JPH11296892A - Light receiving amplifier circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an output noise due to a heat noise in a light receiving circuit for an optical pickup caused accompanying the speed up of a CD-ROM driver. SOLUTION: In the light receiving amplifier circuit in which one end of a photo-diode PD1 is connected to a first input terminal of a differential amplifier Al, and a first resistor Rf11 for converting the photocurrent of the photodiode PD1 to a voltage is connected between the first input terminal and the output terminal of the differential amplifier A1, and a reference voltage is supplied to the second input terminal of the differential amplifier A1 through an offset voltage correcting second resistor Rf12 of the same value as the first resistor Rf11, one end of a capacitor C1 is connected to a connection point between the second input terminal and the second resistor Rf12, and the other end of the capacitor C1 is connected to a fixed potential point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light receiving and amplifying circuit for an optical pickup used for a CD-ROM (compact disk reader) and the like.

[0002]

2. Description of the Related Art A conventional light receiving amplifier circuit for an optical pickup of a CD-ROM has a differential amplifier A5 as shown in FIG.
, A photodiode PD5 is connected to its negative input terminal, a current / voltage conversion resistor Rf51 is connected between the inverting input terminal (-) and the output terminal 12, and the photodiode PD5 is connected.
5 is output by using Rf51.
Voltage conversion is being performed.

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 to the non-inverting input terminal (+) of the differential amplifier is corrected. To the terminal 11 of the reference voltage Vref via the resistor Rf52 for performing the operation, when IB51 = IB52 and Rf51 = Rf52, the output voltage V53 becomes V53 = Vref + Isc5 × R
f51.

[0004]

In the conventional system, FIG.
As shown in (a), a non-inverting input terminal (+) of the differential amplifier A5 is connected to a resistor Rf52 for offset voltage correction, and the thermal noise generated by the resistor R4k (Rf52) T (Ｔ).
f)｝ ^1/2 occurred. Here, k: Boltzmann constant, Rf52: resistance value of Rf52, T: absolute temperature, △
f: bandwidth.

In the noise analysis, in the circuit shown in FIG. 6A, the junction capacitance of the photodiode PD5 does not affect the circuit operation in a low frequency region, and is equivalent to the circuit shown in FIG. The output noise due to the resistor Rf52 is not amplified by the amplifier A5,
f52) T ({f)} remains at ^1/2 .

At present, in a high frequency range of 20 MHz or more required by a CD-ROM driver, FIG.
The circuit of FIG. 6A is equivalent to the circuit of FIG. 6C, and the resistance Rf52 is set by the junction capacitance C5 of the photodiode PD5 connected between the inverting input terminal (-) and the ground (GND).
Is amplified and {4k (Rf52) T (Δf)} ^1/2 × ｛1+ (jωC
5) Output noise represented by × Rf51｝ was generated.

[0007]

According to the first aspect of the present invention, one end of a photodiode is connected to a first input terminal of a differential amplifier, and a first input terminal of the differential amplifier is connected to the first input terminal of the differential amplifier. A first resistor for converting a photocurrent of the photodiode into a voltage is connected between the input terminal and the output terminal, and a second input terminal of the differential amplifier has a second input terminal for correcting an offset voltage having the same value as the first resistor. In a light-receiving amplifier circuit to which a reference voltage is supplied via two resistors, one end of a capacitor is connected to a connection point between the second input terminal and the second resistor,
The other end of the capacitor is connected to a fixed potential point.

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, so that the output noise generated at the output terminal is reduced. You.

The invention according to claim 2 is characterized in that, in the invention according to claim 1, the capacitor has a capacitance value equal to or larger than a junction capacitance of the photodiode. According to this configuration, when the output noise is expressed mathematically, there is a term in which the junction capacitance of the photodiode is on the numerator side and the capacitance of the capacitor is on the denominator side. If so, the noise in that term can be reduced.

[0010] The invention of claim 3 is based on claim 1 or 2.
In the invention, the capacitor is formed 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 ground point.

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, and a first differential amplifier of the first differential amplifier are provided. A resistor connected between an input terminal and an output terminal for converting the photocurrent of the photodiode into a voltage, and a resistor for correcting an offset voltage connected between a reference voltage input point and a second input terminal of the first differential amplifier. This is a light receiving amplification circuit including a transistor circuit. According to this configuration, output noise can be reduced only by the transistor circuit without providing a capacitor.

Further, according to a fifth aspect of the present invention, in the fourth aspect of the present invention, the transistor circuit includes a constant current circuit which multiplies a current for biasing a pair of transistors commonly connected to the emitter constituting the first differential amplifier. And a second differential amplifier.

[0013]

FIG. 1 shows a first embodiment of the present invention. In the figure, A1 is a differential amplifier, the inverting input terminal (-) of which is connected to a photodiode PD1 and a current / voltage conversion resistor Rf11, and the non-inverting input terminal (+) of a differential amplifier. Voltage correction resistor Rf12 having the same value as the resistor Rf11
Is connected.

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. Correction resistor R to which offset voltage Vref is given
f12 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, from which information recorded on the compact disk detected by the photodiode PD1 is output in the form of a voltage.

Here, the inverting input terminal (-) of the amplifier A1
Input current IB11 and non-inverting input terminal (+) input current IB
12 is equal when the positive and negative input circuits of the differential amplifier are matched, the output voltage V13 of the amplifier A1 becomes V13 = V11 + Rf11 × IB11, and the voltage V11 at the point b is V11 = V12 = Vref−Rf12 × IB12 Here, IB12 = IB11, Rf11 = Rf
When V21 = 21 V13 = (Vref−Rf12 × IB12) + Rf11
× IB11 = Vref.

Therefore, Rf11 × IB is determined by Rf12.
A difference voltage (referred to as an offset voltage) between the output voltage represented by 11 and Vref can be canceled. In FIG. 1, when a photocurrent of Isc1 flows through the photodiode PD1, the output voltage of the amplifier A1 becomes V13 = Vref + Rf11 × Isc1. As described above, the offset voltage caused by the input current can be corrected by the action of the resistor Rf12, but the resistor Rf12 is connected to the non-inverting input terminal (+) of the amplifier A1.
As a result, a thermal noise voltage of {4k (Rf12) T ({f)} ^1/2 is input.

As described above, in a 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. Assuming that the junction capacitance is Cj1, the output terminal of the amplifier A1 is {4k (Rf12) T (△ f)} ^1/2 × ｛1+ (jωC
j1) × Rf11}.

By inserting a capacitor C1 between the non-inverting input terminal (+) of the amplifier A1 and the ground GND, the resistance R
The thermal noise generated at f12 is integrated by an integrating circuit of 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 (△ f)} ^1/2 × ｛1 / (jωC
1 × Rf12)｝ × ｛1+ (jωCj1) × Rf1
1) It becomes｝.

In FIG. 1, when the junction capacitance of the photodiode PD1 is Cj1 as described 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 (△ f) ｝ ^1/2 × ｛1 / (jωC
1 × Rf12)｝ × ｛1+ (jωCj1) × Rf1
1) It becomes｝.

Here, C1 = Cj1, Rf11 = Rf1
When 2 is set, {4k (Rf12) T (△ f)} ^1/2 × ｛1 + 1 / (j
ωC1 × Rf12)｝. Here, in the high frequency signal, 1 / (jωC1 ×
Rf12) can be regarded as zero, so the above noise is ｛4k
(Rf12) T ({f)} ^1/2 , and the amplification effect caused by the bypass effect of Cj1 can be canceled. When C1> Cj1, ｛4k (Rf1
2) Output noise can be reduced from T ({f)} ^1/2 .

FIG. 2 specifically shows the differential amplifier A1 of FIG. In FIG. 2, the differential amplifier comprises a pair of NPN type transistors Q1, Q2 and a constant current source transistor Q
12, NPN 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 transistor Q1.
Connected to the collector.

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. The transistor Q
3, Q4 forms 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 the inverting terminal of the differential amplifier, and the photodiode PD1 and the resistor R
f11 is connected. Output terminal 2 is transistor Q
5 and the collector of the transistor Q13.

Next, FIG. 3 shows a configuration of the capacitor C1. A21 is a thick protective film made of an oxide film or the like outside the capacitor, A22 is a thin film layer such as a nitride film or an oxide film constituting a dielectric portion (insulating film) of the capacitor, A23 and A24 are metal layers, and A25 has a high impurity concentration. N-type semiconductor, A26 is an N-type semiconductor layer having a low concentration by an epitaxial layer, A2
Reference numeral 7 denotes a P-type semiconductor substrate, and reference numerals A28 and A29 denote P-type diffusion layers for separating the epitaxial layer of the capacitor portion from other epitaxial layers.

The metal layer A23 constitutes one of the electrodes of the capacitor, and A24 has an ohmic contact with the N-type semiconductor A25 serving as the other electrode of the capacitor and serves as an outlet for the other electrode of the capacitor. A25 and A26 are conductive with 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.

When A24 is connected to the non-inverting input terminal (+) of the differential amplifier A1 in FIG.
c2, a voltage drop of Rf12 × Isc2 is generated at the non-inverting input terminal (+) of the differential amplifier A1.
Causes output fluctuation. A2 when A24 is grounded
7 is also grounded, so Isc2 is A26 → A27 →
This can be prevented from being consumed in the loop of A24 → A25 → A26 and affecting the amplifier circuit.

In FIG. 6A, if Rf52 is removed (Rf52 is set to 0), the non-inverting input terminal (+) and the inverting input terminal (-) of the differential amplifier A5 as shown in (1) above. When the offset voltage between terminals is ignored, V53 =
Vref + Rf51 × IB51 becomes Rf51 × IB5
One output voltage error (offset voltage) occurs. Here, IB51 is the input current of the inverting input terminal (-) of the differential amplifier A5. However, the thermal noise generated by Rf52 is reduced to 0.
It can be.

FIG. 4 shows a second embodiment of the present invention, 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. It is an example. The output noise is reduced by minimizing the noise generated in the transistor circuit A32. Here, A31, Rf3
1, PD3, V31, V32, Isc3 are the first
A1, Rf11, PD1, V11, V12 of the embodiment,
This is a component corresponding to Isc1.

FIG. 5 shows a specific example of a light receiving amplifier circuit having the transistor circuit A32 for offset voltage correction 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 them, the current / voltage block A4
1 has substantially the same configuration as that in FIG. 2 except for the capacitor C1.

Therefore, the transistors Q401, Q40
2, Q412, Q403, Q404, Q405, Q41
Reference numeral 3 denotes the differential amplifier A31 in FIG. The bases of the constant current circuits Q412, Q413, Q414, and Q415 are transistors Q41 having a collector and a base connected.
1 are connected in common with each other, and they constitute a current mirror circuit. When the base current flowing through them is ignored, the collector current I flowing through Q412, Q413, and Q415 is ignored.
41 becomes (Vcc-VBE) / R43. Where V
BE is the base-collector voltage of Q411.

Q414 is composed of n transistors of Q411 or a transistor having an area which is n times as large as the emitter area of Q411, and its collector current is n × I
It becomes 41. Q406 and Q407 are commonly connected to the emitter, and the connection point is connected to the collector of Q414. Q408 and Q409 are commonly connected to a base to form a so-called current mirror circuit. Q408 has a base and a collector connected, is connected to a collector of Q406, and a collector of Q409 is connected to a collector of Q407.

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 form an emitter follower circuit. Q406, Q407, Q414, Q408, Q40
9, 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.

In the second differential amplifier A42 in FIG.
Since the inverting input terminal (-) and the output terminal of the differential amplifier are connected, the second differential amplifier A42 forms a voltage follower circuit. In the first differential amplifier A41 in FIG. 5, when Q401 and Q402 are formed in a monolithic integrated circuit, matched transistor pairs can be formed, so that the collector currents of Q401 and Q402 are considered to be equal. Therefore, the collector current of I41 / 2 becomes Q4
01, Q402. At this time, the base current of Q402 is (1 / hFE4
02) × (I41 / 2).

Therefore, the output voltage of A41 V44 = V42 + R41 × (I41 / 2) × (1
/ HFE402). Here, hFE402 is the current amplification factor of the transistor Q402.

Similarly, in the second differential amplifier A42, Q4
When Q06 and Q407 are matched, Q406 and Q407
Since a collector current of (n × I41) / 2 flows through 407, (1 / hFE406) × (n × I
41/2) flows. Here, hFE4
06 is the current amplification factor of Q406. Base voltage V41 of Q406 = Vfef-R42 × (1 / hFE406)
× (n × I41 / 2). When Q406 and Q407 are matched, V41 = V42 and V42 = Vre
f-R42 × (1 / hFE406) × (n × I41 /
2).

Therefore, V44 = Vref-R42 × (1
/ HFE406) × (n × I41 / 2) + R41 × (I
41/2) × (1 / hFE402). Where Q
Since 406 and Q402 are in the same chip, hFE40
When it is considered that 6 = fFE402 and nR42 = R41, V44 = Vref, and it is possible to cancel the output voltage error (offset voltage) of R41 × (base current of Q402).

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 R4
2 can be reduced to 1 / n1 / ² .

[0037]

According to the first to third aspects of the present invention, for example, the thermal noise generated in the offset voltage correction resistor, which has been a problem with the increase in the speed of the light receiving amplifier circuit used in the pickup of the CD-ROM driver, is reduced. This makes it possible to output a high quality pickup output voltage. Further, 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, there is no thermal noise itself due to the correction resistor.

[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.

FIG. 3 is a structural diagram of a capacitor indicated by C1 in FIG. 1;

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 the second embodiment.

FIG. 6 is a diagram showing a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reference voltage input terminal 2 Output terminal A1 Differential amplifier PD1 Photodiode RF11 Current / voltage conversion resistance Rf12 Offset correction resistance C1 Noise reduction resistance A32 Transistor circuit

Claims (5)

[Claims] A first input terminal of the differential amplifier, one end of the photodiode being connected to the first input terminal and an output terminal for converting a photocurrent of the photodiode into a voltage between the first input terminal and the output terminal of the differential amplifier; A light-receiving amplifier circuit to which a first resistor is connected and a reference voltage is supplied to a second input terminal of the differential amplifier via a second resistor for correcting an offset voltage having the same value as the first resistor; A light-receiving amplifier circuit, wherein one end of a capacitor is connected to a connection point between a terminal and a second resistor, and the other end of the capacitor is connected to a fixed potential point. 2. The light receiving and amplifying circuit according to claim 1, wherein the capacitor has a capacitance value equal to or larger than a junction capacitance of the photodiode. 3. The capacitor comprises an insulating film sandwiched between a metal and a semiconductor, the metal side being connected to a second input terminal,
3. The light-receiving amplifier circuit according to claim 1, wherein the semiconductor side is connected to a ground point. 4. A photoreceiver / amplifier circuit comprising:
A differential amplifier; a photodiode connected between a first input terminal of the first differential amplifier and a fixed potential point; a photodiode connected between a first input terminal and an output terminal of the first differential amplifier; A resistor for converting a photocurrent into a voltage, and a transistor circuit for offset voltage correction connected between a reference voltage input point and a second input terminal of the first differential amplifier. 5. The semiconductor device according to claim 1, wherein the transistor circuit is constituted by a second differential amplifier including a constant current circuit multiplied by a current for biasing a transistor pair connected to the emitter in common, which constitutes the first differential amplifier. The light receiving and amplifying circuit according to claim 4, wherein: