Logarithmic compression circuit

Abstract

This provides a logarithmic compression circuit that can cancel out not only the temperature characteristics of the diode, but also the variations caused by the slope of the V / I curve differing with temperature. [Solution] The logarithmic compression circuit 1 comprises a first amplifier 12, a first diode 13 for logarithmic compression, a second diode 23 for temperature compensation, and a current source 30. The first amplifier 12 amplifies the first input. The first diode 13 is placed in the feedback circuit of the first amplifier 12. The second diode 23 has the same temperature characteristics as the first diode 13 and is placed in a position to cancel out the characteristic change of the first diode 13 due to temperature changes. The current source 30 flows an idling current of the same value to the first diode 13 and the second diode 23, increasing the value of the idling current as the temperature rises and decreasing the value of the idling current as the temperature falls.

Description

Technical Field

[0001] The present invention relates to a logarithmic compression circuit having a function of canceling the temperature characteristics of a diode.

Background Art

[0002] Patent Document 1 and Patent Document 2 disclose logarithmic compression circuits. The logarithmic compression circuit of Patent Document 1 has a diode for logarithmic compression and a diode for temperature compensation. The temperature characteristics of the diode for temperature compensation are the same as those of the diode for logarithmic compression. With this configuration, it is possible to cancel the change in the forward voltage of the diode accompanying the temperature change. The logarithmic compression circuit of Patent Document 2 includes a current source. The current source generates a current proportional to the absolute temperature. Thereby, by compensating for the temperature dependence, it is possible to reduce the change in the light detection amount (exposure value) of the photodiode with respect to the temperature change.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Since a diode has temperature characteristics, its forward voltage changes according to a temperature change. Further, the amount of change in the forward voltage corresponding to the current change (that is, the slope of the V / I curve) also varies depending on the temperature. In the logarithmic compression circuit of Patent Document 1, although it has a function of canceling the change in the forward voltage accompanying the temperature change, it does not have a function of canceling the variation caused by the fact that the slope of the V / I curve varies depending on the temperature. Also, the control of the current source in Patent Document 2 does not cancel the variation caused by the fact that the slope of the V / I curve varies depending on the temperature.

[0005] This invention has been made in view of the above circumstances, and its main objective is to provide a logarithmic compression circuit that can cancel out not only the temperature characteristics of the diode, but also the fluctuations caused by the fact that the slope of the V / I curve differs with temperature. Means and effects for solving the problem

[0006] The problems that this invention aims to solve are as described above, and next, the means for solving these problems and their effects will be explained.

[0007] In view of the present invention, a logarithmic compression circuit having the following configuration is provided. That is, the logarithmic compression circuit logarithmically compresses a first input. The logarithmic compression circuit comprises a first amplifier, a first diode for logarithmic compression, a second diode for temperature compensation, and a current source. The first amplifier amplifies the first input. The first diode is placed in the feedback circuit of the first amplifier. The second diode has the same temperature characteristics as the first diode and is placed in a position to cancel out the characteristic change of the first diode due to temperature changes. The current source flows an idling current of the same value to the first diode and the second diode, increasing the value of the idling current as the temperature rises and decreasing the value of the idling current as the temperature falls.

[0008] As a result, the temperature characteristics of the first diode can be canceled out by using the second diode. Furthermore, by increasing the idling current value as the temperature rises, the change in the forward voltage generated in the first diode can be made closer to constant, thus canceling out fluctuations caused by the slope of the V / I curve differing with temperature. In this way, a logarithmic compression circuit with excellent temperature compensation can be realized.

[0009] In the logarithmic compression circuit described above, it is preferable that the current source changes the idling current such that the slope of the V / I curve when only the idling current flows through the first diode is substantially the same regardless of temperature.

[0010] This allows for accurate cancellation of variations caused by the different slopes of the V / I curve depending on temperature.

[0011] In the logarithmic compression circuit described above, the following configuration is preferable. That is, a first current and a second current obtained by adding a fixed value to the first current are assumed. The current source changes the idling current so that the difference between the forward voltage when the first current flows through the first diode and the forward voltage when the second current flows through the first diode is substantially the same regardless of temperature.

[0012] This allows for the detection of voltage differences without being affected by temperature changes.

[0013] In the logarithmic compression circuit described above, the following configuration is preferable. That is, the logarithmic compression circuit comprises a second amplifier and a comparator. The comparator compares the output of the first amplifier with the output of the second amplifier and outputs the comparison result. The second diode is placed in the feedback circuit of the second amplifier.

[0014] This ensures that the comparison results output by the comparator are independent of temperature.

[0015] In the logarithmic compression circuit described above, the following configuration is preferable. That is, the logarithmic compression circuit includes a second amplifier which is a differential amplifier. The first amplifier is a differential amplifier. The cathode of the second diode and the feedback circuit of the second amplifier are connected to the input terminal of the second amplifier. The first input and the output terminal of the second amplifier are connected to the input terminal of the first amplifier. The output of the first amplifier is used as the output of the circuit.

[0016] This allows the present invention to be applied to logarithmic compression circuits that use two differential amplifiers.

[0017] In the logarithmic compression circuit described above, it is preferable to include a base current cancellation circuit that generates a current for canceling the influence of the base current of the first amplifier and supplies the generated current to the first amplifier.

[0018] Thereby, the influence of the base current can be reduced.

[0019] In the logarithmic compression circuit described above, it is preferable that the current source includes a first resistance change portion having a temperature coefficient in which the resistance value decreases as the temperature rises and increases as the temperature drops.

[0020] Thereby, the current value can be increased as the temperature rises without performing temperature measurement and current change control.

[0021] In the logarithmic compression circuit described above, it is preferable to adopt the following configuration. That is, the current source includes a second resistance change portion having a temperature coefficient in which the resistance value increases as the temperature rises and decreases as the temperature drops. By combining the first resistance change portion and the second resistance change portion, the current value of the idling current is increased as the temperature rises.

[0022] Thereby, even if it is difficult to change the idling current with a desired accuracy using only the first resistance change portion, the idling current can be changed with the desired accuracy by also using the second resistance change portion.

[0023] In the logarithmic compression circuit described above, it is preferable that the current source includes a current mirror circuit and outputs the current copied by the current mirror circuit as the idling current.

[0024] Thereby, the idling current input to the first amplifier can be stabilized, and thus the deterioration of the responsiveness of the first amplifier can be suppressed.

[0025] In the logarithmic compression circuit described above, it is preferable to have the following configuration. That is, the logarithmic compression circuit includes a photodiode. The first input is the output of the photodiode.

[0026] Thereby, in a logarithmic compression circuit that detects light, a configuration excellent in temperature compensation function can be realized.

[0027] In the logarithmic compression circuit described above, it is preferable to include a dark current cancellation circuit that cancels the dark current of the photodiode.

[0028] Thereby, the influence of the dark current can be reduced, and a logarithmic compression circuit with higher temperature compensation accuracy can be realized.

Brief Description of the Drawings

[0029] [Figure 1] Circuit diagram of a logarithmic compression circuit according to an embodiment of the present invention. [Figure 2] A diagram showing Graph 2-1 indicating the temperature characteristics of the first diode and Graph 2-2 indicating the V / I curve after canceling the temperature characteristics. [Figure 3] A diagram showing Graph 3-1 obtained by magnifying a part of Graph 2-2 and Graph 3-2 indicating the variation in the sensitivity of the logarithmic compression circuit. [Figure 4] A diagram showing Graph 4-1 explaining maintaining the slope of the V / I curve by changing the idle current and Graph 4-2 with a constant sensitivity. [Figure 5] Circuit diagram of a logarithmic compression circuit including a base current cancellation circuit. [Figure 6] Detailed circuit diagram of a current source including a first resistance change part. [Figure 7] A diagram showing the detailed circuit diagram of a current source including a second resistance change part and the detailed circuit diagram of a current source including a third resistance change part. [Figure 8] Circuit diagram of a logarithmic compression circuit including a dark current cancellation circuit. [Figure 9] Circuit diagram of a logarithmic compression circuit with a different configuration. [Modes for carrying out the invention]

[0030] Next, embodiments of the present invention will be described with reference to the drawings. First, the logarithmic compression circuit 1 of this embodiment will be described with reference to Figure 1.

[0031] Logarithmic compression circuit 1 is a circuit that converts the amplitude of an input signal into a logarithmic value and outputs it. By using logarithmic compression circuit 1, the dynamic range of the input signal can be compressed, making it possible to handle input signals with large amplitude changes. Hereafter, the input signal to logarithmic compression circuit 1 may be referred to as the first input.

[0032] The logarithmic compression circuit 1 in this embodiment is a circuit for light detection. Therefore, the first input is the photocurrent (photodetection signal) output by the photodiode. Note that the logarithmic compression circuit 1 is applicable to circuits other than those for light detection. For example, the logarithmic compression circuit 1 may be a sensor that detects something other than light, and the circuit may process the signal that is output as a current or voltage as the detection result. Alternatively, the logarithmic compression circuit 1 may be a circuit that processes a current signal or voltage signal output by a circuit other than a sensor.

[0033] As shown in Figure 1, the logarithmic compression circuit 1 comprises, as a basic configuration, a photodiode 11, a first amplifier 12, and a first diode 13.

[0034] The photodiode 11 detects light and generates a photocurrent (Ipd). The cathode side of the photodiode 11 is connected to the first amplifier 12.

[0035] The first amplifier 12 is an amplifier composed of, for example, a bipolar transistor. The first amplifier 12 amplifies the input signal (i.e., the first input) and outputs it. In this embodiment, the first amplifier 12 has one input terminal, but the present invention can also be applied to an amplifier with two input terminals (differential amplifier) ​​(details will be described later).

[0036] The first diode 13 is a diode for logarithmic compression. The first diode 13 is placed in the feedback circuit of the output of the first amplifier 12. Specifically, the anode side of the first diode 13 is connected to the output terminal of the first amplifier 12. Logarithmic compression is achieved with this configuration.

[0037] The logarithmic compression circuit 1 includes a dummy photodiode 21, a second amplifier 22, and a second diode 23 as a compensation configuration.

[0038] The dummy photodiode 21 is a photodiode covered with a light-blocking material and does not generate a photocurrent. The dummy photodiode 21 has the same characteristics as the photodiode 11. Therefore, the dummy photodiode 21 generates the same noise as the photodiode 11 and is affected in the same way as the photodiode 11 when temperature changes occur. In this embodiment, the dummy photodiode 21 is used to cancel the characteristics of the photodiode 11. Note that the dummy photodiode 21 is not an essential component and can be omitted.

[0039] The second amplifier 22 has the same configuration and characteristics as the first amplifier 12. Therefore, the second amplifier 22 generates the same noise as the first amplifier 12 and is affected in the same way as the first amplifier 12 when temperature changes occur. In this embodiment, the second amplifier 22 is used to cancel out the characteristics of the first amplifier 12.

[0040] The second diode 23 is placed in the feedback circuit of the second amplifier 22. The second diode 23 has the same temperature characteristics as the first diode 13. The second diode 23 is a temperature compensation diode and cancels out the temperature characteristics of the first diode 13.

[0041] The logarithmic compression circuit 1 further includes a comparator 14. The comparator 14 has two input terminals and outputs the result of comparing two input signals. The output of the comparator 14 corresponds to the output of the logarithmic compression circuit 1. Specifically, the output terminal of the first amplifier 12 is connected to the inverting input terminal of the comparator 14. The output terminal of the second amplifier 22 is connected to the non-inverting input terminal of the comparator 14. In this embodiment, the comparator 14 outputs a signal of a predetermined voltage if the difference between the two input voltage signals exceeds a threshold, and does not output a signal if the difference is less than or equal to the threshold.

[0042] Here, the circuit consisting of photodiode 11, first amplifier 12, and first diode 13 is substantially identical to the circuit consisting of dummy photodiode 21, second amplifier 22, and second diode 23, except that dummy photodiode 21 is a dummy. Therefore, the difference between the output of the first amplifier 12 and the output of the second amplifier 22 corresponds, in other words, to the photodetection signal of photodiode 11 (more specifically, the signal after voltage conversion of the photocurrent) after canceling out the effects of various noises and temperature changes. In other words, logarithmic compression circuit 1 is a circuit that detects whether the amount of light received exceeds a reference value. Note that the output of logarithmic compression circuit 1 is not limited to this. For example, the photodetection signal of photodiode 11 obtained by taking the difference may be used as the output.

[0043] Furthermore, the logarithmic compression circuit 1 is equipped with a current source 30. The current source 30 generates an idling current (Iid) and supplies it to the logarithmic compression circuit 1. Specifically, the current source 30 supplies an idling current to the first diode 13 and an idling current to the second diode 23. The current values ​​of these idling currents are the same. By supplying an idling current, the responsiveness of the IV conversion in each diode can be increased. The current source 30 increases the current value of the idling current as the temperature rises and decreases the current value of the idling current as the temperature falls. The configuration for changing the current value of the idling current and its purpose will be described later.

[0044] Note that the circuit configuration of the logarithmic compression circuit 1 in this embodiment is just one example, and it can be replaced with another circuit configuration that achieves the same or similar functions.

[0045] Next, with reference to Figures 2 to 4, we will explain the temperature compensation implemented in the logarithmic compression circuit 1.

[0046] Graph 2-1 in Figure 2 shows the V / I curve of the first diode 13. The V / I curve is a curve that correlates the current flowing through the first diode 13 with its forward voltage. Since the first diode 13 has temperature characteristics, it exhibits different V / I values ​​depending on the temperature. Specifically, the forward voltage for the same current decreases as the temperature rises. As mentioned above, the first diode 13 and the second diode 23 have the same temperature characteristics, so their V / I curves are identical. Therefore, by taking the difference in their forward voltages, the temperature characteristics of the first diode 13 can be canceled out.

[0047] The horizontal axis of Graph 2-2 shown in Figure 2 represents the current flowing through the first diode 13. The vertical axis of Graph 2-2 represents the forward voltage shown by the V / I curve, minus the forward voltage at 1000nA for each temperature. As a result, at 1000nA, the value on the vertical axis is naturally 0 regardless of temperature. However, in ranges other than 1000nA, the value on the vertical axis differs depending on the temperature. This is because the slope of the V / I curve differs depending on the temperature. Specifically, the slope of the V / I curve becomes steeper as the temperature increases. This is because the dynamic resistance of the diode increases as the temperature increases due to temperature characteristics.

[0048] In this embodiment, the current values ​​flowing through the first diode 13 and the second diode 23 are different. Specifically, the first diode 13 carries a current that is the sum of the idling current and the photocurrent. The second diode 23 carries only the idling current. In other words, the current values ​​flowing through the two diodes are different. As a result, the characteristics of the logarithmic compression circuit 1 change due to the change in the slope of the V / I curve depending on the temperature.

[0049] For example, as shown in Graph 3-1 of Figure 3, when the temperature is 25°C, the idling current is 1000nA, and the photocurrent is 200nA, the difference in voltage detected by the comparator 14 exceeds the threshold. In other words, at 25°C, the difference between the forward voltage when 1200nA flows through the first diode 13 and the forward voltage when 1000nA flows through the second diode 23 is the voltage equivalent to the threshold.

[0050] However, as shown in Graph 3-1, when the temperature is -40°C, the difference at 1200nA is lower than the voltage equivalent to the threshold, so the output of comparator 14 does not switch. In other words, as shown in Graph 3-2, a current greater than 200nA is required to reach the threshold, so the sensitivity of logarithmic compression circuit 1 changes to a low value. On the other hand, when the temperature is 85°C, the difference becomes equivalent to the threshold with a current value smaller than 1200nA. In other words, as shown in Graph 3-2, a current smaller than 200nA is sufficient to reach the threshold, so the sensitivity of logarithmic compression circuit 1 changes to a high value. In short, the sensitivity of logarithmic compression circuit 1 fluctuates due to the slope of the V / I curve changing with temperature.

[0051] In contrast, in this embodiment, the above fluctuations are suppressed by changing the current source 30 in accordance with the temperature. Specifically, the current source 30 increases the current value of the idling current as the temperature rises. Since the slope of the V / I curve is large when the temperature is high, it is shifted to a high current range where the slope of the V / I curve is small. For example, as shown in graph 4-1 of Figure 4, at 85°C the slope of the V / I curve is larger than at 25°C so it is shifted to a high current range where the slope of the V / I curve is small. As a result, the slope of the V / I curve, which has increased in accordance with the temperature rise, can be canceled out by shifting the current range, and fluctuations in the sensitivity of the logarithmic compression circuit 1 can be suppressed.

[0052] On the other hand, the current source 30 reduces the idling current value as the temperature decreases. At low temperatures, the slope of the V / I curve is small, so it is shifted to a low current range where the slope of the V / I curve is larger. For example, as shown in graph 4-1 of Figure 4, at -40℃ the slope of the V / I curve is smaller than at 25℃, so it is shifted to a low current range where the slope of the V / I curve is larger. In this way, the decrease in the slope of the V / I curve due to the temperature decrease can be canceled out by shifting the current range, so that fluctuations in the sensitivity of the logarithmic compression circuit 1 can be suppressed.

[0053] In detail, it is preferable that the current source 30 changes the idling current so that the slope of the V / I curve when only the idling current flows through the first diode 13 is substantially the same regardless of temperature. This effectively cancels out the fluctuations in the sensitivity of the logarithmic compression circuit 1 caused by the different slopes of the V / I curve depending on the temperature. Alternatively, from another perspective, the current source 30 changes the idling current so that the difference between the forward voltage when a current (first current) flows through the first diode 13 and the forward voltage when a current of a different value (second current) flows through the first diode 13 is substantially the same regardless of temperature.

[0054] As described above, as shown in Graph 4-2 of Figure 4, the sensitivity of the logarithmic compression circuit 1 does not change even when the temperature changes, thus enabling temperature compensation.

[0055] Next, with reference to Figures 5 to 9, details or modified examples of the logarithmic compression circuit 1 will be described.

[0056] Figure 5 shows details of the first amplifier 12 and the second amplifier 22, as well as the base current cancellation circuit 40.

[0057] As shown in Figure 5, the first amplifier 12 and the second amplifier 22 are bipolar transistors. Therefore, it is necessary to supply a base current. However, supplying a base current changes the current flowing through the first diode 13, which changes the sensitivity of the logarithmic compression circuit 1. To suppress this effect, the base current cancellation circuit 40 generates a current of the same magnitude as the base current and flows it in the opposite direction to the base current flowing through the first diode 13, thereby reducing or eliminating the effect of the base current. The base current cancellation circuit 40 is, for example, a current mirror circuit. However, the base current cancellation circuit 40 is not limited to a current mirror circuit, and other known base cancellation circuits can be used.

[0058] Figure 6 shows details of the current source 30.

[0059] As shown in Figure 6, the current source 30 comprises a constant voltage source 31, a differential amplifier 32, a first resistance change unit 33, and a current mirror circuit 34.

[0060] The constant voltage source 31 is connected to the non-inverting input terminal of the differential amplifier 32. The first resistance change unit 33 is connected to the inverting input terminal of the differential amplifier 32. The first resistance change unit 33 has a negative temperature coefficient. Specifically, the first resistance change unit 33 has a temperature coefficient in which the resistance decreases with increasing temperature and increases with decreasing temperature. The output of the differential amplifier 32 is input to the current mirror circuit 34.

[0061] The current mirror circuit 34 is a circuit that copies the current generated by another circuit. In this embodiment, the current mirror circuit 34 copies the current based on the output of the differential amplifier 32 and flows it to the first diode 13 and the second diode 23 as an idling current. By using the current mirror circuit 34, the influence on the other circuit can be reduced. The mirror ratio of the current mirror circuit 34 is arbitrary, but it is preferably less than 1, for example. This makes it possible to create a large current in another circuit and then copy it while reducing the current value when it is copied by the current mirror circuit 34.

[0062] With the above configuration, the resistance decreases as the temperature rises, causing the voltage output by the differential amplifier 32 to increase. As a result, the current value of the current input to and copied by the current mirror circuit 34 increases. Furthermore, by using the current mirror circuit 34, a configuration that changes the idling current due to temperature changes can be realized using only the circuit configuration. Therefore, in this embodiment, a temperature sensor is not required, nor is a program that changes the current value according to the detection result of the temperature sensor. However, a current source 30 may be realized using a temperature sensor and a program.

[0063] The first resistance changing section 33 is, for example, a polysilicon resistor or a carbon resistor. Some polysilicon or carbon resistors have a negative temperature coefficient. The first resistance changing section 33 is, for example, a circuit combining a diode and a diffusion resistor having a positive temperature coefficient. The two may be connected in series, in parallel, or a combination of series and parallel connections. This allows the temperature coefficient, which would be too large with just the diode, to be reduced and adjusted by the diffusion resistor. A Zener diode may be used instead of the diode.

[0064] Figure 7(a) shows the circuit in Figure 6, plus a first current source 35 and a second resistance change unit 36. The first current source 35 generates a current directed toward the non-inverting input terminal of the differential amplifier 32. The second resistance change unit 36 ​​has a positive temperature coefficient. Specifically, the second resistance change unit 36 ​​has a temperature coefficient such that its resistance increases with increasing temperature and decreases with decreasing temperature. The second resistance change unit 36 ​​is positioned between the constant voltage source 31 and the non-inverting input terminal of the differential amplifier 32. As a result, even if it is difficult to change the idling current with the desired precision using only the first resistance change unit 33, the idling current can be changed with the desired precision by using the second resistance change unit 36 ​​in conjunction with it.

[0065] In the circuit shown in Figure 7(b), a second current source 37 is installed instead of the first current source 35, and a third resistance change unit 38 is installed instead of the second resistance change unit 36. The second current source 37 generates a current that flows from the differential amplifier 32 to the third resistance change unit 38. The third resistance change unit 38 has a positive temperature coefficient. Even in this circuit configuration, if it is difficult to change the idling current with the desired precision using only the first resistance change unit 33, the idling current can be changed with the desired precision by using the third resistance change unit 38 in conjunction with it.

[0066] Figure 8 shows a circuit that includes a dark current cancellation circuit 50 in addition to the circuit diagram in Figure 1.

[0067] In low-temperature or room-temperature environments, the dark current is very small. However, as the temperature rises, the dark current increases exponentially. The dark current, like the base current described above, affects the current flowing through the first diode 13. Therefore, by canceling the dark current, changes in the sensitivity of the logarithmic compression circuit 1 can be suppressed. Circuits for canceling dark current are known, but they can be implemented, for example, by a photodiode and current mirror circuit having the same characteristics as the photodiode 11.

[0068] The temperature compensation circuit of this embodiment can be applied to circuits other than the logarithmic compression circuit 1 shown in Figure 1. For example, the temperature compensation circuit of this embodiment is also applied to the logarithmic compression circuit 1 shown in Figure 9.

[0069] In the logarithmic compression circuit 1 shown in Figure 9, a first differential amplifier 16 is used instead of the first amplifier 12. The arrangement of the photodiode 11 and the first diode 13 is the same as in the logarithmic compression circuit 1 shown in Figure 1. Therefore, the first diode 13 is a diode for logarithmic compression. The output of the first differential amplifier 16 corresponds to the output of the logarithmic compression circuit 1.

[0070] Furthermore, a second differential amplifier 26 is placed in place of the second amplifier 22. The second differential amplifier 26 functions as a buffer. In the logarithmic compression circuit 1 of Figure 9, no elements including the second diode 23 are placed in the feedback circuit of the second differential amplifier 26. The anode side of the second diode 23 is connected to the constant voltage source 25. The cathode side of the second diode 23 is connected to the non-inverting input terminal of the second differential amplifier 26. The output terminal of the second differential amplifier 26 is connected to the non-inverting input terminal of the first differential amplifier 16.

[0071] In this configuration, the voltage drop across the second diode 23 and the voltage drop across the first diode 13 cancel each other out, thus canceling out the temperature characteristics. Also, in the logarithmic compression circuit 1 of Figure 9, the current source 30 supplies an idling current to the first diode 13 and the second diode 23. This also cancels out fluctuations caused by the slope of the V / I curve varying with temperature. Thus, the second diode 23 may be placed at any position as long as it cancels out the temperature characteristics of the first diode 13.

[0072] As described above, the logarithmic compression circuit 1 of this embodiment performs logarithmic compression on the first input. The logarithmic compression circuit 1 comprises a first amplifier 12, a first diode 13 for logarithmic compression, a second diode 23 for temperature compensation, and a current source 30. The first amplifier 12 amplifies the first input. The first diode 13 is placed in the feedback circuit of the first amplifier 12. The second diode 23 has the same temperature characteristics as the first diode 13 and is placed in a position to cancel out the characteristic change of the first diode 13 due to temperature changes. The current source 30 flows an idling current of the same value to the first diode 13 and the second diode 23, increasing the value of the idling current as the temperature rises and decreasing the value of the idling current as the temperature falls.

[0073] As a result, the temperature characteristics of the first diode 13 can be canceled by using the second diode 23. Furthermore, by increasing the idling current value as the temperature rises, the amount of change in the forward voltage generated in the first diode 13 can be made closer to constant, thus canceling out fluctuations caused by the slope of the V / I curve differing with temperature. Thus, a logarithmic compression circuit 1 with excellent temperature compensation function can be realized.

[0074] In the logarithmic compression circuit 1 of this embodiment, the current source 30 changes the idling current so that the slope of the V / I curve when only the idling current flows through the first diode 13 is substantially the same regardless of temperature.

[0075] This allows for accurate cancellation of variations caused by the different slopes of the V / I curve depending on temperature.

[0076] In the logarithmic compression circuit 1 of this embodiment, a first current and a second current obtained by adding a fixed value to the first current are assumed. The current source 30 changes the idling current so that the difference between the forward voltage when the first current flows through the first diode 13 and the forward voltage when the second current flows through the first diode 13 is substantially the same regardless of temperature.

[0077] This allows for the detection of voltage differences without being affected by temperature changes.

[0078] The logarithmic compression circuit 1 of this embodiment (Figure 1) comprises a second amplifier 22 and a comparator 14. The comparator 14 compares the output of the first amplifier 12 with the output of the second amplifier 22 and outputs the comparison result. The second diode 23 is placed in the feedback circuit of the second amplifier 22.

[0079] As a result, the comparison result output by comparator 14 will be a value that is independent of temperature.

[0080] The logarithmic compression circuit 1 of this embodiment (Figure 9) includes a second differential amplifier 26. The first differential amplifier 16 is placed in place of the first amplifier 12. The cathode side of the second diode 23 and the feedback circuit of the second differential amplifier 26 are connected to the input terminal of the second differential amplifier 26. The first input and the output terminal of the second differential amplifier 26 are connected to the input terminal of the first differential amplifier 16. The output of the first differential amplifier 16 is used as the output of the logarithmic compression circuit 1.

[0081] This allows the present invention to be applied to logarithmic compression circuits 1 of the type that use two differential amplifiers (first differential amplifier 16, second differential amplifier 26).

[0082] The logarithmic compression circuit 1 of this embodiment includes a base current cancellation circuit 40 that generates a current to cancel the effect of the base current of the first amplifier 12 and supplies it to the first amplifier 12.

[0083] This reduces the influence of the base current.

[0084] In the logarithmic compression circuit 1 of this embodiment, the current source 30 includes a first resistance change section 33 having a temperature coefficient that decreases in resistance as the temperature rises and increases in resistance as the temperature falls.

[0085] This allows the current value to increase in accordance with the rise in temperature without the need for temperature measurement or current change control.

[0086] In the logarithmic compression circuit 1 of this embodiment, the current source 30 includes a second resistance change unit 36 ​​having a temperature coefficient that increases resistance with increasing temperature and decreases resistance with decreasing temperature. The combination of the first resistance change unit 33 and the second resistance change unit 36 ​​increases the current value of the idling current as the temperature rises.

[0087] As a result, even if it is difficult to change the idling current with the desired precision using only the first resistance changing unit 33, the idling current can be changed with the desired precision by using the second resistance changing unit 36 ​​in conjunction with it.

[0088] In the logarithmic compression circuit 1 of this embodiment, the current source 30 includes a current mirror circuit 34, and outputs the current copied by the current mirror circuit 34 as an idling current.

[0089] This stabilizes the idling current input to the first amplifier 12, thereby suppressing the deterioration of the responsiveness of the first amplifier 12.

[0090] The logarithmic compression circuit 1 of this embodiment includes a photodiode 11. The first input is the output of the photodiode 11.

[0091] This makes it possible to achieve a configuration with excellent temperature compensation in the light detection logarithmic compression circuit 1.

[0092] The logarithmic compression circuit 1 of this embodiment includes a dark current cancellation circuit 50 that cancels the dark current of the photodiode 11.

[0093] This reduces the effects of dark current, enabling the realization of a logarithmic compression circuit 1 with even higher temperature compensation accuracy. [Explanation of Symbols]

[0094] 1. Logarithmic compression circuit 11 Photodiode 12. First Amplifier 13. First Diode 16. First differential amplifier (first amplifier) 21 Dummy photodiode 22. Second Amplifier 23. Second Diode 26. Second differential amplifier (second amplifier) 30 current source

Claims

1. In a logarithmic compression circuit that performs logarithmic compression on the first input, A first amplifier that amplifies the first input, A first diode for logarithmic compression is placed in the feedback circuit of the first amplifier, A second diode for temperature compensation is provided, which has the same temperature characteristics as the first diode and is positioned to cancel out the characteristic change of the first diode due to temperature changes. A current source that supplies the same idling current to the first diode and the second diode, increases the value of the idling current as the temperature rises, and decreases the value of the idling current as the temperature falls, A logarithmic compression circuit characterized by comprising the following features.

2. A logarithmic compression circuit according to claim 1, A logarithmic compression circuit characterized in that the current source changes the idling current such that the slope of the V / I curve when only the idling current flows through the first diode is substantially the same regardless of temperature.

3. A logarithmic compression circuit according to claim 1, Assuming a first current and a second current obtained by adding a fixed value to the first current, A logarithmic compression circuit characterized in that the current source changes the idling current such that the difference between the forward voltage when the first current flows through the first diode and the forward voltage when the second current flows through the first diode is substantially the same regardless of temperature.

4. A logarithmic compression circuit according to claim 1, The second amplifier and A comparator that compares the output of the first amplifier and the output of the second amplifier and outputs the comparison result, Equipped with, The logarithmic compression circuit is characterized in that the second diode is placed in the feedback circuit of the second amplifier.

5. A logarithmic compression circuit according to claim 1, It is equipped with a second amplifier which is a differential amplifier, The first amplifier is a differential amplifier, The cathode of the second diode and the feedback circuit of the second amplifier are connected to the input terminal of the second amplifier. The input terminal of the first amplifier is connected to the first input and the output terminal of the second amplifier. A logarithmic compression circuit characterized in that the output of the first amplifier is used as the output of the circuit.

6. A logarithmic compression circuit according to claim 1, A logarithmic compression circuit characterized by comprising a base current cancellation circuit that generates a current to cancel the effect of the base current of the first amplifier and supplies it to the first amplifier.

7. A logarithmic compression circuit according to claim 1, The logarithmic compression circuit is characterized in that the current source comprises a first resistance changing section having a temperature coefficient that decreases in resistance with increasing temperature and increases in resistance with decreasing temperature.

8. A logarithmic compression circuit according to claim 7, The current source includes a second resistance changing section having a temperature coefficient that increases in resistance with increasing temperature and decreases in resistance with decreasing temperature. A logarithmic compression circuit characterized by increasing the current value of the idling current as the temperature rises by combining the first resistance changing section and the second resistance changing section.

9. A logarithmic compression circuit according to claim 1, The logarithmic compression circuit is characterized in that the current source includes a current mirror circuit and outputs the current copied by the current mirror circuit as the idling current.

10. A logarithmic compression circuit according to claim 1, Equipped with a photodiode, A logarithmic compression circuit characterized in that the first input is the output of the photodiode.

11. A logarithmic compression circuit according to claim 10, A logarithmic compression circuit characterized by comprising a dark current cancellation circuit for canceling the dark current of the photodiode.

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