JPS62100038A - Optical transmission circuit - Google Patents

Abstract

PURPOSE:To eliminate the effects of the forward voltage drop of a light emitting element and of the fluctuation of the power supply voltage by applying a pulse voltage having a prescribed amplitude to the base of a driving transistor (TR) so as to vary the drive current. CONSTITUTION:A modulation TR Q2 is in an off-state. A voltage divided by resistors R1-R3 is applied to the base of the driving TR Q1. In opening a switch terminal 2' thrown to vary a light emitting output P, the base voltage VB is expressed in equation I. When the terminal 2' is closed, the voltage VB is expressed in equation II. A drive current IF supplied to a light emitting element LD is expressed in equation III, where VBE is the base-emitter voltage of the TR Q1. The current IF is not effected by the forward voltage drop VF of the element LD and the power voltage Vcc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field 1] The present invention relates to an optical transmission circuit that can set the intensity of optical output in stages.

[Background Art 1 Conventionally, the light emitting element is driven as shown in FIG.
, D and a driving transistor Q1 that drives the light emitting element LD.
For example, a switch SW is connected in parallel to both ends of the resistor R2, and by switching this switch SW, the resistance value connected in series with the light emitting element Ll) can be set. By varying the light emitting element L D
l: A method of varying the flowing drive current TF is often used. In this method, the current I flowing through the light emitting element L[),
Then, the driving transistor Q is on and the switch S is turned on.
When W is open, I p ”(Vcc-Vp-VcFsat)/
'(R++ R2) - (1), and the drive current ■ when the switch SW is on is the resistance R2 of equation (1).
The value is set to 0.

Since the drive 'Ki current I and the light emission output P are approximately proportional, the following relationship exists: ))=klF (k is a proportionality constant) (2). If you want the light emission output to be Pl when the switch SW is closed and the light emission output to be R2 when the switch SW is open, then from equations (1) and (2), P 1 = k (Vcc - VF −Vcasat)/(
R1+RzL=(3)P2=mP.

=k(Vcc-Vp-Vopsat)/R+-
(4), and the resistances R, , R2 may be determined so as to satisfy this relationship. Note that Vcc is the power supply voltage, VF is the forward voltage drop of the light emitting element LD, Vc+=sat is the collector-emitter saturation N pressure of the driving transistor Q1, and m is the magnification. However, since the values of the proportionality constant and forward voltage drop VF vary due to variations in the characteristics of the light emitting element LD, it is not possible to unambiguously determine the values of the resistors R, , R2, and the proportionality constant of the light emitting element LD varies. Constant forward voltage drop ■F
It is necessary to adjust the values of the resistors R, , R2 for each light emitting element LD by varying them according to the values of . For this reason, as the number of switching stages increases, the number of w4W locations increases, which requires more effort.

Furthermore, in this system, the drive current IP fluctuates sensitively to changes in the characteristics of the forward voltage drop (P) and to fluctuations in the power supply voltage Vcc, resulting in a lack of stability.

[Objective of the Invention 1 The present invention has been made in view of the above-mentioned five issues, and its purpose is to reduce the number of 1llll sorting stations, reduce variations in the characteristics of light emitting elements, and reduce power supply voltage fluctuations. The object of the present invention is to provide an optical transmission circuit that operates stably.

[Disclosure of the Invention 1 (Embodiment 1) Fig. 1 is a diagram showing an embodiment of the present invention, in which the power supply terminal ■
A series circuit of the light emitting element LD, a driving transistor Q, and a variable resistor VR which is the emitter resistance of the driving transistor Q is connected to the circuit, and a constant voltage circuit 1 and resistors R1 to R, are connected in parallel to this series circuit. Connect the series circuit of , and connect the connection point between resistor l and resistor R2 to drive transistor Q1.
The amount of light emitted is adjusted by short-circuiting and opening both ends of the resistor R3. Furthermore, a V-key trannostar Q2 that switches based on a PCM code signal for transmission is connected to both ends of the series circuit of the resistor R2 and the resistor R3.

The operation will be explained below. The power supply voltage Vcc applied to the power supply terminal ■ is set to a constant voltage V reg by the constant voltage circuit 1.
The voltage is stabilized and applied to resistors R1 to R3. Now, a case will be described in which the voltage transistor Q2 is in an off state. In this case, a voltage divided by resistors R3 to R1 is applied to the base of the driving transistor Q1.

Therefore, when the switching terminal ■, which is switched to vary the light emission output P, is open, the base voltage VB is V
n=(R2+R3)V reg/ (Rr +R2+
Rt)''(5).In addition, when the switching terminal ■ is short-circuited, VB=R2Vre8/(R, +R,)...(
6), and a light emitting element LD is connected between the collector of the driving transistor Q and the power supply terminal ■, and a variable resistor VR as an emitter resistance is connected to the emitter, so that the light emitting element LD If the flowing drive current is the base-emitter open voltage VBE of the drive transistor Q1, then I P = (V B - V BE) / R, (7)
Therefore, the driving current IF is the power supply voltage Vcc, the light emitting element L
Forward voltage drop of D■Shadow of F? Does not receive e. Furthermore, R,
indicates the resistance value of the variable resistor VR. Here, the light emission output when the light emission output P switching terminal ■ is short-circuited is Po,
If you want the light emission output when open to P4 (2)
, (5), (6), and (7), Pz=k(RzVre
g/(RI+Rz)-VBp)/R4=18)P==
mP 3 =k((R2+R3)Vreg/(Rt+R2+R,)
-VBE)/R-...(9) Also, the magnification theory is In=((R20Rz)Vreg/(R+R2+R
p)-Vnp)/ (R2V reg/ (R+ +
R2)-Vnr:), where the output voltage Vred of the constant voltage circuit 1 is constant, and the base-emitter voltage v
Since n' is also almost constant, the magnification W can be kept constant with the values of the fixed resistances R, ~R.In other words, the magnification I11
is the resistance R ratio measurement v! It has nothing to do with the value of Lk, and it is only necessary to determine the resistances R6 to R3 so that the desired value of the magnification m is achieved, making adjustment easy.
Since the light emitting output P is uniquely determined by the resistance value of 3 etc., even if the switching stages increase? The number of I8 adjustment points will not increase. Furthermore, it is clear that it is sufficient to deal with variations in the proportionality constant by simply adjusting the resistor R1. So far, we have explained when the modulation transistor Q2 is off, but when the modulation transistor Q2 is on, the driving transistor Q
, is lowered by conduction of the modulating transistor pQ2, and the driving transistor Q1 is cut off, so that no driving current IF flows through the light emitting element LD.

By switching the modulating transistor Q2 according to the transmission signal in this way, a pulse voltage of a constant amplitude can be applied to the base of the driving transistor Q, and by opening the short circuit of the switching terminal ■ of the light emission output P, the pulse voltage can be applied to the base of the driving transistor Q. The amplitude of the voltage can be set in stages, and the intensity of the pulsed light can be set in stages.

(Embodiment 2) FIG. 2 is a diagram showing another embodiment of the present invention, in which a modulation CMO is used instead of the modulation transistor Q2 of the first embodiment.
3 logic circuit 2 is used.

In this embodiment, the output voltage V reg of the constant voltage circuit 1 is applied to the power supply terminal VOO of the CMO5 logic circuit 2 for modulation, so when the output of the CMO8 logic circuit 2 is at a high level, the output voltage is almost equal to V reg. Become.

Therefore, the base voltage VB of the driving transistor Q
can be set in the same way as in the first embodiment, and the light emission output P can also be set in the same way.

[Effect of the invention 1] As described above, the present invention connects a series circuit of a light emitting element and a driving transistor to both ends of a power source, and inserts a variable resistor between the emitter of the driving transistor and the negative electrode of the power source. A bias circuit is connected to the base of the driving transistor to apply a constant amplitude pulse voltage according to the transmission signal, and the amplitude can be set in multiple stages, so a constant amplitude pulse voltage is applied to the base of the driving transistor. By biasing the drive transoster and varying the drive current, it is possible to eliminate the effects of forward voltage drop of the light emitting element and variations in power supply voltage, and
Since the drive current can be adjusted to WR9 using a variable resistor, the light emission output can be stabilized, and the light emission output can be uniquely determined by the bias circuit, so even if the number of switching stages increases, the number of adjustment points increases. It has a unique effect.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram of one embodiment of the present invention, Fig. 2 is a circuit diagram of another embodiment of the invention, and Fig. 3 is a circuit diagram of a conventional example. It is. LD is a light emitting element, Q is a driving transistor, VR is a variable resistor, 1 is a constant voltage circuit, 2 is a CMO8 logic circuit, R2
-R8 is a resistor. Agent Patent Attorney Ishi 1) Ai 7 Figure 3 procedural amendment (voluntary) [December 6, 1985]

Claims (1)

[Claims] (1) In an optical transmission circuit that switches the drive current flowing through the light emitting element through a driving transistor to cause the light emitting element to emit pulse light, a series circuit of the light emitting element and the driving transistor is connected to both ends of the power supply, and a series circuit of the driving transistor is connected to both ends of the power supply. Insert a variable resistor between the emitter and the negative pole of the power supply,
An optical transmission circuit comprising: applying a pulse voltage of constant amplitude according to a transmission signal to the base of the driving transistor; and connecting a bias circuit capable of setting the amplitude in multiple stages.