JP4567177B2 - Wideband preamplifier - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a broadband preamplifier, and more particularly to a broadband preamplifier mounted on a light receiving module for optical communication.
[0002]
[Prior art]
In recent years, with the progress of IT (information technology), the demand for communication has increased, and the importance of optical communication has been increasing accordingly. One of 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 (MMIC) 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. Is installed. Thus, in order to satisfy the standard, it is required to amplify the photocurrent up to 800 mV in the band of 100 kHz to 2.5 GHz in the amplifier circuit in the light receiving module. In order to amplify signals over such a wide band with a high degree of amplification, a common source type differential amplifier using HEMT or MESFET is connected in cascade by DC coupling without using a coupling capacitor between circuit stages. Is required.
[0003]
FIG. 3 is a circuit diagram of a conventional light receiving module for optical communication. In the figure, the thick line indicates the package of the light receiving module, and therefore the inside of the thick line is a circuit constituting the light receiving module. In the light receiving module, a light receiving element 1 made of an avalanche photodiode (APD) or the like 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 A differential voltage amplifier 3 to 5 for amplifying a signal and an integrating circuit constituted by a resistor R1 and a capacitor C1 are mounted. Among these, the light receiving element 1 and the capacitor C1 are separate 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 is duty = 50%, and therefore, the output signal of the integrating circuit shows almost the amplitude center of the output signal of the preamplifier 2.
[0004]
[Problems to be solved by the invention]
As described above, in the wideband preamplifier, voltage amplifiers are connected in multiple stages by direct current coupling. Therefore, if an offset occurs in the first stage amplifier, this is successively amplified in each stage. The offset can be reduced by arranging the paired transistors close to each other symmetrically, but in high-frequency circuits, the wiring arrangement to avoid crosstalk between the positive phase signal and the negative phase signal is given priority, and the relative accuracy is reduced. Since it may be sacrificed, the threshold voltage Vth of the pair of transistors may be formed differently, and an offset is likely to occur.
[0005]
FIG. 4 is a diagram showing signal waveforms at various parts of the circuit of FIG. The symbols described on the waveform in FIG. 4 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. If an offset Vos is generated in the voltage amplifier 3 to which this voltage signal and its integral value are input, the output signal includes an offset as shown by waveforms D and E. When the amplification factor of the next-stage amplifier is A, the offset Vos is A · Vos as shown by the waveforms F and G in the next-stage output signal. Since the offset is amplified at each stage in this way, a large offset is output from the most concentrated stage. When this exceeds the output dynamic range, the signal cannot be discriminated. Further, when the signal includes a large offset even if it does not exceed 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 prior art, and its purpose is to provide a signal that can be determined without error so that the offset generated in the first stage amplifier can be canceled in the subsequent stage. It is to be able to provide a wideband preamplifier that outputs.
[0006]
[Means for Solving the Problems]
To achieve the above object, according to the present invention, in a wideband preamplifier including differential amplifiers connected in multiple stages, it is opposite to the positive phase output terminal of the amplifier of the nth (n is a positive integer) stage. Offset level detecting means for detecting an offset appearing between the phase output terminals and canceling the offset by inputting the output signal of the offset level detecting means to the output circuit of the amplifier in the (n + 1) th stage and thereafter. A wideband preamplifier characterized by:
Preferably, the offset level detection means includes an integration circuit connected to each of two output terminals of the n-th stage amplifier, and a differential amplifier circuit to which an integration value of each integration circuit is input. Composed.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a circuit showing an embodiment of the present invention, and the inside of the thick 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 the light receiving element power supply VPD, and the other end is connected to the input terminal of the preamplifier 2 for converting the output current of the light receiving element 1 into a voltage signal. The output of the preamplifier 2 is input to the positive input terminal (+) of the voltage amplifier 3 and is input to an integrating circuit composed of a resistor R1 (200 kΩ) and a capacitor C1 (100 pF). The output signal of this integration circuit is input to the negative input terminal (−) of the voltage amplifier 3.
[0008]
The positive phase output terminal (+) and the negative phase output terminal (−) of the voltage amplifier 3 are connected to the positive input terminal (+) and the negative input terminal (−) of the voltage amplifier 4, respectively. The output terminal (+) and the negative phase output terminal (−) are connected to the positive input terminal (+) and the 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 resistors R4 (50Ω) and R5 (50Ω) through capacitors C4 (0.1 μF) and C5 (0.1 μF), respectively. ). The power supply terminals of the preamplifier 2 and the voltage amplifiers 3 to 5 are connected to the power supplies VDD and VSS, respectively.
[0009]
An offset cancel circuit according to the present invention is connected to the voltage amplifier 4 in parallel. The offset cancel circuit 10 is input with two integration circuits composed of a resistor R 2 (100 kΩ), a resistor R 3 (100 kΩ), capacitors C 2 (100 pF) and C 3 (100 pF), and output signals of these integration circuits. And a differential amplifier circuit composed of field effect transistors 6 and 7 and a constant current source 8. 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. The drain of the transistor 6 (7) is connected to the reverse (positive) phase output terminal (−) [(+)] of the voltage amplifier 4. The sources of the transistors 6 and 7 are commonly connected to the power source VSS via the constant current source 8.
Here, the preamplifier 2 and the voltage amplifiers 3 to 5 are configured by using field effect transistors, and the circuit of the light receiving module for optical communication excludes the light receiving element 1 and the chip capacitors C1 to C3. It is configured on a single chip.
[0010]
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 having the waveform B is input to an integrating circuit composed of a resistor R1 and a capacitor C1. Here, assuming that the input signal is duty = 50%, the waveform C shows a DC voltage that is approximately ½ of the amplitude of the waveform B.
[0011]
The output of the voltage amplifier 3 has a waveform including the offset voltage Vos as waveforms D and E, and each signal is input in parallel to the voltage amplifier 4 in the next stage and the offset cancel circuit 10 according to the present invention.
When the signals of the waveforms D and E are input to the integrating circuit constituted by the resistors R2 and R3 and the capacitors C2 and C3, the integrating circuit outputs the waveforms H and I that are substantially intermediate values of the amplitudes of the waveforms D and E ( (Assuming duty = 50%).
[0012]
That is, the voltage difference between the two waveforms is the offset voltage Vos. This offset voltage Vos is input to the gate electrodes of the transistors 6 and 7. Here, if the amplification degree of the differential amplifier circuit constituted by the transistors 6 and 7 is made to coincide with the amplification degree of the voltage amplifier 4, the offset can be canceled at the output portion of the voltage amplifier 4. In other words, the waveform amplifiers F and G that do not include the offset are input to the voltage amplifier 5.
As a result, a signal without distortion as shown by the waveforms J and K is output from the light receiving module for optical communication, and it is avoided that an erroneous determination is made on the output signal.
[0013]
The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and appropriate modifications can be made without departing from the scope of the present invention. . For example, in this embodiment, the voltage amplifier has a three-stage configuration, but the number of stages may be increased as appropriate depending on 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 an amplifier subsequent thereto, and 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, and 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]
【The invention's effect】
As described above, the wideband preamplifier according to the present invention detects the offset included in the output signal of one amplifier constituting the multistage amplifier, and cancels the offset included in the output signal of the subsequent stage amplifier using this. Therefore, even if a wide-band amplifier circuit is formed by DC coupling, the occurrence of offset can be suppressed and an output voltage with less waveform distortion can be obtained. Therefore, when the light receiving module is configured by using this wideband preamplifier, it is possible to prevent the erroneous determination of the output signal and realize a light receiving module with stable operation.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of an embodiment of the present invention.
FIG. 2 is a waveform diagram of each part of the embodiment shown in FIG.
FIG. 3 is a circuit diagram of a conventional example.
4 is a waveform diagram of each part of the conventional example shown in FIG. 3;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light receiving element 2 Preamplifier 3, 4, 5 Voltage amplifier 6, 7 Transistor 8 Constant current source 10 Offset cancellation circuit

Claims (6)

In a wideband preamplifier including differential amplifiers connected in multiple stages, an offset level for detecting an offset appearing between the positive-phase output terminal and the negative-phase output terminal of the n-th (n is a positive integer) stage amplifier A wideband preamplifier comprising: a detecting unit that cancels an offset by inputting an output signal of the offset level detecting unit to an output circuit of an amplifier in the (n + 1) -th stage or later. The offset level detection means includes an integration circuit connected to each of the two output terminals of the n-th stage amplifier, and a differential amplification circuit to which an integration value of each integration circuit is input. The wideband preamplifier according to claim 1, wherein: The amplification factor of one or a plurality of amplifiers connected in parallel to the offset level detection means (the total amplification factor in the case of a plurality of amplifiers) is equal to the amplification factor of the differential amplifier circuit. 2. The wideband preamplifier according to 2. The wideband preamplifier according to any one of claims 1 to 3, wherein a preamplifier that outputs a unipolar signal is connected to a front stage of the first stage amplifier. 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. The broadband preamplifier according to any one of 1 to 4. 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 include a field effect transistor. The broadband preamplifier according to claim 1.

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