JPH05209788A - Photocurrent detecting circuit - Google Patents

Abstract

PURPOSE:To eliminate the need for a circuit which detects photocurrent at an anode of a photo diode, reduce input capacity, enlarge a band, or improve maximum input level since no bias voltage is reduced due to resistance when photocurrent is large. CONSTITUTION:When the value of a first resistor 1 is made equal to that of a second resistor 2, collector currents of both transistors become equal since the base potentials of a first transistor 3 and a second transistor are equal, thus constituting a current mirror circuit. Since the collector current of a second transistor 4 is equal to photocurrent of a photo diode 5, the photocurrent of the photo diode 5 can be detected by allowing the current to flow to a detection resistor 8.

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 for an optical receiver, which can eliminate the extra circuit from the photodiode to the preamplifier and reduce the input capacitance to extend the band. ..

[0002]

2. Description of the Related Art As a conventional example, as described in Japanese Patent Application No. 63-246942, a resistor is connected to the anode of the photodiode and the voltage applied to this resistor is measured to flow through the photodiode. The current was being detected.

[0003]

However, in the above conventional technique, the input capacitance of the preamplifier is increased by providing the resistance and the measurement terminal or the measurement element, and as a result, the amplification band is limited. Further, when the photocurrent is large, the voltage generated across the resistor increases, so that the bias voltage of the photodiode decreases and the bandwidth of the photodiode itself is limited.

In the present invention, a current mirror circuit is used as the bias circuit of the photodiode, and the current value is detected so that the input capacitance of the preamplifier is not increased, and the bias voltage is applied even when the photocurrent is large. It is to provide a photocurrent detection circuit that does not decrease.

[0005]

The above-mentioned object is to use a current mirror circuit as a bias circuit of a photodiode and detect the current value thereof so that a photocurrent can be supplied without providing an extra circuit at the input of a preamplifier. Can be detected.

[0006]

By using a current mirror circuit for the bias circuit of the photodiode, a current equal to the current flowing in the photodiode can be obtained. By passing this current through the resistor and measuring the voltage applied to the resistor, the value of the photocurrent flowing through the photodiode can be known.

[0007]

FIG. 1 shows an embodiment of the present invention. In FIG. 1, a bias circuit 7 is connected to the emitter of a PNP type first transistor 3 through a first resistor 1, a base and a collector are connected to each other, and a photodiode 5 is provided.
Connected to the cathode. The optical signal is converted into an electric signal in the photodiode 5 and amplified by the preamplifier 6. A second PNP transistor 4
The bias circuit 7 is connected to the emitter of the transistor through the second resistor 2, the base is connected to the base of the first transistor 1, and the collector is grounded through the detection resistor 8.

If the values of the first resistor 1 and the second resistor 2 are made equal, the collector potentials of both transistors become equal because the base potentials of the first transistor 3 and second transistor are equal. Make up.
Since the collector current of the second transistor 4 becomes equal to the photocurrent of the photodiode 5, this current is detected by the detection resistor 8
The photocurrent of the photodiode 5 can be detected by applying the current to the photodiode.

[0009]

According to the present invention, a circuit for detecting a photocurrent is not required in the anode of the photodiode unlike the conventional method, so that the input capacitance can be reduced and the band can be widened. Since the bias voltage does not drop due to the resistance when the photocurrent is large, the maximum input level can be improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional embodiment.

[Explanation of symbols]

1 ... 1st resistance, 2 ... 2nd resistance, 3 ... 1st transistor, 4 ... 2nd transistor, 5 ... Photodiode, 6 ... Preamplifier, 7 ... Bias circuit, 8 ...
Detection resistor, 9 ... Detection terminal

Claims (1)

[Claims] 1. A photocurrent detection circuit for detecting a current of a photodiode by using a current mirror circuit as a bias circuit of the photodiode in an optical receiver.