JP3016782B2 - Optical transmitter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter used for optical fiber communication, and more particularly to an optical transmitter for digital signal transmission. 2. Description of the Related Art Conventionally, as shown in FIG. 2, in this type of optical transmitter, a transistor 5 is connected in parallel to connect and disconnect a drive current of a light emitting element 4 in response to an input digital signal. Is used. Here, in order to vary the current flowing through the light emitting element 4 and adjust the output light power, the variable resistor 2 is inserted in series. [Problems to be Solved by the Invention] The conventional optical transmitter described above has a drawback that the power consumption of the variable resistor increases because the drive current of the light emitting element also flows through the variable resistor. Furthermore, to allow power consumption of, for example, about 300 mW, a variable resistor with a rated power of about 0.5 W must be used, and a variable resistor with large external dimensions is required, and the optical transmitter is downsized. Was a hindrance. SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems caused by the related art, and has as its object to provide an optical transmitter capable of reducing the power consumption of a variable resistor and reducing its size. [Means for Solving the Problems] The optical transmitter of the present invention has a power input terminal to which a DC power supply is connected, a light emitting element, a collector electrode on the anode side of the light emitting element, and an emitter electrode on the cathode side. A first transistor connected thereto, a driving circuit for intermittently intermittently connecting a base current of the first transistor in response to an input digital signal, a collector electrode at the power input terminal, and an emitter at an anode side of the light emitting element. The electrode includes a second transistor connected to each of the transistors and a variable resistor for adjusting a DC bias voltage applied to a base of the second transistor. Embodiment Next, the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment according to the present invention.
In this figure, a first transistor 5 and a light emitting element 4 are connected in parallel, a second transistor 3 is connected in series to this parallel circuit, and a power supply voltage is supplied from a power supply input terminal 1. The change in the internal resistance of the second transistor 3 is used for adjusting the drive current of the light emitting element. On the other hand, the digital signal applied to the signal input terminal 7 interrupts the base current of the first transistor 5 via the drive circuit 6. The DC bias voltage applied to the base of the second transistor 3 is obtained by adjusting the voltage of the power supply voltage by the variable resistor 2. Here, 2SC2954 is used for the transistors 3 and 5, the normal TTL IC 7404 is used for the drive circuit 6, and the power supply voltage is set to DC + 5V.
The driving current of the light emitting diode is set to 50 to 100 mA, assuming that a light emitting diode of 0.8 μm band is used as the light emitting element 4.
To change the output light power by about 3 dB,
Since the DC current amplification factor of the transistor 3 is 30 or more, the base current needs to be about 1.7 to 3.3 mA. If a 1kΩ variable resistor is used, the power consumption here will be about 50mW. [Effects of the Invention] As is clear from the above description, the present invention utilizes the change in the internal resistance of the transistor to adjust the drive current of the light emitting element, thereby reducing the power consumption of the variable resistor to be used by several minutes. In addition to being able to reduce to about one, it is possible to use a small variable resistor accordingly, and the obtained effect is great.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showing an embodiment of an optical transmitter according to the present invention, and FIG. 2 is a circuit diagram showing an example of a conventional optical transmitter. In the figure, 1 is a power input terminal, 2 is a variable resistor, 3 and 5 are transistors, 4 is a light emitting element, 6 is a drive circuit, and 7 is a signal input terminal.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/28 (56) References JP-A-54-122904 (JP, A) JP-A-54-51452 (JP, A) Opened 58-191657 (JP, U) Opened 61-37644 (JP, U) Opened 58-98745 (JP, U) Opened 57-53030 (JP, U)

Claims (1)

(57) [Claims] A power supply input terminal to which a DC power supply is connected, a light emitting element, a first transistor connected to a collector electrode on the anode side of the light emitting element, an emitter electrode connected to the cathode side thereof, and a base of the first transistor. A drive circuit for interrupting a current in response to an input digital signal, a second transistor having a collector electrode connected to the power supply input terminal, and an emitter electrode connected to an anode of the light emitting element, respectively; And a variable resistor for adjusting a DC bias voltage applied to a base of the optical transmitter.