JPH10242774A - Optical reception front end amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical reception front end amplifier having a high speed and sure saturation prevention function and having advantage of circuit integration. SOLUTION: A reception current from a photo diode 6 receiving a small reception light is amplified by a feedback transimpedance circuit consisting of transistors(TRs) 7, 8 and a feedback resistor 10 with a high sensitivity. Furthermore, a reception current from the photo diode 6 receiving a high reception light is given the TR 7, but the saturation of which is avoided by making a TR 9 conductive with a voltage drop across a feedback resistor 10 so as to decrease the feedback amount and to discharge the excess component to ground via an emitter and a collector of the TR 9.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiving amplifier for PCM optical communication and the like, and more particularly to an optical receiving front-end amplifier in a photoelectric conversion circuit which does not saturate when receiving large input light.

[0002]

2. Description of the Related Art It is necessary for an amplifier in an optical receiving section of optical communication to have a sufficient gain for a small optical signal and operate without saturating a large optical signal. As an amplifier, for example, a circuit described in JP-A-64-18304 is known.

FIG. 3 shows an amplifier described in the above publication. The configuration of the amplifier shown in FIG. 1 includes a light receiving element 12 for receiving a signal light and converting it into a current signal, a common emitter type transistor 13 for amplifying the photoelectric conversion output of the light receiving element 12, and a common collector type transistor for generating an output voltage. 14 and
The output voltage of the transistor 14 is fed back to the input of the transistor 13 by a parallel circuit of the resistor 16 and the diode 17 forming a non-linear feedback circuit.

[0004] The amplifier circuit performs the following operation for a high light receiving signal.

As the light receiving level increases, the light receiving element 1
The current supplied from 2 to the base of the transistor 13 increases. When the current increases, the diode 17 conducts and the current shunts to the diode. In this state, the same effect as increasing the feedback amount is obtained, and the current-voltage conversion gain is reduced.
That is, even when the light receiving level is high, the amplification operation is stabilized without saturating the transistor.

[0006]

In a conventional amplifier, in order to prevent saturation of the amplifying element when the light receiving current from the light receiving element increases, a feedback circuit is formed by a combination of a resistor and a diode to prevent the light receiving current from flowing. Is increased so that the amount of feedback from the output side is increased at the time of increase.

However, in the configuration of the above-mentioned conventional amplifier, as a countermeasure against the case where the received light current increases, the diode is turned on to reduce the feedback resistance, thereby preventing saturation as a result. The saturation of the transistor cannot be sufficiently suppressed, and it has not been possible to prevent the saturation at high speed and reliably.

(Object of the Invention) It is an object of the present invention to provide a light receiving front-end amplifier having a saturation preventing function capable of reliably preventing saturation at high speed when receiving large light.

It is another object of the present invention to provide an optical receiving front-end amplifier having a saturation prevention function which has a simple circuit configuration and is advantageous for integration.

[0010]

More specifically, an optical receiving front-end amplifier according to the present invention comprises a grounded emitter transistor connected to the light receiving element, and an emitter follower having an output of the grounded emitter transistor as an input. And a feedback type transimpedance circuit comprising a feedback resistor connected between the input and output of the amplifier, and an emitter and a base connected to the feedback resistor in parallel and a collector connected to a reference potential point. Having a transistor for preventing saturation.

Preferably, the common emitter type transistor is an NPN type transistor, the saturation prevention transistor is a PNP transistor, and the threshold value of the saturation prevention transistor is smaller than the threshold value of the common emitter type transistor.

(Operation) The transistor for preventing saturation lowers the feedback resistance when a large light receiving signal is input, and also discharges a surplus of the light receiving current to the reference potential point side to prevent saturation of the common-input-emitter type transistor.

[0013]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the principle of an optical receiving front-end amplifier according to the present invention.

As shown in FIG. 1, the basic structure of the amplifier of the present invention is a photodiode 1 for converting an optical signal into an electric signal.
An amplifier 2 having at least a common-emitter transistor in an input stage, and an optical receiving front-end amplifier having a feedback resistor 3 connected between the input and output of the amplifier 2, wherein the emitter-base is connected in parallel to the feedback resistor 3. And a saturation preventing transistor 4 whose collector is connected to the reference potential point. The amplifier having the above configuration constitutes a feedback transimpedance circuit whose transfer impedance is represented by a feedback resistor.

When an optical signal is input to a light receiving element, a light receiving current proportional to the intensity of the light flows through the amplifier, and this current is converted into a voltage signal by a feedback resistor. At this time, when the optical signal is small, the PNP transistor arranged in parallel with the feedback resistor does not conduct, so that the feedback resistor directly contributes to the current-voltage conversion operation (voltage signal = light receiving current × feedback resistance) as transimpedance (transmission resistance). .

When the light input increases and the light receiving current × feedback resistance> V _BE (V 2) of the PNP transistor 4, P
Since the NP transistor conducts, the input voltage signal is limited, and the saturation of the front-end amplifier is suppressed due to the relationship of V1> V2. Further, the surplus current is discharged to the ground through the emitter and collector of the PNP transistor to prevent saturation.

Based on the above principle, the amplifier of the present invention realizes high sensitivity characteristics without deteriorating the transfer resistance at the time of small light reception, and prevents the transistor of the front-end amplifier from saturating at the time of large light reception, and Data errors can be avoided.

FIG. 2 is a diagram showing an embodiment of the optical receiving front-end amplifier according to the present invention.

In this embodiment, a photodiode 6 for converting an optical signal into an electric signal, a first-stage transistor 7 of a common-emitter type connected to the photodiode 6, and an emitter follower having a base connected to the collector of the first-stage transistor 7 -Type next-stage transistor 8 and the next-stage transistor 8
In the optical receiving front-end amplifier having the feedback resistor 10 connected from the output load resistor 11 to the base of the first-stage transistor 7, the emitter-base is connected in parallel to the feedback resistor 10, and the collector is connected to the reference potential point. And a transistor circuit including the saturation prevention transistor 9 between the power supply and the reference potential point. The amplifier having the above configuration forms a feedback transimpedance amplifier in which the input impedance is low and the transfer impedance is determined by the feedback resistor.

In the amplifier having the above-described configuration, the relationship between the base-emitter voltages of the first-stage transistor 7 and the saturation preventing transistor 9 is preferably such that the respective base-emitter voltages are V _BE1 and V _BE 2. V _BE 1>
Set as _VBE2 .

The operation of the present embodiment will be described. In the above-described configuration, the photodiode 6 performs a photoelectric conversion operation as a current source with respect to the optical input Pr as an input, and the received light Ir is changed according to the received light. , Ir = k × Pr (k is a constant). This current is amplified by a two-stage transistor as a voltage signal by the input impedance of the amplifier and output as a voltage Vo.

Here, when the light receiving current Ir is sufficiently small, the output voltage Vo is given by: Vo = I
It can be expressed as r × RF.

On the other hand, when the optical signal input is in a high input state, the light receiving current Ir increases, and the light receiving current Ir from the photodiode 1 increases such that the voltage drop on the feedback resistor side approaches the threshold voltage between the base and the emitter of the transistor 7. When output, that is, Ir × (RF + Ro) ≒ Ir × RF> V _BE 1
, The threshold voltage V _BE2 between the base and the emitter of the transistor 9 for preventing saturation is higher than the threshold voltage V _BE1 between the base and the emitter of the first-stage transistor 7 under the above-mentioned condition (V _BE 1> V _BE 2). Since the threshold value is small, the transistor 9 for preventing saturation becomes conductive, and furthermore, the light receiving current Ir
Approaches the saturation region in accordance with the increase of.

In this state, the feedback resistance substantially decreases and the feedback amount increases. In addition, the excess current due to the increase of the light receiving current Ir is discharged to the ground through the emitter-collector of the saturation prevention transistor 9. Therefore, it is possible to reliably prevent the emitter potential of the saturation prevention transistor 9 from rising to a high potential exceeding the threshold voltage V _{BE1 of the} transistor 7. Therefore, the first-stage transistor 7 has its base-emitter voltage Vi
BEV _BE is clipped to about 2 and the output Vo due to the saturation of the feedback amplifier constituting the feedback transimpedance circuit
Is prevented from occurring.

In the above description of the embodiment, the first-stage transistor 7 is an NPN transistor and the saturation prevention transistor 9 is a PNP transistor.
Since the threshold value V _{BE of the} PN transistor is larger than the threshold value of the PNP transistor, the above condition V _BE1 > V _BE 2 is satisfied by the transistors formed on the same wafer.

Even if both transistors have substantially the same characteristics with respect to the threshold value, the operation of discharging the excess current of the received light current Ir from the collector of the transistor 9 to the ground before the saturation of the saturation prevention transistor starts. Therefore, it is apparent that the saturation operation of the first-stage transistor 7 functions effectively effectively.

[0028]

According to the present invention, a feedback type transimpedance circuit configuration which is the most effective for realizing high sensitivity is employed, and only a transistor for preventing saturation is connected. As the amount can be increased and the excess received light current can be discharged to the ground side, and saturation can be prevented quickly and reliably, high sensitivity and large received light signals can be received, and a wide dynamic range is secured. it can.

As a preferable means for realizing the present invention, the above-described condition of the threshold value of the transistor can be easily satisfied by forming a circuit in the same wafer, which is extremely advantageous for integration.

[0030]

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of an optical reception front-end amplifier according to the present invention.

FIG. 2 is a diagram showing an embodiment of an optical reception front-end amplifier according to the present invention.

FIG. 3 is a diagram showing a conventional optical receiving amplifier.

[Explanation of symbols]

 1, 6, 12 Photodiode 2 Amplifier 3, 10, 16 Feedback resistor 4, 9 Saturation prevention transistor 5 Feedback transimpedance circuit 7, 13 First stage transistor 8, 14 Next stage transistor 11, 15 Load resistance 17 Diode

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H04B 10/04 10/06

Claims (3)

[Claims] 1. An optical receiving front-end amplifier having a feedback transimpedance circuit including an amplifier having at least a grounded-emitter transistor at an input stage and a feedback resistor between an input and an output, receiving a current of a light receiving element as an input. An optical receiving front-end amplifier, comprising: a transistor for preventing saturation in which an emitter and a base are connected in parallel to a feedback resistor and a collector is connected to a reference potential point. 2. An amplifier comprising a light receiving element, a common emitter transistor connected to the light receiving element, and an emitter follower transistor having an output of the common emitter transistor as an input, and an input / output of the amplifier. An optical reception front-end amplifier, comprising: a feedback resistor connected thereto; and a transistor for preventing saturation having an emitter and a base connected in parallel to the feedback resistor and having a collector connected to a reference potential point. 3. The common-emitter type transistor is an NP-type transistor.
3. The optical receiving front-end amplifier according to claim 2, wherein the transistor is an N-type transistor, the transistor for preventing saturation is a PNP transistor, and a threshold value of the transistor for preventing saturation is smaller than a threshold value of the common-emitter type transistor.