JP2002171147A - Wide band preamplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate no distortion due to an offset voltage even if a wide band preamplifier is constructed by DC-coupling of an amplifier. SOLUTION: An optical current of a photodetector 1 is converted into voltage by means of the preamplifier 2, and the voltage signal is amplified by voltage amplifiers 3, 4 and 5 which are structured in three stages, An offset cancellation circuit 10 is connected in parallel with the voltage amplifier 4. A forward and a reverse output signal of the voltage amplifier 3 are inputted into two integration circuits. A differential output voltage of the two integration circuits is an offset. The offset is amplified by a differential circuit consisting of transistors 6 and 7, and then is inputted into an output section of the voltage amplifier 4. By making the amplification factor of the voltage amplifier 4 and that of the differential circuit consisting of the transistors 6 and 7 consistent with each other, the offset can be cancelled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a broadband preamplifier, and more particularly to a wideband preamplifier mounted on a light receiving module for optical communication.

[0002]

2. Description of the Related Art In recent years, with the development of IT (information technology), the demand for communication has been increased, and accordingly, the importance of optical communication has been increasing. One of the key devices that support optical communication is a light receiving module. The light-receiving module includes a light-receiving element such as an avalanche photodiode, and an IC that converts a minute photocurrent detected by the light-receiving element into a voltage signal and amplifies the signal to a logic level conforming to the standard.
(MMIC). Therefore, in order to satisfy the standard, the photocurrent must be 1
It is required to amplify up to 800 mV in a band of 00 kHz to 2.5 GHz. In order to amplify a signal over such a wide band with a high amplification degree, HEMT or M
Common source type differential amplifier using ESFET etc.
It is necessary to cascade connection by DC coupling without using a coupling capacitor between circuit stages.

FIG. 3 is a circuit diagram of a conventional light receiving module for optical communication. In the figure, the bold line indicates the package of the light receiving module, and accordingly, the circuits inside the bold line are circuits constituting the light receiving module. In the light receiving module, a light receiving element 1 such as an avalanche photodiode (APD) for detecting the intensity of light, a preamplifier 2 for converting a photocurrent generated in the light receiving element 1 into a voltage signal, and an output of the preamplifier 2 The differential type voltage amplifiers 3 to 5 for amplifying a signal, and an integrating circuit including a resistor R1 and a capacitor C1 are mounted. Among them, the light receiving element 1 and the capacitor C1 are individual components, and the preamplifier 2, the voltage amplifiers 3 to 5 and the resistor R1 are configured on one chip. It is assumed that the input optical signal has a duty = 50%, so that the output signal of the integration circuit substantially indicates the amplitude center of the output signal of the preamplifier 2.

[0004]

As described above, in a wideband preamplifier, since voltage amplifiers are connected in multiple stages by DC coupling, when an offset occurs in the first stage amplifier,
This will be amplified one after another in each stage. The offset can be reduced by symmetrically arranging the paired transistors in close proximity.However, in a high-frequency circuit, the wiring arrangement for avoiding crosstalk between the positive-phase signal and the negative-phase signal is prioritized and the relative accuracy is reduced Since the transistors may be sacrificed, the threshold voltages Vth of the paired transistors may be formed differently, and offset is likely to occur.

FIG. 4 is a diagram showing signal waveforms at various parts of the circuit of FIG. The symbols described on the waveforms in FIG. 4 correspond to the symbols given to each part in FIG. When light enters the light receiving element 1, a current having a waveform A flows according to the optical signal, and a voltage having a negative waveform B is output from the preamplifier 2. If an offset Vos is generated in the voltage amplifier 3 to which this voltage signal and its integrated value are input, the output signal includes an offset as shown in waveforms D and E. When the amplification factor of the amplifier at the next stage is A, the offset Vos at the output signal of the next stage is A · Vos as shown by waveforms F and G. As described above, since the offset is amplified in each stage, a large offset is output from the highest stage. When this exceeds the output dynamic range, it becomes impossible to determine the signal. Further, when the signal contains a large offset without exceeding the output dynamic range, as shown in waveforms F and G; waveforms J and K, waveform distortion occurs and causes erroneous determination. An object of the present invention is to solve the above-described problems of the related art, and an object of the present invention is to make it possible to cancel an offset generated in a first-stage amplifier in a subsequent stage so that a signal that can be determined without error is provided. The purpose is to provide a broadband preamplifier for outputting.

[0006]

According to the present invention, there is provided a broadband preamplifier including a differential amplifier connected in multiple stages, the nth (n being a positive integer) stage of the present invention. Offset level detection means for detecting an offset appearing in the output circuit of the amplifier of the above (1), and canceling the offset by inputting the output signal of the offset level detection means to the output circuits of the (n + 1) th and subsequent amplifiers. A broadband preamplifier is provided. Preferably, the offset level detecting means includes an integrating circuit connected to each of two output terminals of the n-th stage amplifier, and a differential amplifier circuit to which an integrated value of each integrating circuit is input. Be composed.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG.
Is a circuit showing an embodiment of the present invention, and the inside of the bold line in the figure is a circuit constituting a light receiving module for optical communication.
In the drawing, VDD, VSS, and VPD indicate power sources, respectively. One end of the light receiving element 1, which is an avalanche photodiode, is connected to a power supply VPD for the light receiving element, and the other end is connected to an input terminal of a preamplifier 2 that converts an output current of the light receiving element 1 into a voltage signal. The output of the preamplifier 2 is input to a positive input terminal (+) of the voltage amplifier 3, and a resistor R1 (200 kΩ) and a capacitor C1 (1
00pF). The output signal of this integration circuit is input to the negative input terminal (-) of the voltage amplifier 3.

A positive output terminal (+) and a negative output terminal (−) of the voltage amplifier 3 are connected to a positive input terminal (+) and a negative input terminal (−) of the voltage amplifier 4 respectively. Are connected to a positive input terminal (+) and a negative input terminal (-) of the voltage amplifier 5, respectively. The negative-phase output terminal (−) and the positive-phase output terminal (+) of the voltage amplifier 5 are connected to the resistors R4 (50Ω) and R5 (50Ω) via capacitors C4 (0.1 μF) and C5 (0.1 μF), respectively. ). The power terminals of the preamplifier 2 and the voltage amplifiers 3 to 5 are connected to power sources VDD and VSS, respectively.

An offset canceling circuit according to the present invention is connected to the voltage amplifier 4 in parallel. The offset cancel circuit 10 includes a resistor R3 (100 kΩ), a resistor R4 (100 kΩ), and a capacitor C2 (100pΩ).
F) and two integrating circuits composed of C3 (100 pF), and a differential amplifying circuit composed of field-effect transistors 6, 7 and a constant current source 8 to which output signals of these integrating circuits are inputted. And That is, one end of the resistor R2 (R3) is connected to the negative (positive) input terminal (-) [(+)] of the voltage amplifier 4, and the other end is connected to the gate of the transistor 6 (7) and the capacitor. It is grounded via C2 (C3), and the drain of transistor 6 (7) is connected to the opposite (positive) phase output terminal (-) [(+)] of voltage amplifier 4. The sources of the transistors 6 and 7 are commonly connected to a power supply VSS via a constant current source 8. Here, the preamplifier 2 and the voltage amplifiers 3 to 5 are configured using field-effect transistors, and the circuit of the optical communication light receiving module except for the light receiving element 1 and the chip capacitors C1 to C3. It is configured on one chip.

Next, the circuit operation of the embodiment of the present invention shown in FIG. 1 will be described in detail together with the waveforms of the respective parts shown in FIG. The symbols described on the waveform in FIG. 2 correspond to the symbols given to the respective parts in FIG. When light enters the light receiving element 1, a current having a waveform A flows according to the optical signal, and a voltage having a negative waveform B is output from the preamplifier 2. The signal of the waveform B is input to an integrating circuit constituted by the resistor R1 and the capacitor C1. Here, the input signal is
Assuming = 50%, waveform C shows a DC voltage that is approximately の the amplitude of waveform B.

The output of the voltage amplifier 3 has a waveform including the offset voltage Vos as shown by waveforms D and E, and the respective signals are input in parallel to the voltage amplifier 4 at the next stage and the offset cancel circuit 10 according to the present invention. You. When the signals of the waveforms D and E are input to the integration circuit formed by the resistors R2 and R3 and the capacitors C2 and C3, the integration circuit
The waveforms H and I having substantially the intermediate value of the amplitude of E are output (dut
assuming y = 50%).

That is, the voltage difference between the two waveforms becomes the offset voltage Vos. This offset voltage Vos is input to the gate electrodes of the transistors 6 and 7. Here, if the amplification of the differential amplifier circuit constituted by the transistors 6 and 7 is made to match the amplification of the voltage amplifier 4, the offset can be canceled at the output section of the voltage amplifier 4. That is, the signals of the waveforms F and G not including the offset are input to the voltage amplifier 5. As a result, the waveforms J and K
As a result, a signal having no distortion as shown in (1) is output, and erroneous determination is not made on the output signal.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and appropriate modifications can be made without departing from the spirit of the present invention. Things. For example, in this embodiment, the voltage amplifier has a three-stage configuration, but the number of stages may be increased as appropriate in relation to the amplification factor. Further, the voltage amplifier to which the offset cancel circuit is connected in parallel is not limited to the second stage, and may be a subsequent amplifier. Further, the offset cancel circuit may be connected so as to straddle a plurality of stages of amplifiers. Further, the constants of the resistor and the capacitor are not limited to the values in the embodiment, but can be changed as appropriate. Further, in the embodiment, the circuit is configured using the field effect transistor, but a bipolar transistor may be used instead of the field effect transistor.

[0014]

As described above, the wideband preamplifier according to the present invention detects the offset contained in the output signal of one amplifier constituting the multistage amplifier, and uses this to detect the offset contained in the output signal of the subsequent amplifier. Therefore, even if a wide-band amplifier circuit is formed by DC coupling, the occurrence of the offset can be suppressed, and an output voltage with less waveform distortion can be obtained. Therefore, when a light receiving module is configured by using this broadband preamplifier, erroneous determination of an output signal can be prevented and a light receiving module with stable operation can be realized.

[Brief description of the drawings]

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

FIG. 2 is a waveform chart of each part of the embodiment shown in FIG.

FIG. 3 is a circuit diagram of a conventional example.

FIG. 4 is a waveform chart of each part of the conventional example shown in FIG.

[Explanation of symbols]

 Reference Signs List 1 light receiving element 2 preamplifier 3, 4, 5 voltage amplifier 6, 7 transistor 8 constant current source 10 offset cancel circuit

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H03F 3/68 H03F 3/68 Z F term (reference) 5J066 AA01 AA11 AA12 AA56 CA13 CA21 CA62 CA91 FA09 HA02 HA09 HA19 HA25 HA29 HA44 KA02 KA05 KA31 MA08 MA22 PD02 SA01 TA01 TA06 5J069 AA01 AA11 AA12 AA56 CA13 CA21 CA62 CA91 FA09 HA02 HA09 HA19 HA25 HA29 HA44 KA02 KA05 KA31 MA08 MA22 SA01 TA01 TA06 5J090 AA01 AA11 AA12 CA19 HA19 CA21 CA19 CA21 HA44 KA02 KA05 KA31 MA08 MA22 SA01 TA01 TA06 5J091 AA01 AA11 AA12 AA56 CA13 CA21 CA62 CA91 FA09 HA02 HA09 HA19 HA25 HA29 HA44 KA02 KA05 KA31 MA08 MA22 SA01 TA01 TA06 5J092 AA01 AA11 AA12 CA29 HA19 CA21 KA05 KA31 MA08 MA22 SA01 TA01 TA06 UL03

Claims (6)

[Claims] 1. A wideband preamplifier including a differential amplifier connected in multiple stages, comprising an offset level detecting means for detecting an offset appearing in an output circuit of an n-th (n is a positive integer) amplifier, A wide-band preamplifier wherein an offset signal is canceled by inputting an output signal of the offset level detecting means to an output circuit of an amplifier of the (n + 1) th and subsequent stages. 2. The offset level detecting means comprises an integrating circuit connected to each of two output terminals of the n-th stage amplifier, and a differential amplifier circuit to which an integrated value of each integrating circuit is input. The wideband preamplifier according to claim 1, wherein 3. The method according to claim 1, wherein an amplification factor of one or more amplifiers connected in parallel to said offset level detection means (total amplification factor in the case of a plurality of amplifiers) is equal to an amplification factor of said differential amplifier circuit. 3. The wideband preamplifier according to claim 2, wherein: 4. The wideband preamplifier according to claim 1, wherein a preamplifier for outputting a unipolar signal is connected to a stage preceding the first stage amplifier. 5. The method according to claim 1, wherein the plurality of amplifiers, or the plurality of amplifiers and the differential amplifier circuit, or the plurality of amplifiers, the differential amplifier circuit and the preamplifier are formed on one chip. Claims 1-4
The wideband preamplifier according to any one of the above. 6. The plurality of amplifiers, or the plurality of amplifiers and the differential amplifier circuit, or the plurality of amplifiers, the differential amplifier circuit and the preamplifier are configured to include a field effect transistor. The wideband preamplifier according to any one of claims 1 to 5, wherein: