JPH0345032A - Broad band dynamic optical reception circuit - Google Patents

Abstract

PURPOSE:To operate the circuit stably starting from one-shot optical pulse by constituting the circuit of 1st-3rd transistors(TRs), a feedback resistor, 1st-3rd resistors and plural diodes connected in series, and forming a diode array so as to be turned off when no light is made incident in a photodetector. CONSTITUTION:Emitters are connected together so as to constitute a differential circuit of 1st TR 20 and a 2nd TR 21 and both the emitters connect to a ground level. In such a case, the number of diodes 34 is selected so that the on-level of a series diode array 35 is higher than a base-emitter level of a 3rd TR 28. When a signal with a small current to flow to a photodetector is present, the gain is dependent on the feedback resistor, and when a signal with a large current to flow to the photodetector is present, a base level of the 1st TR of the differential amplifier circuit rises when the optical current reaches a prescribed value or over and excess optical current to the diode array is bypassed when the base level reaches a prescribed level. Thus, the circuit is operated from 1st-short optical pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a circuit for an optical receiving module used for optical transmission through an optical fiber, and particularly to a wide dynamic optical receiving circuit thereof.

(Prior Art) As a circuit of an optical receiving module used for optical transmission using an optical fiber, a circuit shown in FIG. 4 is shown in Japanese Patent Application Laid-Open No. 61-41213. In this circuit, a light receiving element ■ is connected between the input side of the inverting amplifier (1) and the ground potential, and a feedback resistor ■, gate, and drain are connected between the input side and the output side of the inverting amplifier (G). The connected field effect transistors (hereinafter referred to as FETs) (1) are connected in parallel. Instead of this FET@), a Schottky diode or the like may be used.

If the circuit is configured as above, even if light enters the light receiving element ■, the FET ■ will not operate and the feedback resistor ■ will not operate when the signal is small.
The gain is determined only by On the other hand, when the signal is large, the FET
(2) operates, the equivalent feedback resistance value decreases, and wide dynamic range is achieved.

However, in this circuit configuration, when the FET (2) begins to operate, the amount of feedback increases, so that peaking appears in the frequency characteristics, which may cause oscillation. In order to suppress this peaking, a capacitor must be inserted in parallel with the feedback resistor (2), resulting in a narrow band.

As another example of a circuit, Japanese Unexamined Patent Publication No. 144034/1983 shows a circuit shown in FIG. This circuit has a configuration in which a level detection circuit 0 is connected to the output side of a feedback amplifier 0, and the output signal of the level detection circuit 0 drives the transistor (2) to bypass the photocurrent of the light receiving element (2). However, in this circuit, the loop path for bypassing the photocurrent becomes long, so if a limiter is applied from the first optical pulse, the delay time becomes longer and stable operation becomes impossible. In other words, with this method,
AGC circuit (automatic gain circuit) for steady optical input
It can only work as .

A circuit similar to this is shown in Japanese Patent Laid-Open No. 10543/1983. In this circuit, A
In order to perform GC operation (automatic gain control operation), a capacitor (10) is connected as shown in FIG. 6, thereby smoothing the signal. Therefore, in this circuit,
It cannot be operated from the first light pulse.

As another example, JP-A-58-168343 discloses a device in which a diode (11) is connected in parallel with a transistor (8) to bypass the current at the time of a large signal, as shown in Figure 7. something is shown. Since this circuit has no feedback loop, it operates as a limiter from the t-th optical pulse. However, since the diode is connected in parallel between the diode (11) and the base-emitter of the transistor ■, the diode (11)
The photocurrent is bypassed and the sensitivity deteriorates. Furthermore, even in the case of a large signal, most of the current cannot be made to flow through the diode (11), and the transistor (2) becomes saturated, resulting in a slow speed.

Furthermore, as shown in FIG. 8, JP-A-58-214821 discloses that a load resistor (12) is connected in series with the light receiving element (1), and a Zener diode (13) is connected in parallel with the load resistor (12). It shows what to insert and apply the limiter. This circuit has a worse S/N ratio than the circuit using feedback amplifier 0 shown in FIG. 5 and cannot provide a wide band, so it cannot be used for high speed applications.

(Problems to be Solved by the Invention) As described above, there have conventionally been various types of optical receiving module circuits used for optical transmission through optical fibers. However, both circuits have problems such as unstable operation, narrow band, and inability to operate from the first optical pulse.

This invention was made to solve the above problems, and can be operated from the first light pulse.
It works stably and the bandwidth is wide enough.

It is an object of the present invention to configure an optical receiving circuit that also has a large dynamic range.

[Structure of the invention]

(Means for Solving the Problems) A wide dynamic optical receiver circuit includes a first and a second transistor whose emitters are connected to each other to form a differential circuit, and a ground potential between the two emitters. means for applying a bias potential to the base of the second transistor; a first resistor inserted between the collector of the first transistor and the power supply; a second resistor inserted between the collector of the transistor and the power source;
a third transistor whose base is connected to the collector of the first transistor and whose collector is connected to the power source; and an emitter of the third transistor. a feedback resistor inserted between the base of the first transistor;
A third resistor inserted between the emitter of the third transistor and the ground potential, and a plurality of series-connected diodes connecting the base of the first transistor to the ground potential, and the light receiving element The number of diodes is determined so that the diodes are turned off when no light is incident on the diodes.

(effect) When configured as above, at small signals.

The gain is determined by the feedback resistance. Furthermore, at the time of a large signal, the first and second transistors form a differential circuit, so when the photocurrent exceeds a certain value, the base potential of the first transistor increases, and this base potential becomes a constant potential. Then, the excess photocurrent is bypassed to the diode string. Therefore, the amount of feedback does not change depending on the signal size, and instead of automatic gain control due to feedback, it operates as a limiter, and operates from the first optical pulse. Optical reception is stable, has a wide band, and has a wide dynamic range. It can be a circuit.

(Example) Hereinafter, the present invention will be described based on an example with reference to the drawings.

FIG. 1 shows an optical receiving circuit that is one embodiment of the present invention.

In this circuit, the emitters of a first transistor (20) and a second transistor (21) are connected to each other so that a differential circuit is configured, and both emitters are connected to a current source (22). Connected to ground potential via. On the other hand, a first resistor (24) is connected between the collector of the first transistor (20) and the power input terminal (23), and a first resistor (24) is connected between the collector of the second transistor (21) and the power input terminal (23).
) A second resistor (25) is inserted between the base of the second transistor (21) and a current source (26) that provides a bias potential to balance the differential circuit.
is connected.

Further, the first resistor (24) is connected to the collector of a third transistor (28) which serves as an emitter follower, and the collector of the first transistor (20) is connected to the base of this third transistor (28). It is connected. The emitter of this third transistor (28) is grounded via a third resistor (29), and the emitter of this third transistor (28) and the first transistor (2
A feedback resistor (30) is inserted between the base of 0) and the base of 0). The light receiving element (32) connected to the power input terminal (23)
) is connected to the base of the first transistor (20). Further, the base of the first transistor (20) is connected to the light receiving element (32) and a diode string (35) consisting of a plurality of diodes (34) connected in series. is grounded through.

In the above circuit, the base potential VB8 of the first transistor (20) when there is no signal is the *g input terminal (23
) potential is V. . , the current flowing through the current source (22) is ■.

, the resistance value of the first resistor (24) connected to the collector of the first transistor (20) is R1, and the potential between the base and emitter of the third transistor (28) is VBf! Let x be.

is given by Here, the on-potential of the diode (34) constituting the series diode string (35) is set to VB[! Binding,
In other words, the on-potential of the series diode array (35) is higher than that of the VB circuit.

V at < n X V B11! The number of diodes (34) nt& is set so that.

This is, for example, V(H, H=5v, IO=2mA
, R,=0.5 and Ω, -Vax=3.8v, and when VBE2=0.7v, n
= 6, then 6 x VBB = 4.2v, which satisfies the above condition.

With the above configuration, when the photocurrent IP is a small signal and the resistance value of the feedback resistor (30) is R, the potential v0 of the output terminal (33) is determined by:

When the photocurrent IP becomes large, that is, it is fixed to the second transistor in the differential circuit, and the base potential BB□ of the first transistor (20) increases.
rises. And this first transistor (20
) exceeds the on-potential of the series diode string (35), the excess photocurrent flows through the series diode string (35).
35) and is clamped.

Therefore, the core that constitutes the circuit as described above and the photocurrent I
Even if P changes, the amount of feedback does not change, thereby making it possible to provide a wide-band, wide-dynamic optical receiving circuit that operates stably without oscillation.

Note that in the above embodiment, the resistor is connected to the collector of the second transistor, but the collector of the second transistor may be directly connected to the power input terminal without connecting the resistor.

Further, in the above embodiment, the first. Although the emitter of the second transistor is connected to the ground potential through a current source, similar results can be obtained by connecting a resistor instead of this current source.

Next, another embodiment of this optical receiving circuit is shown in FIG.
In particular, two diodes (36) are inserted in series between the emitter of the third transistor (28) and the third resistor (29) that connects it to the ground potential, and the first and second transistors ( 20) and (21) so that the gain of the differential circuit can be larger than that of the circuit shown in FIG. Note that in the circuit of this example, a fourth transistor having a resistance value equal to the resistance value of the feedback resistor (30) is connected to the base of the second transistor (21).
The resistor (37) is connected to the first transistor (20).
The circuit configuration is symmetrical to the base side.

FIG. 3 shows the first and second transistors (20), (
21), fourth and fifth transistors (39) and (40), each serving as an emitter follower, are inserted.

Note that in each of the above embodiments, the transistor used may be either an NPN type or a PNP type.

〔Effect of the invention〕

The emitters of the first and second transistors are connected to each other to form a differential circuit, a resistor is inserted between the collector of each transistor and the power supply, and the base is connected to the collector of the first transistor. A feedback resistor is inserted between the emitter of the third transistor and the base of the first transistor, and the base of the third transistor is turned off when no light is incident on the light receiving element. When connected to ground potential through a diode string consisting of a number of diodes connected in series, the feedback resistor determines the gain when the current flowing through the photodetector is small, and the current flowing through the photodetection element is At the time of a large signal, when the photocurrent exceeds a certain value, the base potential of the first transistor of the differential circuit increases, and when this base potential reaches a constant potential, the excess photocurrent is bypassed to the diode string. Therefore, due to the limiter operation, it is possible to create an optical receiving circuit that operates from a single optical pulse, is stable, has a wide band, and has a wide dynamic range.

[Brief explanation of drawings]

Figures 1 to 3 are detailed explanatory diagrams of this invention.
The figure is a diagram of a wide dynamic optical receiver circuit which is one embodiment of the circuit, and FIGS. 2 and 3 are diagrams of other different embodiments.
FIGS. 4 to 8 are diagrams of conventional wide dynamic optical receiver circuits, respectively. 20... First transistor 21... Second transistor 22... Ammeter 23... Power input terminal 2
4...First resistor 25...Second resistor 28.
...Third transistor 30...Feedback resistor 32...Light receiving element 33...
...Output terminal 34...Diode 35...
diode string

Claims (1)

[Scope of Claims] First and second transistors forming a differential circuit whose emitters are connected to each other; a current source connected between both of the emitters and a ground potential; and the second transistor. means for applying a bias potential to the base of the transistor; a first resistor inserted between the collector of the first transistor and the power source; and a first resistor inserted between the collector of the second transistor and the power source. a third resistor whose base is connected to the collector of the first transistor and whose collector is connected to the power source; a light receiving element inserted between the power source and the base of the first transistor; a feedback resistor inserted between the emitter of the third transistor and the base of the first transistor; and a third resistor inserted between the emitter of the third transistor and ground potential. and a diode array, which is composed of a plurality of series-connected diodes that connect the base of the first transistor to a ground potential, and whose number is determined so that it is turned off when no light is incident on the light receiving element. A wide dynamic optical receiver circuit comprising: