JPH0716071B2 - Semiconductor light emitting element output stabilization circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to the field of optical transmitters and relates to a method for stabilizing output of a semiconductor light emitting device by high-speed negative feedback.

[Prior art and its problems]

Negative feedback circuits on electric circuits and electronic circuits are used in various ways for the purpose of stabilizing the output of semiconductor light emitting devices such as semiconductor lasers and light emitting diodes. FIG. 1 shows a conventional example. A part of the output of the semiconductor light emitting element (11) is received by the monitor photodetector (12), and the signal is fed back to the drive section. At this time, the operation of the semiconductor light emitting element (11) and the photodetector (12) requires current drive in the former case, and the photodetection signal is a current signal in the latter case due to its physical characteristics. That is, these optical semiconductor elements operate in the current mode. However, since the input signal of the circuit is generally the voltage signal from the voltage input signal source (17), when negative feedback is applied to the output of the semiconductor light emitting element, the voltage amplifier (13) is used as shown in FIG. Voltage-current converter for driving light-emitting element by voltage-current conversion after voltage amplification (14)
And a current for converting the light detection signal to a voltage signal.
It has a voltage converter (16) and a voltage amplifier (15) in a negative feedback loop. The loop bandwidth but is most several KH _z can stabilize the output variation of the semiconductor light-emitting device in the low-speed region, that is, sufficient to follow a temperature change or the like, a method that is most widely used.

Recently, there has been an attempt (FAST-APC) to provide negative feedback to a semiconductor light emitting device at high speed. It is the FA that causes the intensity distortion and phase distortion of the light emitting element, which is a problem especially in analog signal transmission.
It is intended to reduce it by using ST-APC. Loop bandwidth when you Hodokoso negative feedback to the video signal at this time for example signal bandwidth 4 mH _z from being required the same degree of broadband performance is required review and speed of the negative feedback configuration.

[Object of the Invention]

The present invention has been made in consideration of the above problems, and an object thereof is to provide a circuit configuration most suitable for high-speed negative feedback of a semiconductor light emitting device.

SUMMARY OF THE INVENTION The present invention relates to a semiconductor light emitting element, a photodetector for outputting a part of output light of the semiconductor light emitting element as a detection current, and a voltage / current conversion for converting a voltage input signal into a current signal. And a low-input-impedance current amplifier whose input side is electrically connected to the voltage / current converter and the photodetector, and whose output side is electrically connected to the semiconductor light-emitting element. There is provided a semiconductor light emitting device output stabilizing circuit characterized in that no current amplifier is provided between the detector and the input side of the low input impedance current amplifier.

[Advantages of the Invention] According to the present invention, not only a high-speed and high-gain negative feedback loop can be configured to enable high-quality optical transmission, but also its circuit configuration is very simple, which facilitates IC integration. Thus, an inexpensive optical transmitter can be provided.

Examples of the Invention Hereinafter, the present invention will be described in detail with reference to examples. FIG. 2 shows a block diagram of the present invention. A feature of the present invention is to make an effort to reduce the components in the loop as much as possible and to reduce the distortion. That is, the voltage-current converter (26) is provided between the voltage input signal source (27) and the signal adder (28) and is output outside the loop, so that the voltage in the loop in FIG.
The current converter (14) and current / voltage converter (16) are not required, and all the element operations in the loop are current-driven. No current amplifier was provided between the output of the photodetector (22) and the signal adder (28), and the current detected by the photodetector (22) was added directly to the output of the voltage-current converter (26). This is because consideration is given to improving the frequency characteristics of the closed loop without disposing elements having frequency characteristics on the feedback path from the semiconductor light emitting element (21) to the adder (28). Further, the current amplifier (29) between the current adder (28) and the semiconductor light emitting element (21) is a low input impedance type current amplifier. This is because the actual circuit example of this configuration shown in FIG. 3 has a semiconductor light emitting element (31) and a photodetector (32), and is the most common emitter-grounded amplifier (36) as a voltage / current conversion element. Considering an adder (38) that only connects signals as a current adder, and an amplifier (39) that has a grounded base as the first stage as a current amplifier with a low input impedance. The voltage at the collector terminal of the transistor changes with the input signal. Therefore, if the input impedance of the current amplifier is high, an unnecessary signal due to the voltage signal may be generated there, which may cause a malfunction as a loop.

In this case, what low input impedance is required is a value sufficiently smaller than the input impedances of the photodetector and the voltage-current conversion transistor, and the circuit is about the input impedance of the grounded base circuit.
However, the grounded base circuit has no current gain,
An emitter grounding circuit is placed in the latter stage, and current amplification is performed here. When the emitter grounding circuit is used instead of the base grounding circuit, its input impedance cannot be used because it is larger than that of the base grounding circuit by the current amplification factor. However, a current amplifier, for example, a parallel feedback type amplifier (49) shown in FIG.

In addition, as shown in FIG. 5, another transistor (60) is arranged outside the loop to drive the semiconductor light emitting device, and only the error signal is transmitted in the loop (Japanese Patent Publication No. 58- 41675) can also apply the circuit configuration of the present invention. Also, the circuit components are not limited to transistors, but FETs
It is clear that this is also possible.

The same parts in FIGS. 3 to 5 are designated by the same reference numerals.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional example, FIG. 2 is a diagram showing an embodiment of the present invention, and FIGS. 3 to 5 are specific circuit diagrams of the embodiment shown in FIG. 11,21,31 …… Semiconductor light emitting device 12,22,32 …… Photodetector 16,26,36 …… Voltage / current converter 17,27,37 …… Voltage input signal source 18,28,38… … Current adder 19,29,39,49 …… Low input impedance current amplifier

Claims (4)

[Claims] 1. A semiconductor light emitting device, a photodetector for outputting a part of output light of the semiconductor light emitting device as a detection current, a voltage-current converter for converting a voltage input signal into a current signal, and A low-input-impedance current amplifier whose input side is electrically connected to the voltage / current converter and the photodetector, and whose output side is electrically connected to the semiconductor light-emitting element; and A semiconductor light emitting element output stabilizing circuit, in which a current amplifier is not provided between the input side of a current amplifier having a low input impedance and the input side. 2. The voltage / current converter comprises a grounded-emitter transistor, a base side is connected to a voltage input signal source, and a collector side is connected to the photodetector and the input side of the low input impedance current amplifier. The semiconductor light emitting element output stabilizing circuit according to claim 1, wherein 3. The low input impedance current amplifier is composed of a base-grounded transistor, and the emitter side is connected to the collector side of the transistor forming the voltage-current converter and the one electrode side of the photodetector. The semiconductor light emitting element output stabilizing circuit according to claim 1 or 2, wherein the side is electrically connected to the semiconductor light emitting element. 4. The semiconductor light emitting element output stabilizing circuit according to claim 1, wherein the low input impedance current amplifier is a parallel feedback type amplifier.