JP3116884B2 - Transimpedance amplifier for optical receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preamplifier used in an optical receiver, and more particularly, to a transimpedance amplifier for an optical receiver (hereinafter simply referred to as "a") for converting a current signal converted by a light receiving element into a voltage signal. Transimpedance amplifier.)

[0002]

2. Description of the Related Art A transimpedance amplifier is used as a preamplifier in an optical receiver, and converts a current signal converted by a light receiving element into a voltage signal. It is desired that the optical receiver can cope with various transmission distances and can cope with an increase in capacity. Therefore, as characteristics of the transimpedance amplifier, low noise for receiving a small signal, high gain for obtaining an equalized amplitude, and a wide band for large capacity are required. Furthermore, low power consumption,
In order to realize miniaturization, cost reduction, and high-speed operation, integration into an integrated circuit is essential. In addition, it is necessary to realize a variable gain function and a large input protection function for preventing saturation and waveform deterioration caused by low voltage driving and high gain.

In a transimpedance amplifier, the input parasitic capacitance of a light receiving element and the like in addition to the parasitic capacitance of a transistor also causes band degradation. Therefore, the effect of the input parasitic capacitance must be reduced in order to increase the bandwidth. .

FIG. 9 is a circuit diagram showing a first conventional example of a transimpedance amplifier. Hereinafter, description will be made based on this drawing.

[0005] The transimpedance amplifier of this conventional example is called a parallel feedback type and is most often used. This transimpedance amplifier includes an amplifier 80 and a feedback resistor Rf connected in parallel to the amplifier 80. The transimpedance gain is determined by the resistor R
The high cutoff frequency is determined by the resistance value of f, and has a relationship inversely proportional to the resistance value of the resistor Rf and the input parasitic capacitance. Therefore, it is difficult to achieve both high gain and wide band. The effect of the input parasitic capacitance is also large.

FIG. 10 is a circuit diagram showing a second conventional example of a transimpedance amplifier. Hereinafter, description will be made based on this drawing.

The transimpedance amplifier of this prior art is called a common base type, and has a transistor Q1 connected to a common base, a load resistor R1 connected to the collector of the transistor Q1, and a constant resistor connected to the emitter of the transistor Q1. It comprises a current source 82, a constant voltage source 84 connected to the base of the transistor Q1, and the like. The transimpedance gain is determined by the resistance value of the load resistor R1. Also, since the base is grounded, it is easy to reduce the input impedance, so that it is hardly affected by the input parasitic capacitance. The high cutoff frequency has a relationship that is inversely proportional to the resistance value of the resistor R1 and the parasitic capacitance of the transistor. Therefore, when a high-performance transistor having a small parasitic capacitance is used, it is advantageous for achieving both high gain and wide band as compared with the parallel feedback type.

[0008]

However, a grounded base type transimpedance amplifier has the following problems.

To increase the input dynamic range in order to prevent saturation, the constant current source 82 is connected to the input terminal IN, so the constant current value of the constant current source 82 must be set to a value equal to or greater than the maximum input current. is there. However, increasing the constant current value has a limit because it causes an increase in power consumption and the like. Therefore, it is difficult to expand the input range at the time of large input.

When the resistance of the resistor R1 is increased to increase the gain, the voltage drop between the power supply terminal 86 and the collector of the transistor Q1 increases, so that the transistor Q1 is easily saturated. To prevent the saturation of the transistor Q1, it is necessary to increase the power supply voltage Vcc. However, increasing the power supply voltage Vcc goes against technical trends such as low-voltage driving and is almost impossible in practice. Therefore, it is difficult to increase the transimpedance gain.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a transimpedance amplifier having a wide input dynamic range and a high transimpedance gain in a grounded base transimpedance amplifier suitable for broadening the bandwidth and increasing the gain. Is to provide.

[0012]

A transimpedance amplifier for an optical receiver according to the present invention comprises a first transistor grounded to a base, a variable current source connected to the emitter of the transistor, and a collector connected to the transistor. A first resistor for the load, a power supply terminal connected to the collector of the transistor via the resistor, a constant voltage source connected to the base of the transistor, and an emitter connected to the emitter of the transistor. An input terminal, an output terminal connected to the collector of the transistor, and a current value of the variable current source that is provided between the output terminal and the variable current source according to an output voltage of the output terminal. And a current source control circuit. This transistor is a bipolar transistor, but may be either an NPN type or a PNP type. Further, an FET (field effect transistor) may be used instead of the bipolar transistor. In this case, the emitter, the collector, and the base are replaced with a drain, a source, and a gate, respectively. When an FET is used, either an n-channel type or a p-channel type may be used.

For example, the current source control circuit keeps the current value constant for the output voltage below a certain value, and increases the output voltage for the output voltage above the certain value. The current value is increased correspondingly.

[0014]

FIG. 1 is a circuit diagram showing one embodiment of a transimpedance amplifier according to the present invention.
Hereinafter, description will be made based on this drawing. However, the same parts as those in FIG.

The transimpedance amplifier of this embodiment includes a transistor Q1 whose base is grounded, a variable current source 10 connected to the emitter of the transistor Q1, a load resistor R1 connected to the collector of the transistor Q1, Constant voltage source 84 connected to the base of transistor Q1, input terminal IN connected to the emitter of transistor Q1, output terminal OUT connected to the collector of transistor Q1, output terminal OUT and variable current source 10
And a current source control circuit 12 provided between the two. The variable current source 10 has a function of changing the current value Ix by the control signal Sc. The current source control circuit 12 outputs a control signal Sc for changing the current value Ix according to the output voltage Vo of the output terminal OUT.

A current source control circuit 12 is provided in a feedback loop from the output terminal OUT to the variable current source 10. The current source control circuit 12 and the variable current source 10 realize a large input protection function. The large input protection function has an input / output transfer characteristic as shown in FIG. 2, and the output voltage Vo increases to V ₁
, The current value Ix starts to increase. Then, FIG.
As shown by the solid line, the rise of the output voltage Vo is suppressed.
As a result, the transimpedance amplifier of the present embodiment operates at a high gain at a small input and can operate at a large input, thereby achieving a high gain and a wide dynamic range.

The current of the variable gain function shown in FIG.
Voltage transfer characteristics are bipolar transistors or FETs
This is suitable for high-speed control because it can be easily realized by a level shift circuit, a resistor voltage dividing circuit, and the like.

[0018]

FIG. 4 is a circuit diagram showing a first embodiment of a transimpedance amplifier according to the present invention. Less than,
Description will be made based on this drawing. However, the same parts as those in FIG.

The input terminal IN is connected to the emitter of the transistor Q1 and the variable current source 10. The collector of the transistor Q1 is connected to the output terminal OUT and the resistor R1.
Is connected to the power supply terminal 86 via the. Output terminal OU
A current source control circuit 12 is connected to T, and the current source control circuit 12 and the variable current source 10 realize a large input protection function.

The variable current source 10 includes a resistor R2 and a transistor Q2. The resistor R2 is connected between the input terminal IN and the ground GND. The transistor Q2 has a collector connected to the input terminal IN and an emitter connected to the ground GND. The current source control circuit 12 includes a transistor Q3 and a resistor R3. The transistor Q3 has a collector connected to the power supply terminal 8
6. The base is connected to the output terminal OUT, and the emitter is connected to the ground GND via the resistor R3, thereby forming an emitter follower circuit. The emitter of the transistor Q3 is connected to the base of the transistor Q2.

Next, the operation of the transimpedance amplifier of this embodiment will be described with reference to FIGS.

[0022] Iin input current, the current value of the variable current source 10 Ix, the power supply voltage V _CC, an output voltage Vo, the resistor R1
Is R ₁ , and the collector current of transistor Q2 is I
Assuming that c, Vo = Vcc−Ic · R ₁ ... Iin + Ic = Ix. Thus, the input / output transfer characteristics are as follows. Vo = Iin · R ₁ + Vcc−Ix · R ₁ ···

When the base potential of the transistor Q2 is low and the transistor Q2 is in the off state and the current value of the variable current source 10 is a small input at which Ix = I _B0 (constant), Vo = Iin · R ₁ + Vcc−I _B0 R ₁ ..., And operates with the transimpedance gain R ₁ , and the output voltage Vo increases linearly with an increase in the input current Iin. Further, since the base-emitter voltage V _BE of the transistor Q3 is constant, the base voltage Vx of the transistor Q2 also increases with the input current Iin.

The input current Iin is increased to I _1, the output voltage V
When the o is V _1, the transistor Q2 is turned on and the base potential of the transistor Q2 becomes high. Then
The current value of the variable current source 10 increases. In operation in this region, the input / output transfer characteristic of the large input protection function is Vo> V ₁ and the transconductance g for simplicity.
Assuming that m is constant, Ix = gm (Vo−V ₁ ) + I _BO . Therefore, the input / output transfer characteristic at Vo> V ₁ is
From the equations and formulas, Vo = {1 / (1 + gm · R 1)} (Iin · R 1 + Vcc-I BO · R 1 + gm · R 1 · V 1) becomes .... Therefore, the transimpedance amplifier of the present embodiment has the input / output transfer characteristic shown in FIG. 3 for suppressing the output amplitude, and thus functions as a non-linear transimpedance amplifier.

FIG. 5 shows signal waveforms of the input current, the current value of the variable current source, and the output voltage in the operation described above.

[0026]

FIG. 6 is a circuit diagram of a transimpedance amplifier according to a second embodiment of the present invention. Hereinafter, description will be made based on this drawing. However, the same parts as those in FIG.

In this embodiment, the bipolar transistors Q1 to Q3 used in the first embodiment (FIG. 4) are replaced with FETs Q11 to Q3.
13 is replaced. The transimpedance amplifier of the present embodiment can also obtain the same effects as the first embodiment.

[0028]

Third Embodiment FIG. 7 shows a circuit diagram of a third embodiment of the transimpedance amplifier according to the present invention. Hereinafter, description will be made based on this drawing. However, the same parts as those in FIG.

This embodiment is different from the first embodiment in the configuration of the current source control circuit 22. In the current source control circuit 22, a resistor R is connected between the emitter of the transistor Q3 and the resistor R3.
4 are connected. Therefore, the base voltage Vx of the variable current source 10 can be adjusted by the resistors R3 and R4. Accordingly, it is possible to set the voltages V ₁ to a large input protection starts operating, you set the output amplitude freely. Also in this embodiment, the same effects as those of the first embodiment can be obtained, and the output amplitude can be freely set, so that connection to the next-stage circuit becomes easy.

[0030]

Fourth Embodiment FIG. 8 is a circuit diagram showing a transimpedance amplifier according to a fourth embodiment of the present invention. Hereinafter, description will be made based on this drawing. However, the same parts as those in FIG.

In this embodiment, the input impedance is reduced by providing a common emitter circuit including the transistor Q4 and the resistor R5. The transistor Q4 has a base connected to the input terminal IN, a collector connected to the base of the transistor Q1, and an emitter grounded. The resistor R5 is connected between the collector of the transistor Q4 and the power supply terminal 86. With this configuration, since the input impedance is reduced, the influence of the input parasitic capacitance is reduced, and the band can be widened. Since the large input protection function operates in the same manner as in the first embodiment, it has an effect that a wider band can be provided in addition to the effect of the first embodiment. The transistor Q of the current source control circuit 32
Reference numeral 5 denotes a short circuit between the collector and the base, which replaces the resistor R4 in the third embodiment.

[0032]

According to the transimpedance amplifier of the present invention, the following effects can be obtained.

The first effect is that the input range at the time of large input can be expanded. The reason is that the large input protection function consisting of the variable current source and the current source control circuit operates at the time of a large input, so that even if the input current increases, the current value of the variable current source increases following the input current. This is because a waveform can be output even during a large input.

The second effect is that a high transimpedance gain can be achieved. The reason is that the current value of the variable current source does not depend on the maximum input current and can be set to a small value, so that the load resistance that determines the transimpedance gain can be increased.

A third effect is that the large input protection function is pulled in at a high speed, so that a burst signal can be received. The reason is that the current value of the variable current source is controlled so as to follow the output voltage, so that it can follow the variation of the input current for each bit. Furthermore, since the large input protection function can be realized by the level shift circuit and one transistor, high-speed control with little influence of circuit delay can be performed.

The fourth effect is that low voltage driving is possible. The reason is that both the grounded-base transistor circuit and the variable current source require only one transistor, so that the output voltage amplitude can be set to a small amplitude by the large input protection function, so that operation can be performed even at an ultra-low voltage of 2 V or less. Because it is possible.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a transimpedance amplifier according to the present invention.

FIG. 2 is a graph showing input / output transfer characteristics of a large input protection function in the transimpedance amplifier of FIG.

FIG. 3 is a graph showing input / output transfer characteristics in the transimpedance amplifier of FIG.

FIG. 4 is a circuit diagram showing a first embodiment of the transimpedance amplifier of the present invention.

FIG. 5 is a time chart illustrating an operation of the transimpedance amplifier of FIG. 4;

FIG. 6 is a circuit diagram showing a transimpedance amplifier according to a second embodiment of the present invention.

FIG. 7 is a circuit diagram showing a third embodiment of the transimpedance amplifier of the present invention.

FIG. 8 is a circuit diagram showing a fourth embodiment of the transimpedance amplifier of the present invention.

FIG. 9 is a circuit diagram showing a first conventional example of a transimpedance amplifier.

FIG. 10 is a circuit diagram showing a second conventional example of a transimpedance amplifier.

[Explanation of symbols]

 Reference Signs List 10 variable current source 12, 22, 32 current source control circuit 84 constant voltage source 86 power supply terminal Q1 to Q5, Q11 to Q13 transistor R1 to R5 resistor IN input terminal OUT output terminal

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/28

Claims (6)

(57) [Claims] 1. A first transistor having a grounded base, a variable current source connected to an emitter of the transistor, a first resistor for a load connected to a collector of the transistor, and a resistor connected to a collector of the transistor. A power supply terminal connected to the collector of the transistor, a constant voltage source connected to the base of the transistor, an input terminal connected to the emitter of the transistor, and an output terminal connected to the collector of the transistor. A current source control circuit that is provided between the output terminal and the variable current source and that changes the current value of the variable current source according to the output voltage of the output terminal. Dexterous transimpedance amplifier. 2. The current source control circuit holds the current value constant for the output voltage equal to or less than a certain value, and increases the output voltage for the output voltage equal to or more than the certain value. 2. The transimpedance amplifier for an optical receiver according to claim 1, wherein the current value is increased correspondingly. 3. The variable current source includes a second transistor and a second resistor, the current source control circuit includes a third transistor and a third resistor, and the second transistor includes A collector is connected to the input terminal, an emitter is grounded, the second resistor is connected between the collector and the emitter, and the third transistor has a base connected to the collector of the first transistor, The transimpedance amplifier for an optical receiver according to claim 1, wherein a collector is connected to the power supply terminal, and an emitter is grounded via the third resistor and connected to a base of the second transistor. 4. The variable current source includes a second transistor and a second resistor, and the current source control circuit includes a third transistor, a third resistor, and a fourth resistor, In the second transistor, a collector is connected to the input terminal, an emitter is grounded, the second resistor is connected between a collector and an emitter, and a base of the third transistor is the first transistor. The collector is connected to the power supply terminal, the emitter is connected to the fourth resistor, the fourth resistor is grounded via the third resistor, and the fourth The transimpedance amplifier for an optical receiver according to claim 1, wherein a connection point between a resistor and the third resistor is connected to a base of the second transistor. 5. The transimpedance amplifier for an optical receiver according to claim 1, wherein the constant voltage source is removed, and a fourth transistor and a fifth resistor are provided. The fourth transistor has a collector connected to the power supply terminal via the fifth resistor, connected to a base of the first transistor, an emitter grounded, and a base connected to the input terminal. Transimpedance amplifier for optical receiver. 6. The transistor according to claim 1, wherein the transistor has a gate replacing the base, a source replacing the collector, and a drain replacing the emitter.
The transimpedance amplifier for an optical receiver according to 2, 3, 4 or 5.