JP2000134045A - Voltage-to-current conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance linearity of the voltage-to-current conversion circuit that supplies a current in proportion to an input voltage. SOLUTION: This voltage-to-current conversion circuit has an operational amplifier circuit 10, a bipolar transistor(TR) Q1, a resistor R0, a current mirror type constant current source circuit 20, a negative feedback signal line 30 that negatively feeds back a potential of a node N1 to the operational amplifier circuit 10, and a voltage correcting circuit 40. Then the current mirror type constant current source circuit 20 outputs an output current Iout proportional to an input voltage Vi applied to the operational amplifier 10. The voltage correcting circuit 40 applies a correcting voltage to improve deterioration in the linearity of the voltage-to-current conversion circuit attended on deviation in a voltage V0 at the node N1 from a specified voltage to the operational amplifier circuit 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage / current conversion circuit for supplying a current proportional to a voltage, and more particularly to a voltage / current conversion circuit having improved linearity.

[0002]

2. Description of the Related Art Voltage / current conversion circuits are used in various fields. One of the applications of such a voltage / current conversion circuit
For example, a driver circuit for a laser diode is known.

FIG. 4 is a circuit diagram of a conventional voltage / current conversion circuit used for such a laser diode driver circuit. The voltage / current conversion circuit 1 illustrated in FIG. 4 includes an operational amplifier circuit 10 and a first bipolar transistor Q1.
A current mirror type constant current source circuit 20 connected to the collector of the first bipolar transistor Q1, a first resistor R0 connected to the emitter of the first bipolar transistor Q1, and a first bipolar transistor A negative feedback signal line for negatively feeding back a voltage at a first node, which is a connection point between the emitter of Q1 and the first resistor R0, to an inverting (-) input terminal of the operational amplifier circuit 10; A non-inverting (+) input terminal as a first input terminal of the operational amplifier circuit 10 has an input voltage Vi to be converted.
Is applied.

The basic operation principle of the voltage / current conversion circuit using the operational amplifier circuit 10 will be described. The operational amplifier 10 is connected to the input voltage Vi to be converted applied to the non-inverting (+) input terminal and the signal line 30 for negative feedback.
Inversion (-) base of the first bipolar transistor Q1 from the output terminal of the operational amplifier circuit 10 as the voltage difference [Delta] V = Vi -V ₀ to the voltage V ₀ which node N1 which is negatively fed back to the input terminal becomes 0 and outputs the base current I _b to. According to the base current _Ib , the first bipolar transistor Q
1 of emitter current I _e flows through the first resistor R0, to generate a voltage V ₀ to the node N1. Current I _F flowing through the resistor R0 via the first bipolar transistor Q1 from the output terminal of the operational amplifier circuit 10 is represented by the following formula.

[0005] _{I F = Vi / R 0 ··} (1) However, I _F is the current flowing from the output terminal of the operational amplifier circuit 10 to a resistor R0, Vi is the input voltage applied to the operational amplifier circuit 10 And R ₀ is the resistance of resistor R0.

In this example, the current _IF is supplied from the emitter of the first bipolar transistor Q1 to the first resistor R1.
It is equal to the current IR ₀ flowing to ₀ (or the emitter current I _{e of} the first bipolar transistor Q1). That is,
It is _{I F = IR 0 = I e} . Therefore, the following equation 2 is established.

IR ₀ = Vi / R ₀ = I _b + h _FE × I _b = I _b (1 + h _FE ) (2) where h _FE is the current amplification factor of the transistor Q1.

[0008] The first bipolar transistor Q1
A constant current (collector current I _C) To supply
The current mirror type constant current source circuit 20 is a current mirror type constant current source circuit.
As a basic characteristic of the current source circuit, the first bipolar transistor
The current flowing through the collector of the transistor Q1 (this is the collector current
I)_COutput current I equal to_outTo the other output
Output from child. This output current I_outChanges the input voltage Vi
This is the converted current. Note that the collector current I_CIs Emi
Current I_eLike I_C= H_FE× I_bStipulated as
It is. That is, it is represented by the following equation.

I _out = I _C = h _FE × I _b (3)

From equations 1 to 3, the following equation is obtained.

I _out = (Vi / R ₀ ) × (h _FE / (1 + h _FE )) (4)

As described above, according to the voltage / current conversion circuit of FIG. 4, an output current I _out proportional to the input voltage Vi can be obtained.

Note that the output current I _out can be defined from Equation 3 as follows.

I _out = I _S (1 + V _ce / V _EA ) exp (V _be / V th) (5) where I _S is the junction saturation current of the transistor Q 1, and V _ce is the collector / emitter of the transistor Q 1 V _EA is the early voltage (Ea) of the transistor Q1.
rly) is the voltage, _V be is the voltage between the base and the emitter of the transistor Q1, Vth is a threshold voltage Vth of the transistor Q1.

[0015]

However, according to the experiment, the linearity of the voltage and current of the voltage-current conversion circuit illustrated in FIG. 4 becomes a curve CV3 as shown in FIG. It does not have the characteristic of the curve CV <b> 1 showing a typical linearity. Note that the curve CV1 is a characteristic of an ideal voltage-current conversion circuit in which the characteristic does not decrease (degrade), in other words, the linearity is perfect, and the input voltage Vi and the output current _Iout are completely proportional. And shows linearity.

If the characteristics of the voltage / current conversion circuit are not linear, for example, when such a voltage / current conversion circuit is used for a driver circuit of a laser diode, a desired laser light emission characteristic cannot be obtained. Encounter.
In the above-described example, the characteristic is greatly deteriorated particularly with an increase in the input voltage Vi. Therefore, when the voltage / current conversion circuit of FIG. 4 is used as a driver circuit for a laser diode, the output characteristics of the laser when a high voltage is applied do not become the desired values.

An object of the present invention is to provide a voltage / current conversion circuit which has excellent linearity and can operate stably for a long time. In other words, an object of the present invention is to improve the linearity of a voltage / current conversion circuit.

[0018]

The inventor of the present application has considered the decrease in linearity (or deviation from ideal characteristics or deterioration) of the voltage / current conversion circuit described with reference to FIG.
The cause was found as follows. Input voltage Vi
Changes, the voltage V _{0 of the} node N1 also changes, and the collector-emitter voltage _{Vce of the} transistor Q1 changes. As a result, as is apparent from Equation 5, the output current I _out shifts and the linearity of the voltage-current conversion circuit decreases. That is, when the collector-emitter voltage _Vce of the transistor Q1 decreases, the depletion layer of the transistor Q1 expands. In particular, since the base width of the bipolar transistor Q1 is narrow, the collector current I _C also decreases. Since the same output current I _out and the collector current I _C is output from the current mirror type constant current source circuit 20, the output current I _out with a decrease of the collector current I _C decreases. Inventor adds the voltage correction circuit for correcting the decrease in voltage V ₀ at the node N1 as described above, it devised a present invention based on the concept of improving the linearity of the voltage-current conversion circuit.

Therefore, according to the present invention, there is provided a current source circuit connected to the first power supply terminal and supplying a current corresponding to the current supplied to the first current supply terminal to the second current supply terminal. A first transistor connected between the first current supply terminal and a first node, and a first resistor connected between the first node and a second power supply terminal An element, an operational amplifier circuit to which an input voltage is applied to one input terminal, the other input terminal is connected to the first node, and an output terminal is connected to a control terminal of the first transistor; And a correction circuit for correcting the voltage of the first node.

Specifically, the correction circuit includes a second transistor and a second resistance element connected in series between a third power supply terminal and a fourth power supply terminal; A third resistance element and a third transistor connected in series between the node and the fourth power supply terminal, wherein a reference voltage is applied to a control element of the second transistor; The control terminal of the third transistor is connected to the second terminal.
Is connected to a connection midpoint between the transistor and the second resistance element.

Preferably, there is provided a reference voltage control circuit for controlling the reference voltage according to the input voltage.

More preferably, a power supply voltage is applied to the first and third power supply terminals, a ground potential is applied to the second and fourth power supply terminals, and the current source circuit is A current mirror type current source circuit including a plurality of transistors, wherein the first and second transistors are NPN-type bipolar transistors, the third transistor is a PNP-type bipolar transistor, Is a non-inverting input terminal, and the other input terminal is an inverting input terminal.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment A voltage / current conversion circuit according to a first embodiment of the present invention will be described with reference to FIG. The voltage / current conversion circuit illustrated in FIG. 1 is similar to the voltage / current conversion circuit illustrated in FIG. 3, except that a voltage correction circuit 40 is added to the circuit illustrated in FIG.

The voltage / current conversion circuit 1A illustrated in FIG.
Is an operational amplifier circuit 10, an NPN-type bipolar transistor (hereinafter, referred to as a first transistor) Q1 as a current conversion transistor, and a current mirror type constant current source connected to the collector of the first bipolar transistor Q1. Circuit 20 and a first bipolar transistor Q
A first resistor (or a resistance element) R0 as a current / voltage conversion resistor connected to the emitter of the first bipolar transistor Q1, and an emitter of the first bipolar transistor Q1 and the first resistor R0
A negative feedback signal line 30 for inputting a voltage at a first node N1 which is a connection point with the negative input terminal of the operational amplifier circuit 10, and a voltage correction circuit 40. Operational amplifier circuit 1
An input voltage Vi to be converted is applied to a non-inverting input terminal as a first input terminal of 0.

The current mirror type constant current source circuit 20 has three
It has a known circuit configuration using two PNP-type bipolar transistors, is supplied with a voltage from a power supply voltage source Vcc, and supplies a constant current from a first output terminal to the collector of the first bipolar transistor Q1. As shown, they are connected as shown. From the operating principle of the current mirror type constant current source circuit 20, the first bipolar transistor Q
A current having the same value as the current (collector current I _C ) flowing through one collector is output as an output current I _out .

The voltage / current conversion circuit 1A illustrated in FIG.
Is basically the same as the voltage / current conversion circuit 1 described with reference to FIG. 4, except that the voltage correction circuit 40 is added. Reference is made to the content with 40 added.

The voltage correction circuit 40 has a correction reference voltage V
an NPN-type bipolar transistor (hereinafter referred to as a second transistor) Q2 having _ref applied to its base;
Of the transistor Q2 having one end connected to the emitter thereof.
A PNP-type bipolar transistor (hereinafter, referred to as a third transistor) Q3 having a base connected to a connection point (node N2) between the emitter of the second transistor Q2 and the second resistor R2, One end is connected to the emitter of the third transistor Q3, and the other end is connected to the operational amplifier 1
A third resistor R3 connected to a connection point (node N3) between the 0 inverting input terminal and the negative feedback signal line 30;
The power supply voltage Vcc is applied to the collector of the second transistor Q2, and the other end of the second resistor R2 and the collector of the third transistor Q3 are commonly connected and grounded.

The operation of the voltage correction circuit 40 will be described. In the voltage correction circuit 40, when the correction reference voltage _Vref is applied to the base of the second transistor Q2, the emitter current I _e (Q2) according to the current amplification factor h _FE (Q2) of the second transistor Q2. Flows. Therefore, the second resistor R
2, the voltage drop V _drop (R ₂ ) = I _e (Q2) × R
(The R ₂ is the resistance of the second resistor R2) ₂ is generated. The voltage at the node N2 due to this voltage drop V _drop (R ₂ ) is applied to the base of the third transistor Q3,
Of the transistor Q3, the emitter current I _e (Q corresponding to the current amplification factor of the third transistor Q ₃ h _FE (Q3)
3) flows to the third resistor R3. Therefore, the voltage drop V _drop (R ₃ ) = I _e (Q3) in the third resistor R3.
× R ₃ (R ₃ is the resistance value of the third resistor R3) occurs. As a result, current i flows from node N3 toward the emitter of third transistor Q3.

The current i is generated via the negative feedback signal line 30.
As a result, negative feedback is applied to the inverting input terminal of the operational amplifier circuit 10.
Potential V of node N1₀Is the voltage drop V_drop(R_Three) = I
_e(Q3) × R_ThreeJust means lowering. That
As a result, the inverting input of the operational amplifier circuit 10 illustrated in FIG.
Negative feedback voltage V to input terminal₀Lower voltage (V₀-V_drop
(R_Three)) Is the operational amplifier of the operational amplifier 10 of FIG.
Negative feedback is provided to 10 inverting input terminals. As a result,
Difference between two input voltages in width circuit 10 = Vi− (V₀
-V_drop(R_Three)) Is the voltage difference illustrated in FIG.
V₀The first bipolar in FIG.
Base current I of transistor Q1_bIs the first illustrated in FIG.
Base current I of one bipolar transistor Q1_bThan
growing. This first bipolar transistor Q1
Base current I_bIncrease of the first bipolar transistor
Collector current I of Q1 _CIncrease. As a result,
Output from the other output terminal of the current mirror type constant current source circuit 20.
Output current I_outIs also the first bipolar transistor
Collector current I of the_CIncrease with
You. As a result, the output voltage of the voltage / current conversion circuit 1A of FIG.
Style I_outIs the decrease in linearity illustrated in curve CV3 in FIG.
Is improved, and approaches the ideal characteristic curve CV1.

The basic items of the voltage / current conversion circuit 1A other than the above-described operation to which the voltage correction circuit 40 is added are the same as those described with reference to FIG. That is, before the voltage correction circuit 40 is added, the operating principle of the voltage / current conversion circuit 1 illustrated in FIG. 4 is also applied to the voltage / current conversion circuit 1A illustrated in FIG. It was in the voltage-current conversion circuit 1A basically, based on the formula 1-5 as described above, the non-inverting input current mirror output current I _out corresponding to the input voltage Vi applied to the terminal of the operational amplifier circuit 10 Output from the second output terminal of the constant current source circuit 20. However, in the voltage / current conversion circuit 1A of FIG. 1, as described above, the voltage correction circuit 40 is added.
The linearity is improved.

Experimental Example In the voltage / current conversion circuit 1A of FIG. 1, the resistance value R _{0 of} the first resistor R0 = 150Ω, the resistance value R _{2 of} the second resistor R2 = 20 KΩ, and the third resistor R3 Resistance value R ₃ = 4
00 KΩ, power supply voltage V _cc = 5 V, correction reference voltage V _ref
= 1.5V (fixed at almost the middle value of the range of input voltage Vi)
The simulation result in the case of
Shown in The curve CV2 of the experimental value based on the present embodiment is
It is close to the ideal characteristic curve CV1.
Compared with the characteristic curve CV3 in the case of the current conversion circuit 1, the linearity is considerably improved.

When the illustrated voltage / current conversion circuit 1A of FIG. 1 is realized as an integrated circuit, the operational amplifier circuit 10, the current mirror type constant current source circuit 20, the second transistor Q2 and the third transistor Transistor Q3
Is housed in an IC circuit, but it is desirable that the first resistor R0, the second resistor R2, and the third resistor R3 be provided outside the IC package.

Second Embodiment FIG. 3 illustrates a voltage / current conversion circuit 1B according to a second embodiment of the present invention. The voltage / current conversion circuit 1B is obtained by adding a voltage correcting voltage generation circuit 50 to the voltage / current conversion circuit 1A illustrated in FIG. The voltage compensating voltage generating circuit 50 is connected to the second transistor Q2 of the voltage compensating circuit 40 in accordance with the input voltage Vi applied to the operational amplifier circuit 10.
_Is a circuit for changing the correction reference voltage V _ref applied to the base of the control circuit.

In the voltage-current conversion circuit 1A illustrated in FIG. 1, the correction reference voltage _Vref is fixed at 1.5 V as illustrated as the curve CV2 in the above experimental example.
As the _distance from the correction reference voltage _Vref increases, it deviates from the ideal characteristic curve CV1. In the example illustrated in FIG. 2, in a region where the input voltage Vi is lower than the correction reference voltage _Vref , the deviation from the ideal characteristic is negligibly small, and when the input voltage Vi becomes considerably higher than the correction reference voltage _Vref. The deviation from the ideal characteristic becomes large. In order to improve the limit of the correction caused by fixing the above-described correction reference voltage _Vref , the voltage correction voltage generation circuit 50 is provided with an input voltage Vi.
The correction reference voltage _Vref applied to the base of the second transistor Q2 is changed in accordance with the magnitude of.

In this embodiment, since the deviation from the ideal characteristic curve is small in a region where the input voltage Vi is low, when the input voltage Vi is a predetermined value, for example, 1.5 V or more, the correction is made according to the increase of the input voltage Vi. The reference voltage _Vref is somewhat increased. Of course, the correction can be performed for the entire range of the input voltage Vi. Thus, the voltage generation circuit 5 for voltage correction
0, the voltage / current conversion circuit 1B of FIG.
Can perform voltage-current conversion having linearity very close to the characteristic of the ideal characteristic curve CV1 in FIG.

According to the second embodiment of the voltage / current conversion circuit of the present invention, it is possible to obtain substantially the same linear voltage / current conversion characteristics as the curve CV1 in FIG.

The implementation of the voltage / current conversion circuit of the present invention is not limited to the circuit configuration described in the above embodiment, but may take various modifications. For example, the conductivity of the first bipolar transistor Q1, the second transistor Q2, and the third transistor Q3 can be reversed from that shown. For example, the first transistor Q1 can be changed from NPN type to PNP type, the second transistor Q2 can be changed from NPN type to PNP type, and the third transistor Q3 can be changed from PNP type to NPN type.

FIGS. 1 and 3 show an example in which the input voltage Vi is applied to the non-inverting input terminal of the operational amplifier circuit 10. The input voltage Vi is applied to the inverting input terminal, and the negative feedback signal line 3 is applied.
A voltage from 0 may be applied to the non-inverting input terminal.

In the above example, the case where the voltage / current conversion circuit of the present invention is used for a driver circuit of a laser diode has been described. However, it can be used as various voltage / current conversion circuits.

[0040]

According to the present invention, the linearity of the voltage / current conversion circuit can be remarkably improved by adding a simple circuit such as a voltage correction circuit, more preferably a voltage correction voltage generation circuit. .

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment of a voltage / current conversion circuit according to the present invention.

FIG. 2 is a graph illustrating characteristics of the voltage / current conversion circuit illustrated in FIG. 1 and characteristics of a conventional voltage / current conversion circuit.

FIG. 3 is a circuit diagram of a voltage-current conversion circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a voltage / current conversion circuit as a background art.

[Explanation of symbols]

 1, 1A, 1B voltage / current conversion circuit 10 operational amplifier circuit 20 current mirror type constant current source circuit 30 negative feedback signal line 40 voltage correction circuit Q2 to Q3 second -Third bipolar transistor R2-R3-second-third resistor 50-voltage correction voltage generation circuit Q1-first bipolar transistor R0-first resistor (for current-voltage conversion) Resistor)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J090 AA03 CA21 FA01 GN01 HA08 HA25 KA00 KA01 KA09 KA11 MA13 MA21 SA00 TA02 5J091 AA03 CA21 FA01 HA08 HA25 KA00 KA01 KA09 KA11 MA13 MA21 SA00 TA02 UW08

Claims (4)

[Claims] A first current supply circuit connected to a first power supply terminal for supplying a current corresponding to a current supplied to the first current supply terminal to a second current supply terminal; A first transistor connected between the supply terminal and the first node; a first resistance element connected between the first node and a second power supply terminal; An operational amplifier circuit to which an input voltage is applied to a terminal, the other input terminal is connected to the first node, and an output terminal is connected to a control terminal of the first transistor; And a voltage-current conversion circuit having the following. A second transistor and a second resistor connected in series between a third power supply terminal and a fourth power supply terminal; the first node and the second node; A third resistor element and a third transistor connected in series between the third transistor and a fourth power supply terminal; a reference voltage is applied to a control element of the second transistor; 2. The voltage / current conversion circuit according to claim 1, wherein a control terminal of the transistor is connected to a connection point between the second transistor and the second resistance element. 3. The voltage control circuit according to claim 2, further comprising a reference voltage control circuit for controlling said reference voltage in accordance with said input voltage.
Current conversion circuit. 4. A power supply voltage is applied to the first and third power supply terminals, a ground potential is applied to the second and fourth power supply terminals, and the current source circuit includes a plurality of transistors. The first and second transistors are NPN-type bipolar transistors, the third transistor is a PNP-type bipolar transistor, and the one input terminal is 4. The voltage / current conversion circuit according to claim 2, wherein the input terminal is a non-inverting input terminal, and the other input terminal is an inverting input terminal.