JPH057039A - Field effect transistor logic circuit - Google Patents

Abstract

PURPOSE:To obtain a field effect transistor logic circuit for driving a laser diode, by mounting a circuit which compensates a bias current according to temperature change of a laser diode. CONSTITUTION:The gate voltage of a current source FET 14 of a bias circuit obtained from a reference voltage generating circuit constituted of an FET 16 whose gate electrode and source electrode are connected with a node 42, and a diode 17. Since the gate bias of the FET 14 is obtained from the reference voltage generating circuit, the voltage drop across a resistor 15 is increased when the threshold voltage of the FET 16 shifts to the negative side on account of temperature rise, so that a bias current can be increased according to temperature rise by increasing the voltage between the gate and the source of the FET 14. The field effect transistor logic circuit in the invention disclosed here can be integrated on a chip of a laser circuit driving circuit, so that manufacturing cost can be reduced as compared with an optical output control method in usual optical communication systems.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field effect transistor logic circuit, and more particularly to a temperature compensation circuit for bias current of a laser diode driving IC.

[0002]

2. Description of the Related Art Since GaAs semiconductor has electron mobility several times faster than Si and a semi-insulating substrate can be easily obtained, the parasitic capacitance of the circuit can be reduced during integration. Energetic research and development have been carried out in various places because of the idea that high-speed logic operation is possible. Some GaAs semiconductors have been put on the market, and expectations are particularly increasing for SSI for ultrahigh-speed optical communication of 10 Gbps or more, which is required for the next-generation optical communication system.

In an optical communication system, as shown in FIG. 2, an electric signal is converted into an optical signal by a laser diode 3, transmitted using an optical fiber 5, and converted into an electric signal again by an avalanche photodiode 6. However, a constant threshold current and a modulating current are required to drive the laser diode in this system.

The threshold current of the laser diode tends to increase due to temperature rise due to heat generation of the laser diode, etc. In order to obtain a constant optical output even if the temperature changes, it corresponds to the threshold current fluctuation. It is necessary to control the bias current.

IC7 for driving this laser diode
3 supplies a modulation current composed of a differential circuit 1 and a bias circuit 2 and biased to a constant current to the laser diode 3 as shown in FIG. 3, and the gate of the FET of the bias circuit monitors the optical output of the laser diode. There has been a method of controlling the light output by increasing the gate voltage when the light output decreases and decreasing the gate voltage when the light output increases.

In FIG. 3, 11, 12, 13, and 14 are depletion type n-channel MESFETs, and 15 is a resistance as a level shift element. The drain electrode of the FET 11 is connected to the power supply terminal 100, the gate electrode is connected to the input terminal 20, the source electrode is connected to the node 41, the drain electrode of the FET 12 is connected to the output terminal 30, and the gate electrode is connected to the input terminal 21. Connected, the source electrode is connected to the contact 41, the drain electrode of the FET 13 is connected to the node 41, and the gate and source electrodes are the power supply terminal 1
01, and these FETs 11, 12 and 1
Reference numeral 3 constitutes the differential circuit 1. The FET 14 has a drain electrode connected to the output terminal 30, a gate electrode connected to the optical output feedback circuit 4, and a source electrode connected to the power supply terminal 101.

[0007]

In the laser diode drive circuit shown in FIG. 3, in order to increase the bias current as the temperature of the laser diode rises, an element for monitoring the optical output and the element for feeding it back to the drive circuit are provided. However, it has a drawback that the cost is high and the size becomes large when it is modularized.

It is an object of the present invention to provide a field effect transistor logic circuit for driving a laser diode, which is equipped with a circuit for compensating the bias current according to the temperature fluctuation of the laser diode.

[0009]

According to the field effect transistor logic circuit of the present invention, a drain electrode is connected to a first power supply terminal, a gate electrode is connected to a first input terminal, and a source electrode is connected to a first node. And a second MESFET having a drain electrode connected to the output terminal, a gate electrode connected to the second input terminal and a source electrode connected to the first node, and the drain electrode being the first MESFET. A third differential MESFET having a gate and a source electrode connected to a second power supply terminal and a drain electrode connected to the first output terminal and a second gate electrode connected to the first output terminal. A fourth MESFET having a source electrode connected to the second power supply terminal, a drain electrode connected to a third power supply, and a gate and source electrode connected to the second node. A load element having one end connected to the fifth MESFET connected thereto and the other end connected to the second power supply terminal, and an anode connected to the second node and a cathode connected to the second node. And a reference voltage generation circuit including a first diode connected to the power supply terminal.

[0010]

In the field-effect transistor logic circuit according to the present invention, the gate potential of the bias circuit is obtained from the reference voltage generating circuit to increase the gate-source voltage of the bias circuit current reduction FET as the temperature rises. If the current of the FET is increased and it is set to control with the maximum voltage by the diode clamp when the temperature is higher than the maximum temperature specified by the laser diode, the fluctuation of the optical output due to the temperature rise can be suppressed and the operating temperature will be higher than the specified value. If so, it is possible to suppress the bias current and prevent excess current from flowing to the laser diode.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of a field effect transistor logic circuit according to the present invention. In this embodiment, the gate voltage of the current source FET 14 of the bias circuit is the FET 16 in which the drain electrode is connected to the power supply terminal 102, the gate and source electrodes are connected to the node 42, and one end is connected to the node 42 and the other end is It is obtained from a reference voltage generating circuit including a diode 17 connected to the power supply terminal 102. Other configurations are
The configuration is the same as that of the circuit shown in FIG. 3, and the same elements are denoted by the same reference numerals.

Now, when "H" level is input to the input terminal 20 and "L" level is input to the input terminal 21, the output terminal 30
The current flowing through the output terminal 30 decreases, while when the “L” level is input to the input terminal 20 and the “H” level is input to the input terminal 21, the current flowing to the output terminal 30 increases. Further, a bias current connected to the output terminal 30 is flowing. The amount of current at this time is determined by the gate bias of the FET 14.

This gate bias is obtained from the reference voltage generating circuit, and the threshold voltage of the FET 16 shifts to the negative side due to the temperature rise, so that the voltage drop in the resistor 15 increases, so that the gate voltage of the FET 14 increases. By increasing the source-to-source voltage, the bias current can be increased as the temperature rises. Further, at a temperature equal to or higher than the standard value, the diode 17 prevents the gate-source voltage higher than a predetermined value from being applied to the FET 14, so that an excessive current can be controlled so as not to flow into the laser diode. As a result, the optical output of the laser diode driven by the amount of current in this bias circuit is not affected by temperature fluctuations.

[0014]

In the field effect transistor logic circuit according to the present invention, since the gate bias of the bias circuit current source FET is obtained from the reference voltage generating circuit, it can be integrated on the chip of the laser diode driving circuit, and therefore, It is possible to reduce the manufacturing cost as compared with the optical output control method for a normal optical communication system.

[Brief description of drawings]

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

FIG. 2 is a diagram showing an optical communication system.

FIG. 3 is a circuit diagram illustrating a conventional example.

[Explanation of symbols]

1 differential circuit 2 bias circuit 3 laser diode 4 optical output monitor circuit 5 optical fiber 6 avalanche photodiode 7 laser diode drive IC 8 preamplifier 11, 12, 13, 14, 16 depletion type F
ET 15 resistance 17 diode 20,21 input terminal 30 output terminal 41,42 node 100,101,102 power supply terminal

Claims (1)

Claim: What is claimed is: 1. A first MESFET having a drain electrode connected to a first power supply terminal, a gate electrode connected to a first input terminal and a source electrode connected to a first contact, and a drain. A second electrode having an electrode connected to the output terminal, a gate electrode connected to the second input terminal, and a source electrode connected to the first contact.
A differential circuit comprising a MESFET and a third MESFET whose drain electrode is connected to the first contact and whose gate and source electrodes are connected to the second power supply terminal, and the drain electrode is connected to the first output terminal. Second gate electrode connected
A fourth MESFET having a source electrode connected to the second power supply terminal and a drain electrode connected to a third power supply and a gate and source electrode connected to the second node. Of the MESFET, one end of which is connected to the second node and the other end of which is connected to the second power supply terminal, the load element and the anode are connected to the second node, and the cathode is connected to the second power supply terminal. A field effect transistor logic circuit comprising: a reference voltage generating circuit including a first diode.