JP3642675B2 - Receiver amplifier circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light receiving amplification circuit for an optical pickup used in a CD-ROM (compact disk reader) or the like.
[0002]
[Prior art]
As shown in FIG. 6A, a conventional optical receiver circuit for optical pickup of a CD-ROM uses a differential amplifier A5 to connect a photodiode PD5 to its negative input terminal, and an inverting input terminal (-) and an output terminal. 12, a current / voltage conversion resistor Rf51 is connected, and the photocurrent Isc5 output from the photodiode PD5 is subjected to current / voltage conversion using Rf51.
[0003]
A non-inverting input terminal (+) of the differential amplifier is used to correct the offset voltage Rf51 × IB51 generated when the bias current IB51 of the differential amplifier is supplied from the output side via the current / voltage conversion resistor Rf51. When IB51 = IB52 and Rf51 = Rf52, the output voltage V53 is V53 = Vref + Isc5 × Rf51 when connected to the terminal 11 of the reference voltage Vref via the resistor Rf52.
[0004]
[Problems to be solved by the invention]
In the conventional method, as shown in FIG. 6A, an offset voltage correction resistor Rf52 is connected to the non-inverting input terminal (+) of the differential amplifier A5, and thermal noise {4k (Rf52) generated by the resistor is generated. T (Δf)} ^1/2 . Here, k is a Boltzmann constant, Rf52 is a resistance value of Rf52, T is an absolute temperature, and Δf is a bandwidth.
[0005]
In the noise analysis, in the circuit of FIG. 6A, the junction capacitance of the photodiode PD5 does not affect the circuit operation in the low frequency region, and is equivalent to the circuit shown in FIG. The output noise is not amplified by the amplifier A5 and remains {4k (Rf52) T (Δf)} ^1/2 .
[0006]
Currently, in the high frequency region of 20 MHz or more required by the CD-ROM driver, the circuit of FIG. 6A is equivalent to the circuit of FIG. 6C and is connected between the inverting input terminal (−) and the ground (GND). The noise due to the resistor Rf52 is amplified by the junction capacitance C5 of the photodiode PD5.
{4k (Rf52) T (Δf)} ^1/2 × {1+ (jωC5) × Rf51}
The output noise represented by
[0007]
[Means for Solving the Problems]
In order to solve the above-described problem, in the invention of claim 1, one end of a photodiode is connected to the first input terminal of the differential amplifier, the other end of the photodiode is grounded, and the first input of the differential amplifier is connected. A first resistor for converting the photocurrent of the photodiode into a voltage is connected between the terminal and the output terminal, and the same voltage is generated with respect to the offset voltage generated by the current flowing through the first input terminal and the first resistor. a light receiving amplifying circuit in which the second reference voltage through a resistor is supplied to the second input terminal of said differential amplifier for causing, connecting one end of said second input terminal and the capacitor to the connection point of the second resistor The other end of the capacitor is connected to a ground potential point.
[0008]
According to this configuration, the noise due to the thermal noise of the second resistor generated at the second input terminal of the differential amplifier is integrated by the second resistor and the capacitor, thereby reducing the output noise generated at the output terminal.
[0009]
The invention of claim 2 is characterized in that, in the invention of claim 1, the capacitor has a capacitance value larger than a junction capacitance of the photodiode. According to this configuration, when expressed output noise in mathematically, the junction capacitance of the photodiode becomes numerator, the capacitance of the capacitor is present term becomes denominator, if the capacitance of the capacitor is greater than the junction of the diodes , The noise of the term can be reduced.
[0010]
According to a third aspect of the present invention, in the first or second aspect of the present invention, the capacitor is configured by sandwiching an insulating film between a metal and a semiconductor, the metal side is connected to the second input terminal, and the semiconductor side is connected to a grounding point. It is characterized by.
[0011]
According to a fourth aspect of the present invention, a first differential amplifier, a photodiode connected between a first input terminal of the first differential amplifier and a fixed potential point, a first input terminal of the first differential amplifier, A first resistor connected between the output terminals for converting the photocurrent of the photodiode into a voltage, and an offset voltage due to the first resistor connected between the reference voltage input point and the second input terminal of the first differential amplifier. The transistor circuit has the same configuration as that of the first differential amplifier, and a constant current obtained by multiplying the current for biasing the common emitter-connected transistor pair constituting the first differential amplifier. A second differential amplifier having a circuit; and a second resistor connected between an input terminal of the second differential amplifier and the reference voltage input point . The first differential amplifier is an emitter. Is the first constant current A first current mirror circuit grounded through the first current mirror circuit; an input side connected to the collector of the first transistor and an output side connected to the collector of the second transistor; and the output side of the first current mirror circuit And an emitter follower transistor whose base is connected to the connection point of the collector of the second transistor and whose emitter is connected to the output terminal. On the other hand, in the transistor circuit for offset voltage correction, the emitter is the first constant current source. Third and fourth transistors grounded via a second constant current source that outputs a constant current of n times; a first transistor whose input side is connected to the collector of the third transistor and whose output side is connected to the collector of the fourth transistor 2 current mirror circuit; base connected to collector of fourth transistor and output side of second current mirror circuit And an emitter follower transistor whose emitter is connected to the base of the fourth transistor and the first transistor; an input terminal to which a reference voltage is applied and the base of the third transistor; / N times the second resistance having a resistance value.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention is shown in FIG. In the figure, A1 is a differential amplifier, a photodiode PD1 and a current / voltage conversion resistor Rf11 are connected to the inverting input terminal (−), and a current / voltage conversion is connected to the non-inverting input terminal (+). An offset voltage correcting resistor Rf12 having the same value as the resistor Rf11 and a capacitor C1 are connected.
[0014]
The current / voltage conversion resistor Rf11 is connected between the inverting input terminal (−) and the output terminal of the amplifier A1, and converts the photocurrent of the photodiode PD1 into a voltage. The correction resistor Rf12 to which the offset voltage Vref is applied is inserted between the reference voltage terminal 1 and the non-inverting input terminal (+) of the amplifier A1. Reference numeral 2 denotes an output terminal of the photoreceiver / amplifier circuit, and the recording information on the compact disc detected by the photodiode PD1 is output in the form of voltage.
[0015]
Here, the inverting input terminal (−) input current IB11 and the non-inverting input terminal (+) input current IB12 of the amplifier A1 are equal when the positive and negative input circuits of the differential amplifier are matched. The output voltage V13 of the amplifier A1 is
V13 = V11 + Rf11 × IB11
The voltage V11 at point b is
V11 = V12 = Vref−Rf12 × IB12
It becomes. Here, when IB12 = IB11 and Rf11 = Rf21, V13 = (Vref−Rf12 × IB12) + Rf11 × IB11 = Vref.
[0016]
Therefore, a difference voltage (referred to as an offset voltage) from the output voltage Vref represented by Rf11 × IB11 can be canceled by Rf12. In FIG. 1, when a photocurrent called Isc1 flows through the photodiode PD1, the output voltage of the amplifier A1 is
V13 = Vref + Rf11 × Isc1
It becomes. As described above, the offset voltage caused by the input current can be corrected by the action of the resistor Rf12. However, the non-inverting input terminal (+) of the amplifier A1 is {4k (Rf12) T (Δf)} ^{1 /} by the resistor Rf12. ^A thermal noise voltage of ² is input.
[0017]
As described above, in the high frequency signal band handled by the CD-ROM driver, the junction capacitance of the photodiode PD1 bypasses between the inverting input terminal (−) of the differential amplifier A1 and the ground GND. If the junction capacitance is Cj1, the output terminal of the amplifier A1 is
{4k (Rf12) T (Δf)} ^1/2 × {1+ (jωCj1) × Rf11}
The output noise can be expressed as follows.
[0018]
Thermal noise generated by the resistor Rf12 by inserting the capacitor C1 between the non-inverting input terminal (+) of the amplifier A1 and the ground GND is integrated by an integration circuit having a time constant (Rf12 × C1) composed of Rf12 and C1. The output noise due to the thermal noise of the resistor Rf12 generated at the output terminal is
{4k (Rf12) T (Δf)} ^1/2 × {1 / (jωC1 × Rf12)} ×
{1+ (jωCj1) × Rf11)}
It becomes.
[0019]
In FIG. 1, when the junction capacitance of the photodiode PD1 is Cj1 as shown above, the output noise caused by the thermal noise of the resistor Rf12 due to the addition of the capacitor C1 is
{4k (Rf12) T (Δf)} ^1/2 × {1 / (jωC1 × Rf12)} ×
{1+ (jωCj1) × Rf11)}
It becomes.
[0020]
Here, when C1 = Cj1, Rf11 = Rf12,
{4k (Rf12) T (Δf)} ^1/2 × {1 + 1 / (jωC1 × Rf12)}
It becomes. Here, since 1 / (jωC1 × Rf12) can be regarded as zero in the high-frequency signal, the noise becomes {4k (Rf12) T (Δf)} ^1/2 and cancels the amplification effect caused by the bypass effect of Cj1. It becomes possible. Further, when C1> Cj1, it is possible to reduce output noise from {4k (Rf12) T (Δf)} ^1/2 .
[0021]
FIG. 2 specifically shows the differential amplifier A1 of FIG. In FIG. 2, the differential amplifier includes a pair of NPN transistors Q1 and Q2, a constant current source transistor Q12, PNP transistors Q3 and Q4, and NPN transistors Q5 and Q13. The emitters of the transistors Q3 and Q4 are connected to the power supply line 3 of the voltage Vcc, and the emitters are commonly connected. The collector and base of the transistor Q3 are connected to the collector of the transistor Q1.
[0022]
On the other hand, the collector of the transistor Q4 is connected to the collector of the transistor Q2 and the base of the emitter follower transistor Q5. Transistors Q3 and Q4 form a current mirror circuit. The base of the transistor Q1 is a non-inverting input terminal of the differential amplifier, and is connected to the terminal 1 via the resistor Rf12. On the other hand, the base of the transistor Q2 is an inverting terminal of the differential amplifier, and the photodiode PD1 and the resistor Rf11 are connected. The output terminal 2 is connected to the emitter of the transistor Q5 and the collector of the transistor Q13.
[0023]
Next, FIG. 3 shows a configuration of the capacitor C1. A21 is a thick protective film formed by an oxide film or the like outside the capacitor, A22 is a nitride film constituting the dielectric part (insulating film) of the capacitor, a thin film layer such as an oxide film, A23 and A24 are metal layers, and A25 is a high impurity concentration. An N-type semiconductor, A26 is an N-type semiconductor layer having a low concentration due to an epitaxial layer, A27 is a P-type semiconductor substrate, and A28 and A29 are P-type diffusion layers for separating the epitaxial layer of the capacitor portion from other epitaxial layers. .
[0024]
The metal layer A23 constitutes one of the electrodes of the capacitor, and A24 makes an ohmic contact with the N-type semiconductor A25 which becomes the other electrode of the capacitor and serves as an outlet for the other electrode of the capacitor. A25 and A26 are electrically connected by the same N-type semiconductor, and A26 forms a PN junction with A27 which is a P-type semiconductor. Since the PN junction performs the same operation as the photodiode in the light receiving element, a photocurrent is generated from A26 to A27.
[0025]
When the photocurrent is Isc2 when A24 is connected to the noninverting input terminal (+) of the differential amplifier A1 in FIG. 1, the voltage drop of Rf12 × Isc2 is set to the noninverting input terminal (+) of the differential amplifier A1. And output fluctuation due to Isc2. Since A27 is also grounded when A24 is grounded, Isc2 is consumed in the loop of A26 → A27 → A24 → A25 → A26, and it is possible to prevent the amplifier circuit from being affected.
[0026]
In FIG. 6A, if Rf52 is removed (Rf52 is set to 0), as shown in the above (1), between the non-inverting input terminal (+) and the inverting input terminal (−) of the differential amplifier A5. When the offset voltage is ignored, V53 = Vref + Rf51 × IB51, and an output voltage error (offset voltage) of Rf51 × IB51 is generated. Here, IB51 is an input current of the inverting input terminal (−) of the differential amplifier A5. However, the thermal noise generated by Rf52 can be reduced to zero.
[0027]
FIG. 4 shows a second embodiment of the present invention, which is an example in which a transistor circuit A32 for correcting the output voltage error between the non-inverting input terminal (+) of the amplifier A31 and the reference voltage Vref is inserted. . The output noise is reduced by minimizing the noise generated in the transistor circuit A32. Here, A31, Rf31, PD3, V31, V32, and Isc3 are components corresponding to A1, Rf11, PD1, V11, V12, and Isc1 of the first embodiment, respectively.
[0028]
FIG. 5 is a specific example of a photoreceiver amplifier circuit including the offset voltage correcting transistor circuit A32 in FIG. In FIG. 5, A41 is a current / voltage conversion block of the light receiving amplifier circuit, and A42 is an offset voltage correction block. Among these, the current / voltage block A41 has substantially the same configuration as the portion excluding the capacitor C1 in FIG.
[0029]
Therefore, the transistors Q401, Q402, Q412, Q403, Q404, Q405, and Q413 constitute the differential amplifier A31 in FIG. The bases of the constant current circuits Q412, Q413, Q414, and Q415 are connected in common with the base of the transistor Q411 that connects the collector and the base, and they constitute a current mirror circuit, and Q412, Q413, The collector current I41 flowing through Q415 is (Vcc-VBE) / R43. Here, VBE is the voltage between the base and collector of Q411.
[0030]
Q414 is composed of n transistors of Q411 or a transistor having an area n times as large as the emitter area of Q411, and its collector current is n × I41. Q406 and Q407 are connected in common to the emitter, and the connection point is connected to the collector of Q414. Q408 and Q409 are commonly connected to the base to form a so-called current mirror circuit, and Q408 is connected to the collector of Q406 while the base and collector are connected, and the collector of Q409 is connected to the collector of Q407.
[0031]
The connection point between the collector of Q407 and the collector of Q409 is connected to the base of Q410, and Q410 and constant current circuit Q415 constitute an emitter follower circuit. Q406, Q407, Q414, Q408, Q409, Q410, and Q415 constitute a second differential amplifier. The base of Q406 is a non-inverting input terminal (+), the base of Q407 is an inverting input terminal (−), and the emitter of Q410. Is an output terminal.
[0032]
In the second differential amplifier A42 in FIG. 5, since the inverting input terminal (−) and the output terminal of the differential amplifier are connected, the second differential amplifier A42 constitutes a voltage follower circuit. In the first differential amplifier A41 in FIG. 5, when Q401 and Q402 are configured in a monolithic integrated circuit, a matched transistor pair can be configured, so that the collector currents of Q401 and Q402 are considered to be equal. Therefore, it is considered that a collector current of I41 / 2 flows through the transistors Q401 and Q402. At this time, the base current of Q402 is (1 / hFE402) × (I41 / 2).
[0033]
Therefore, the output voltage V44 of A41 becomes V41 = V42 + R41 × (I41 / 2) × (1 / hFE402) by the base current. Here, hFE402 is a current amplification factor of the transistor Q402.
[0034]
Similarly, when Q406 and Q407 are matched in the second differential amplifier A42, since a collector current of (n × I41) / 2 flows in Q406 and Q407, (1 / hFE406) × (n × I41 / 2) base current flows. Here, hFE406 is the current amplification factor of Q406. The base voltage of Q406 is V41 = Vfef−R42 × (1 / hFE406) × (n × I41 / 2). When Q406 and Q407 are matched, V41 = V42 and V42 = Vref−R42 × (1 / hFE406) × (n × I41 / 2).
[0035]
Therefore, V44 = Vref−R42 × (1 / hFE406) × (n × I41 / 2) + R41 × (I41 / 2) × (1 / hFE402). Here, since Q406 and Q402 are in the same chip, it is considered that hFE406 = fFE402 and when nR42 = R41, V44 = Vref, and the output voltage error (offset voltage) corresponding to R41 × (base current of Q402) is canceled. Is possible.
[0036]
Since R42 = R41 / n, R42 can be reduced by setting n> 1. Since the thermal noise generated in R42 is proportional to (R42) ^1/2 , the above configuration makes it possible to reduce the thermal noise generated in R42 to 1 / n ^1/2 .
[0037]
【The invention's effect】
According to the first to third aspects of the present invention, for example, it is possible to reduce the thermal noise generated in the offset voltage correction resistor, which has been a problem as the speed of the light receiving amplifier circuit used for the pickup of the CD-ROM driver is increased. Thus, a pickup output voltage with good quality can be output. According to the fourth aspect of the present invention, since the offset voltage correction transistor circuit is provided instead of the offset voltage correction resistor, the second resistance corresponding to the first to third aspects can be reduced, and the heat generated by the second resistance can be reduced. There can be no or very little noise.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of the present invention.
FIG. 2 is a circuit diagram showing a specific example of the amplifier in FIG. 1;
FIG. 3 is a structural diagram of a capacitor indicated by C1 in FIG.
FIG. 4 is a circuit diagram of a second embodiment of the present invention.
FIG. 5 is a circuit diagram showing a specific circuit configuration of a second embodiment.
FIG. 6 is a diagram showing a conventional example.
[Explanation of symbols]
1 Reference Voltage Input Terminal 2 Output Terminal A1 Differential Amplifier PD1 Photodiode RF11 Current / Voltage Conversion Resistor Rf12 Offset Correction Resistor C1 Noise Reduction Resistor A32 Transistor Circuit

Claims (4)

One end of a photodiode is connected to the first input terminal of the differential amplifier and the other end of the photodiode is grounded, and the photocurrent of the photodiode is converted to a voltage between the first input terminal and the output terminal of the differential amplifier. A first resistor for conversion is connected, and a second resistor having the same value as the first resistor is used to generate the same voltage as the offset voltage generated by the current flowing through the first input terminal and the first resistor. A light receiving amplifier circuit in which a reference voltage is supplied to a second input terminal of the differential amplifier;
One end of a capacitor is connected to a connection point between the second input terminal and the second resistor, and the other end of the capacitor is connected to a ground potential point. The light receiving amplifier circuit according to claim 1, wherein the capacitor has a capacitance value larger than a junction capacitance of the photodiode. 3. The light receiving amplifier circuit according to claim 1, wherein the capacitor is configured by sandwiching an insulating film between a metal and a semiconductor, a metal side is connected to a second input terminal, and a semiconductor side is connected to a grounding point. . A first differential amplifier;
A photodiode connected between the first input terminal of the first differential amplifier and a fixed potential point;
A first resistor connected between a first input terminal and an output terminal of the first differential amplifier for converting the photocurrent of the photodiode into a voltage;
A transistor circuit connected between the reference voltage input point and the second input terminal of the first differential amplifier for correcting the offset voltage by the first resistor;
The transistor circuit has the same configuration as that of the first differential amplifier, and a second differential amplifier including a constant current circuit that multiplies a current for biasing a common emitter-connected transistor pair constituting the first differential amplifier. And a second resistor connected between the input terminal of the second differential amplifier and the reference voltage input point ,
The first differential amplifier has first and second transistors whose emitters are grounded via a first constant current source; a first transistor whose input side is connected to the collector of the first transistor and whose output side is connected to the collector of the second transistor. a current mirror circuit and, is composed of an emitter follower transistor output side and the emitter is the base connected to the connection point of the collector of the second transistor of the first current-mirror circuit is connected to said output terminal, while the offset voltage correction The transistor circuit includes a third and a fourth transistor whose emitter is grounded via a second constant current source that outputs a constant current n times that of the first constant current source; A second current mirror circuit connected on the output side to the collector of the fourth transistor; a base connected to the collector of the fourth transistor; An emitter follower transistor connected to the output side of the two current mirror circuit and having an emitter connected to the base of the fourth transistor and the first transistor; connected between an input terminal to which a reference voltage is applied and the base of the third transistor And a second resistor having a resistance value 1 / n times that of the first resistor .

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