JP3217806B2 - Light detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photodetection circuit used for an optical memory, a laser beam printer, and the like.

[0002]

2. Description of the Related Art Conventionally, increasing the speed of a photodetector circuit is an important point for optical application equipment. On the other hand, in a photodetection circuit using feedback, a stable operation of the circuit is also required by increasing the open loop gain. However, in order to satisfy the above two conditions at the same time, the collector resistance at the common emitter must be small in the former case and large in the latter case. There is a problem.

As a first conventional example, FIG. 7 shows a typical circuit having the above-mentioned problem. This impedance conversion as an emitter follower by direct volume Q ₂ output of the collector of the common emitter of the transistor Q _1, is carried out that feedback by resistor Rf. In this case, due to the presence of capacitance Cbc between the base and collector of Q _1, it can not be a large value of collector resistance Rc ₁ is to speed. If Rc ₁ is made large, that is, if Rc ₁ / Re ₁ is made large, the feedback in C ₁ by Cbc becomes large.

FIG. 8 shows a second conventional example. In this case, Rb ₁ together with Rf biases the base of Q ₁ , but in such a circuit configuration, the offset of Vout usually has a negative coefficient with respect to temperature (but , the temperature coefficient of Rb ₁ and Rf is a are equal).

[0005] In addition, other conventional examples related to the above-mentioned problems include those disclosed in JP-A-1-264313 and JP-A-2-84803. Description is omitted.

[0006]

The first conventional example is a circuit example in which cascode connection is not used. in this case,
Q ₁ is directly the output of the emitter grounded the collector side R
Since undergoing in c _1, large if the Rc ₁ was used, it is the response of the deterioration due Cbc problem, not suitable for circuits that must take a large Rf.

In the case of the second conventional example, the offset of Vout has a circuit configuration having a negative coefficient with respect to the temperature, and is not suitable for the case of amplifying from a DC power supply.

Further, these two circuits (see FIGS. 7 and 8) both have a large temperature coefficient in the offset,
In addition, there is a problem that a sufficient band cannot be obtained because the feedback resistor is easily affected by the capacitance between the terminals.

[0009]

According to the present invention, a cascode circuit is provided which amplifies the output of a photodiode with a transistor whose emitter is grounded and receives the output of its collector at a grounded base. To the emitter follower circuit, and provide positive feedback from the collector side of this emitter follower circuit via a capacitor.
The connection to be applied and the emitter side of the emitter follower circuit
A connection for applying a negative feedback via a resistor from, for each base first-stage transistor of the cascode circuit
A bias resistor and a forward bias diode were connected in series between the base of the transistor whose emitter was grounded in the cascode circuit and the ground.

According to the second aspect of the present invention, a photo diode
Ground one terminal and connect the other terminal to Darlington
Connected to the base of the first stage of the transistor
Receive transistor with emitter follower and connect to Darlington
The emitter and ground of the second transistor
A resistor and a diode are connected in series between
Casco received at the collector side of the transistor with the base grounded
The cascode circuit is connected to the output side of the cascode circuit.
Connect the emitter follower circuit and
Apply positive feedback from the collector side of the road via a capacitor
Connection and whether the emitter side of the emitter follower circuit
Connection to apply negative feedback through a feedback resistor from the
To the first transistor in the
Photodiode bias and initial transistor base
Biasing is performed via the feedback resistor .

[0011]

According to the first and second aspects of the present invention, the cascade
To achieve broadband by de circuit, the emitter follower circuit
Positive and negative feedback using emitter and collector
Casco which was not able to be applied because of the photodiode
DC circuit to the base of the first stage transistor
Ground through a negative feedback resistor.
Combination of bias resistor and bias diode
Temperature compensation can be performed .

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows the I / O of the output of the photodiode.
2 shows a circuit configuration of a V-conversion first-stage amplifier. That is, the cascode circuit 1 including the transistors Q ₁ and Q ₂ is connected to the output side of the photodiode PD.
A biasing diode Db ₁ of the bias resistor rb ₁ and the forward direction are connected in series with between the base and the ground of the cascode circuit transistor Q ₁ whose emitter is grounded one. An emitter follower circuit 2 is connected to the output side of the cascode circuit 1. A part of the output of the emitter follower circuit 2 is connected to a feedback circuit 3 for feeding back to the base of the transistor Q1 in the _first stage of the cascode circuit 1. In this case, the feedback circuit 3, a signal extracted via a resistor rfn connected to the emitter of the transistor Q _3, signal and said cascode taken out through a capacitor Cfp connected to the collector of the transistor Q ₃ The feedback is made to the base of the transistor Q1 in the _first stage of the circuit 1.

[0013] In such a configuration, PD is reverse biased to Vcc side, the anode side NPN transistors Q _1, Q ₁ of the first stage cascode circuit 1 is composed of two Q ₂ 'base It is connected to the. In Q ₂ of the second stage of the cascade connection, the base resistance rb _2, r
It is biased at bb ₂ and is grounded at an impedance Cb ₂ that is sufficiently low in terms of AC. The third stage Q ₃ from the collector side of Q ₂ is emitter-follower, the emitter side provides negative feedback via rfn, the collector side provides positive feedback via Cfp, and the first stage Q ₁ Connected to the base. In this case, the base bias of Q ₁ is are carried out by the resistance rfn negative feedback, a bias resistor rb _1, a bias diode Db _1.

[0014] current output from the PD, the current amplification is performed with the emitter grounded Q _1, enter at low input impedance due to the grounded base of Q _2, is output after being converted into a high impedance. When the emitter is grounded, the response speed is affected when the load impedance on the collector side is large. However, since the cascode connection operates between Q ₁ and Q ₂ with low impedance,
The response of the Q ₁ may be a good thing. Also, Q
The output of the common base ₂ is dropped by a resistor rc ₂ to be taken out as a voltage signal, and impedance-converted by an emitter follower of Q ₃ for output. Therefore, since Q ₂ be inserted large resistance rc ₂ by the base ground is fast response, Q ₂ is no, i.e., excellent in high gain, yet high speed as compared with the case of by Q ₁ emitter grounded only I have.

In the feedback, r in FIG.
fn has a small inter-terminal capacitance Cfn, lowers the closed-loop gain in the high frequency range, and narrows the band. Therefore, by connecting a small resistor rc ₃ to the collector of Q ₃ and applying a positive feedback via the capacitor Cfp, it is possible to compensate for a drop in high-frequency gain.

Furthermore, the resistors and the transistor has a temperature coefficient, had had a particularly negative effect on the offset in practical applications, by biasing the rb ₁ and Db ₁ to the base of Q _1, the rb ₁ Just select the size,
The temperature coefficient of the offset of the output signal of the circuit can be adjusted. Further, as shown in FIG. 2 in front of Q _1, when fitted with a emitter-follower of the transistor Q ₄ are, the biasing diode Db insert one or more, if in order to compensate the resistance r Good.

As a specific example, comparing with the conventional example by giving numerical values, in the above-mentioned second conventional example (see FIG. 8), rf (= rfn) = 100 kΩ, and the terminal-to-terminal capacitance Cf of rf If (= Cfp) = 0.1 pF, the value of the band fc is fc = 30 MHz at best. In contrast, in FIG. 1 of the present embodiment, by applying a positive feedback from the collector of Q _3, can be compensated for more than fc = 100 MHz.

Here, a specific example based on FIG. 1 will be described with reference to numerical values. All terminal capacitances are 0.1
pF, Vcc = 12v, rb1 = 10.5kΩ, rc2 = 5
6 kΩ, rc ₃ = 100 Ω, re ₃ = 10 kΩ, rfn = 10
0 kΩ, Cfp = 9 pF, Cb2 = 10 μF, Cpd = 3 p
F (27 ° C, temperature coefficient of resistance 2200ppm / ° C)
In this case, the band fc is fc ≒ 70 MHz and the transimpedance is 93 kΩ. On the other hand, when no compensation is performed with rc ₃ = 0Ω and Cfp = 0 pF, fc ≒ 20 MHz.

Rb ₁ = 10 kΩ, 10.5 kΩ, 1
Assuming 1 kΩ, the value of the offset voltage of Vout is as shown in Table 1 below for each temperature T ° C.

[0020]

[Table 1]

Thus, when rb ₁ = 10.5 kΩ, V
It can be seen that the temperature change of the offset of out becomes very small.

Next, a second embodiment of the present invention will be described with reference to FIG. Here, Q ₃ is the opposite polarity transistors that are used in emitter follower, that is, Q _1,
Against NPN transistor Q _2, Q ₃ are those showing a circuit configuration in which consist PNP transistor. Also in this case, as in FIG. 1, a wider band can be obtained for positive feedback by inserting Cfn from the collector side.

Next, a third embodiment of the present invention will be described with reference to FIG. Here, transistors Q ₁ , Q ₂ , Q ₃
Are all configured by PNP, corresponding to FIG. In this case, similarly to FIG. 1, it is also possible to widen the band by applying positive feedback or to perform temperature compensation using a diode and a resistor.

Next, a fourth embodiment of the present invention will be described with reference to FIG. Here, a circuit configuration when the photodetector PD is connected to the ground side is shown. In this case, in order to apply a large reverse bias to the PD, FIG.
As shown in FIG. 6, the transistor Q ₄ (only one stage in FIG. 6) of the emitter follower is connected in several stages, or a diode such as De ₁ is connected to the emitter side of Q ₁ . Such a connection of elements can be inserted into the positive feedback circuit of FIG.
, The resistor re ₁ and the diode De ₁ are connected to the emitter of Q ₁ instead. However, that case, if the resistance re ₁ is increased, the voltage gain becomes small in Q _2, it is necessary to take care because it becomes difficult to conduct adequate compensation.

[0025]

According to the first and second aspects of the present invention, a photo die is provided.
High reverse bias as an I / V conversion circuit of Aether
Because it is easy to apply like a common emitter circuit
Base bias that could not be
A DC amplifier as an I / V conversion circuit for the photodiode
Temperature compensation that is required to
The output of the photodiode is a current source
First stage generated due to high output impedance
Band limitation due to the collector capacitance of the transistor
Compensation for widening the bandwidth is performed. And this
Each of these functions is not implemented by a separate circuit element.
Ku, single circuit elements are for cum to the realization of a plurality of functions
Therefore, the circuit configuration can be simplified .

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing a state in which an emitter follower circuit is provided in a stage preceding a first transistor of the cascode circuit of FIG. 1;

FIG. 3 is a circuit diagram showing a second embodiment of the present invention.

FIG. 4 is a circuit diagram showing a third embodiment of the present invention.

FIG. 5 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 6 is a circuit diagram showing a modification of the fourth embodiment.

FIG. 7 is a circuit diagram showing a first conventional example.

FIG. 8 is a circuit diagram showing a second conventional example.

[Explanation of symbols]

Reference Signs List 1 photodiode 2 emitter follower circuit 3 feedback circuit PD photodiode Q ₁ transistor Q ₃ transistor rb ₁ bias resistor Db ₁ forward bias diode rc ₃ resistor rfn resistor Cfp capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-182906 (JP, A) JP-A-63-184408 (JP, A) JP-A-2-84803 (JP, A) 178910 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03F 3/08 H03K 17/78

Claims (2)

(57) [Claims] 1. A cascode circuit which amplifies the output of a photodiode with a transistor whose emitter is grounded and receives the output of its collector at a base ground. An emitter follower circuit is connected to the output side of the cascode circuit. From the collector side of the
Connection and the emitter follower circuit
Connect the negative feedback from the emitter side via a resistor.
This is performed on the base of the first-stage transistor of the cascode circuit, and a bias resistor and a forward bias diode are connected in series between the base of the transistor whose emitter is grounded and the ground of the cascode circuit. A photodetector circuit characterized in that: 2. One of the terminals of the photodiode is grounded.
The first stage of a transistor with the other terminal in Darlington connection
Connected to the base of the
The tiger in the second stage connected to the receiver Darlington with a follower
A resistor and a diode are connected between the emitter of the transistor and ground.
Connected in series, and the collector side of the transistor
A cascode circuit is provided at the base ground.
Connect the output emitter follower circuit to the output side of the code circuit.
Connected from the collector side of this emitter follower circuit.
Connection to apply positive feedback via a capacitor,
Negative return from the emitter side of the tap follower circuit via the feedback resistor
Connection, the first tier of the Darlington connection
For the photodiode and the via of the photodiode.
And the bias of the base of the first transistor
A photodetection circuit characterized in that the detection is performed via a feedback resistor .