JPH10335957A - Amplifier for optical reception - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the chip size of an amplifier for optical reception. SOLUTION: When an input optical signal is inputted to an amplifier for optical reception, a photoelectric conversion element 11 generates a corresponding current signal Iin and supplies the signal Iin to an input terminal IN. A transimpedance preamplifier 20, having a transistor 23 which functions as an amplifier and a feedback resistor 26, generates a voltage signal by converting the current Iin into a voltage by performing voltage amplification. A waveform- shaping circuit 40 shapes the waveform of the voltage signal and fetches an output voltage Vout from a voltage output terminal. When the modification of the waveform of the output voltage Vout is performed, the circuit constants of a resistor 42 and a capacitor 43 in the waveform-shaping circuit 40 are adjusted. Since the circuit constant adjustment is not performed only on the capacitor 43, the chip size of the amplifier for optical reception can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical receiving amplifier used for an optical receiver of an optical communication system and outputting a voltage corresponding to an input optical signal.

[0002]

2. Description of the Related Art Conventionally, an optical receiver used in an optical communication system has a function of converting an input optical signal into an electric signal.
For example, it includes a photoelectric conversion element that generates a current signal according to the intensity of an input optical signal, and an optical receiving amplifier that includes a preamplifier. The preamplifier used in the optical receiver is required to have characteristics such as low noise, a wide dynamic range, a wide band, and a high gain, and an amplifier generally called a transimpedance amplifier is used. A typical transimpedance preamplifier has a feedback resistor connected between an input and an output.
The following equation (1) holds between the input current Iin input from the input section and the output voltage Vout output from the output section. Here, the resistance value of the feedback resistor is Rf. Vout = −Iin · Rf (1) When a transimpedance type preamplifier is applied to an optical receiver, the band characteristic BW of the photoelectric conversion element is, for example, a PIN photodiode (hereinafter referred to as PIN-PD) of a photoelectric conversion element.
Assuming that CT is the sum of the junction capacitance of the preamplifier, the input capacitance of the amplifier in the preamplifier, and the stray capacitance added for mounting, and A is the voltage gain of the amplifier, the following equation (2) is obtained. BW = A / (2π · Rf · CT) (2)

[0003]

However, the conventional optical receiving amplifier using the transimpedance type preamplifier has the following problems. In the transimpedance type preamplifier, it is general to adopt a method of equivalently reducing the transimpedance in a region where the input signal current is large (hereinafter, referred to as a large signal region) in order to improve the dynamic range characteristics. is there. In this case, the resistance Rf in the equation (2) decreases as the input signal current Iin increases. Here, since the sum CT of the capacitance and the voltage gain A are constant, the band characteristics thereof become wider as the resistance value Rf decreases. Eventually, there is a problem that peaking occurs and distortion occurs in the output waveform of the amplifier. Therefore, an optical receiving amplifier that suppresses waveform distortion particularly in a large signal region without deteriorating dynamic range characteristics and has improved waveform characteristics over the entire region is desired.

FIG. 2 is a circuit diagram showing an example of a conventional optical receiving amplifier with an improved output waveform. This optical receiving amplifier circuit has a PIN in which an anode is connected to an input terminal IN.
It comprises a photodiode (hereinafter referred to as PIN-PD) 1 and an npn transistor 2 whose base is connected to the input terminal IN. The cathode of PIN-PD1 is connected to the power supply voltage Vcc. The transistor 2 forms an emitter follower, and the collector of the transistor 2 is connected to the power supply voltage Vcc and the emitter is connected to the emitter resistor 3.
Grounded. The emitter of the transistor 2 is connected to the base of a transistor 4 that serves as an amplifier. The collector of transistor 4 is connected to power supply voltage Vcc via load resistor 5. A connection node N1 between the collector of the transistor 4 and the resistor 5 is connected to the input terminal IN by a feedback resistor 6 having a resistance value of Rf. This node N1 is connected to the output terminal OUT, one end of a resistor 7 having a resistance value of Re, and one electrode of a capacitor 8 having a capacitance value of Ce. The resistor 7 and the capacitor 8 are in parallel, and the other end of the resistor 7 and the other electrode of the capacitor 8 are connected to a constant current source 9. A bypass diode 10 having a forward direction from the input terminal IN to the constant current source 9 is connected between the input terminal IN and the constant current source 9.

[0005] In the optical receiving amplifier of FIG.
In a region where the current Iin inputted from the input terminal is small (hereinafter, referred to as a small signal region), the output voltage Vout is also small, and the current Id flowing through the bypass diode can be ignored relative to the current Iin. Therefore, the band characteristic BW is limited by the equation (2). On the other hand, in a region where the input current Iin is large (hereinafter, referred to as a large signal region), the output voltage Vout also increases, and the current Iout flowing in the forward direction of the bypass diode 10 is increased.
d cannot be ignored. At this time, a part of the current Iin is bypassed by the bypass diode 10, and the saturation of the transistor 4 serving as an amplifier is avoided. In this case, the transimpedance value for setting the gain in the transimpedance preamplifier having the transistors 2 and 4 and the feedback resistor 6 starts to change, and the impedance value is changed by the resistance value Rf of the feedback resistor 6 and the bypass diode 10. It becomes equal to the parallel resistance with the resistance value Rd. At this time, the band characteristic BW is limited by the resistance value Re of the resistor 7 and the capacitance value Ce of the capacitor 8 as in the following equation (3). BW = 1 / (2π · Re · Ce) (3) Therefore, the time constant composed of the resistance value Re and the capacitance value Ce is changed to the time constant composed of the resistance value Rf and the capacitance sum CT of the equation (2). By matching, the waveform characteristics can be improved in a wide dynamic range from a small signal region to a large signal region.

Here, when the input signal area switches from the small signal area to the large signal area, that is, when the bypass diode 1
When 0 is clamped, the maximum current value of the current flowing through the bypass diode 10 is Imax, and the constant current source 9 is
Is the current flowing through the feedback resistor 6 when the bypass diode 10 clamps, Iclp is the current flowing through the feedback resistor 6 when the bypass diode 10 clamps, Vclp is the voltage across the feedback resistor 6 at this time, and Assuming that the inter-terminal voltage is Vd, the inter-terminal voltage Vd is given by the following equation (4). Vd = Vclp + Ve (4) Maximum current value Imax flowing through bypass diode 10 =
Assuming that the current is 1.0 mA, the constant current source current value Ie is set to be larger than Imax. For example, if it is set to 1.5 mA, the terminal voltage Ve is given by equation (5). Ve = Re · Ie = Re · 1.5 (mA) (5) That is, as shown in the equation (4), the inter-terminal voltage Vd is set by the inter-terminal voltage Vclp and the inter-terminal voltage Ve. However, the terminal voltage Ve becomes constant by the current value Ie of the constant current source 9 regardless of the input current value according to the equation (5). Therefore, it is the terminal voltage Vclp that sets the input current value to be bypassed to the bypass diode in order to avoid the saturation of the transistor 4 serving as an amplifier. The terminal voltage Vclp is expressed by the following equation (6).

Vclp = Iclp · Rf (6) The feedback resistor 6 is also a resistor that determines the gain of the transimpedance type preamplifier, and the relationship between the resistance value Rf and the output voltage Vout is (7) Expression. Vout = − (Iin−Id) · Rf (7) In consideration of the relations of the above equations (4) to (7), the resistance value Rf
, Re are set. For example, if the inter-terminal voltage Vclp is 0.4 V, the inter-terminal voltage Vd is approximately 0.8 V. Therefore, according to the equations (4) and (5), the resistance value Re becomes Re =
266.7Ω. In order to improve the waveform characteristics in a wide dynamic range from a small signal region to a large signal region, it has been described that the time constant of Expression (3) is made the same as the time constant of Expression (2). And the resistance value R of the resistor 7
e cannot be set large alone. Therefore, the time constant is adjusted by the capacitance value Ce. However, when the preamplifier is mounted on a chip, it is difficult to set the capacitance value Ce to a large value because there is a limitation on the chip size and the like. There were challenges.

[0008]

According to a first aspect of the present invention, an input optical signal is photoelectrically converted and a voltage corresponding to the input optical signal is output from a voltage output terminal. The optical receiving amplifier includes the following photoelectric conversion element, transimpedance preamplifier, variable gain circuit, and waveform shaping circuit. The photoelectric conversion element generates a current signal according to the intensity of the input optical signal when a predetermined bias voltage is applied. The transimpedance type preamplifier has an input section, an output section, and a feedback resistor connected therebetween, and converts a current signal input from the input section into a voltage signal by an amplification operation using the feedback resistor. Has a function. The variable gain circuit includes a bypass diode that bypasses a current flowing through a feedback resistor, and a constant current source connected to an output terminal of the bypass diode, and is a circuit that controls a gain in amplification of a transimpedance preamplifier. is there. The waveform shaping circuit is a circuit for shaping the waveform of the voltage signal, and is connected between the output terminal to which the feedback resistor is connected and the output terminal of the bypass diode, and sets an input current when the bypass diode conducts. A first resistor, a second resistor connected between the first resistor and the voltage output terminal, and a capacitor connected in parallel with the first resistor and the second resistor in series. doing.

A second invention relates to an optical receiving amplifier,
It is configured using the following photoelectric conversion element, transimpedance preamplifier, variable gain circuit, and waveform shaping circuit. The photoelectric conversion element is a device to which a predetermined bias voltage is applied and generates a current signal according to the intensity of the input optical signal. Transimpedance preamplifier
It has an input section, an output section, and a feedback resistor connected between them, and has a function of converting the current signal input from the input section into a voltage signal by an amplification operation using the feedback resistor. . The variable gain circuit has the same bypass diode and constant current source as the first invention, and is a circuit for controlling the gain in the amplification of the transimpedance preamplifier. The waveform shaping circuit has a resistor connected between the output section of the transimpedance preamplifier to which the feedback resistor is connected and the voltage output terminal, and a capacitor connected in parallel to the resistor, and a voltage signal This is a circuit that performs waveform shaping.

According to the first and second aspects of the present invention, since the optical receiving amplifier is configured as described above, when the input optical signal is input, the photoelectric conversion element causes the current signal corresponding to the intensity of the input optical signal to be changed. Is generated and given to the input terminal. The transimpedance type preamplifier performs voltage amplification and generates a voltage signal obtained by converting a current signal into a voltage. The waveform shaping circuit shapes the waveform of the voltage signal, and outputs an output voltage from the voltage output terminal. The variable gain circuit controls the gain in the preamplifier by bypassing the current by the bypass diode. Here, in the first invention, in the waveform shaping circuit having the first resistor, the second resistor, and the capacitor connected in parallel to the first resistor and the second resistor, the second resistor can be connected without changing the resistance value of the first resistor. By adjusting both the capacitance and the capacitance, the waveform characteristics for the voltage signal are changed. On the other hand, in the second invention, in the waveform shaping circuit having a resistor connected between the voltage output terminal and the output section of the transimpedance preamplifier and a capacitor connected in parallel to the resistor, both the resistance and the capacitance are provided. The adjustment changes the waveform characteristics for the voltage signal. Therefore, the above problem can be solved.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIG. 1 is a circuit diagram of an optical receiving amplifier according to a first embodiment of the present invention. This optical receiving amplifier has an input terminal IN
Is connected to an anode, a cathode is connected to a power supply Vcc, and a bias voltage is applied to the photoelectric conversion element PI.
An N-PD 11, a transimpedance type preamplifier 20 for performing current-voltage conversion to generate a voltage signal, a variable gain circuit 30 for controlling the gain of the preamplifier 20, and a voltage output terminal for shaping the waveform of the voltage signal And a waveform shaping circuit 40 that outputs from OUT. The preamplifier 20 includes an npn transistor 21 having a base serving as an input unit connected to an input terminal IN. The transistor 21 forms an emitter follower, and the transistor 21
Is connected to the power supply voltage Vcc, and the emitter of the transistor 21 is grounded via the emitter resistor 22. The emitter of the transistor 21
It is connected to the base of an npn transistor 23 serving as an amplifier. The collector of transistor 23 is connected to power supply voltage Vcc via load resistor 24, and the emitter of transistor 23 is grounded. Transistor 23
Is connected to a connection node N1 between the collector of the NPN transistor and the resistor 24, the base of an npn transistor 25 forming an emitter follower. The collector of transistor 25 is connected to power supply voltage Vcc. A feedback resistor 27 having a resistance value of Rf is connected between a node N2, which is an output section of the transimpedance preamplifier 20, and an input terminal IN.

The variable gain circuit 30 has a bypass diode 31 whose anode is connected to the input terminal IN. The cathode of the bypass diode 31 is connected to the node N2
, And grounded via a constant current source 32. The waveform shaping circuit 40 includes a constant current source 32 and a node N2.
And a first resistor 41 connected between
5, a second resistor 42 connected between the node N2 and a capacitor 4 connected in parallel with the resistors 41 and 42.
3 is comprised. A connection point between the emitter of the transistor 25 and the resistor 42 and the capacitor 43 is connected to a voltage output terminal OU.
Connected to T. Next, the operation of the optical receiving amplifier will be described. When an input optical signal is input, PIN-PD
Numeral 11 generates a current signal Iin corresponding to the intensity of the input optical signal and supplies it to the input terminal IN. The transimpedance preamplifier 20 including the transistors 21 and 25 constituting the emitter follower, the transistor 23 functioning as an amplifier, and the feedback resistor 26 performs voltage amplification by the transistor 23 to generate a voltage signal obtained by converting the current Iin. The waveform shaping circuit 40 shapes the waveform of the voltage signal, and extracts the output voltage Vout from the voltage output terminal. Here, the variable gain circuit 30 controls the gain of the preamplifier 20.

In correspondence with the conventional optical receiving amplifier of FIG.
The current value of the constant current source 32 is Ie, and the resistance value of the resistor 41 is Re.
, The voltage between the terminals of the bypass diode 31 and the forward current are Vd and Id, respectively.
Assuming that the saturation current of No. 1 is Is and the thermal voltage is Vt, the current Iin
And the output voltage Vout, the following equations (8) to (10) are established. Vout = − (Iin−Id) · Rf (8) Id = Is · (exp (Vd / Vt−1)) (9) Vd = Vout + Re · Ie (10) Small signal In a region, that is, a region where the current Iin inputted from the input terminal IN is small, the output voltage Vout is also small, and the current Iin
d is negligible for the current Iin. Therefore, equation (8) can be approximated to equation (11). Vout = −Iin · Rf (11) The band characteristic BW in the case of operating as described above is represented by the following equation (2): the gain A of the preamplifier 20, the resistance Rf of the feedback resistor 26, and the capacitance Limited by sum CT. On the other hand, when the input signal switches from the small signal region to the large signal region, the current flowing through the feedback resistor 26 starts to be bypassed by the bypass diode 31 and the impedance in the transimpedance preamplifier 20 switches, so that the transistor 23 Saturation is avoided. The band characteristic BW at this time is represented by the resistance value Re of the resistor 41, where Ro is the resistance value of the resistor 42 and Ce is the capacitance value of the capacitor 43.
And the resistance value Ro and the capacitance value Ce limit the expression (12).

BW = 1 / (2π · (Ro + Re) · Ce) (12) Here, the resistance value Re of the resistor 41 is as described in the above (4) to (4).
It is inevitably determined from equation (7). Therefore, by setting the resistance value Ro and the capacitance value Ce such that the time constant (Ro + Re) · Ce in the equation (12) is equal to the time constant in the equation (2), the signal from the small signal area to the large signal area is set. Waveform characteristics can be improved over a wide dynamic range. As mentioned above,
In the optical receiving amplifier according to the first embodiment, the resistor 4 that sets the input current when the bypass diode 10 conducts is used.
1; a resistor 42 in series with the resistor 41;
And a capacitor 43 in parallel with the waveform shaping circuit 30.
Is composed. Therefore, the time constant can be adjusted by the resistance Ro of the resistor 42 and the capacitance Ce of the capacitor 43. For example, if Re = Ro in the equation (12), the equation (12) can be substantially expressed by the following equation (13). BW = 1 / (2π · Re · 2Ce) (13) Comparing the expressions (3) and (13), the same band characteristic B is obtained.
In order to obtain W, it is only necessary to set the capacitance value to half the value of Ce in the equation (13), and it is possible to achieve both a reduction in chip size and an improvement in waveform characteristics.

Second Embodiment FIG. 3 is a circuit diagram of an optical receiving amplifier according to a second embodiment of the present invention. This optical receiving amplifier has an input terminal IN
And a transimpedance preamplifier 60 that performs a current-to-voltage conversion and generates a voltage signal, with a PIN-PD 51 similar to that of the first embodiment in which an anode is connected to the power supply Vcc and a cathode is connected and a bias voltage is applied. A variable gain circuit 70 for controlling the gain of the preamplifier 60;
And a waveform shaping circuit 80 for shaping the waveform of the voltage signal and outputting it from the voltage output terminal OUT. The preamplifier 60 has the same configuration as the preamplifier 20 of the first embodiment, and includes an npn-type transistor 61 whose input terminal IN is connected to a base serving as an input unit. The transistor 61 forms an emitter follower. The collector of the transistor 61 is connected to the power supply voltage Vcc, and the emitter of the transistor 61 is grounded via the emitter resistor 62. The emitter of the transistor 61
Further, the base of an npn-type transistor 63 serving as an amplifier is connected. The collector of the transistor 63
The transistor 63 is connected to the power supply voltage Vcc via the load resistor 64, and the emitter of the transistor 63 is grounded. A connection node N1 between the collector of the transistor 63 and the resistor 64 is connected to an npn transistor 6 forming an emitter follower.
5 bases are connected. The collector of transistor 65 is connected to power supply voltage Vcc. A feedback resistor 66 having a resistance value of Rf is connected between a node N2 which is an output section of the transimpedance type preamplifier 60 and an input terminal IN.

The variable gain circuit 70 is the same as the variable gain circuit 30 of the first embodiment, and has a bypass diode 71 whose anode is connected to the input terminal IN. The cathode of the bypass diode 71 is connected to the node N2 and grounded via the constant current source 72. The waveform shaping circuit 80 is a waveform shaping circuit 40 according to the first embodiment.
Unlike the first embodiment, it is constituted by one resistor 82 and a capacitor 83. The resistor 82 is a resistor 42 according to the first embodiment.
The node N2 and the transistor 65
Connected between the emitters. The capacitor 83 is connected in parallel to the resistor 82. That is, the feedback resistor 66
And the resistor 82 are connected to the constant current source 72 at the node N2. The connection point between the emitter of the transistor 65, the resistor 82 and the capacitor 43 is connected to the voltage output terminal OUT. The basic operation of this optical receiving amplifier is the same as that of the first embodiment.
The IN-PD 51 generates a current signal Iin corresponding to the intensity of the input optical signal and supplies the current signal Iin to the input terminal IN. A transimpedance preamplifier 60 having transistors 61 and 26 constituting an emitter follower, a transistor 63 functioning as an amplifier, and a feedback resistor 66 performs voltage amplification by the transistor 63 to generate a voltage signal obtained by converting the current Iin. The waveform shaping circuit 80 shapes the waveform of the voltage signal, and extracts the output voltage Vout from the voltage output terminal. The variable gain circuit 70 controls the gain by changing the impedance of the preamplifier 60.

This optical receiving amplifier is the same as the first embodiment in which the resistance value Re of the resistor 41 in the first embodiment is set to zero. Therefore, if Re = 0, equation (12) becomes equation (14). BW = 1 / (2π · Ro · Ce) (14) Therefore, by adjusting both the resistance Ro and the capacitance Ce, it is possible to improve the waveform characteristics as in FIG. Therefore, an increase in chip size can be prevented. Note that the present invention is not limited to the above embodiment, and various modifications are possible. The circuit configurations of the preamplifiers 20 and 60 are shown in FIGS.
However, the present invention is not limited to this. For example, the npn transistors 21, 23, 25, 61, 63, and 65 may be replaced with pnp transistors.

[0018]

As described above in detail, according to the first aspect, a photoelectric conversion element, a transimpedance preamplifier for converting a current signal into a voltage signal by an amplification operation using a feedback resistor, and A variable gain circuit and a waveform shaping circuit are provided in an optical receiving amplifier, and the waveform shaping circuit is connected in series to a first resistor and a first resistor that set an input current when a bypass diode bypasses a current. The second resistor and the capacitor connected in parallel to the first resistor and the second resistor, the waveform characteristics can be improved by adjusting both the capacitor and the second resistor. Size can be reduced. According to the second aspect, the waveform shaping circuit is configured by the resistor connected between the output section of the transimpedance type preamplifier and the voltage output terminal, and the capacitor in parallel with the resistor. Can be improved by adjusting both the capacitance and the resistance, and the chip size can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an optical receiving amplifier according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional optical receiving amplifier.

FIG. 3 is a circuit diagram of an optical receiving amplifier according to a second embodiment of the present invention.

[Explanation of symbols]

11, 51 PIN-PD 20, 60 Transimpedance type preamplifier 26, 66 Feedback resistor 30, 70 Variable gain circuit 31, 71 Bypass diode 32, 72 Constant current source 40, 80 Waveform shaping circuit 41 First resistor 42 First 2 resistance 43,83 capacitor 82 resistance IN input terminal OUT voltage output terminal N2 output section Iin current signal

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H04B 10/04 10/06

Claims (2)

[Claims] 1. An optical receiving amplifier for photoelectrically converting an input optical signal and outputting a voltage corresponding to the input optical signal from a voltage output terminal, wherein a predetermined bias voltage is applied, and a predetermined bias voltage is applied. A photoelectric conversion element for generating a current signal, an input section, an output section, and a feedback resistor connected between the input section and the output section, and the current signal input from the input section is amplified by an amplification operation using the feedback resistor. A transimpedance preamplifier for converting the signal into a signal; a bypass diode for bypassing a current flowing through the feedback resistor; and a constant current source connected to an output terminal of the bypass diode. A variable gain circuit that controls a gain in amplification; and a connection between the output section to which the feedback resistor is connected and an output terminal of the bypass diode. A first resistor for setting an input current when the bypass diode bypasses a current; a second resistor connected between the first resistor and the voltage output terminal; Resistance and second
And a capacitor connected in parallel with the resistor, and a waveform shaping circuit for shaping the waveform of the voltage signal. 2. An optical receiving amplifier for photoelectrically converting an input optical signal and outputting a voltage corresponding to the input optical signal from a voltage output terminal, wherein a predetermined bias voltage is applied, and a predetermined bias voltage is applied. A photoelectric conversion element for generating a current signal, an input section, an output section, and a feedback resistor connected between the input section and the output section, and the current signal input from the input section by an amplification operation using the feedback resistor. A transimpedance preamplifier that converts the voltage into a voltage signal; a bypass diode that bypasses a current flowing through the feedback resistor; and a constant current source connected to the bypass diode. A variable gain circuit that controls a gain, an output section of the transimpedance preamplifier connected to the feedback resistor, and the voltage output terminal. Connected in parallel with a resistor connected with said resistors are constituted by a capacitor, and a waveform shaping circuit for performing waveform shaping of the voltage signal, the optical receiver amplifier, characterized in that it includes.