JPH09232877A - Preamplifier for optical communication - Google Patents

Abstract

PROBLEM TO BE SOLVED: To expect a stable operation even if power source voltage is lowered and to provide the new form of a front end amplifier which is capable of operat ing by even single power source voltage. SOLUTION: Almost all the photoelectric current ip generated by receiving light by a photo diode (PD) 2 becomes iRF flowing in a feedback resistance RF when incident light intensity is weak, between the source and drains of the FETs 6 connected with the feedback resistance RF in parallel it becomes in a conductive state as incident light intensity is increased, the ip is shunted into iRF and iFET, transimpedance is equivalently lowered and the drop of the output potential by the feedback resistance RF is suppressed. Further, when incident light intensity is increase, a front end amplifier is operated up to larger optical input by flowing gate bias current in the gate of the input stage FET of an amplifier 4 and shunting the photoelectric current ip into iRF, iFET and ig .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a preamplifier for optical communication using a field effect transistor (hereinafter referred to as FET), and particularly to a preamplifier for optical communication suitable for single low power supply voltage operation. There is something about.

[0002]

2. Description of the Related Art In a conventionally known preamplifier for optical communication, a depletion type FET (hereinafter referred to as DF) is used.
ET) was used, and +5 V or -5.2 V was used, and two or more power source voltages were used to operate with multiple power sources.

For example, Otsuka et al., 1988 IEICE Spring National Convention Proceedings, 1st Volume, 396 pages, Lecture No. B-645, the band consists of one amplification stage consisting of five FETs, two resistors, and four diodes. A transimpedance amplifier of 3.8 GHz is realized. Alternatively, in the 1988 10th GaAs-IC symposium technical digest page 15-18 by Hatta et al., A preamplifier in which another FET is cascode-connected to the load of the input FET achieves a band of 2.4 GHz.

[0004]

In the case of a single-power-supply preamplifier for optical communication using a D-FET, in order to increase the gain, it is used in a multistage amplifier in which amplifiers are connected in multiple stages, or in Hatta et al. F to the load of the first stage FET of the amplifier
Cascode connection for connecting ETs in series is required. However, when amplifiers are connected in multiple stages, there is a problem that each amplifier easily oscillates and its operation becomes unstable. On the other hand, when the latter cascode connection is used, if the power supply voltage is lowered in order to reduce power consumption, the source-drain voltage (Vds) of each cascode-connected FET becomes small, Since the operating point enters from the saturation region to the linear region, it becomes difficult to balance the gain and the dynamic range.

An object of the present invention is to provide a novel form of a preamplifier for optical communication, which can be expected to operate stably even if the power supply voltage is lowered and can operate even with a single power supply voltage.

[0006]

SUMMARY OF THE INVENTION The present invention is a transimpedance type optical device having an FET having a current / voltage conversion function for converting a current signal generated in a light receiving element in response to an optical signal into a voltage signal as a main constituent element. In the preamplifier for communication, the input FET is an E-FET, and its gate bias causes a part of the signal current from the light receiving element at the time of a large optical signal incident to flow into the FET as a bias current to increase the gain of the amplification stage. It must be reduced. For this reason the FE
A forward biased diode was connected to the source electrode of T, and the level shift diode of the output source follower of the amplifier was one stage. Also, the drain and source of the FET are connected in parallel to the feedback resistance of the amplifier, and the control signal generated from the output signal of the amplifier is gated so that the current is bypassed between the drain and source of the FET when a large optical signal is input. To enter.

Further, in order to keep the gain of the amplification stage at a required value when a small optical signal is input, another FET cascode-connected with the load of the input stage FET is inserted.

The gate bias condition Vgs of the first-stage FET of the amplifier is determined by the output potential fed back through the feedback resistor and the source potential of the first-stage FET. When the optical input signal is zero or small, the gate potential is set to be slightly higher than the source potential, so that the optical signal current from the light receiving element does not substantially flow to the gate and most of it passes through the feedback resistor. To the output. At this time, input F
Since the operating point of ET is in the saturation region, the same voltage gain as that set by the cascode-connected FET can be obtained.

When the optical signal is gradually increased, the output potential decreases accordingly. With reference to this decrease, the gate potential of the FET inserted in parallel with the feedback resistor is increased, and the current bypasses between the drain and source. As a result, the value of transimpedance is equivalently reduced. As a result, the amount of feedback becomes large, and the saturation of the output of the amplifier can be suppressed.

When the light input is further increased, the signal current from the light receiving element is further increased, and the gate potential of the input FET also rises accordingly. On the other hand, since the forward potential of the diode is substantially fixed as the source potential, the gate bias voltage Vgs gradually increases. F
The gate-source of ET is equivalently regarded as a diode connected in the forward direction, so that finally the gate bias current starts to flow as the forward current of the diode, and the optical signal current is connected in parallel with the feedback resistor. It is divided into the current flowing between the drain and source of the FET and the gate bias current. Even if the optical signal further increases and the signal current further increases, most of the increase flows as the bias current of the input FET, so that an increase in the current flowing back to the output via the feedback resistor is suppressed. Under these conditions, the operating point of the input FET is not in the saturation region,
Since it is in the linear region, the transconductance Gm also decreases at the same time.

On the other hand, the current-voltage conversion characteristic of the amplifier is given as a voltage of a value obtained by multiplying the value of the current flowing through the feedback resistance by the value of the feedback resistance. First, when the current flows through the FET connected in parallel with the feedback resistance. The conversion gain is reduced. Then, when a part of the signal current is shunted to the input FET, the amplification gain of the input FET is reduced and the open loop gain of the amplifier is also reduced. As a result, it is possible to suppress saturation of the output of the preamplifier and secure a wide dynamic range.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a block diagram of a circuit showing such an embodiment.
2), amplifier 4, feedback resistance RF, parallel FET
6 is a preamplifier for optical communication, which includes a transimpedance control circuit 8. As shown in the figure, the photocurrent ip generated by the photodiode (PD) 2 upon receiving light.
Is a feedback resistance RF when the incident light intensity is weak.
While the i _RF flowing between the source and the drain of the FET6 connected in parallel with the feedback resistor RF in accordance with the incident light intensity increases becomes conductive, ip is split into i _RF and i _FET, equivalently trans The impedance is lowered and the drop of the output potential due to the feedback resistance RF is suppressed.

When the incident light intensity is further increased,
Amplifier gate bias current flows through the gate of the input stage FET 4, the photocurrent ip i _RF, i _FET, by diverting the i _g, made to work preamplifier to a greater optical input.

As described above, in the transimpedance type preamplifier for optical communication, a part of the optical signal current from the light receiving element is connected in parallel to the feedback resistance FET or the input FE.
By shunting to the gate of T, equivalent reduction of transimpedance and suppression of voltage gain of the amplifier are realized,
Enables a wide dynamic range.

[0015]

FIG. 1 shows a preamplifier for optical communication according to an embodiment of the present invention. In this embodiment, a GaAs-MESFET is used as the field effect transistor. In addition to this, an injection resistor and a GaAs Schottky barrier diode monolithically formed on the GaAs substrate by using ion implantation are used. Where GaAs-MES
Two types of FET are used, an E-FET having a threshold voltage of +50 mV and a D-FET having a threshold voltage of -400 mV.

In the circuit shown in FIG. 1, Q ₁ has a gate width of 120 μ as an FET constituting the amplification stage of the preamplifier.
m, gate length 0.5 μm E-FET, Q ₂ , Q ₃ , Q
₄ is a D-FET having a gate width of 50, 10, 60 μm and a gate length of 0.8 μm. In this circuit, the gate widths of Q ₂ , Q ₃ and Q ₄ are set so that the gate bias Vgs of Q ₁ becomes + 0.35V when there is no signal. A resistor of 1 kΩ for applying a bias voltage to the gate is inserted between the source and gate of Q ₃ .

The feedback resistance RF is 20 kΩ, and a FET of 50 μm width and Q _RF are connected in parallel. This Q _RF
Is controlled by an output signal from a transimpedance control circuit composed of Q ₁₁ , Q ₁₂ , R ₁ , R ₂ and R ₃ .

An InGaAs photodiode was used as a light receiving element, and an optical receiver using this preamplifier was manufactured and evaluated. The light receiving surface of the photodiode is 100μ
A current corresponding to an optical signal in the 1.3 μm band is output at mφ. The sensitivity including the coupling efficiency of the photodiode is
0.80 A / W was obtained.

When the preamplifier shown in FIG. 1 is operated at a power supply voltage V _D of +3.3 V, the preamplifier uses only a conventional D-FET having a voltage gain of 32 dB and a band of 550 MHz and a power supply voltage of +5. Performance equivalent to that of an amplifier operated at 0.0 V or -5.2 V was obtained. Further, it was confirmed that the operation was performed even when the power supply voltage V _D was lowered to 2.8V. As an optical receiver, a transimpedance of 19 kΩ and a transimpedance band of 180 MHz were obtained.

In the preamplifier according to this embodiment, -30d
In the region where the light incident intensity is weaker than Bm, the current flowing through the gate of Q ₁ is 0.35 μA at 25 ° C. and about 1.5 μA at 100 ° C., and the gate current does not substantially flow. Therefore, the effect of shot noise on the minimum receiving sensitivity of the preamplifier is small, and it is 155.52 Mbps.
If this receiver is evaluated using the signal
Defining the optical input that gives 10 ⁻¹⁰ as sensitivity, −36.
A sensitivity of 5 dBm was obtained.

When the light incident intensity is gradually increased, Q _RF connected in parallel to the feedback resistor RF from the vicinity of the incident light intensity of -18 dBm (average incident light intensity of -21 dBm for a modulation signal having a duty ratio of 50%). Is turned on, the transimpedance begins to decrease, and when the incident light intensity becomes brighter than +1 dBm (average incident light intensity-2 dBm for a modulation signal with a duty ratio of 50%), Q connected in parallel with the feedback resistor RF is connected. _RF alone cannot carry all of the photocurrent. Thus, the gate potential for Q ₁ starts to rise, the photocurrent begins to flow to the gate for Q _1.

FIG. 4 shows the average incident light intensity (the horizontal axis represents the photocurrent i).
shows the change in the gate current for Q ₁ with respect to that) shows the p, 5 are those of investigating changes in the gate potentials for Q _1. Since the gate of Q ₁ has a Schottky diode characteristic, the photocurrent is 4 mA (peak light intensity 6.5 d
Bm, 50% modulation signal up to 3.5 dBm),
Bias voltage V even when a large amount of gate current flows
gs is about 0.65V, and the light incident intensity is weak,
Or even if compared with the state of no signal, 0.65-0.3
It is suppressed to a fluctuation of about 5 = 0.30V.

In the present invention, it is important that the source potential of Q ₁ does not fluctuate greatly even when a large amount of light is input. The source electrode of Q ₁ is a constant voltage source or a constant voltage source as shown in FIG. It is preferably connected to a forward diode having a voltage characteristic.

The optical receiver using the preamplifier thus manufactured is operated at a power supply voltage of 3.3 V and a temperature of 25 ° C., and an output when a 1-0 signal of 155.52 Mbps and 0 dBm is input. The waveform is shown in FIG. It can be seen that the distortion of the waveform is small and there is no problem even if an optical signal of 0 dBm enters.

Next, according to the present embodiment, in order to confirm how low the power supply voltage can be operated, all FETs including Q ₁ are constructed by using D-FETs having a threshold voltage of -400 mV. The characteristics of the preamplifier having the same configuration as the circuit shown in FIG. 1 were examined. This preamplifier has a power supply voltage of + 5.0V
When operated with a voltage gain of 31 dB, bandwidth 600 MHz
Although a good characteristic of z was obtained, the power supply voltage was +3.2.
When it is lowered to V, it does not operate, and it does not operate unless at least 0.4 V of power supply voltage is applied in comparison with the present invention.

This is because the E-FET and the D-FET have different knee voltages (in the Vds-Ids characteristic of the FET, the drain voltage at the transition from the linear region to the saturation region), and the E-FET has a lower knee voltage. This is because it exhibits voltage characteristics and is suitable for low voltage operation. Further, the gate potential of the input FET that determines the dynamic range / amplitude of the output signal is only the voltage drop of one forward diode (about 0.6V), and the dynamic range /
To increase the amplitude, (1) Q ₁ of the gate - flow (it is necessary to increase the 4 times) enormous drain current in Q ₁ to a large source voltage Vgs to the + side, or, ( 2) A method of increasing the number of forward diodes connected to the source side of Q ₁ to 2 is conceivable, but (1)
The method of (2) causes an increase in current consumption, and the method of (2),
Unless the power supply voltage is increased by a voltage drop corresponding to one diode, the power supply does not operate, which is disadvantageous for low voltage operation. None of the methods surpasses the characteristics of the preamplifier configured using the E-FET of the present application.

As described above, in order to obtain the preamplifier for optical communication which operates at a low voltage while securing the dynamic range / amplitude of the output signal, the input FET of the amplification circuit of the preamplifier is composed of E-FET. However, by adopting a configuration in which the gate potential of the input FET is formed by the sum of the gate bias Vgs and the source potential, it is possible to obtain a preamplifier for optical communication that operates stably even under a low voltage. Operation at a power supply voltage of 2.8V was confirmed. Further, at the time of a small signal, only a gate current that does not reduce the sensitivity of the receiver flows to the input FET, and at the time of a large signal, a part of the output current of the light receiving element flows to the gate of the input FET as a gate bias current. Since the operation was realized by using the E-FET and the dynamic range was expanded, the signal from -36.5 dBm to a maximum of 0 dBm or more as an optical receiver should be identified with an error rate of 1 x 10 ^-10 or less. I was able to.

The present invention can be applied to other than the above-mentioned embodiments. The circuit can be applied to a transimpedance type amplifier using an ordinary publicly known cascode connection as shown in FIG. 2 in which an FET is not connected in parallel to a feedback resistor, a simple source grounded amplifier, and the like. Regarding the FET, the threshold voltage of the E-FET is not limited to +50 mV and can be variously adjusted.
The material is not only GaAs-MESFET, but also Si junction type FET (J-FET), AlGaAs / GaAs
System or AlInAs / InGaAs system HEMT
It is possible to use various things such as.

[0029]

As described above, according to the present invention, the input FET of the amplification circuit of the preamplifier is composed of an E-FET,
By adopting a configuration in which the gate potential is the sum of the gate bias Vgs and the source bias voltage Vs, it is possible to obtain a preamplifier for optical communication that operates at a low voltage while securing the dynamic range / amplitude of the output signal. it can. Further, the input E-FET is allowed to flow into the gate as a bias current only to the extent that the sensitivity of the receiver is lowered at the time of a small signal, and a part of the output current of the light receiving element is used as a gate bias current at the time of a large signal as the input FET. Therefore, it is possible to obtain a preamplifier for optical communication having a wide dynamic range with low voltage operation.

[Brief description of drawings]

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

FIG. 2 is a diagram showing another embodiment of the present invention.

FIG. 3 is a diagram showing an example of an embodiment of the present invention.

FIG. 4 is a diagram showing an average photocurrent dependency of a gate current of an input FET in an example of the present invention.

FIG. 5 is a diagram showing an average photocurrent dependency of a gate potential of an input FET in an example of the present invention.

FIG. 6 is a diagram showing an output waveform (155.52 Mbps, 0 dBm) of the preamplifier in the embodiment of the present invention.

[Explanation of symbols]

 2 Photodiode (PD) 4 Amplifier 6 FET in parallel with feedback resistor RF 8 Transimpedance control circuit RF feedback resistor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/06

Claims (8)

[Claims] 1. A preamplifier for optical communication comprising a light receiving element for receiving an optical signal and generating a signal current corresponding to the optical signal, and a current / voltage conversion circuit for converting the signal current into a voltage signal. A preamplifier for optical communication, wherein the input transistor of the current / voltage conversion circuit is constituted by an enhancement type field effect transistor (FET). 2. In the preamplifier for optical communication, when a large amount of light is input, a part of the output current of the light receiving element is set to the current / current.
The preamplifier for optical communication according to claim 1, wherein the preamplifier for the optical communication is supplied to the gate of the input FET of the voltage conversion circuit. 3. The preamplifier for optical communication according to claim 2, wherein the source electrode of the input FET is connected to a predetermined voltage source or the anode electrode of a diode biased in the forward direction. 4. The first-stage amplifier circuit portion of the current / voltage conversion circuit is composed of two cascode-connected field effect transistors.
The preamplifier for optical communication according to any one of 1. 5. The preamplifier for optical communication according to claim 1, wherein the current / voltage conversion circuit is composed of a transimpedance type amplifier having a feedback resistor. 6. The drain of the field effect transistor is connected to one end of the feedback resistor, the source is connected to the other end of the feedback resistor, and the gate is connected to a feedback amount control means having the output of the current / voltage conversion circuit as a control input. 5. The preamplifier for optical communication according to claim 4. 7. The preamplifier for optical communication according to claim 1, which is operated by a single power supply voltage whose absolute value is less than 5V. 8. The preamplifier for optical communication according to claim 1, wherein the FET is a GaAs-MESFET.