JPH031730A - Optical transmission circuit - Google Patents

Abstract

PURPOSE:To output an optical signal with high quality for a long period by inhibiting a current flowing to a couple of transistors(TRs) being the components of a differential amplifier circuit while the constant current circuit is interrupted in the absence of an input signal. CONSTITUTION:A switching circuit 7 detects the presence of an input signal to a differential signal generating circuit 6 to control the interruption of a voltage applied to the base of a TR 4. That is, TRs 2, 3 being the components of a differential amplifier circuit connecting to the TR 4 and the constant current circuit including a light emitting element 1 receive a current in the presence of an input signal and no current flows to them in the absence of the signal. Thus, there is no difference caused in the characteristic change of the TRs 2, 3 due to uneven heating or the like. Moreover, aging is also advanced equally and the pulse width distortion of an output optical signal wave is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to optical transmission circuits. More specifically, the present invention relates to a novel configuration of an optical transmission circuit used in the field of digital communication using optical signals as a medium.

BACKGROUND OF THE INVENTION FIG. 5 is a circuit diagram showing the configuration of an optical transmitter circuit.

As shown in FIG. 5, this optical transmitter circuit generates a differential signal from an input signal with a pair of transistors 2 and 3 whose emitters are common and grounded via a transistor 4 and a resistor 5. The differential signal generating circuit 6 supplies complementary voltage signals to the bases of the transistors 2 and 30, respectively, and the light emitting element l is connected to the collector of the transistor 2 as a load. Note that the collector of the transistor 3 and the anode of the light emitting element 1 are coupled to a current source.

In the optical transmitter circuit configured as described above, the current flowing between the collector and emitter of transistor 4 flows through the current path configured by transistors 2 and 3 in accordance with the fixed bias voltage applied to the base of transistor 4. . Here, complementary voltages are applied to the bases of transistors 2 and 3 in response to the input signal, and the current flowing through the light emitting element 2 changes in response to the input signal, so the light emitting element It emits light with a light output corresponding to the signal.

Problems to be Solved by the Invention Conventional circuits of this type have a configuration in which current flows through the differential amplifier circuit even when the input signal is in a no-signal state. That is, as shown in Figure 5, in the conventional optical transmitter circuit, a fixed bias was applied to the base of the transistor 4 constituting the constant current circuit, so that regardless of the presence or absence of an input signal, the current flowing to the differential amplifier circuit The current is always kept constant. Since the light emitting element 1 is turned off in a state where there is no signal by human power, the current flowing through the transistor 2 and the light emitting element 1 is zero. On the other hand, the entire current between the current source and ground flows through the transistor 3.

In other words, a signal type in which a relatively large number of no-signal states occur or a signal type in which a no-signal state continues for a relatively long time (
For example, in the case of a burst signal, the time during which current flows through transistor 3 is clearly longer than that of transistor 2, and the amount of heat generated by transistor 3 is also greater.

For this reason, a difference occurs in the change in characteristics due to heat generation between the transistors 2 and 3, and pulse width distortion of the optical waveform is likely to occur due to an off-cent shift in the base power of the two transistors. Furthermore, there is a problem in that variations occur over time and pulse width distortion of the optical waveform becomes common.

Therefore, the present invention solves the problems of the prior art described above, reduces offsets caused by continuous no-signal state or bias in duty ratio, and outputs higher quality optical signals over a long period of time. The purpose is to provide a new optical transmission circuit configuration.

Means for Solving the Problems, that is, according to the present invention, a differential amplifier circuit includes a pair of transistors, one of which is connected to a light emitting element as a load, and a differential signal is generated from an input signal.
A differential signal generating circuit configured to apply a complementary voltage to each base of a pair of transistors, and a constant current circuit controlling a current flowing through the differential amplifier circuit.
The signal transmission circuit includes means for detecting the presence or absence of an input signal to the optical transmission circuit and means for controlling the constant current circuit, and when there is no input signal, the constant current circuit is cut off and the difference is controlled. An optical transmission circuit is provided, characterized in that it is configured so that no current flows through a pair of transistors forming a dynamic amplification circuit.

The main feature of the optical transmission circuit according to the present invention is that the current flowing through the light emitting element drive circuit is controlled depending on the presence or absence of an input signal, and the current flowing through the drive circuit is cut off when there is no input signal. There is.

Therefore, when there is one word in the input signal to this optical transmitter circuit, the duty cycle is generally 50%, so current flows equally through the pair of transistors in the light emitting element drive circuit. When there is no signal, no current flows through any transistor.

In this way, there is no difference in the changes in the characteristics of the two due to heat generation, and there is little difference in changes over time, so it is possible to suppress pulse width distortion of the optical waveform.

Hereinafter, the present invention will be described in more detail with reference to the drawings, but the following disclosure is only one example of the present invention and does not limit the technical scope of the present invention in any way.

Embodiment FIG. 1 is a circuit diagram showing the basic configuration of an optical transmitter circuit according to the present invention.

As shown in FIG. 1, this circuit is similar to the conventional optical multiplication circuit shown in FIG. 5 with respect to the drive circuit for the light emitting element. That is, a differential signal generation circuit 6 that generates a differential signal from a human-powered optical signal, and a pair configured to apply a complementary voltage signal generated by the differential signal generation circuit 6 to their respective bases. The transistor 2.3 is composed of a transistor 2.3 and a light emitting element 1 connected as a load of the transistor 2, and the emitters of the transistor 2.3 are connected in common.
It is grounded via a transistor 4 and a resistor 5.

Further, the collector of the transistor 3 and the anode of the light emitting element 1 are commonly connected to a current source.

The unique structure of this circuit lies in the circuit that includes the switching circuit 7. That is, the switching circuit 7 is configured to detect the presence or absence of an input signal to the differential signal generation circuit 6 and control the on/off of the voltage applied to the base of the transistor 4.

FIGS. 2(a), 0:1), and (C) are diagrams for explaining the operation of the optical transmission circuit according to the present invention shown in FIG. 1.

As shown in FIG. 2(a), the input signal here is a burst signal, and there is a period in which there is no signal input.

In response to this input signal, the switching circuit 7 shown in FIG. 1 outputs ON when there is an input signal, and outputs OFF when there is no input signal, as shown in FIG. 2(b). The voltage is applied to the base of the transistor 4.

That is, when there is an input signal, the switching circuit 7 becomes an ON output, and the voltage is V. N is applied to the base of transistor 4 to turn transistor 4 on. At this time, the current io generated in the constant current circuit is (VON > VBE)
/ Given by R. However, VBE is the pace emitter voltage of transistor 4, R is the resistance value of resistor 5, and V
O) I > VBE holds true.

On the other hand, when the input signal becomes a no-signal state, the switching circuit 7 becomes an OFF output, and VOFF (for example, ground level) is applied to the base of the transistor 4 to turn off the transistor 4 and cut off the current flowing through the constant current circuit. Therefore, the current flowing through the differential amplifier circuit including transistors 2.3 also becomes zero.

As described above, in the circuit according to the present invention, the transistor 4
As shown in Figure 2 (C), current flows through the constant current circuit including the transistor 2.3 and the light emitting element 1 connected to the circuit when there is an input signal, and no current flows when there is no signal. There will be no.

The duty cycle of the transmission signal is generally 50%, and when there is an input signal to the transmission circuit as described above, current flows equally through transistors 2 and 3. Furthermore, when there is no signal from human power, no current flows through any of the transistors, so there is no difference in the characteristics of the transistors 2.3 due to nonuniform heat generation or the like.

Moreover, aging progresses evenly, and pulse width distortion of the output optical signal waveform can be suppressed.

FIG. 3 is a circuit diagram showing a specific configuration example of the optical transmission circuit according to the present invention as described above.

The basic configuration of this circuit is the same as that already shown in FIG. 1, and the same components are given the same reference numbers.

The circuit shown in FIG. 3 uses an LED 8 as a light emitting element, a dual output line receiver 9 as a differential signal generating circuit, and applies the outputs of the line receiver 9 to the bases of transistors 2 and 3, respectively. is configured to be

The switching circuit 7 is configured as follows.

That is, this switching circuit 7 is composed of a peak hold circuit configured with an operational amplifier 10, a resistor 11, and a capacitor 12 that receives an input signal, and a comparator 13 whose non-inverting input is connected to the output of this peak hold circuit. There is.

A predetermined reference voltage is supplied to the inverting input of the comparator 13, and the output of the comparator 13 is the output of the switching circuit 7. That is, the comparator 13 outputs 2 depending on the level of the peak hold circuit output.
Level V. It is configured to output either one of N% VOFF and input it to the base of a transistor 4 constituting a constant current circuit.

4(a), (b), (C) and (d) are waveform diagrams illustrating the operation of the circuit shown in FIG. 3 configured as described above.

As shown in FIG. 4(a), the input signal here is an intermittent pulse signal, and there is a period in which there is no signal input at all.

From such an input signal, the peak hold circuit of the switching circuit 7 described above generates a signal having a waveform as shown in FIG. On the other hand, the comparator 13 has V r e
A reference voltage is supplied as shown as f. Therefore, when there is an input signal, the output of the peak hold circuit is higher than the reference voltage V r a r, and when the input signal disappears, the output voltage decreases according to the CR time constant of the resistor 11 and capacitor 12, and the output voltage of the peak hold circuit is higher than the reference voltage V r a r. r becomes lower than @f.

Therefore, as shown in FIG. 4(C), the comparator 13
has two levels V depending on the presence or absence of an input signal. , and V. Output either one of PP. The output of the comparator 13 is applied to the base of a transistor 40 constituting a constant current source circuit, and the transistor 4 is turned on or off depending on the presence or absence of an input signal. thus,
As shown in FIG. 4(d), the current flowing through the constant current source circuit is interrupted depending on the presence or absence of an input signal. Therefore, when there is no input signal, the current flowing through transistors 2 and 3 is cut off. be done.

As explained in detail of the invention, when there is a signal in the input signal, current flows through the differential amplifier circuit, but when there is no signal, no current flows, so this is a signal format in which no-signal states occur relatively frequently. When used in an optical communication transmission circuit that uses a signal state with a relatively long period of no signal, there will be no difference in characteristics changes due to heat generation or aging of the two transistors that make up the above differential amplifier circuit. Therefore, offset deviation is less likely to occur, and pulse width distortion of the optical waveform can be suppressed.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a basic configuration example of an optical transmitter circuit according to the present invention, and FIGS. 2(a), (b), and (C) are circuit diagrams of the optical transmitter circuit shown in FIG. FIG. 3 is a circuit diagram showing a specific configuration example of the optical transmission circuit according to the present invention, and FIGS. 4(a), (b), (C) and ( d) C. This is a diagram explaining the operation of the optical transmission circuit shown in FIG. 3, and FIG. 5 is a circuit diagram showing the general configuration of the conventional optical transmission circuit. [Main reference numbers] 1..Light emitting element, 2-4..Transistor, 5.11..Resistor, 6..Differential signal generation circuit, 7..Switching circuit, 8..Light emitting element, 9.. Line receiver, 10... operational amplifier, 12... capacitor, 13... comparator

Claims (1)

[Claims] A differential amplifier circuit consisting of a pair of transistors, one of which is connected to a light emitting element as a load, and a differential signal that is generated from an input signal and complementary to each base of the pair of transistors. In an optical transmission circuit, the optical transmission circuit includes a differential signal generation circuit configured to apply a specific voltage, and a constant current circuit that controls a current flowing through the differential amplifier circuit. comprising means for detecting and means for controlling the constant current circuit, and when there is no input signal, the constant current circuit is cut off so that no current flows through the pair of transistors forming the differential amplifier circuit. An optical transmission circuit characterized in that it is configured as follows.