Preamplifier

Details Preamplifier Preamplifier

2000-10-24

2001-12-11

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including atomic particle or radiant energy impinging on a...

C330S110000, C250S2140AG

Reexamination Certificate

active

06329881

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photo receiver used in an optical transmission system and more particularly to a preamplifier capable of preventing an output waveform from distorting when a large light is input.
The present application claims the priority of Japanese Patent Application No. Hei11-303170 filed on Oct. 25, 1999, which is hereby incorporated by reference.
2. Description of the Related Art
In an optical transmission system, in order to convert a received input light into a signal voltage and in order to output the signal voltage, a circuit is generally used in which the received input light is converted into a current via a photodiode and then is amplified by a preamplifier so that a required signal voltage output is obtained.
A conventional preamplifier used for this purpose is disclosed in Japanese Patent Application Laid-open No. Hei 4-306904.
In the conventional preamplifier, as shown in
FIG. 16
, a photodiode
104
is connected between an input terminal
102
and a bias source V
H
of a differential amplifier
101
, a reference voltage V
ref
is connected to a reference input terminal
107
, a feedback resistor R
f
is connected between an inverting output terminal
103
and the input terminal
102
, an FET (Field Effect Transistor) Q
100
is connected in parallel with the feedback resistor R
f
, a level converting circuit
106
is connected to a positive phase output terminal
105
and an output of the level converting circuit
106
is connected to a gate of the FET Q
100
.
In operation of the conventional preamplifier, when an input light is received, an optical current I
ph
flows into the input terminal
102
via the photodiode
104
. With this operation, an output voltage V
out
occurs at the inverting output terminal
103
. At this time, feedback is applied via the feedback resistor R
f
, and thereby a constant amplifying gain can be obtained. When the input light level is excessive, an output occurs from the level converting circuit
106
caused by the output voltage from the positive phase output terminal
105
, therefore, the FET Q
100
becomes conductive and a feedback resistance value becomes small. As a result, the amplifying gain lowers and the output voltage V
out
is prevented from increasing, therefore, the differential amplifier
101
is prevented from saturating.
Also, there is disclosed an another conventional preamplifier used for the above-mentioned purpose is disclosed in Japanese Patent Application Laid-open No. Hei 4-306905.
In the another conventional preamplifier, as shown in
FIG. 17
, a photodiode
104
is connected between an input terminal
102
and a bias source V
H
of a differential amplifier
101
, a reference voltage V
ref
is connected to a reference input terminal
107
, a feedback resistor R
f
is connected between an inverting output terminal
103
and the input terminal
102
, an FET (Field Effect Transistor) Q
101
is connected between the input terminal
102
and a ground potential V
BB
, a level converting circuit
106
is connected to a positive phase output terminal
105
and an output of the level converting circuit
106
is connected to a gate of the FET Q
101
.
In operation of the another conventional preamplifier, when an input light is received, an optical current I
ph
flows into the input terminal
102
via the photodiode
104
. With this operation, an output voltage V
out
occurs at the inverting output terminal
103
. At the time, a feedback is applied via the feedback resistor R
f
, and thereby a constant amplifying gain can be obtained. When the input light level is excessive, an output occurs from the level converting circuit
106
caused by the output voltage from the positive phase output terminal
105
, therefore, the FET Q
101
becomes conductive and the optical current I
ph
is distributed to the ground potential V
BB
. As a result, amplifying gain lowers and the output voltage V
out
is prevented from increasing, therefore, the differential amplifier
101
is prevented from saturating.
With the conventional preamplifiers shown in FIG.
16
and
FIG. 17
, when the input light exceeds a predetermined level, the output voltage V
out
is prevented from increasing, therefore, the differential amplifier
101
can be prevented from saturating.
However, at this time, since the level converting circuit
106
detects increment in a polarity direction of a positive phase output voltage and then generates an output, one side of a waveform of the output voltage V
out
is clipped in accordance with an operation of the FET Q
100
or the FET Q
101
.
FIG. 18
shows an example of the output voltage when an input current is bypassed in the conventional preamplifiers shown in FIG.
16
and FIG.
17
.
In
FIG. 18
, a |voltage| designates a potential difference of both ends of the feedback resistor R
f
after bypassing an over-current, a V
0
designates a voltage when an optical current I
ph
is minimum, an I
ph
XR
f
designated by a dotted line designates a voltage when the optical current I
ph
is maximum and the differential amplifier
101
is not saturated. Actually, when the optical current I
ph
is large, the output voltage is limited in accordance with the operation of the FET Q
100
or the Q
101
. As a result, therefore, the |voltage| varies asymmetrically up and down as designated by a solid line.
As above described, in the conventional preamplifier circuit, there is a problem in that an output waveform is distorted when the optical input level is excessive.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a preamplifier capable of preventing an output waveform from distorting though an optical input level of an optical receiver is large.
According to a first aspect of the present invention, there is provided a preamplifier including:
a current feedback circuit including an amplifier for amplifying an input optical current and a feedback resistor connected between an input and an output of the amplifier;
an average detecting circuit for detecting an average of an output voltage of the current feedback circuit; and
a current controlling circuit for distributing the input optical current in accordance with a detected average of the output voltage.
In the foregoing, a preferable mode is one wherein the current feedback circuit includes a grounded-emitter transistor amplifier for amplifying the input optical current, an emitter follower transistor amplifier for amplifying an output from the grounded-emitter transistor amplifier and a feedback resistor connecting an output of the emitter follower transistor amplifier with an input of the grounded-emitter transistor amplifier.
Also, a preferable mode is one wherein a bypass circuit for clipping a voltage more than a predetermined threshold bi-directionally is connected in parallel with the feedback resistor of the current feedback circuit.
Also, a preferable mode is one wherein the bypass circuit includes two diodes connected in parallel in an opposite direction to each other.
Also, a preferable mode is one wherein the current controlling circuit is a current mirror circuit for distributing the input optical current in accordance with the output voltage of the average detecting circuit.
Also, a preferable mode is one wherein the average detecting circuit includes an integrating circuit for smoothing the output voltage of the current feedback circuit and an emitter follower circuit for amplifying the output voltage smoothed by the integrating circuit.
Also, a preferable mode is one wherein a comparing circuit is provided at an output of the average detecting circuit and outputs an average voltage exceeding a reference voltage.
Furthermore, a preferable mode is one wherein the reference voltage is generated by a non-input current feedback circuit having a same configuration of the current feedback circuit.
With the above configurations, the preamplifier includes a current feedback circuit for amplifying an input optical current, an av

Affiliated with

Also associated with

Rating

Preamplifier does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Preamplifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Preamplifier will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2593096