JP3469727B2 - Current-voltage conversion 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 current / voltage conversion circuit for converting the output current of a photodiode into a voltage.

[0002]

2. Description of the Related Art In a CD reproducing apparatus, reading of data on an optical disk is performed by irradiating the optical disk with laser light and converting it into an electric signal corresponding to the intensity of light reflected by the optical disk. Conventionally, a photodiode is used as an element that converts this reflected light into an electric signal, and the photodiode generates an output current according to the intensity of the reflected light. The output current of the photodiode is converted into voltage. In general, the signal processing circuit performs signal processing in the voltage mode, so that an I / V conversion circuit for converting the output current of the photodiode into a voltage is required. When such a photodiode and an I / V conversion circuit are integrated on the same substrate, it is configured as shown in FIG.

In FIG. 3, first, the photodiode P
A case will be described where D is not irradiated with light and the photodiode PD does not generate an output current. The reference voltage Vref is applied to the base of the transistor 1 via the resistor 2. When the base voltage Vb3 of the transistor 3 is higher than the base voltage Vb1 of the transistor 1, since the transistors 1 and 2 are differentially connected, the collector current of the transistor 1 decreases and the collector current of the transistor 3 increases. Then, the output current of the current mirror circuit 4 that inverts the collector current of the transistor 1 decreases, and thus the base current of the transistor 5 forming the emitter follower circuit decreases. Since the transistor 5 current-amplifies this base current, the emitter current of the transistor 5 also decreases due to the decrease in the base current. The emitter current of the transistor 5 is fed back to the base of the transistor 3 via the feedback resistor 6. The feedback current is converted into a voltage by the feedback resistor 6, and the base voltage Vb3 of the transistor 3 drops due to the drop of the emitter current of the transistor 5. Therefore, transistor 3
Is the base voltage Vb3 of the transistor 1
When it becomes higher than 1, the base voltage Vb3 tends to decrease.

On the contrary, the base voltage Vb3 of the transistor 3
Is lower than the base voltage Vb1 of the transistor 1, the collector current of the transistor 1 increases and the collector current of the transistor 2 decreases. Therefore, the output current of the current mirror circuit 4 increases and the base current of the transistor 5 increases. When the base current of the transistor 5 increases, the emitter current of the transistor 5 also increases and the base voltage Vb3 of the transistor 3 rises. Therefore, transistor 3
Is the base voltage Vb3 of the transistor 1
When it becomes lower than 1, the base voltage Vb3 tends to increase.

That is, as described above, negative feedback is applied to the base of the transistor 3, so that the circuit of FIG. 2 is in a balanced state, and the base voltage Vb3 of the transistor 3 becomes equal to the base voltage Vb1 of the transistor 1. here,
If the value of the resistor 6 connected to the base of the transistor 3 is the same as the value of the resistor 2, the base currents of the transistors 1 and 3 are equal in the equilibrium state, and the voltage drop across the resistor 6 causes the reference voltage Vref from the output terminal OUT. Occurs.

In the equilibrium state, the photodiode PD
When light is emitted to the photodiode, a current I flows through the photodiode PD. Since the base current of the transistor 3 is constant in the balanced state, the output current I of the photodiode PD all flows through the resistor 6. When the value of the resistor 6 is R due to the voltage drop of the resistor 6, the output voltage (Vr
ef + I × R) occurs.

[0007]

By the way, the resistor 2 is connected to the base of the transistor 1, but the resistor 2 is connected in order to reduce the offset voltage.
That is, if the resistor 2 is not connected, the base voltage of the transistor 3 becomes equal to the reference voltage Vref by the action of negative feedback. At that time, a voltage drop occurs due to the current flowing through the feedback resistor 6, and the output voltage of the output terminal OUT is Vref + α + I × R, where α is the voltage corresponding to this voltage drop. Therefore, as shown in FIG. 3, the resistor 2 having the same value as the feedback resistor 6 is used.
To connect the reference voltage Vref from the output terminal OUT.
And a voltage (I × R) obtained by converting the input current into a voltage is generated. As a result, the voltage α corresponding to the voltage drop of the feedback resistor 6 is removed and the offset voltage is reduced.

However, in the circuit of FIG. 3, the thermal noise of the resistor 2 has been a problem. Generally, thermal noise is generated from the resistor, and the thermal noise of the resistor 2 is also applied to the base of the transistor 1. In the circuit of FIG. 3, due to the action of negative feedback,
It operates to equalize the base voltages of transistors 1 and 3. When the base voltage of the transistor 1 fluctuates due to the thermal noise of the resistor 2, the base voltage of the transistor 3 also fluctuates due to the action of negative feedback. Further, this fluctuation appears at the output terminal OUT via the feedback resistor 6. Therefore, there is a problem in that noise due to thermal noise increases due to the connection of the resistor 2.

[0009]

According to the present invention, there is provided a first transistor having a base to which a reference voltage is applied, a second transistor differentially connected to the first transistor, and the first transistor.
And a load connected to the collector of the second transistor,
An emitter follower circuit to which an output signal generated from the load is supplied, and a feedback resistor for returning the output signal of the emitter follower circuit to the base of the second transistor are provided, and an input current is applied to one end of the feedback resistor. In the current-voltage conversion circuit for obtaining an output voltage from the other end of the feedback resistor, a third transistor having the same characteristics as the second transistor, a constant current circuit connected to the emitter of the third transistor, and A current mirror circuit for inverting the base current of the third transistor and supplying it to the base of the second transistor; and a low-pass filter for removing noise in the base current of the third transistor. To do.

The low-pass filter is the third filter.
It is characterized in that it is a capacitor connected between the base of the transistor and the power supply voltage. Further, the output current of the constant current circuit is set to 1/2 of the operating current of the first and second transistors, and the mirror ratio of the current mirror circuit is set to 1. Furthermore, the output current of the constant current circuit is N times the operating current of the first and second transistors, and the mirror ratio of the current mirror circuit is 1 /
(2 × N).

According to the present invention, when the current-voltage conversion circuit is in a balanced state, the same current as the base current of the second transistor is generated by the third transistor, the constant current circuit and the current mirror circuit, and the second transistor is generated. , The base current of the second transistor is compensated and the offset can be reduced. At that time, since the noise of the base current of the third transistor is removed by the low-pass filter, the noise in the output voltage can also be reduced.

[0012]

1 is a diagram showing an embodiment of the present invention, 7 is a constant current source for generating an operating current Ie of transistors 1 and 3 which are differentially connected, and 8 is a second transistor 3 The transistor 9 has the same characteristics as that of the transistor 9. The emitter of the transistor 8 has a current Ie which is half the output current Ie of the constant current source 7.
/ 2 is a constant current source for generating / 2, 10 is a diode-connected transistor, the collector of which the base current of the transistor 8 flows through, 11 is a mirror connection with the transistor 10, and 11 is a transistor for supplying the collector current to the base of the transistor 3. , 12 are transistors 10
Is a capacitor connected between the collector of V and the power supply voltage Vcc. In addition, in FIG. 1, as shown in FIG.
Not connected. The same elements as those in FIG. 3 are designated by the same reference numerals as those in FIG.

The photodiode PD is not irradiated with light,
If no output current is generated from the photodiode PD,
The operation of making the base voltages of the transistors 1 and 3 of FIG. 1 equal by the action of the negative feedback is the same as the conventional one. When the base voltages of the transistors 1 and 3 become equal to each other, the output current Ie of the constant current source 7 changes to the transistors 1 and 3
And the emitter current of each is Ie / 2.
Becomes Since the emitter current of the transistor 3 is Ie,
If the current amplification factor of the transistor 3 is β, the base current Ib3 of the transistor 3 is

[0014]

[Equation 1]

[0015] On the other hand, the output current of the constant current source 9 is
It is Ie / 2, which is half the output current Ie of the constant current source 7. The current Ie / 2 flows through the transistor 8, and the transistor 8 generates a base current according to its emitter current. Since the transistor 8 has the same characteristics as the transistor 3, the current amplification factor of the transistor 8 can also be β. Therefore, the base current Ib8 of the transistor 8 is

[0016]

[Equation 2]

[0017] Base current Ib of transistor 8
8 is inverted by a current mirror circuit consisting of transistors 10 and 11. Since the resistance ratio of the emitter resistors RE10 and RE11 of the transistors 10 and 11 is set to 1: 1, the mirror ratio of the current mirror circuit is set to 1: 1. Since the mirror ratio is set in this way, the collector current Ic11 of the transistor 11 is Ic11 =
It becomes Ib8 and is supplied to the base of the transistor 3.
Therefore, in the equilibrium state, as is apparent from the equations (1) and (2), the base current of the transistor 3 is entirely supplied from the transistor 11. Since the base current of the transistor 3 does not flow from the feedback resistor 6 at all, the base current of the transistor 3 does not cause the voltage drop of the feedback resistor 6, and the base voltage of the transistor 3 is generated at the output terminal OUT as it is. That is, when no output current is generated from the photodiode PD, the reference voltage Vref is generated at the output terminal OUT.

When the photodiode PD is irradiated with light in the above-mentioned equilibrium state, the output current I is output from the photodiode PD.
Occurs. Since the base current Ib3 of the transistor 3 is supplied from the transistor 11, all the current I flows through the feedback resistor 6. Therefore, an output voltage (Vref + I × R) is generated at the output terminal OUT. Thus, the base resistance of the transistor 1 is removed, and instead, the base current of the transistor 3 is changed to the constant current circuit 9, the transistors 8 and 1.
Since it is generated by 0 and 11 and the base current of the transistor 3 is compensated, the reference voltage Vref is applied to the output terminal OUT.
And the voltage I × R obtained by the voltage conversion are generated, and the offset voltage is not generated. Further, since the base resistance of the transistor 1 is removed, unnecessary thermal noise can be reduced. Therefore, the reference voltage V
Even if ref is applied, it is possible to prevent the generation of the offset voltage at the output terminal OUT.

By the way, noise is generated from the transistor 8. When a current flows in the transistor 8, so-called gr noise is generated in the transistor 8 because the amount of generation and recombination of minority carriers fluctuates with time. Then, this gr noise is generated in the baseline of the transistor 8. However, since the capacitor 12 is connected to the base of the transistor 8, only the baseline noise component passes through the capacitor 12. Due to the action of the capacitor 12, no noise component is supplied to the base and emitter of the transistor 10,
As a result, no noise is generated from the transistor 11. Therefore, it is possible to prevent a noise component due to the gr noise of the transistor 8 from being generated at the output terminal OUT.

FIG. 2 is a diagram showing another embodiment of the present invention. The difference from FIG. 1 is that the resistance ratio between the resistors RE10 and RE11 is 2: 1 and that the constant current source 13 is used instead of the constant current source 9 to generate the same output current as the constant current source 7. It is in. In FIG. 2, the output current of the constant current source 13 is
It is the same as the output current of the constant current source 7 and is Ie. Since the current Ie flows through the transistor 8, the base current Ib8 of the transistor 8 becomes

[0021]

[Equation 3]

It becomes Base current Ib of transistor 8
8 is inverted by a current mirror circuit consisting of transistors 10 and 11. Since the resistance ratio of the emitter resistors RE10 and RE11 of the transistors 10 and 11 is set to 2: 1, the mirror ratio of the current mirror circuit is set to 0.5. Due to this mirror ratio, the collector current Ic11 of the transistor 11 is

[0023]

[Equation 4]

And is supplied to the base of the transistor 3. Therefore, as is clear from the equations (1) and (4), the base current of the transistor 3 is entirely supplied from the transistor 11. Note that the combination of the magnitude of the output current of the constant current circuit 9 and the current mirror ratio of the transistors 10 and 11 is such that the collector current of the transistor 11 is Ie.
If it becomes / (2 × β), it is not limited to the combination of FIGS. The collector current of the transistor 11 is I
When the output current of the constant current circuit 9 is N times the output current of the constant current circuit 7 so that e / (2 × β), the mirror ratio is set to 1
/ (2 × N) Therefore, like the circuit of FIG.
Offset due to the base current of the transistor 3 can be prevented. Further, since the capacitor 12 is connected to the collector of the transistor 8, noise caused by the gr noise of the transistor 8 can be reduced.

[0025]

According to the present invention, a third transistor having the same characteristics as the second transistor is prepared, and a current having a predetermined ratio to the operating currents of the first and second transistors is supplied to the third transistor. Generates the same current as the base current of the second transistor when balancing the voltage conversion circuit,
Since it is supplied to the base of the second transistor, the base current of the second transistor is compensated and the offset can be reduced. Further, since the noise of the base current of the third transistor caused by the gr noise is removed by the low-pass filter, this noise is prevented from being supplied to the second transistor and the feedback resistor, and the noise in the output voltage can be reduced.

[Brief description of drawings]

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

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

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

1 transistor 3 transistors 4 Current mirror circuit 5 transistors 6 Feedback resistor 7 constant current circuit 8 transistors 9, 13 constant current source 10 transistors 11 transistors 12 condenser

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H03F 1/00-3/72

Claims (4)

(57) [Claims] 1. A first transistor having a base to which a reference voltage is applied, and a second transistor differentially connected to the first transistor.
A transistor, a load connected to the collectors of the first and second transistors, an emitter follower circuit to which an output signal generated from the load is supplied, and an output signal of the emitter follower circuit to the base of the second transistor. A feedback resistor for feeding back, wherein an input current is supplied to one end of the feedback resistor and an output voltage is obtained from the other end of the feedback resistor in a current-voltage conversion circuit having the same characteristics as the second transistor. A transistor, a constant current circuit connected to the emitter of the third transistor, and a base current of the third transistor,
A current-voltage conversion circuit comprising a current mirror circuit supplied to the base of a transistor, and a low-pass filter for removing noise in the base current of the third transistor. 2. The current-voltage conversion circuit according to claim 1, wherein the low-pass filter is a capacitor connected between the base of the third transistor and a power supply voltage. 3. The output current of the constant current circuit is 1/2 of the operating current of the first and second transistors, and the mirror ratio of the current mirror circuit is 1. Current-voltage conversion circuit. 4. The output current of the constant current circuit is N times the operating current of the first and second transistors, and the mirror ratio of the current mirror circuit is 1 / (2 × N). The current-voltage conversion circuit according to claim 1.