JPH11145913A - Preamplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a preamplifier with which an optical. reception signal can be normally received without the occurrence of waveform distortion even when prebias is contained. SOLUTION: An optical signal is converted to a current signal by a photodiode(PD) 1 and converted/amplified to a voltage signal by a trans- impedance type amplifier circuit 12. When the input signal from the PD 1 is accompanied with the prebias, the offset level of an automatic offset control(AOC) 11 is shifted to timing corrected just for the prebias by a reset signal 6. In the case of containing the prebias more than a fixed amount and in the case of containing no prebias, the offset '0' level of an output signal from the amplifier circuit 12 is different. Therefore, the optimum threshold voltage in each case is set by a first automatic threshold control(ATC) 8 and a second ATC 9 and while receiving a control signal from a prebias detection circuit 7, the outputs of the first and second ATC are selected by a selector 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preamplifier capable of amplifying and receiving a signal without causing waveform distortion even when receiving an input current when receiving an optical signal with offset light (pre-bias) in the input signal. It is about.

[0002]

2. Description of the Related Art Conventionally, a PON handling a burst signal
(Passive Optical Network)
In the system, many receivers (receiving circuits) for the optical power difference between individual burst signals have been developed and put into practical use.

[0003] However, with the increase in the number of subscribers and the reduction in the price of the transmitter / receiver, a reduction in the cost of optical components is also required.

[0004] Conventional backbone transmission equipment and PON at the beginning of development
In the system, an LD (Laser D) for transmitting an optical signal is used.
iode) also has a relatively high performance.
Also, technically, a desired optical output signal could be transmitted without applying a bias voltage to the LD.

[0005]

On the other hand, in order to reduce the cost of the transmitter / receiver, there is a tendency to use inexpensive optical components. Otherwise, there is a problem that a desired optical output waveform cannot be obtained.

As a result, a signal having an optical output with offset light (pre-bias) is transmitted in advance, and when the optical signal is converted into a current signal by a PD (Photo Diode) on the receiver side, an offset is generated. It becomes the input current. Further, not all of the burst signal transmitters are accompanied by the pre-bias as described above, and the input condition is applied to a plurality of specific transmitters. There is a need.

An object of the present invention is to provide a preamplifier that can receive signals normally without causing waveform distortion even if the optical reception signal includes a prebias.

[0008]

According to the present invention, a PD is provided.
The optical signal is converted into a current signal by (Photo Diode), and is converted and amplified into a voltage signal by a transimpedance amplifier formed by an operational amplifier and a feedback resistor.

At this time, if the input signal from the PD is accompanied by an input signal step (pre-bias), an AOC (Automatic Offset) is generated by the reset circuit.
Control) is shifted to a timing corrected by the pre-bias.

The offset '0' level of the output signal of the transimpedance amplifier differs between the case where the pre-bias is included in a certain amount or more and the case where the pre-bias is not included. For this reason, the first ATC (Automatic T
threshold control) and the second ATC to set the optimum threshold voltage in each case, and receive the control signal from the pre-bias detection circuit to select the output of the first ATC and the second ATC by the switch. I do.

[0011]

FIG. 1 is a diagram showing a configuration of an embodiment of a preamplifier according to the present invention.

In FIG. 1, a photodiode (PD)
1 converts the optical signal into an electric current signal. The transimpedance amplifier 12 (hereinafter, core amplifier) formed by the operational amplifier 3 and the feedback resistor 2
It is converted and amplified into a voltage signal. This voltage signal is a signal inverted from the input current signal.

In order to cancel the output offset of the core amplifier 12, an automatic offset control (AO
C) The input DC offset level is feedback-controlled by 11. The AOC 11 includes a peak detection unit 5 that detects a peak value of a core amplifier output, and an AOC control unit 4 that adjusts a DC offset level based on a control signal from the peak detection unit 5.

When the optical input signal to the PD 1 is accompanied by an offset light in advance, the offset amount corresponding to the offset light is
2, the output of the core amplifier is distorted. As a countermeasure, the reset circuit 6 generates a self-reset signal to control the peak detection unit 5 of the AOC circuit 11.

Since the absolute value level controlled by the AOC 11 is different between the DC offset level when the pre-bias is included and the DC offset level when the pre-bias is not included, an automatic threshold control (ATC) corresponding to each level is performed. ) A circuit is required.

For example, in the circuit diagram of FIG. 1, the first ATC 8
Is a circuit type in which the threshold voltage is set higher than the DC offset level, and the second ATC 9 is set lower than the DC offset level.

The switching between the outputs of the ATCs 8 and 9 is selected by a selector 10 in response to a control signal from the pre-bias detection circuit 7. The pre-bias detection circuit 7 may detect whether the rising of the current signal (the first falling of the core amplifier 12) is larger or smaller than a specified level when light is first input to the PD 1 from the light non-input state. .

Next, the operation of the circuit of FIG.
This will be described with reference to FIG. In the time chart of FIG. 2, a signal obtained by converting an optical signal into an electric signal (current signal) by the PD 1 is represented as an input current (a). This input current includes a pre-bias and causes an offset. The reset circuit 6 generates a self-reset signal (b) corresponding to the rising edge of the pre-bias, and the self-reset signal controls the peak detector 5 of the AOC circuit 11 to reset the offset level corresponding to the pre-bias. Detected as '0' level.

As a result, the offset cancel signal (c) is extracted by the same amount as the offset level. The input signal (current) to the core amplifier 12 at this time is the post-cancellation input current (d) in FIG. 2, and the output of the core amplifier 12 is obtained as the core amplifier output voltage (e) in FIG. 2 according to this input current.

On the other hand, when the pre-bias is not included in the input signal, the time chart is as shown in FIG.
When there is no pre-bias, the reset circuit 6 operates at the rising edge of the first bit of the input signal (a).
The High level of the bit is detected as the offset '0' level.

As a result, the offset cancel signal (c) extracts the same amount as the High level (1st bit), and the High level of the core amplifier output operates as the offset '0' level. Therefore, the output signal (voltage) of the core amplifier 12 is output above the offset '0' level, and it can be seen that the output operation level is different from that when the pre-bias is included.

As can be seen from the time charts of FIGS. 2 and 3, when the input signal includes a pre-bias and when the input signal does not include a pre-bias, the output level of the core amplifier with respect to the offset “0” level is different. It is necessary to set the voltage. In the circuit diagram of FIG.
C8 and ATC9 are prepared, and different threshold voltages are set respectively.

The ATC 8 has a function to detect the offset '0' level and the bottom value of the output of the core amplifier 12 and to set the threshold voltage at an intermediate level between them. This operation is shown in FIG. 2, and is receivable when the input signal has a pre-bias.

On the other hand, the ATC 9 is designed to detect the offset '0' level and the peak value of the output of the core amplifier 12 and to set the threshold voltage to an intermediate level between them. This operation is shown in FIG. When the input signal has no pre-bias, the signal can be received.

The output of these ATCs 8 and 9 is detected by a pre-bias detection circuit 7 as to whether or not a pre-bias is present.
0 selects either output signal. As can be seen from the time charts of FIGS. 2 and 3, it is clear that there is no problem in the output logic in any case.

Next, another embodiment of the preamplifier of the present invention will be described with reference to FIG.

This preamplifier is the same as that shown in FIG. 1 except that a saturation prevention circuit 13 is provided in parallel with the feedback resistor 2. Normally, the transimpedance preamplifier becomes saturated when the current signal input to the core amplifier 12 is excessively large, causing output waveform distortion. In order to avoid this saturation phenomenon, the current flowing into the feedback resistor is adjusted by a saturation prevention circuit to prevent waveform distortion.

In the saturation prevention circuit 13 shown in FIG. 4, a diode is connected in parallel with the feedback resistor 2, and the voltage amplitude generated in the feedback resistor is adjusted to the forward voltage of the diode (normally 0.7V to 0.7V).
0.8V).

[0029]

According to the preamplifier of the present invention, even if a pre-bias is included in the optical reception signal, it is possible to receive the signal normally without causing waveform distortion. The reason is that an AOC circuit is provided which performs a self-reset corresponding to the pre-bias, detects an offset corresponding to the pre-bias, and has an offset cancel function.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a preamplifier of the present invention.

FIG. 2 is a time chart illustrating an operation of the preamplifier of FIG. 1 when an input signal includes a prebias;

FIG. 3 is a diagram when the input signal does not include a pre-bias;
6 is a time chart showing the operation of the preamplifier.

FIG. 4 is a diagram showing another embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 PD 2 feedback resistor 3 operational amplifier 4 AOC controller 5 peak detector 6 reset circuit 7 pre-bias detector 8, 9, ATC 10 selector 11 AOC circuit 12 core amplifier 13 saturation prevention circuit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H03F 3/08 3/34

Claims (7)

[Claims] 1. A photoelectric conversion means for converting an optical signal into a current signal, a current-voltage conversion means for converting the current signal into a voltage signal, and a feedback control of a DC offset level of an input of the current-voltage conversion means, Means for canceling the output offset of the current-to-voltage conversion means, and detecting the output of the current-to-voltage conversion means using different threshold voltage values depending on whether or not the optical signal includes a pre-bias. A preamplifier. 2. The means for canceling the output offset comprises: a peak detector for detecting a peak value of an output of the power voltage converter, and an automatic offset for adjusting the DC offset level by a control signal from the peak detector. A control unit, and a reset circuit that generates a self-reset signal when the output of the photoelectric conversion unit changes from a true offset level 0 level. When the reset signal is generated, the peak detection unit is reset and controlled. 2. The preamplifier according to claim 1, wherein an offset 0 level due to a self-reset is detected, and an offset cancel signal having the detected offset 0 level as a value is generated in the automatic offset control unit. 3. The first automatic threshold control circuit having a threshold voltage corresponding to an output voltage of the current-to-voltage converter when the optical signal includes a pre-bias. A second automatic threshold control circuit having a threshold voltage corresponding to the output voltage of the current-to-voltage converter when the optical signal does not include a pre-bias; and determining whether the optical signal has a pre-bias. A pre-bias detection circuit for detecting, and a control signal output from the pre-bias detection circuit,
3. The preamplifier according to claim 2, further comprising: a select circuit that selects an output of each of the first and second threshold control circuits. 4. The preamplifier according to claim 1, wherein said photoelectric conversion means is a photodiode, and said current-voltage conversion means is a transimpedance amplifier. 5. The preamplifier according to claim 4, wherein said transimpedance amplifier comprises an operational amplifier and a feedback resistor. 6. The preamplifier according to claim 5, wherein a saturation prevention circuit is connected in parallel with said feedback resistor. 7. The preamplifier according to claim 6, wherein said saturation prevention circuit is a diode.