where this power source has a capacitance C d has an input capacitance C dfor the logarithm 100. Reference symbol list 100 logarithms 101 Input amplifiers 102 logarithmic component 103 Feedback capacity C fb 103a first partial feedback capacitance C fb1 103b second partial feedback capacitance C fb2 103c third partial feedback capacitance C fb3 110 Circuit device 111 Reference current source 111a first reference current source 111b second reference current source 112 Reference transistor 112a first reference transistor 112b second regeneration transistor 113 switching amplifiers 113a first switching amplifier 113b second switching amplifier 114 switches 114a first switch 114b second switch D input diode C d Input capacity I Ph Input current I lim Reference current I lim1 first reference current I lim2 second reference current V out Output voltage

Claims

[1] Logarithmizer (100) comprising an input amplifier (101), a logarithmic component (102), and a feedback capacitor (103) which is designed to stabilize the logarithm (100), characterized by , that the logarithm (100) comprises a switching device (110), and the feedback capacitance (103) is divided into a plurality of n partial feedback capacitances (103a, 103b, 103c) connected in parallel, wherein a first partial feedback capacitor (103a) is a permanently stabilizing component of the logarithm converter (100), and wherein a number of n-1 of the n partial feedback capacitors (103a, 103b, 103c) are switchable partial feedback capacitors (103b, 103c), wherein the n-1 partial feedback capacitors (103b, 103c) can be switched on separately as stabilizing components in addition to the first partial feedback capacitor (103a), and where the switching device (110) is configured to individually switch the n-1 switchable partial feedback capacitors (103b, 103c) depending on an input current (I Ph ) of the logarithm (100) to be automatically switched on or off and thus depending on the input current (I Ph ) of the logarithm (100) automatically increases or decreases an effective feedback capacity (103) with which the logarithm (100) is stabilized, wherein the effective feedback capacitance (103) corresponds to a sum of the first partial feedback capacitance (103a) and all switched-on switchable partial feedback capacitances (103b, 103c), and the switching device (110) comprises • at least one reference current source (111, 111a, 111b), • at least one reference transistor (112, 112a, 112b), • at least one switching amplifier (113a, 113b), and • at least one switch (114a, 114b). [2] Logarithmizer (100) according to claim 1, characterized by , that the switching device (110) is configured to switch the n-1 switchable partial feedback capacitors (103b, 103c) individually to a respective predetermined switching point. [3] Logarithmizer (100) according to any one of the preceding claims, characterized by , that at least one reference current source (111, 111a, 111b) is set up to provide a respective value for a respective reference current (I lim , I lim1 , I lim2 to adjust. [4] Logarithm converter (100) according to any one of the preceding claims, characterized by, that each of the switching amplifiers (113a, 113b) is connected via each of the switches (114a, 114b) to each of the n-1 switchable partial feedback capacitors (103b, 103c). [5] Logarithmizer (100) according to any one of the preceding claims, characterized by , that the at least one switching amplifier (113a, 113b) is configured to use the respective switch (114a, 114b) to control the respective switchable partial feedback capacitance (103b, 103c) of the n-1 switchable partial feedback capacitances (103b, 103c) as a function of the input current (I Ph ) to switch. [6] Logarithm converter (100) according to any one of the preceding claims, characterized by, that the respective switch (114a, 114b) is configured to switch the switchable partial feedback capacitor (103b, 103c) of the n-1 switchable partial feedback capacitors (103b, 103c) connected to this switch (114a, 114b) depending on a respective output voltage of the switching amplifier (113a, 113b) connected to this switch (114a, 114b). [7] Logarithm converter (100) according to any one of the preceding claims, characterized by , that the respective switching amplifier (113a, 113b) is configured to switch the respective switchable partial feedback capacitor (103b, 103c) to the respective predetermined switching point by means of the respective switch (114a, 114b). [8] Logarithmizer (100) according to claim 7, characterized by , that the respective switching point is determined by the respective set value for the reference current (I) lim , I lim1 , I lim2 ) is specified. [9] Logarithmizer (100) according to claim 8, characterized by , that the respective switching point is determined by a respective amplification of the at least one switching amplifier (113a, 113b). [10] Logarithmizer (100) according to any one of claims 8 to 9, characterized by , that the respective switching point is determined by a respective determinable offset of the at least one switching amplifier (113a, 113b). [11] Logarithmizer (100) according to any one of claims 2 to 10, characterized by , that a first switching point for a second partial feedback capacitor (103b) differs from a second switching point for a third partial feedback capacitor (103c). [12] Method for automatically adjusting the size of an effective feedback capacity of a logarithm (100) according to one of the preceding claims, wherein the feedback capacity (103) is divided into a plurality of partial feedback capacities (103a, 103b, 103c), and wherein to a fixed first partial feedback capacitor (103a) at least one switchable second partial feedback capacitor (103b) depending on an input current (I Ph ) of the logarithm (100) is automatically switched on or off by means of a switching device (110) and the switching device (110) comprises • at least one reference current source (111, 111a, 111b), • at least one reference transistor (112, 112a, 112b), • at least one switching amplifier (113a, 113b), and • at least one switch (114a, 114b). [13] Method according to claim 12, characterized by , that the switchable second partial feedback capacitor is switched on when the input current (I Ph ) is equal to or greater than at least one given value for a reference current (I) lim , I lim1 , I lim2 ). [14] Method according to claim 13, characterized by, that at least one predetermined value for the reference current (I) lim , I lim1 , I lim2 ) is set using a respective reference current source (111, 111a, 111b). [15] Method according to claim 14, characterized by that if the input current (I Ph ) less than a first value for a first reference current (Ilim, I) lim1 ) is which is set by means of a first reference current source (111, 111a), the logarithmizer (100) is stabilized only with the first partial feedback capacitance (103a). [16] Method according to any one of claims 14 to 15, characterized by , that according to a respective i-th value set by means of an i-th reference current source (111) for an i-th reference current (I lim) and according to a gain and / or an offset of at least one k-th switching amplifier (113) connected to the i-th reference current source (111) and a resulting k-th output voltage of the k-th switching amplifier (113), a respective k-th switching point is determined, with i≥1 and k≥1, wherein When the k-th switching point is reached by means of a k-th switch (114) which is connected to the k-th switching amplifier (113), a (k+1)-th switchable partial feedback capacitor (103b, 103c) is switched. [17] Method according to claim 16, characterized by , that when the k-th output voltage exceeds the k-th switching point, the k-th switch (114) adds a (k+1)-th switchable partial feedback capacitor (103b, 103c), and when the k-th output voltage falls below the k-th switching point, the k-th switch (114) switches off the (k+1)-th switchable partial feedback capacitor (103b, 103c). [18] Method according to any one of claims 12 to 17, characterized by , that if at least one of the switchable partial feedback capacitors (103b, 103c) is added to the first partial feedback capacitor (103a), the size of the effective feedback capacitor is increased, wherein the size of the effective feedback capacitance corresponds to the sum of the first partial feedback capacitance (103a) and all switched-on switchable partial feedback capacitances (103b, 103c). [19] Method according to any one of claims 12 to 18, characterized by , that if at least one of the switchable partial feedback capacitors (103b, 103c) is switched off again, the size of the effective feedback capacitor is reduced, where the size of the effective feedback capacitance corresponds to the sum of the first partial feedback capacitance (103a) and all the switchable partial feedback capacitances (103b, 103c) that are still switched on.

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