JPH0951232A - Photocurrent detection circuit - Google Patents

Abstract

(57) Abstract: An object of the present invention is to provide a photocurrent detection circuit having a high-speed response characteristic. A photocurrent detection circuit of the present invention includes a power supply (3
2), a first transistor (30) whose emitter is connected to a power supply (32), and a first transistor (3)
0) a photoelectric converter (38) connected to the base, a voltage detection circuit (36) and a first resistor (34) each connected to the collector of the first transistor (30),
The photocurrent detection circuit that amplifies the output current of the photoelectric converter (38) by the first transistor (30) and detects it by the voltage detection circuit (36) is the first transistor (3).
0) and is configured to supply a voltage approximately equal to the operating base-emitter voltage of the first transistor (30) by shorting its base and collector. It comprises a transistor (46). In addition, the first transistor (30) from the second transistor (46) via the second resistor (44).
Bias current is supplied to the base of.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocurrent detection circuit, and more particularly to a photocurrent detection circuit used in an optical receiver capable of high-speed response.

[0002]

2. Description of the Related Art FIG. 2 shows the configuration of a conventional photocurrent detection circuit. This circuit is a circuit for detecting the presence / absence of an optical signal input in an optical receiver of an optical transmission system, and includes a current detection transistor 10, a power supply 12 connected to the emitter of the current detection transistor 10, and a current detection transistor 10 respectively. Load resistor 14 connected to the collector of
And the voltage detection circuit 16 and the photodiode 18 whose cathode is connected to the base of the current detection transistor 10.
A capacitor 20 connected between the base of the current detection transistor 10 and the ground, and a photodiode 18
And an amplifier 22 connected to the anode of the.

The photodiode 18 converts an optical signal into an electric signal. The voltage detection circuit 16 is configured to output a photocurrent detection signal when detecting a voltage above a certain level. The amplifier 22 amplifies the signal current output from the anode of the photodiode 18. Capacitor 20
Acts to lower the AC impedance of the cathode of the photodiode 18.

In the conventional photocurrent detection circuit configured as described above, when an optical signal is input to the photodiode 18, the optical signal is converted into an electrical signal and input to the base of the current detection transistor 10. This base current is amplified by the current detection transistor 10 and flows into the load resistor 14. At this time, the voltage detection circuit 16 detects a voltage across both ends of the load resistor 14 when a current of a certain level or more flows, and outputs a photocurrent detection signal. On the other hand, when there is no light input to the photodiode 18, the current detection transistor 10 is completely off, and the base-emitter voltage is almost 0V.

The time T until the current detection transistor 10 is turned on after the optical signal is input to the photodiode 18.
Is as follows, where Von is the on-voltage of the current detection transistor 10 and I is the photocurrent. T = Von · C / I Here, C is the capacitance of the capacitor 20, and includes the parasitic capacitance of the current detection transistor 10 and the photodiode 18. For example, I = 1 μA, Von = 0.7 V, C = 10
At 0 pF, T = 70 μs.

[0006]

However, in the above-described conventional photocurrent detection circuit, the capacitor 20 needs to be charged while the on-voltage of the current detection transistor 10 rises to Von, and therefore the response is increased accordingly. There was a problem that time was delayed.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above conventional problems, and an object of the present invention is to provide a photocurrent detection circuit capable of high-speed response.

[0008]

To achieve the above object, a first transistor whose emitter is connected to a power source, a photoelectric converter connected to the base of the first transistor, and a first transistor are provided. In the photocurrent detection circuit, which includes a voltage detection circuit connected to the collector of the photoelectric conversion device, amplifies the output current of the photoelectric converter with the first transistor, and detects this with the voltage detection circuit The base of the operating state of the first transistor by shorting its base and collector.
A second transistor that outputs a voltage approximately equal to the emitter-to-emitter voltage and a bias resistor that connects the bases of the first and second transistors to each other are provided, and the second transistor is connected to the first transistor via the bias resistor. It is configured to supply a bias current to the base.

The second transistor generates a predetermined voltage, and supplies the bias current to the base of the first transistor through the bias resistor even when the photoelectric current is not output from the photoelectric converter. The predetermined voltage is substantially equal to the base-emitter voltage of the first transistor during operation, and the base-emitter voltage of the first transistor is lower than the voltage during operation by the amount of the voltage drop due to the bias resistance. It is kept at a rather low voltage. Therefore, when the photocurrent starts to flow, the base-emitter voltage of the first transistor starts to rise, but since the rising start voltage is the voltage described above, there is a time limit when the first transistor becomes conductive (ON). It gets shorter.

[0010]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows the configuration of the photocurrent detection circuit according to the embodiment. This circuit includes a current detection transistor 30, a power supply 32 connected to the emitter of the current detection transistor 30, and a load resistor 34 connected to the collector of the current detection transistor 30.
And a voltage detection circuit 36 and a photodiode 38 whose cathode is connected to the base of the current detection transistor 30.
, A capacitor 40 connected between the base of the current detection transistor 30 and the ground, and a photodiode 38.
An amplifier 42 connected to the anode of, and a bias resistor 44 connected to the base of the current detection transistor 30,
An auxiliary transistor 46 having characteristics substantially equal to those of the current detection transistor 30 and having a base and a collector short-circuited;
It is composed of an idle resistor 48 connected to the base-collector short-circuit point of the auxiliary transistor 46.

The photodiode 38 converts an optical signal into an electric signal (photocurrent). The voltage detection circuit 36 detects the terminal voltage of the load resistor 34 based on the photocurrent and outputs a photocurrent detection signal. The amplifier 42 is a photodiode 38.
The transmission signal output from the anode of is amplified. The capacitor 40 reduces the AC impedance of the cathode of the photodiode 38. The auxiliary transistor 46 has its base and collector short-circuited, and supplies an appropriate idle current via the idle resistor 48. Further, since the base of the current detection transistor 30 and the base of the auxiliary transistor 46 are connected by the bias resistor 44, the bias current is always supplied to the current detection transistor 30.
That is, the auxiliary transistor 46 includes the bias resistor 44.
A bias current is supplied to the base of the current detection transistor 30 through the through current, so that the base-emitter voltage when there is no light input is kept at a value slightly lower than the ON voltage.

In the photocurrent detection circuit of the present embodiment configured as described above, the time Tb until the current detection transistor 30 is turned on after the optical signal is input to the photodiode 38 is the time Tb of the current detection transistor 30. When the ON voltage is Von, the base-emitter voltage when the photocurrent of the current detection transistor 30 is not flowing is Vb, and the photocurrent is I, the following results. Tb = (Von−Vb) · C / I Here, C is the capacitance of the capacitor 40 and also includes the parasitic capacitance of the current detection transistor 30 and the photodiode 38. For example, I = 1 μA, Von = 0.7 V, C = 10
If 0 pF, bias current 10 nA, and Vb = 0.65 V, then Tb = 5 μs. At this time, the value of the bias resistor 44 is 5 Mohm.

As described above, according to this circuit, the photocurrent detection speed can be increased about 10 times. In the optical receiver, the photocurrent is generally a small value, and the bias current needs to be smaller than that. This circuit can stably apply this minute bias current with a simple circuit, and according to this, a photocurrent detection circuit with high-speed response can be realized at low cost.

[0014]

As described above, in the photocurrent detection circuit according to the present invention, high-speed response is possible, and since the characteristics of the current detection transistor 30 and the auxiliary transistor 46 are substantially equal, the temperature changes. However, there is an advantage that the bias current becomes constant and the operation becomes stable.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a photocurrent detection circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a photocurrent detection circuit according to a conventional technique.

[Explanation of symbols]

 30 ... Current detection transistor 32 ... Power supply 34 ... Load resistance 36 ... Voltage detection circuit 38 ... Photodiode 40 ... Capacitor 42 ... Amplifier 44 ... Bias resistance 46 ...・ Auxiliary transistor 48 ・ ・ ・ Idle resistance

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/06

Claims (2)

[Claims] 1. A first transistor whose emitter is connected to a power supply, a photoelectric converter connected to the base of the first transistor, and a voltage detection circuit connected to the collector of the first transistor. A photocurrent detection circuit for amplifying an output current of the photoelectric converter by the first transistor and detecting the amplified current by the voltage detection circuit, having a characteristic substantially equal to that of the first transistor, and A second transistor that outputs a voltage substantially equal to the base-emitter voltage of the first transistor in the operating state by shorting the collector; and a bias resistor that connects the bases of the first and second transistors to each other. A bias current is supplied from the second transistor to the base of the first transistor via the bias resistor. A photocurrent detection circuit characterized by being configured to supply. 2. The photocurrent detection device according to claim 1, wherein the second transistor is connected to the other end of an idle resistor whose one end is connected to the reference potential at the terminal point of the base collector. circuit.