JPH0133057B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a driving circuit for a light emitting diode used for multiplexed optical transmission.

When handling analog signals in optical transmission, distortion will increase if the output light is directly amplitude modulated (AM) with the signal, but once the carrier wave is frequency modulated (FM) with the signal.
However, if the output light is intensity-modulated using the modulated carrier wave, the dynamic range can be expanded. Signal transmission using optical fibers is suitable for noisy locations such as in-vehicle audio equipment, but if one optical fiber cable is assigned to one signal, the number of cables increases as the number of signals increases.

Therefore, consider multiplexed optical transmission as shown in FIG. The example shown in the figure is an attempt to multiplex transmit stereo R and L signals using only one optical fiber 1, where 2 is the oscillation/modulation section of carrier wave ₁ , and 3 is the oscillation/modulation section of a different carrier wave ₂ . Part 4 is carrier wave ₁ which is FM modulated with signal R and carrier wave ₂ which is FM modulated with signal L.
6 is a light receiving diode, 7 is a preamplifier, and 8 and 9 are center frequencies of ₁ and _2, respectively.
The bandpass filters 10 and 11 are FM detection sections that demodulate the modulated carrier waves ₁ and ₂ and extract R and L signals.

FIG. 2 shows a general circuit example of the LED driver section 4 used in the multiplex optical transmission system shown in FIG.
In the figure, A ₁ and A ₂ are variable gain amplifiers that amplify the modulated carrier waves ₁ and ₂ , C ₁ and C ₂ are DC blocking (coupling) capacitors, Q ₁₂ is a driving transistor,
R ₁ to R ₃ and C ₃ are resistors and capacitors that constitute a bias circuit, and by this bias circuit, the DC bias current flowing through the transistor Q ₁₂ is set in advance to the optimum DC current flowing through the light emitting diode 5. This circuit consists of a carrier f ₁ which is FM modulated with signal R and a carrier f ₂ which is FM modulated with signal L.
and drives the driving transistor _Q12 to produce a luminance according to the level of the mixed signal (f ₁ + f ₂ ).
By driving the LED 5, it functions as a multiple brightness modulator for FM modulated carrier waves _f1 and _f2 .

However, in this circuit, if the signal levels of the carrier wave f ₁ and carrier wave f ₂ components contained in the output light from the LED 5 become uneven, the signal with the lower level will cause a deterioration in the transmission quality, and in some cases In some cases, this may cause problems such as not being able to transmit.
The amplitudes of carrier wave f ₁ and carrier wave f ₂ are each equal,
In addition, the amplitude upper limit level is set to a value that approximately divides the brightness range (allowable upper limit value) of the LED 5 into four equal parts.
That is, at the maximum value of the combined amplitude of carrier waves f ₁ and f _2, LED 5
is illuminated with a brightness approximately at the allowable upper limit, and the carrier wave f ₁ ,
The gain of amplifiers A ₁ and A ₂ must be adjusted strictly so that the brightness of LED 5 is approximately zero (light off) at the minimum value of the combined amplitude of f ₂ , which is a problem that is difficult to realize in manufacturing. It was hot.

The present invention aims to improve this point, and its characteristics are that the optimum DC current flowing through the light emitting diode for brightness modulation is branched into n branches and given to n drive transistors as DC bias currents. , the DC bias currents are set to substantially the same value, and n transmission AC signals of different frequency bands are respectively applied to the n driving transistors to commonly drive the light emitting diodes. It's at the point where it becomes like that.

This will be explained in detail below with reference to illustrated embodiments. FIG. 3 shows one embodiment of the present invention.
This corresponds to the LED driver section 4 in the figure. _Q1 ,
Q ₂ are first and second driving transistors to which FM-modulated carrier waves f ₁ and f ₂ are respectively applied. A bias circuit is formed between these transistors Q ₁ and Q ₂ by resistors R ₁ to R ₃ and capacitor C ₃ having the same constant, and their DC bias current I _E
is 1/ of the optimal DC current I _F flowing through the light emitting diode 5.
It is set to 2. Transistors Q ₁ and Q ₂ in advance
If a DC bias current of I _E = I _F /2 is passed, if the modulated carrier waves f ₁ and f ₂ are above a certain level, the signal components of both signals contained in the output light of LED 5 will be at almost the same level. . That is, both signal components are balanced. This is because f ₁ and f ₂ are both FM modulated carrier waves, so transistors Q ₁ and Q ₂
drives the LED 5 by a switching operation, that is, a repeated operation of saturation and cutoff regions.
This is because the output light is subjected to brightness modulation of 50% by f ₁ and 50% by f ₂ for a total of 100%. This eliminates the need for adjustment and allows the transmission quality to always be stably maintained in a good state.

As described above, according to the present invention, the optimal DC current flowing through the light emitting diode is divided into n branches (n is an integer of 2 or more) and is applied to each of the n driving transistors as a DC bias current, and the DC bias current are set to approximately the same value, so when multiplexing optical transmission of n AC signals in different frequency bands through one optical fiber, the light emission for brightness modulation is This has the advantage that both components in the output light can be effectively balanced by driving the diodes equally.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of a multiplex optical transmission system, FIG. 2 is a circuit diagram showing an example of a conventional LED driver section, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. In the figure, 4 is an LED driver section, 5 is a light emitting diode, and Q ₁ and Q ₂ are first and second driving transistors.

Claims (1)

[Claims] 1. A light emitting diode drive circuit that multiplexes and optically transmits n transmitted alternating current signals (n is an integer of 2 or more) each having a different frequency band, in which the optimum direct current flowing through the light emitting diode for brightness modulation is divided into n branches. and applying a DC bias current to each of the n driving transistors, setting the DC bias currents to substantially the same value, and applying different AC signals to be transmitted to the n driving transistors, respectively. A light-emitting diode drive circuit, characterized in that the light-emitting diode drive circuit commonly drives the light-emitting diodes.