The present invention generally relates to an optical communication device, and, more particularly, to a power saving optical communication device and an amplifier of a receiving circuit which converts a current signal according to received light to a voltage signal.
An optical communication device for performing data communication is put to practical use, for example, using infrared rays. The optical communication device includes a receiving circuit for converting received light to a current signal. The receiving circuit includes an amplifier for converting the current signal to a voltage signal and a comparator for converting the voltage signal to a digital signal. In order to improve the receiving accuracy, the receiving circuit sets the current-to-voltage conversion rate in the amplifier and sets the clamping operation point of the current signal.
FIG. 1 is a schematic circuit diagram of a conventional receiving circuit 50. The anode of a photodiode PD is connected to ground GND, and its cathode is connected to an input terminal of an amplifier 11 via an input terminal Pin. The photodiode PD generates a diode current IPD that corresponds to received light. The amplifier 11 converts the diode current IPD to a voltage Vout. A resistor Rf and a clamping circuit 12 are connected in parallel between the I/O terminals of the amplifier 11. The output voltage Vout of the amplifier 11 is supplied to the positive input terminal of a comparator 13 via an output terminal Pout and is supplied to the clamping circuit 12. The output voltage Vout may also be amplified by an amplifier having plural stages to compensate for an insufficient gain.
The clamping circuit 12 may be an npn type bipolar transistor Tr1. The transistor Tr1 has a base for receiving the output voltage Vout, a collector for receiving the voltage of a power supply Vcc, and an emitter connected to the input terminal Pin.
The comparator 13 receives the output voltage Vout supplied to its positive input terminal and a threshold voltage Vth supplied to its negative input terminal and converts the output voltage Vout to a digital signal. The digital signal is supplied to an internal circuit (not illustrated) of the optical communication device as a reception signal RX. The threshold voltage vth varies in accordance with the output voltage Vout.
The photodiode PD, as shown in FIG. 2, generates the diode current IPD that corresponds to the received light. The amplifier 11 converts the diode current IPD to the output voltage Vout. At this time, the output voltage Vout is given by the following equation.
Vout=IPDxc3x97Rf
In other words, the output voltage Vout can be obtained by multiplying the diode current IPD by the resistance of the resistor Rf. The current-to-voltage conversion rate (so-called transformer impedance) of the amplifier 11 is substantially equal to the resistance of the resistor Rf. The comparator 13 converts the output voltage Vout to the digital signal (reception signal) RX.
When the diode current IPD increases, the inter-terminal voltage of the resistor Rf increases. When the diode current IPD exceeds a predetermined value and the inter-terminal voltage of the resistor Rf exceeds the voltage VBE between the base and emitter of the transistor Tr1 (IPDxc3x97Rf greater than VBE), the transistor Tr1 is turned on. Hereupon, the voltage of the power supply Vcc is supplied to the input terminal of the amplifier 11 via the transistor Tr1, the inter-terminal voltage of the resister Rf drops, and the output voltage Vout of the amplifier 11 is substantially clamped to the voltage VBE between the base and the emitter. Thus, when the output voltage Vout increases, the output voltage Vout is clamped to the predetermined clamping voltage VCL (VBE) by the clamping circuit 12.
The transformer impedance and the operation point of the clamping circuit 12 are set by the single resistor Rf. However, when the resistor Rf has a relatively high resistance in order to improve the transformer impedance, following disadvantages (a) and (b) arise.
(a) Disadvantage in High-speed Communication
The operation delay time of the transistor Tr1 is prolonged as the resistance of the resistor Rf increases. Accordingly, when the level of the received light and the diode current IPD are high, the clamping operation of the clamping circuit 12 is delayed. As a result, as shown in FIG. 2, when the output voltage Vout rises, an overshoot is generated and the signal waveform of the output voltage Vout is disturbed. Further, because of the large resistance of the resistor Rf, the falling edge of the output voltage Vout becomes slow, and the comparator 13 outputs a reception signal having a long H-level width.
(b) Disadvantage when a Direct Current Component is Contained in the Diode Current IPD
When natural light is contained in the received light, as shown in FIG. 3, the diode current IPD contains a direct current component IPD-DC. In other words, the diode current IPD is offset by the direct current component IPD-DC. In this case, the output voltage Vout tend to be clamped by the direct current component IPD-DC. That is, the output voltage Vout that should not be clamped is clamped. Accordingly, the output voltage Vout is not obtained accurately and the comparator 13 outputs an erorrneous reception signal RX.
Optical communication devices are installed in electronic devices, such as personal computers, PDA (personal digital assistants), and digital still cameras. To reduce the power consumption of such optical communication devices, an optical communication device that automatically adjusts transmission output levels according to certain factors, such as the communication distance and communication state is proposed. FIG. 4 is a schematic block diagram of a conventional optical communication device 60. The optical communication device 60 includes a receiving circuit 210a and a transmitting circuit 210b. The receiving circuit 210a has a photodiode 211, an amplifier 212, and a comparator 213. The transmitting circuit 210b has a current driver 214 and a light-emitting diode 215.
The current driver 214 converts a transmission signal TX from an internal circuit to a current signal and amplifies the current signal to generate a transmission current Idrv. The light-emitting diode 215 repeats emission and extinction according to the transmission current Idrv. When the emission level of the photodiode 211 is high, an emission control unit 216 determines that the communication distance is short or the communication state is good and controls the current driver 214 so that the emission level of the light-emitting diode 215 decreases. When the received light level is low, the emission level control unit 216 determines that the communication distance is far or the communication state is not preferable and controls the current driver 214 so that the emission level of the light-emitting diode 215 increases. Such control reduces the power consumption of the optical communication device 210.
Specifically, the emission level control unit 216 includes an emission level detection circuit 216a, a control circuit 216b, an arithmetic circuit 216c, and an emission level adjustment circuit 216d. The emission level detection circuit 216a receives a voltage signal VA of the amplifier 212 and supplies a detection signal SG1 that corresponds to the level of the voltage signal VA to the control circuit 216b. The arithmetic circuit 216c receives the detection signal SG1 via the control circuit 216b and calculates the level of the received light. The arithmetic circuit 216c further determines the communication distance and the communication state based on the received light level and determines the emission level and emission timing of the light-emitting diode 215. The control circuit 216b supplies a control signal SG2 to the emission level adjustment circuit 216d based on the determined emission level and emission timing. The emission level adjustment circuit 216d supplies an adjustment signal SG3 to the current driver 214 in accordance with the control signal SG2. The current driver 214 generates the transmission current Idrv while adjusting the self amplification factor in accordance with the adjustment signal SG3.
However, it takes time to calculate the emission level based on the received light level. In particular, the level of the received light easily varies according to the communication distance, the angle of receiving plane, and disturbances. In such a case, calculation of the emission level takes a very long time and high-speed processing of the optical communication device 210 is prevented.
It is an object of the present invention to provide a receiving circuit which generates an accurate voltage signal based on received light.
It is a second purpose of the present invention to provide an optical communication device for enabling high-speed processing while adjusting transmission output levels.
In a first aspect of the present invention, there is provided a receiving circuit including a light receiving element for generating a current signal that corresponds to received light. A current distribution circuit is connected to the light receiving element and distributes the current signal to first and second current signals in accordance with a predetermined distribution ratio. A first amplifier is connected to the current distribution circuit and converts the first current signal to a first voltage signal. A second amplifier is connected to the current distribution circuit and converts the second current signal to a second voltage signal. A current control circuit is connected to the light receiving element and the second amplifier and controls the amount of current of the first current signal in accordance with the second voltage signal.
In a second aspect of the present invention, there is provided a receiving circuit including a light receiving element for generating a current signal that corresponds to received light. A first amplifier is connected to the light receiving element and converts a first current signal that is a part of the current signal to a first voltage signal. A second amplifier is connected to the light receiving element and converts a second current signal that is a remaining part of the current signal to a second voltage signal. The first and second amplifiers include first and second transistors, connected to the light receiving element, the transistors having a size ratio that determine a distribution ratio of the first and second current signals. A current control circuit is connected to the light receiving element and the second amplifier and controls the amount of current of the first current signal in accordance with the second voltage signal.
In a third aspect of the present invention, there is provides a receiving circuit including a light receiving element and a first transistor and a first resistor connected in series between the light receiving element and a predetermined power supply. A second transistor and a second resistor are connected in series with each other and connected in parallel with the first transistor and the first resistor. The gates of the first and second transistors are connected to a reference voltage. A third transistor is connected in parallel with the second transistor and the second resistor. The gate of the third transistor is connected to a node between the second transistor and the second resistor.
In a fourth aspect of the present invention, there is provided an optical communication device including a transmitting circuit and a receiving circuit. The transmitting circuit includes a current driver for converting a transmission signal to a current signal and amplifying the current signal in accordance with a predetermined amplification factor, and a light-emitting diode, connected to the current driver, for emitting light in accordance with the amplified current signal. The receiving circuit includes a light receiving element for generating a reception current that corresponds to received light, an amplifier, connected to the light receiving element, for converting the reception current to a voltage signal, and a comparator, connected to the amplifier, for converting the voltage signal to a digital reception signal. A voltage holding circuit is connected to the amplifier and holds reception information including a peak voltage of the voltage signal. A transmission current control circuit is connected between the voltage holding circuit and the current driver, receives the reception information, and generates a control signal for controlling the predetermined amplification factor of the current driver based on the reception information.
In a fifth aspect of the present invention, there is provided a waveform shaping circuit including a current generation circuit for generating a current signal in response to a pulse signal and a differential circuit for converting the pulse signal to a differential waveform signal. A complementary current generation circuit is connected to the differential circuit and the current generation circuit and generates a complementary current signal that complements the waveform of the current signal in response to the differential waveform signal to generate a waveform-shaped pulse signal.
In a sixth aspect of the present invention, there is provided a waveform shaping circuit including first and second transistors connected in series between a potential of a pulse signal and a predetermined potential, the first transistor responsive to the pulse signal, and the second transistor responsive to a first reference voltage signal. A differential circuit converts the pulse signal to a differential waveform signal. Third and fourth transistors are connected in series between the potential of the pulse signal and the predetermined potential, the third transistor responsive to the differential waveform signal, and the fourth transistor responsive to a second reference voltage signal.