JPH0666605B2 - Transimpedance circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a transimpedance circuit.

[Conventional technology and its problems]

Conventionally, a transimpedance circuit used for an optical data link or the like has a configuration as shown in FIGS. 2 and 3.

FIG. 2 is a transimpedance circuit with two-stage transistors. The collector output of the signal input transistor Q ₁ is given to the base of the signal output transistor Q ₂ . The emitter follower output signal of the transistor Q ₂ is given to the base of the transistor Q ₁ via the feedback circuit formed of the resistor Rf.

In this first conventional circuit, the direct current (DC) bias point of the input is set by the feedback resistor Rf, and when an input signal is input and the input current is I _IN , the voltage corresponding to Rf · I _IN is output. Appear in. However, since the DC bias point of this circuit is only the base-emitter voltage V _BE of one transistor Q ₂ , it can be generally used for optical data links that require constant current and high speed operation. I can't. For example, when connecting an optical fiber with a length of 1 m to several tens of m, the maximum power required is about 80 μW,
This corresponds to about 2V _BE for the DC bias point, so the DC bias point is insufficient with V _BE alone. Therefore, it is necessary to prepare transimpedance circuits having different circuit configurations or different circuit constants depending on whether the optical fiber is long or short.

The second conventional circuit shown in FIG. 3 overcomes the drawbacks of the first conventional circuit and is composed of three stages of transistors. That is, the emitter of the first-stage transistor Q ₁ for signal input is connected to the emitter resistor R _E1 as an emitter follower, and the second-stage transistor Q ₁ is connected.
₃ bases are connected. The emitter of the signal output transistor Q ₂ is connected to the emitter resistor R _E2 as an emitter follower and the feedback resistor Rf.

Also in this second conventional circuit, the input DC bias point is set by the feedback resistor Rf, and when an input signal is input, a voltage corresponding to Rf · I _IN appears on the output side when the input current is I _IN . In this circuit, the base-emitter voltage of the transistors Q ₁ and Q ₂ with two DC bias points is 2V _BE
Therefore, it can be used for an optical data ring having a long optical fiber.

However, in this circuit, 3
Since it is composed of the transistors in stages, the peaking of the frequency characteristic is likely to occur. In order to eliminate this peaking, for example, a capacitance may be connected in parallel with the feedback resistor Rf, but if this is done, the frequency characteristics will deteriorate as compared with the first conventional circuit.

As a circuit that suppresses the deterioration of frequency characteristics,
There is an amplifier circuit described in No. 15. This circuit configuration is shown in FIG. This is a capacitor connected in parallel with the feedback resistor. As a capacitor, static capacitance between the base and collector of the transistor (PN junction base-collector diode).
Is used. Since this diode has a small capacitance compared to the case where the capacitance between the base and the emitter is used, it is possible to suppress ringing absorption and waveform blunting of the output waveform. However, this circuit uses a mirror circuit for current amplification, and when viewed as a whole circuit, it has a circuit configuration of three stages including an input stage, a mirror circuit, and an output stage, and circuit delay is the same as that of the second conventional circuit. does not change. Also, the DC bias point is V _BE , which has the same problem as the first conventional circuit.

Therefore, an object of the present invention is to provide a transimpedance circuit that can increase the DC bias point of an input signal, improve the frequency characteristics, and increase the dynamic range.

[Means for solving problems]

A transimpedance circuit according to the present invention includes a first transistor having a base to which a signal is input and a collector connected to a power supply through an additional resistance; and a mirror diode having an anode connected to the collector of the first transistor. A mirror circuit connected between the emitter of the first transistor and the cathode of the mirror diode, and a second transistor for output whose base is connected to the cathode of the mirror diode and whose emitter is grounded via a resistance element. And a feedback circuit for feeding back the output signal from the second transistor to the base of the first transistor.

[Action]

According to the structure of the present invention, the diode connected to the collector of the input first transistor acts to raise the operating point of the DC bias. Further, since the circuit is composed of two-stage transistors like the first conventional circuit, the frequency characteristic is not deteriorated, and the mirror circuit stabilizes the operating point of the diode and enables its high speed operation. Acts like.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. 1 of the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is a circuit diagram of a transimpedance circuit according to an embodiment. The circuit of this embodiment has a first transistor Q ₁ for signal input and a second transistor Q ₂ for signal output, and a feedback resistor Rf is connected between the input terminal and the output terminal. 2 is the same as the conventional circuit shown in FIG. The difference is that a mirror diode D is connected between the collector of the transistor Q _{1 and} the base of the transistor Q ₂ , and a mirror circuit is connected between the emitter of the transistor Q ₁ and the cathode of the diode D. Here, the mirror circuit is composed of transistors Q ₄ and Q ₅ , and the collector of the transistor Q ₄ is connected to the emitter of the transistor Q _{1 and} the collector of the transistor Q ₅ is connected to the cathode of the diode D. The collector and base of the transistor Q ₄ and the base of the transistor Q ₅ are commonly connected, and their emitters are both grounded.

Next, the operation of the above embodiment will be described.

As described above, in the circuit of this embodiment, the anode of the diode D is connected to the base of the transistor Q _{1 and} the base of the transistor Q ₂ is connected to the cathode of the diode D 1. Therefore, 2V due to the diode D and the transistor Q ₂ is applied between the base and collector of the transistor Q _1.
BE can be obtained, and the DC bias point of the input is 2V _BE . Therefore, the circuit of this embodiment can be used for general purposes even when the level of the DC bias required here is different because the length of the optical fiber applied in the optical data link is different.

On the other hand, since the circuit of this embodiment includes the diode D, a delay due to this is inevitably generated. However, in this embodiment, since the mirror circuit including the transistors Q ₄ and Q ₅ is provided, this causes the diode D to be self-biased, and the above delay is greatly suppressed. Therefore, the operating point of the diode D is stabilized and high speed operation is possible.

Further, in this embodiment, the input impedance is the same as that of the conventional circuit shown in FIG. 2, and the frequency characteristics are the same.

The present invention is not limited to the above embodiment, but various modifications can be made.

In particular, various modes are possible for the specific configuration of the mirror circuit. Further, its application is not limited to the optical data link.

〔The invention's effect〕

As described above in detail, according to the transimpedance circuit of the present invention, since the diode is connected to the collector of the first transistor for input, the operating point of the DC bias becomes high. Further, since the circuit is composed of two-stage transistors like the first conventional circuit, the frequency characteristic is not deteriorated, and the mirror circuit stabilizes the operating point of the diode and enables its high speed operation. . Therefore, there is an effect that the frequency characteristic is improved and the dynamic range can be increased while increasing the input DC bias point.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a transimpedance circuit according to an embodiment of the present invention, and FIGS. 2, 3, and 4 are circuit diagrams of conventional transimpedance circuits. Q ₁ …… First transistor, Q ₂ …… Second transistor, D …… Mirror diode, Q ₄ , Q ₅ …… Mirror circuit transistor.

Claims (2)

[Claims] 1. A first transistor having a base to which a signal is input and a collector connected to a power supply through an additional resistor,
A mirror diode having an anode connected to the collector of the first transistor, a mirror circuit connected between the emitter of the first transistor and a cathode of the mirror diode, and a base connected to the cathode of the mirror diode. And an output second transistor whose emitter is grounded via a resistance element, and a feedback circuit for returning an output signal from the second transistor to the base of the first transistor. Transimpedance circuit to do. 2. The mirror circuit includes a third transistor having a collector connected to the emitter of the first transistor, and a fourth transistor having a collector connected to the cathode of the mirror diode. The transimpedance circuit according to claim 1, wherein a collector and a base of the third transistor and a base of the fourth transistor are commonly connected.