JP5176917B2 - Preamplifier - Google Patents

Description

  The present invention relates to a preamplifier having a differential input / output type differential amplifier among preamplifiers for converting a current signal obtained by converting an optical signal into a voltage signal in optical communication.

  In recent years, a communication apparatus using an optical fiber has been required to increase the bit rate, and communication apparatuses having a bit rate of 1 Gbps (gigabit per second) or more are spreading. In trunk systems and the like, communication devices having higher bit rates such as 10 Gbps, 40 Gbps, and 100 Gbps are being put into practical use. Of the communication devices, an optical receiver has a function of converting an optical input signal into an electric signal and outputting the electric signal, and an optical signal that has been digitized and encoded according to a specific communication protocol is input to the optical receiver.

  Here, the digitized optical signal has a certain frequency band determined by the bit rate and the communication protocol. The higher the bit rate, the higher the upper limit of the frequency band of the digitized optical signal, and the lower the lower limit of the frequency band, the greater the number of 0 or 1 consecutive bits. In order to receive a stable optical signal, an amplifier circuit such as a pre-amplifier that amplifies the electrical signal output from the photoelectric conversion circuit or the photoelectric conversion circuit in the optical receiver is flat in the entire frequency band of the optical signal. It is particularly important to have a good frequency characteristic. For example, it is desirable to be flat in the range of 100 kHz to 30 GHz at 40 Gbps and in the range of 25 kHz to 7.5 GHz at 10 Gbps.

  Generally, a light receiving element such as a photodiode (hereinafter referred to as PD) or an avalanche photodiode (hereinafter referred to as APD) is used in the photoelectric conversion circuit of the optical receiver. In the conventional optical receiver, a low-pass filter having a single characteristic is provided in an amplifier circuit that amplifies an electric signal from the photoelectric conversion circuit in order to remove a DC component of the electric signal. In addition, in order to secure a dynamic range when a large optical signal is received, a bypass circuit that bypasses an excessive current is connected to the PD or APD (see, for example, Patent Document 1).

JP 2000-349571 A

  FIG. 10 shows a configuration diagram of a preamplifier in a conventional optical receiver. In FIG. 10, 1 is a PD, 2 is a power supply for applying a reverse bias to PD1, 3 is a bypass circuit for bypassing a part of the input current from PD1 as a bypass current, and 5 is a first connected to the anode of PD1. Feedback resistor 6 is a differential amplifier having a differential amplifier connected to the anode of PD 1, 8 is a first resistor representing the input impedance of the differential amplifier 6, and 9 is a negative phase input terminal of the differential amplifier 6. And a capacitor provided between the ground and the ground. Reference numeral 12 denotes a current signal output from the PD1. 21 is a second resistor connected to the negative phase output terminal of the differential amplifier 6, 22 is a third resistor connected to the positive phase output terminal of the differential amplifier 6, and 23 is the second resistor 21 and the third resistor. A second feedback resistor 24 connected between the first resistor 22 and the negative phase input terminal of the differential amplifier 6, and 24 is an intermediate potential generating circuit composed of a second resistor 21 and a third resistor 22. . Reference numeral 25 denotes a kind of DC feedback circuit formed by the intermediate potential generating circuit 24 and the second feedback resistor 23.

  Next, the operation of the conventional preamplifier will be described. The PD 1 whose cathode is connected to the constant voltage source 2 converts the optical signal into a current signal 12 and outputs it to the differential amplifier 6. The differential amplifier 6 amplifies the current signal 12 with a gain determined according to the resistance value of the feedback resistor 5 and then outputs the signal as a positive phase signal and a negative phase signal. According to the magnitude of the current signal 12 from the PD 1, the bypass circuit 3 overloads a part of the current signal as a bypass current, for example, in a DC manner before the current signal is input to the differential amplifier 6. Prevents input and stabilizes preamplifier operation.

  Further, a midpoint potential generation circuit 24 attached to the output of the differential amplifier 6 generates a midpoint potential between the negative phase output and the positive phase output of the differential amplifier 6. This midpoint potential is input as a threshold voltage to the negative phase input terminal of the differential amplifier 6 via the second feedback resistor 23. That is, an appropriate threshold value is determined by feedback. If the threshold voltage to be fed back includes a component in the frequency range of the signal to be amplified by the preamplifier, the gain of the differential amplifier is lowered or an oscillation operation by positive feedback is caused. Therefore, the negative phase input terminal of the differential amplifier 6 is grounded in an AC manner by the capacitor 9 to prevent the generation of voltage amplitude in the frequency band of the preamplifier.

  The bypass circuit 3 is configured using a resistor and an active element such as a transistor or a non-linear element such as a diode. Since the impedance of these active elements or nonlinear elements has frequency characteristics, the conventional preamplifier has a problem that the frequency flatness in the signal band is impaired in the bypass circuit 3 connected to the input signal. there were.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain a preamplifier excellent in frequency flatness.

The preamplifier according to the present invention includes a signal input terminal that inputs a current signal generated by a light receiving element, a bypass circuit that is connected to the signal input terminal and extracts the current signal according to the current signal, and the current A differential amplifier for amplifying a signal or a signal obtained by amplifying the current signal, a threshold voltage generator for generating a threshold voltage for determining a reference point of the differential amplifier, and a multiple filter circuit for correcting frequency response characteristics And the output of the threshold voltage generator is connected to the input of the differential amplifier, the multiple filter circuit is connected between the output of the threshold voltage generator and the input of the differential amplifier, and It is characterized by having two or more different time constants.

In the preamplifier according to the present invention, the threshold voltage generator has one input connected to the negative phase output of the differential amplifier and the other input connected to the positive phase output of the differential amplifier. It is characterized by comprising a circuit.

In the preamplifier according to the present invention, the threshold voltage generator generates the threshold voltage by branching the current signal generated by the light receiving element or the signal obtained by amplifying the current signal. It is characterized by comprising.

The preamplifier according to the present invention further includes a transimpedance amplifier that converts the current signal generated by the light receiving element into a voltage signal, and an input of the transimpedance amplifier is connected to the signal input terminal, and the transimpedance amplifier Is connected to the input of the differential amplifier.

In the preamplifier according to the present invention, the multiple filter circuit includes a first stage filter constituted by a first capacitor, and a second resistor and a second capacitor connected in series with each other. The first capacitor has one end connected between the output of the threshold voltage generator and the input of the differential amplifier, the other end of the first capacitor connected to ground, and One end of the second stage filter is connected to one end of the first capacitor, and the other end of the second stage filter is connected to the ground.

In the preamplifier according to the present invention, the first-stage filter is a low-pass filter having a cutoff frequency f1, the second-stage filter is a low-pass filter having a cutoff frequency f2, and the cutoff frequency f1 is a preamplifier before correction. The cut-off frequency f2 is equal to or less than the frequency at which the amplitude response of the preamplifier before correction ends up with the decrease in frequency.

The preamplifier according to the present invention is an integrated circuit in which circuit components of the preamplifier other than the multiplex filter circuit are integrated into one chip, and the multiplex filter circuit is connected to the outside of the integrated circuit. It is what.

  According to the present invention, a preamplifier excellent in frequency flatness can be obtained.

Embodiment 1 FIG.
A first embodiment according to the present invention will be described below. Note that the present invention is not limited to the embodiments.

  FIG. 1 is a diagram illustrating a schematic configuration of a preamplifier according to the first exemplary embodiment of the present invention. In the figure, 1 is a PD, 2 is a power supply for applying a reverse bias voltage to the PD, 3 is a bypass circuit for bypassing a part of the input current from the PD1 as a bypass current, 4 is a transimpedance amplifier connected to the anode of the PD1, 6 is a first differential amplifier connected to the output of the transimpedance amplifier 4; 7 is a DC feedback circuit that generates a reference voltage based on the DC voltages of the positive and negative phases of the first differential amplifier 6; Is a first resistor representing the input impedance of the first differential amplifier 6, 9 is a first capacitor connected between the negative phase input of the first differential amplifier 6 and the ground, and 10 is one end at the first. A second resistor 11 connected to the negative phase input of the differential amplifier 6 and the first capacitor, and a second capacitor 11 connected between the other end of the second resistor 10 and the ground. Reference numeral 12 denotes a current signal output from the PD1. One end of the second resistor 10 and one end of the second capacitor 11 are connected and connected in series, the other end of the second capacitor 11 is connected to the ground, and the other end of the second resistor 10 is connected to the first capacitor. 9 is connected to one end. The other end of the first capacitor 9 is connected to the ground.

  Further, in the bypass circuit 3, 13 is a transistor inserted between the anode of PD1 and the ground, 14 is a collector resistance connected to the collector of the transistor, 15 is a base resistance connected to the base of the transistor, and 16 is a transistor. Is an emitter resistor connected to the emitter. In the DC feedback circuit 7, 17 is a second differential amplifier to which the positive and negative phase output terminal voltages of the differential amplifier 6 are input, and 18 is an output terminal of the second differential amplifier 17 and the first differential amplifier. This is a second feedback resistor inserted between the differential amplifier 6 and the negative phase input terminal. A first feedback resistor 26 feeds back a part of the output voltage signal of the transimpedance amplifier 4 to the input.

  Next, the operation of the preamplifier according to the first embodiment will be described. A bias voltage of about +3.3 V is applied to the PD 1 from the power source 2, and a 40 Gbps NRZ (Non Return to Zero) optical signal received by the PD 1 with this bias voltage applied is generated by an optical / electrical conversion action. It is converted into a current signal 12. This current signal is proportional to the power of the input optical signal and is a current that flows from the PD into the input terminal of the preamplifier. This current signal is amplified with a gain determined by the transimpedance amplifier 4 and the feedback resistor 5 and output as a voltage signal. Here, when the power of the optical signal received by the PD 1 is remarkably large, the output direct current value also increases proportionally, so that the bias voltage of the input terminal of the transimpedance amplifier deviates from the optimum operating point, and the transimpedance Distortion occurs in the amplification characteristics of the amplifier. In order to prevent such a bias voltage shift due to a direct current signal, a bypass circuit 3 is connected between the input terminal of the transimpedance amplifier 4 and the ground on the anode side of the PD 1. The bypass circuit 3 is a circuit for forcibly extracting a part of the current signal 12 by the transistor 13. Since the signal proportional to the direct current flowing through the transimpedance amplifier 4 is connected to the base of the transistor 13 via the base resistor 15, the current drawn by the transistor 13 increases in proportion to the increase in the current signal 12. The input signal level to the impedance amplifier 4 can be kept constant, and deterioration of the output waveform due to circuit saturation can be prevented.

  The output of the transimpedance amplifier 4 is input to the positive phase input terminal of the first differential amplifier 6 in the subsequent stage. The first differential amplifier 6 is an amplifier that amplifies a potential difference generated between a voltage signal input to the positive phase input terminal and a threshold voltage input to the negative phase input terminal and outputs the amplified signal as a differential signal. . In the first differential amplifier 6, if the average voltage and the operating amplitude between the positive phase and the negative phase are not equal, the gain of the differential amplifier is reduced and the pulse width is distorted. Feedback is applied via the DC feedback circuit 7 so as to change the threshold voltage of the differential input terminal of the differential amplifier 6 in accordance with the potential difference between the phases. The DC feedback circuit includes a second differential amplifier 17 that outputs a voltage proportional to the potential difference between the positive phase and the negative phase of the differential output, and a second differential amplifier that reduces the output voltage of the differential amplifier to an appropriate level. This is a threshold voltage generating means configured to output a threshold voltage so that the average potential difference between the positive phase and the negative phase of the output terminal of the first differential amplifier 6 becomes 0V. By operating this circuit, the differential amplifier can maintain an optimal operation state balanced between the positive phase and the reverse phase regardless of the strength of the optical signal. Since the DC feedback circuit 7 is a circuit that generates a threshold voltage as a reference, its operating speed must be sufficiently lower than the frequency band of the main signal. For this reason, the first capacitor 9 is connected and operates at a frequency equal to or lower than the low-frequency cutoff frequency determined between the first capacitor 8 and the first resistor 8 of the differential amplifier.

  The above operation is the same as that of the conventional technique. However, in the present embodiment, the point that the second resistor 10 and the second capacitor 11 are added is different from the conventional technique shown in FIG. Yes. The operation will be described below.

  When the bypass circuit 3 is connected to the PD 1 as in the preamplifier shown in FIG. 1, the bypass circuit 3 is connected in parallel to the signal input portion of the transimpedance amplifier 4. The bypass circuit should originally operate only at low frequencies outside the signal frequency band, but it is difficult to keep the impedance component of the bypass circuit sufficiently high across the entire signal band, and the frequency response characteristics are low. There are often situations where the frequency differs from the high frequency.

  In FIG. 2, when the flatness is impaired due to the influence of the bypass circuit and there is no effect of the filter circuit, that is, all of the first capacitor 9, the second resistor 10, and the second capacitor 11 are connected. The frequency response characteristic of the preamplifier circuit in the case of not performing is shown. In this case, a level difference of 15 dB occurs between 100 kHz and 10 GHz. That is, since the bypass circuit 3 illustrated has a low impedance in the high frequency range, the high frequency component of the optical signal 12 does not easily flow to the differential amplifier 6, and the amplitude response of the preamplifier in the high frequency range is low. ing. In this example, the amplitude response is substantially constant in a region of about 100 kHz or less and a region of about 10 MHz or more.

On the other hand, FIG. 3 shows a state in which 100 nF is mounted as the first capacitor 9 while the bypass circuit is not connected or the bypass circuit does not have frequency characteristics, and the second resistor 10 and the second capacitor 11 are mounted. It is the figure which showed the frequency response characteristic as the whole preamplifier when not doing. The frequency characteristic in FIG. 3 is that the DC feedback circuit 7 is effective only in the low frequency range and the threshold value is adjusted by the low-pass filter constituted by the first resistor 8, that is, the input impedance of the differential amplifier 6 and the first capacitor 9. Can be obtained. If the output impedance of the second differential amplifier 17 is high, the contribution of the resistor 18 need not be considered.
The first resistor 8 depends on the circuit configuration inside the differential amplifier, but has a size of about 100Ω, for example. A frequency response that is substantially flat from about 100 kHz to 10 GHz, which is higher than the frequency f ₁ (= 1 / (2π · R1 · C1)) determined by the resistance value R1 of the first resistor 8 and the capacitance C1 of the first capacitor 9. A characteristic is obtained, and the amplitude response characteristic monotonously decreases as the frequency decreases below about 100 kHz.

FIG. 4 shows a state in which 100 nF is mounted as the first capacitor 9 and the second resistor 10 and the second capacitor 11 are not mounted in a state where the flatness is impaired due to the influence of the bypass circuit. It is the figure which showed the frequency response characteristic of a preamplifier. Peaking of 10 dB or more occurs in the vicinity of 100 kHz, and a serious bit error occurs when a broadband digital signal such as 40 Gbps is handled.
The cause of peaking is that a simple low-pass filter having only one time constant cannot correct even a frequency response characteristic of a profile having steps without changing monotonously with frequency as shown in FIG. It is.

  When the frequency response characteristics of the bypass circuit are difficult to handle, the values of the first capacitor, the second resistor, and the second capacitor are set appropriately to reduce the frequency response characteristics of the complex profile. It can be corrected.

When correcting the frequency response characteristic as shown in FIG. 2 caused by the frequency response characteristic of the bypass circuit, first, a characteristic (roll-off characteristic) that is generated from 100 kHz to 10 MHz and has a large response in the low frequency range. To correct, select 2.3 nF as the first capacitor.
Since the first resistor 8 and the first capacitor 9 are connected in parallel between the input terminal of the differential amplifier 6 and the ground, the resistance value of the first resistor 8 is set to R1, the first capacitor When the capacity is C1, a frequency of about 700 kHz or higher which is higher than the frequency f ₁ (= 1 / (2π · R1 · C1)) determined by the resistance value R1 of the first resistor 8 and the capacitance C1 of the first capacitor 9 is cut off. Will be. Therefore, the feedback circuit 7 becomes sufficiently effective at a frequency of less than about 700 kHz, and the amplitude response of the preamplifier as a whole can be greatly reduced in a frequency range of less than about 700 kHz.

  However, if the second resistor 10 and the second capacitor 11 are not connected as a single-stage low-pass circuit as described above, the peaking shown in FIG. 3 occurs, or the frequency band close to the lower limit of the necessary band is generated. The amplitude response becomes too low, and flat frequency characteristics necessary for communication cannot be obtained.

Therefore, as the second capacitor, a large-capacitance capacitor of 100 nF or more is selected in order to make the low-frequency cutoff frequency of the entire preamplifier circuit less than 100 kHz and to flatten the necessary frequency band. Further, 200Ω is selected as the second resistor 10. In this case, the frequency f ₂ (= 1 / (2π · R2 · C2)) determined by the resistance value R2 of the second resistor 10 and the capacitance C2 of the second capacitor 11 is cut off. It becomes. Therefore, when the effects of the first-stage low-pass filter formed by the first resistor 8 and the first capacitor 9 and the second-stage low-pass filter formed by the second resistor 10 and the second capacitor 11 are combined, In the frequency range from about 10 kHz to about 100 kHz, the feedback signal is cut off mainly by the effect of the second-stage low-pass filter, and in the frequency range of about 100 kHz or more, the feedback signal is cut off mainly by the effect of the first-stage low-pass filter. It will be. A feedback characteristic that cancels the complex frequency non-flatness of the preamplifier can be obtained only by sharing the main frequency regions that exhibit the effects of the first-stage low-pass filter and the second-stage low-pass filter. As a result, it is possible to correct a preamplifier whose frequency characteristic is not monotonous, such as the stepped frequency characteristic shown in FIG.

In order to share the main frequency regions that exhibit the effects of the first-stage low-pass filter and the second-stage low-pass filter, it is important to connect the second resistor 10 having a finite resistance value R2. In order to explain this, FIG. 5 shows the frequency characteristics of the preamplifier when the circuit constant of the low-pass filter is changed while the characteristics of the bypass circuit are flat. FIG. 6 shows the frequency characteristics of the preamplifier when the circuit constant of the low-pass filter is changed while the characteristics of the bypass circuit are not flat.
When the second resistor is 0Ω, that is, when the second resistor is not attached, the first capacitor and the second capacitor are simply connected in parallel, and only one capacitor having a combined capacitance of 102.3 nF is connected. It is not a synthesis of the filter characteristics with different time constants according to the present invention, which is equivalent to the state. When the second resistance is 0Ω, the frequency characteristic of the preamplifier fed back through the low-pass filter as shown in FIG. 5 monotonously decreases below about 100 kHz close to the frequency f1, and the bypass shown in FIG. The frequency response in a state where the circuit is connected has a frequency response characteristic having peaking as in FIG.

  Therefore, when the second resistance is increased sequentially to 100Ω and 200Ω, the peaking level shown in FIG. 6 gradually decreases, and a flat characteristic with little peaking at 100 kHz to 10 GHz at 200Ω can be obtained. That is, when 2.3 nF as the first capacitor, 200 Ω as the second resistor, and 100 nF as the second capacitor are mounted, it changes in two steps as shown in FIG. 5 when the second resistance is 200 Ω. A filter having a frequency characteristic is obtained, and a preamplifier circuit having the most optimal frequency response characteristic can be configured.

  A flat frequency response characteristic can be obtained in this way by the low-pass filter characteristic determined by the first resistor and the first capacitor, and the different low-pass filter determined by the second resistor and the second capacitor. The filter characteristics are combined, and the filter characteristics of the second capacitor are adjusted by the value of the second resistor, whereby the multiple high-pass filter characteristics having a complicated shape as in the case where the second resistance in FIG. 5 is not 0Ω. This is because even the frequency response characteristic of a difficult-to-handle profile as shown in FIG. 2 can be corrected and flattened.

The value of the cut-off frequency f1 determined by the resistance value R1 of the first resistor 8 and the capacitance C1 of the first capacitor 9 is equal to or less than the frequency at which the amplitude response of the preamplifier begins to increase as the frequency decreases. It is desirable. This is because the first-stage low-pass filter transmits a signal significantly in a frequency range equal to or lower than the frequency before and after the frequency f1 (approximately 25% to 400% of f1).
The resistance value R2 of the first resistor 10 and the value of the capacitance C2 of the first capacitor 11 are preferably equal to or less than the frequency at which the amplitude response of the preamplifier has finished increasing with decreasing frequency. . This is because the signal is significantly transmitted even in the second-stage low-pass filter in the frequency range below the frequency f2 (approximately 25% to 400% of f2). Note that since f2 <f1, both the first-stage low-pass filter and the second-stage low-pass filter allow the signal to pass through significantly in the frequency range below the frequency before and after f2.

According to the present invention, in order to connect a multiple filter circuit having two or more different characteristics between the input terminal of the differential amplifier and the threshold voltage generating means, a complicated frequency response characteristic is obtained by the multiple filter circuit. The effect is obtained by correcting and having a flatter frequency response characteristic.
Although it is conceivable to add a circuit in the vicinity of the bypass circuit 3 as a place for correcting the frequency response characteristic, adding a circuit in the vicinity of the signal output of the PD in this way generally deteriorates the noise characteristic. . In the present invention, since the circuit is added to the threshold voltage circuit far from the PD, there is an effect that even if the frequency response characteristic is corrected in such a complicated circuit form, the noise characteristic is not adversely affected.

  There are factors other than the bypass circuit 3 that impair the frequency flatness. When the preamplifier is actually manufactured as an integrated circuit (IC), the circuit parameters do not always operate in an ideal state over a wide frequency band, so that frequency flatness is often impaired. Further, depending on the use of the preamplifier, a special frequency response characteristic that drops the response at a specific frequency may be required. According to the present invention, a preamplifier adapted to such various needs can be configured.

  When a DC feedback circuit is used as the threshold voltage generating means, there is an effect that the preamplifier as a whole can have an ideal frequency response characteristic while maintaining the output balance of the differential amplifier.

  Furthermore, according to the present invention, when a photocurrent generated by a photodiode is used as a current signal and a bypass circuit is connected to the photodiode, a current signal output in response to an increase in the input optical signal is extracted by the bypass circuit, While maintaining the current amplitude input to the transimpedance amplifier, the non-flatness of the frequency response characteristic generated in both the transimpedance amplifier and the bypass circuit and also in the differential amplifier can be corrected and flattened by the multiple filter circuit. Play.

  Further, according to the present invention, the multiple filter circuit includes the first capacitor, the resistor connected in parallel to the first capacitor, and the second capacitor, thereby combining the value of the capacitor and the resistor. Thus, there is an effect that an arbitrary filter characteristic can be generated at low cost. By connecting a multiple low-pass filter to the output of the feedback circuit, the frequency flatness damaged by the bypass circuit or the like can be easily corrected.

  Although the preamplifier used for 40 Gbps has been described above, it is needless to say that the value of the multiple filter may be designed according to the frequency required for flatness at other bit rates and the non-flatness of the preamplifier. Yes.

Embodiment 2. FIG.
FIG. 7 shows the configuration of the preamplifier according to the second exemplary embodiment of the present invention.
Constituent elements that are the same as those in the first embodiment are given the same numbers, and descriptions thereof are omitted.
In the first embodiment, the current signal 12 is converted into a voltage signal by the transimpedance amplifier 4 and then input to the differential amplifier 6. However, in this embodiment, the current signal is directly input to one of the differential amplifiers 6. Connected to the input terminal.

  In the present embodiment, by using the differential amplifier 6 as a device for converting a current signal into a voltage signal, the non-flatness of the frequency response characteristics generated in both the differential amplifier 6 and the bypass circuit is multiplexed filter circuit. The effect of correcting and flattening can be achieved. Even in this configuration, the same effects as those of the first embodiment are obtained. Further, the configuration of the preamplifier can be simplified.

Embodiment 3 FIG.
A configuration according to the third embodiment of the present invention is shown in FIG. Constituent elements that are the same as those in the first embodiment are given the same numbers, and descriptions thereof are omitted.
In the first embodiment, the DC feedback circuit is used as the threshold voltage generating means connected to the input section of the differential amplifier. However, in the present embodiment, the feedforward circuit is generated by branching the current signal.

FIG. 8 is a schematic diagram showing a preamplifier using a feedforward circuit as the threshold voltage generating means. In FIG. 8, reference numeral 19 denotes a feedforward circuit that branches the output signal of the transimpedance amplifier 4 and generates a threshold voltage. The configuration is the same as that of the first embodiment except that a feedforward circuit 19 is used instead of the DC feedback circuit 7.
For example, the feedforward circuit 19 directly inputs the output signal of the transimpedance amplifier 4 to the positive phase input terminal of the differential amplifier 6 and outputs the output of the transimpedance amplifier 4 to the negative phase input terminal of the differential amplifier 6. The operation point of the differential amplifier 6 is optimized by the operation of inputting the average voltage of the signal.

  The multiple filter circuit constituted by the first capacitor 9, the second resistor 10, and the second capacitor 11 has the same effect as the first embodiment. Further, in such a configuration of the feedforward circuit, the input impedance 8 of the differential amplifier can be easily increased. In this case, the first capacitor 9 having a smaller capacity than that of the first embodiment is used. There is also an effect that it can be used. Further, by using the feedforward circuit as the threshold voltage generating means, the capacitance value of the first capacitor can be a relatively smaller value than that of the DC feedback circuit, and the circuit scale can be reduced.

Embodiment 4 FIG.
In the inventions according to the first to third embodiments, the preamplifier is configured by discrete components. In the present embodiment, the parts other than the light receiving element and the multiple filter circuit of the preamplifier according to the third embodiment are integrated into an IC. In FIG. 9, 20 is one IC, and circuit components inside the IC 20 shown in FIG. 9 are mounted on one IC 20. All parts other than the photodiode 1, the power source 2, the first capacitor 9, the second resistor 10, and the second capacitor 11 are made into one chip, thereby reducing the size, cost, and convenience. However, there is an effect that the deterioration of the frequency response characteristic due to the parasitic impedance of the IC can be corrected similarly to the first embodiment by the multiple filter circuit connected to the outside of the IC.

  By adopting such a configuration, there is an effect that desired frequency flatness can be obtained without substantially losing advantages such as downsizing and cost reduction due to the IC. By connecting the multiple filter circuit to the outside of the IC, there is an effect that both cost reduction by the IC and tuning by the multiple filter circuit can be realized. Also in the present embodiment, the same effects as in the first to third embodiments are obtained.

  In the description of the present embodiment, an example in which the light receiving element of the preamplifier according to the third embodiment and the portion other than the multiple filter circuit are integrated is shown as an example. However, the light receiving element of the preamplifier according to the first embodiment. It is also possible to similarly implement the embodiment in which the portion other than the multiple filter circuit is integrated into the IC, and the embodiment in which the light receiving element of the preamplifier according to the second embodiment and the portion other than the multiple filter circuit are integrated into the IC. Similar effects can be obtained.

  In the first to fourth embodiments, the light receiving element is a PD, but an APD has the same effect.

  In the first to fourth embodiments, the feedback circuit can be configured by an intermediate potential generation circuit made of a resistor and a feedback resistor. However, when the feedback circuit is configured by using active elements as in the feedback circuit 7 shown in FIG. 1, it is not necessary to consider the impedance of the intermediate potential generation circuit, so that the filter constant can be selected more freely. It is possible to correct non-flatness of various amplitude responses.

  In the first to fourth embodiments, a new resistor (not shown) is connected in parallel with the first capacitor 9 in order to adjust the resistance value of the first resistor 8 as a constant of the multiple filter. You may connect.

  In the first to fourth embodiments, a multiple filter including three or more stages of filters may be used to correct more complicated frequency characteristics.

It is a figure which shows schematic structure of the preamplifier concerning Embodiment 1 of this invention. It is a figure which shows the frequency response characteristic of the preamplifier in the state where the flatness of the frequency characteristic is impaired by the bypass circuit. It is a figure which shows the frequency response characteristic of the preamplifier at the time of adjusting a threshold value using the low-pass filter with a single time constant in the state where the flatness of the frequency characteristic is not impaired by the bypass circuit. It is a figure which shows the frequency response characteristic of the preamplifier at the time of adjusting a threshold value using the low-pass filter with a single time constant in the state where the flatness of the frequency characteristic is impaired by the bypass circuit. It is a figure which shows the frequency response characteristic of the preamplifier at the time of adjusting a threshold value using the multiple filter circuit in Embodiment 1 in the state where the flatness of the frequency characteristic is not impaired by the bypass circuit. 6 is a diagram illustrating frequency response characteristics of a preamplifier when adjustment is performed by a multiple filter circuit according to Embodiment 1. FIG. FIG. 3 is a diagram illustrating a schematic configuration of a preamplifier according to a second embodiment. FIG. 4 is a diagram illustrating a schematic configuration of a preamplifier according to a third embodiment. FIG. 6 is a diagram illustrating a schematic configuration of a preamplifier according to a fourth embodiment. It is a figure which shows schematic structure of the conventional preamplifier.

Explanation of symbols

1 PD, 2 power supply, 3 bypass circuit, 4 transimpedance amplifier, 6 differential amplifier, 7 DC feedback circuit, 8 first resistor, 9 first capacitor, 10 second resistor, 11 second capacitor, 12 Current signal.

Claims (7)

A signal input terminal for inputting a current signal generated by the light receiving element;
A bypass circuit connected to the signal input terminal and extracting the current signal in response to the current signal;
A differential amplifier for amplifying the current signal or a signal obtained by amplifying the current signal ;
A threshold voltage generator for generating a threshold voltage for determining a reference point of the differential amplifier;
A pre-amplifier comprising a multiple filter circuit for correcting frequency response characteristics ,
The output of the threshold voltage generator is connected to the input of the differential amplifier,
The preamplifier, wherein the multiple filter circuit is connected between the output of the threshold voltage generator and the input of the differential amplifier and has two or more different time constants. The threshold voltage generator includes a DC feedback circuit having one input connected to the negative phase output of the differential amplifier and the other input connected to the positive phase output of the differential amplifier. The preamplifier according to claim 1. The threshold voltage generator includes a feedforward circuit that branches the signal generated by a light receiving element or a signal obtained by amplifying the current signal to generate the threshold voltage. The preamplifier according to claim 1. The preamplifier further includes a transimpedance amplifier that converts the current signal generated by the light receiving element into a voltage signal,
The input of the transimpedance amplifier is connected to the signal input terminal,
4. The preamplifier according to claim 1, wherein an output of the transimpedance amplifier is connected to an input of the differential amplifier. The multiple filter circuit is
A first stage filter composed of a first capacitor;
A second stage filter composed of a second resistor and a second capacitor connected in series with each other;
One end of the first capacitor is connected between the output of the threshold voltage generator and the input of the differential amplifier,
The other end of the first capacitor is connected to ground,
One end of the second stage filter is connected to one end of the first capacitor,
Preamplifier according to any one of claims 1 to 4 and the other end of the second stage filter is characterized in that it is connected to the ground. The first stage filter is a low-pass filter having a cutoff frequency f1,
The second stage filter is a low-pass filter having a cutoff frequency f2.
The cut-off frequency f1 is equal to or lower than the frequency at which the amplitude response of the preamplifier before correction starts to increase as the frequency decreases,
The cutoff frequency f2 before correction of the preamplifier of the preamplifier according to any one of claims 1 to 5 amplitude response is equal to or less than the frequency which increases ends with frequency reduction. Circuit components of the preamplifier than the multiple filter circuit is an integrated circuit that is one chip, one of the claims 1 to 6, characterized in that connected the multiplexing filter circuit external to the integrated circuit A preamplifier according to claim 1.

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