JP2011216951A - Current control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a current control circuit extracting a signal current at a small scale and with a low power consumption.SOLUTION: A current control circuit 100 has a photodiode PD as a power source, a preparation circuit 102, a control circuit 104, and a removal circuit 106. The control circuit 104 supplies a first current to the preparation circuit 102 in a preparation period, and supplies a second current to a path connecting the removal circuit 106 and the photodiode PD in a reception period. In the preparation period, the preparation circuit 102 is charged by a direct current component included in the first current supplied from the current source. In the reception period, the removal circuit 106 separates an alternate current component (signal current) from the current by subtracting the direct current component from the second current supplied from the current source.

Description

  The present invention relates to a current control circuit that photoelectrically converts an optical signal and extracts a signal current.

  The optical signal is photoelectrically converted into a current signal (I) by an optical device such as a photodiode. The current signal (I) is converted into a voltage signal (V) by a current-voltage conversion device such as a trans-impedance amplifier (hereinafter referred to as “TIA”).

  Usually, the incident light to the photodiode includes not only an optical signal component but also a disturbance light component due to room light or sunlight. If the incident light is photoelectrically converted as it is, not only the optical signal component but also the disturbance light component is photoelectrically converted, so a large dynamic range of the TIA must be ensured. Since the disturbance light component is an unnecessary component, it is desirable to remove the disturbance light component from the incident light and generate a current signal corresponding to only the optical signal component.

JP-A-4-32307

  FIG. 6 is a circuit diagram of a conventional AC amplifier. FIG. 6 corresponds to FIG. This AC amplifier includes photodiodes FD1 and FD2 and a current mirror circuit ICM1. A current I1 is generated from the photodiode FD1 and is mirrored as a current I3. The current I1 includes a direct current component corresponding to the disturbance light component and an alternating current component corresponding to the optical signal component. Due to the filter effect of the capacitor C2, the current I3 is only a DC component (disturbance light component). On the other hand, a current I2 equal to the current I1 is generated from the photodiode FD2. Since the output current I4 of the current mirror circuit ICM1 = I2 (AC component + DC component) −I3 (DC component), only the AC component (optical signal component) is output from the current mirror circuit ICM1.

  Since this circuit is provided with two photodiodes, the circuit scale tends to increase. Further, since the current I1 and the current I2 including a direct current component (disturbance light component) flow simultaneously, it is not preferable in terms of power consumption.

  FIG. 2 of Patent Document 1 also discloses an AC amplifier that uses only one photodiode. However, the AC amplifier of FIG. 2 has three sets of current mirror circuits, and is also difficult to reduce in size. In addition, since a current including a direct current component flows through a plurality of paths simultaneously, power consumption tends to increase.

  The present invention has been completed in view of the above problems, and a main object thereof is to provide a current control circuit that extracts a signal current with a small scale and low power consumption.

  The current control circuit according to the present invention has a current source, a preparation circuit, a removal circuit, and a path for supplying the first current from the current source to the preparation circuit in the preparation period and connecting the removal circuit and the current source in the reception period. A control circuit is provided for supplying a second current from the current source. The preparation circuit is charged with the direct current component included in the first current during the preparation period, and the removal circuit extracts the alternating current component by subtracting the direct current component retained by the charge from the second current during the reception period. To do.

  According to such an aspect, in a situation where a current in which an AC component is superimposed on a DC component is supplied, the AC component is extracted during the reception period by measuring and holding the magnitude of the DC component during the preparation period. It becomes easy to do. In addition, since the direct current component alternately flows through the preparation circuit and the removal circuit, it becomes easy to suppress power consumption of the entire circuit. In particular, it is effective when separating a DC component generated by a disturbance and a signal current indicated as an AC component. “Supplying from the current source” does not mean that the direction of current flow is limited only to the outflow direction when viewed from the current source, but also includes the inflow direction.

  The preparatory circuit may charge the capacitor with a DC component, and the removal circuit may separate the AC component by subtracting the DC component generated by the voltage held by the capacitor from the current. In addition, the control circuit connects the current source and the preparation circuit during the preparation period and cuts off the connection between the preparation circuit and the removal circuit, and connects the current source and the removal circuit and connects the preparation circuit and the removal circuit during the reception period. May be.

  The current source may be a light receiving element. The light receiving element may generate first and second currents according to the irradiation light.

  The direct current component may be a current component generated by ambient light. The alternating current component may be a current component generated by an optical signal. Further, a voltage conversion circuit for converting the AC component into a voltage may be provided after the removal circuit.

  According to the present invention, it becomes easy to realize a current control circuit capable of extracting a signal current with a small scale and low power consumption.

It is a circuit diagram of the current control circuit in the first embodiment. It is a time chart which shows the change of the input / output in a current control circuit. It is a circuit diagram of the current control circuit in a 2nd embodiment. It is a circuit diagram of the current control circuit in a 3rd embodiment. It is a circuit diagram of the current control circuit in a 4th embodiment. It is a circuit diagram of the conventional AC amplifier.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[First Embodiment]
FIG. 1 is a circuit diagram of a current control circuit 100 in the first embodiment. The current control circuit 100 includes a preparation circuit 102, a control circuit 104, a removal circuit 106, a conversion circuit 108, and a photodiode PD (light receiving element). The photodiode PD is connected to either the preparation circuit 102 or the removal circuit 106 by the switch SW1, and functions as a current source. The preparation circuit 102 and the removal circuit 106 are connected or disconnected by the switch SW2. The control circuit 104 controls the switches SW1 and SW2.

  The preparation circuit 102 includes a transistor Tr1 and a capacitor C. The transistor Tr1 in this embodiment is a MOS (Metal Oxide Semiconductor), and the gate and drain are diode-connected. The gate of the transistor Tr1 is grounded via the capacitor C1. The source of the transistor Tr1 is grounded, and the drain is connected to the photodiode PD via the terminal A and the switch SW1.

  The removal circuit 106 includes a transistor Tr2. The gate of the transistor Tr1 and the gate of the transistor Tr2 are connected via the switch SW2. The source of the transistor Tr2 is grounded. The drain of the transistor Tr2 is connected to the photodiode PD via the terminal B and the switch SW1, and is also connected to the conversion circuit 108. The transistors Tr1 and Tr2 are transistors configured such that the same source / drain current flows with respect to the same gate voltage.

  Conversion circuit 108 includes a TIA 110 and a resistor R. The current I4 flowing through the conversion circuit 108 is converted into a current voltage by the TIA 110, and becomes an output voltage VOUT.

  The current control circuit 100 includes at least two modes of “preparation mode” and “reception mode”. The preparation mode is a mode for detecting the intensity of disturbance light and recording it. On the other hand, the reception mode is a mode for extracting an optical signal component (alternating current component) from incident light. The preparation mode and the reception mode are switched alternately in a very short time of 1 second or less. In the present embodiment, description will be made assuming that switching is performed every 100 msec.

  In the preparation mode, the switch SW1 is connected to the terminal A on the preparation circuit 102 side, and the switch SW2 is turned off. In the reception mode, the switch SW1 is connected to the terminal B on the removal circuit 106 side, and the switch SW2 is turned on.

  The current control circuit 100 is assumed to be mounted on a portable information terminal or the like. For example, data such as music and images is transmitted as an optical signal at a store and received by a portable information terminal, whereby content can be downloaded using the optical signal. However, when receiving an optical signal, disturbance light such as sunlight or room light is also detected. Therefore, it is desirable that only the original optical signal component can be extracted by removing such disturbance light component. In particular, since the amount of sunlight is much larger than the amount of light of the optical signal, it is necessary to remove disturbance light components at the current stage before the voltage VOUT is generated by the conversion circuit 108. Hereinafter, circuit operations in the preparation mode and the reception mode will be described.

<Preparation mode>
The control circuit 104 connects the switch SW1 to the A terminal on the preparation circuit 102 side, and turns off the switch SW2. A current I1 flows through the preparation circuit 102 due to the incident light of the photodiode PD. The current generated by the photodiode PD includes a disturbance light component (DC component) and an optical signal component (AC component). The capacitor C1 functions as a low-pass filter and is charged with an ambient light component (DC component). The removal circuit 106 does not operate because it is separated from the photodiode PD and the preparation circuit 102.

<Reception mode>
The control circuit 104 connects the switch SW1 to the B terminal on the removal circuit 106 side, and turns on the switch SW2. As a result, the photodiode PD and the conversion circuit 108 are directly connected. Since the incident light of the photodiode PD includes a disturbance light component and an optical signal component, the current I3 flowing through the path to the removal circuit 106 also includes a disturbance light component (DC component) and an optical signal component (AC component). . On the other hand, since the switch SW2 is turned on, the gate potential of the transistor Tr2 becomes equal to the potential of the capacitor C1. As a result, a current I2 corresponding to a DC component (disturbance light component) flows through the transistor Tr2. The current I4 flowing through the conversion circuit 108 = I3−I2. Since the current I3 includes a disturbance light component (DC component) and an optical signal component (AC component), and the current I2 includes only a disturbance light component (DC component), the current I4 includes only an optical signal component (AC component). Will be. The conversion circuit 108 performs current-voltage conversion on the current I4 including only the AC component and outputs a voltage signal VOUT. If amplification is not necessary, the conversion circuit 108 may be a simple resistance circuit.

  FIG. 2 is a time chart showing input / output changes in the current control circuit 100. Incident light includes an optical signal component (AC component) and a disturbance light component (DC component). Strictly speaking, the disturbance light component does not have to be direct current, but can be regarded as direct current in the preparation mode period (hereinafter referred to as “preparation period”) and the reception mode period (hereinafter referred to as “reception period”). Any low frequency component may be used. Further, the optical signal component (alternating current component) needs to be a high frequency component that can be regarded as alternating current in the preparation period. For this purpose, it is necessary to appropriately adjust the length of the preparation period. Specifically, it can be adjusted by the switching timing by the control circuit 104 or the capacitance of the capacitor C1.

  In FIG. 2, time t0 to time t1 correspond to the preparation period, time t1 to time t2 correspond to the reception period, and time t2 to t3 correspond to the preparation period. The current control circuit 100 in the present embodiment alternately repeats the preparation mode and the reception mode every 100 msec. Actually, there is a case where an idle period is provided to save power during the period from the end of the reception period to the start of the next preparation period.

  In the preparation mode, the switch SW1 is connected to the A terminal and the switch SW2 is turned off. At this time, a current I1 including a DC component and an AC component flows. On the other hand, since the removal circuit 106 is separated, the currents I2 to I4 become zero and the output voltage VOUT also becomes zero. The capacitor C1 of the preparation circuit 102 is charged by the disturbance light component (DC component) of the current I1.

  In the reception mode, the switch SW1 is connected to the B terminal and the switch SW2 is turned on. Since no current is supplied to the preparation circuit 102, the current I1 becomes zero. On the other hand, since the photodiode PD is connected to the removal circuit 106 and the conversion circuit 108, a current I3 including a direct current component and an alternating current component flows. Further, a current I2 including only a direct current component flows. As a result, an AC component, that is, a current I4 including only the optical signal component flows to the conversion circuit 108, and a voltage signal VOUT including only the optical signal component is output.

  According to the current control circuit 100 in the present embodiment, the current I4 including only the optical signal component is supplied to the conversion circuit 108, so that the dynamic range of the TIA 110 can be used without waste. Even when an optical signal is received in an environment where sunlight is strong, disturbance light components (DC components) can be effectively removed before current-voltage conversion. Further, since only one photodiode PD is used, it can be configured at low cost and on a small scale.

  In the preparation mode, the direct current component appears in the current I1, and in the reception mode, the direct current component appears in the currents I2 and I3. Since the current I1 and the current I2 do not flow at the same time, the power consumption is easily suppressed. Instead of always enabling the two transistors Q3 and Q4 as in the conventional example described with reference to FIG. 6, the power consumption is reduced by operating the transistors Tr1 and Tr2 alternately. The transistors Tr1 and Tr2 may be bipolar transistors instead of MOS.

  The capacitor C1 functions as both a low-pass filter and a voltage holding device. For this reason, it is necessary to determine the capacitance of the capacitor C1 according to the design conditions such as the length of the reception period and the cut-off frequency as the low-pass filter. Note that the capacitor C1 may be unused if such design conditions can be satisfied by the gate capacitance of the transistor Tr1.

Since the environment in which ambient light changes at high speed is usually difficult to imagine, the DC component current hardly changes during the transition from the preparation mode to the reception mode, and the intended effect can be obtained. Further, since it is not necessary to shift from the preparation mode to the reception mode at high speed, the delay to the switches SW1 and SW2 by the capacitor C1 does not affect the operation of the current control circuit 100.
Although FIG. 2 shows a mode in which the preparation mode and the reception mode are repeated, the repetition is not an essential condition, and the reception mode may be executed after the preparation mode and the processing may be terminated as it is.

[Second Embodiment]
FIG. 3 is a circuit diagram of the current control circuit 112 in the second embodiment. The configuration of the current control circuit 112 in the second embodiment is the same as the configuration of the current control circuit 100 in the first embodiment, except for the positional relationship between the switch SW2 and the capacitor C1. In the second embodiment, the transistor Tr1 is connected to the capacitor C1 via the switch SW2.

  In the preparation mode, the switch SW1 is connected to the terminal A, and the switch SW2 is turned on. Thereby, the current I1 flows and the capacitor C1 is charged. At this time, since the gate voltage is also applied to the transistor Tr2, a current I2 including a DC component flows. In the reception mode, the switch SW1 is connected to the terminal B, and the switch SW2 is turned off. A current I2 including a DC component flows due to the voltage held by the capacitor C1. Compared with the first embodiment, the current I2 flows even in the preparation mode, so the power consumption increases. However, in the reception mode, the current I1 can be made zero, so that the power consumption is higher than that in the conventional configuration described with reference to FIG. Can be suppressed.

[Third Embodiment]
FIG. 4 is a circuit diagram of the current control circuit 114 in the third embodiment. The current control circuit 114 in the third embodiment has a configuration in which a transistor Tr3 is added to the current control circuit 100 in the first embodiment.

  In the case of the current control circuit 100 according to the first embodiment, there is a possibility that the high frequency characteristics of the circuit may be degraded due to the influence of the parasitic capacitance of the transistor Tr2. In order to cope with such a problem, in the third embodiment, a transistor Tr3 whose gate and drain are short-circuited is added to the drain side of the transistor Tr2. Since the transistor Tr3 can be designed with a small size, it functions effectively in suppressing the parasitic capacitance component and improving the frequency characteristics.

[Fourth Embodiment]
FIG. 5 is a circuit diagram of the current control circuit 116 in the fourth embodiment. In the first to third embodiments, the photodiode PD is described as a type in which the anode is grounded. In the fourth embodiment, as shown in FIG. 5, it is possible to use a photodiode PD of the type used by connecting the cathode to the power supply VCC.

  The present invention has been described based on the embodiments. It will be understood by those skilled in the art that the embodiments are illustrative, and that various modifications and changes are possible within the scope of the claims of the present invention, and that such modifications and changes are also within the scope of the claims of the present invention. By the way. Accordingly, the description and drawings herein are to be regarded as illustrative rather than restrictive.

  100, 112, 114, 116 Current control circuit, 102 preparation circuit, 104 control circuit, 106 removal circuit, 108 conversion circuit, 110 TIA, PD photodiode, R resistance, SW1, SW2 switch, Tr1-Tr3 transistors.

Claims (7)

A current source, a preparation circuit and a removal circuit;
A control circuit for supplying a first current from the current source to the preparation circuit in a preparation period and supplying a second current from the current source to a path connecting the removal circuit and the current source in a reception period; With
The preparation circuit is charged with a direct current component included in the first current in the preparation period,
The current removal circuit, wherein the removal circuit separates an alternating current component from the second current by subtracting the charged direct current component from the second current in the reception period. The preparatory circuit charges a capacitor with the DC component,
The current control circuit according to claim 1, wherein the removal circuit subtracts the direct current component generated by the voltage held by the capacitor from the second current.   The control circuit connects the current source and the preparation circuit during the preparation period and disconnects the connection between the preparation circuit and the removal circuit, and connects the current source and the removal circuit during the reception period. The current control circuit according to claim 1, wherein the preparation circuit and the removal circuit are connected.   The current control circuit according to claim 3, wherein the current source is a light receiving element, and generates the first and second currents in accordance with irradiation light.   The current control circuit according to claim 4, wherein the direct current component is a current component generated by ambient light.   The current control circuit according to claim 4, wherein the alternating current component is a current component generated by an optical signal.   7. The current control circuit according to claim 1, further comprising a voltage conversion circuit for converting the voltage of the AC component at a subsequent stage of the removal circuit.

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