JPS58222630A - Optical tansmission line - Google Patents

Abstract

PURPOSE:To prevent an overcurrent from flowing to a light emitting element, by comparing an input signal having a time constant T2 inputted to a peak detecting circuit having a time constant T1 (T2<T1) with a reference voltage and controlling the light emitting element with this comparison output. CONSTITUTION:A video signal from a TV camera 1 is inputted to an amplifier 4 of a clamp circuit 3, and an output of the amplifier 4 is detected by a peak detector having a time constant T1 comprising a diode 5, a capacitor 6 and a resistor 7. This signal is fed back to the amplifier 4 via a comparator 8 to which the reference voltage VR1 is inputted and inputted to a comparison circuit 12 which clamps the input video signal and to which a reference voltage VR2 in a protection circuit 11 is inputted. The level of the input video signal is fluctuated in a period earlier than the time constant T1 and not absorbed at the peak detecting circuit, a protection circuit 3 detects it, a collector current of a transistor(TR) 9 is limited to prevent the over-current of the light emitting element 10 driven with an output signal of the clamp circuit 3.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to an analog and q optical transmitter circuit for video signal transmission;
In particular, the present invention relates to a light emitting element drive circuit that prevents overcurrent from flowing to a light emitting element.

(b) Prior Art and Immediate Issue In an optical transmission circuit that transmits a baseband video signal using a light emitting element, a clamp circuit is used to maintain the operating conditions of the light emitting element to a certain level.

However, since the transmission frequency band of the baseband video signal is several 10H to several MH, the time constant of the peak detection circuit of the clamp circuit needs to be several H2 or less. For this reason, slow fluctuations in the DC level of the baseband video signal (hereinafter referred to as video signal) are compressed by the clamp circuit, but when the video signal is transmitted from the TV camera to the optical transmission circuit, the power of the TV camera is turned on. However, there are disadvantages in that the video signal undergoes rapid DC level fluctuations due to transient phenomena during the OFF period, and that the light emitting elements of the optical transmitter circuit are deteriorated or damaged by abnormal currents caused by the transient phenomena.

(e) Object of the Invention In order to solve the above-mentioned drawbacks, it is an object of the present invention to provide a novel optical transmitter circuit that can prevent overcurrent from flowing through a light emitting element.

(d) Structure of the Invention In order to achieve the above object, the present invention provides an optical transmission circuit equipped with a clamp circuit in which an input signal is peak-detected by a peak detection circuit with a time constant T1. An input signal with a time constant T, (<T+) is compared with a reference voltage in a comparator, and the drive current of the light emitting element is controlled by the output of the nuclear comparator.

(e) Examples of the Invention The present invention will be explained below based on the drawings. This figure shows an embodiment of the present invention, in which the video signal shown in FIG. ■
(The waveform in Figure 2 ■) is a diode 5, a capacitor 6, a resistor 7 (at this time, the capacitor 6, the resistor 70, the time constant Ts=C
Let it be R. ) is subjected to peak detection and shaped into waveform O (waveform 0 in FIG. 2).

The waveform O is compared with a reference voltage in a comparator 8 having a reference voltage VR1, and the comparison voltage is fed back to the amplifier 3 to clamp the input waveform @.

Now, if the video signal output from the TV camera 1 fluctuates at a cycle faster than the time constant T+=CR of the peak detection circuit, the fluctuation cannot be absorbed by the peak detection circuit, and furthermore, the value is larger than the steady value. Drive transistor 9 if available
As a result, an overcurrent is caused to flow through the light emitting element 10.

In the present invention, the following measures are taken to prevent overcurrent in the light emitting element 1° due to fluctuations in the video signal.

The dynamic waveform ■' of the video signal becomes the 00 waveform O' in FIG. 2 at the 0 point in FIG.
The increased amount is inverted as shown in waveform 0' in FIG.
input to the base of

Limits the collector current of transistor 9. As a result, the component 0' that cannot be tracked by the clamp circuit in the waveform of the fluctuation of the video signal is absorbed by the comparator 12 via the diode 13, and the collector current of the transistor 9 is limited, so that the light-emitting element 10 has a constant current. A current flows, and the video signal (2) takes on a stable waveform at point (2) in Figure 1 as shown in (2) in the fJ2 diagram.

Regarding the fluctuation waveform of the video signal waveform [present], the waveform 0゛° is also shaped accordingly, and this waveform 0'' is shaped into the waveform 0'' by the protection circuit 11. As a result, as above, During the period of waveform 0'', the current of the light emitting element is limited, and a steady current flows through the light emitting element IO.

(f) Effects of the Invention According to the present invention, there is an advantage that overcurrent can be prevented from flowing into the light emitting element due to fluctuations in the video signal.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the present invention, and FIG. 2 shows operating waveforms of FIG. 1.

Claims (1)

[Claims] In an optical transmission circuit having a clamp circuit in which an input signal is peak-detected by a peak detection circuit with a time constant TI via an amplifier, a time constant T input to the peak detection circuit
An optical transmitter circuit characterized in that an input signal output from the amplifier (<T+) is compared with a reference voltage of a comparator 5, and a driving current of a light emitting element is controlled by the output of the nuclear comparator.