JPS6391720A - Current generator circuit - Google Patents

Abstract

PURPOSE:To obtain a current generator circuit which can easily improve the accuracy of a low-pass filter and convert a reference voltage by connecting a capacitor to the input of a voltage control current source and impressing a control voltage to said capacitor with the aid of a switched capacitor. CONSTITUTION:When switches S1 and S2 are closed to the side of an input terminal, charges in proportion to the control voltage Vi which uses an E2 as a reference potential are accumulated in the capacitor Cs. If said switches are closed over to the side of the voltage control current source, the charges accumulated the capacitor Cs flow out with an E1 as a reference. Where an action frequency is fs, the switched capacitor functions as a resistance equivalent to 1/(Cs.fs), and the time constants of a circuit are determined by the ratio of Ct to Cs and the fs. Consequently the action frequency fs of the switch can be set at high accuracy. Since the ratio of capacities in an integrated circuit is determined by an area ratio, it is easily attainable at high accuracy. Since a control voltage is transferred to the side of the voltage control current source as the both end voltages of the capacitor Cs, it is not affected by the reference potential.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a current generating circuit that generates a current proportional to an input voltage.

(Prior art and problems) FIG. 3 shows a conventional current generating circuit that generates a current proportional to an input terminal, where VCVS is a voltage controlled current source,
○ is the output terminal of the voltage controlled current source, I vcvs is the input terminal of the voltage controlled current source, C5 is the capacitor, Rt is the resistor,
■ is the input terminal of the current generation circuit, E.

is the reference potential of the voltage controlled current source.

The control voltage applied to the input terminal I is applied to the input I vcvs of the voltage controlled current source vcvs via a low pass filter constituted by a resistor Rt and a capacitor C2, and the output terminal 0 of the voltage controlled current source is applied to the input I vcvs of the voltage controlled current source vcvs. A current proportional to the voltage applied to vcvs is output. However, in such a configuration, the frequency characteristics of the low-pass filter are determined by the product of the resistor R5 and the capacitor Ct. For example, if this is built into an integrated circuit, the absolute value accuracy of the element is not sufficient, so the frequency characteristics of the low-pass filter are determined by the product of the resistor R5 and the capacitor Ct. There was a drawback that a low-pass filter could not be realized.

FIG. 4 shows a conventional current generation circuit when the voltage-controlled current source and the control signal have different reference potentials, where LVC is a level conversion circuit, I is an input reference voltage terminal, and E2 is the reference potential on the input side. . That is, when the reference potential of the current generation circuit and the reference potential on the input side are different, a level conversion circuit LVC is required to convert the control voltage to a voltage based on the reference potential of the current generation circuit, which reduces the circuit size. In addition to this, there are also drawbacks such as an increase in errors due to the passage through the level conversion circuit.

(Means for Solving the Problems) The present invention provides a current generating circuit in which the precision of a low-pass filter is improved and the conversion of a reference potential is easy.

The present invention connects a capacitor to the input of a voltage controlled current source,
A control voltage is applied to this capacitor using a so-called switched capacitor in which the capacitor is alternately connected between multiple terminals via a switch,
This method differs from conventional technology in that it does not use a time constant circuit using a resistor and a capacitor or a level conversion circuit.

(Embodiment) FIG. 1 shows a first embodiment of the present invention, in which SC is a switched capacitor, S, , St are switches, and C5 is a capacitor for the switched capacitor.

Switches s, , s2 in the switched capacitor alternately connect the input terminal side and the voltage-controlled current source side by means of a control signal (not shown). Switch SI.

When S2 is connected to the input terminal side, a charge proportional to the control voltage V with R2 as the reference potential is applied to the capacitor C.
When the switch SI+ s2 is closed to the voltage-controlled current source side, the charge stored in the capacitor C1 flows out to the capacitor Ct with reference to El. The value of each element is expressed by the same symbol as that attached to each element, the operating frequency of the switches S, S2 is fs, and the input of the voltage controlled current source, that is, the voltage across the capacitor Ct is v vcv.
The operation will be described in detail below.

When capacitor C5 is connected to the input terminal side, C3 is charged to V, accumulates charge QA=C,・V, and C5
is connected to the capacitor Ct side, the charge Q8 stored in C8 becomes QB =vvcvs·Cs. Therefore, when the switch SI+ s2 performs an alternating switching operation f5 times per second, the charge of T5 increases by Ct per second (QA Qa)
Move to. Current is defined as the amount of charge (coulombs) that moves per second, and if the unit of C is farad, the unit of voltage is volt, and the unit of current is ampere, then (QA Qi ) ・ fs is generated through a switched capacitor.
= (Vi −Vvcvs) ・C−・f, this means that a current of ampere has flowed. According to Ohm's law, current=voltage/resistance, it can be seen that C3・T3 corresponds to 1/resistance. In other words, the switched capacitor is 1
/ Functions as a resistance equivalent to (C, -f,).

Therefore, the time constant deviation of the circuit of this embodiment is determined by the ratio of C5 and C6 and f. The operating frequency f5 of the switch can be set with high precision, and since the capacitance ratio in the integrated circuit is determined by the area ratio, its value can be easily realized with high precision.

Since the control voltage is transferred to the voltage-controlled current source side as the voltage across the capacitor C6, it is not affected by the reference potential.
Furthermore, no errors occur due to level conversion.

As is clear from this result, it is easier to improve the precision of the circuit compared to the conventional technique, and a circuit that is not affected by the reference potential between the input terminal side and the voltage control current J side can be obtained.

Large Current ■1 FIG. 2 shows a second embodiment showing a specific example of the configuration of a voltage-controlled current source, where A is an operational amplifier, T1 is a transistor, and R8 is a resistor.

The operational amplifier OPA operates as a voltage follower with the + side input terminal as an input, the voltage of the resistor R8 is the same voltage as the + side input terminal, and the collector side of the transistor Tr has V
A current of vcvs/R- flows. That is, output terminal O
A current proportional to the input voltage flows through.

(Effects of the Invention) As explained above, according to the present invention, the characteristics of the low-pass filter provided in front of the voltage-controlled current source are
It can be set with high precision by the capacitance ratio CL/C of t and C-, and is particularly suitable for applications requiring matching between a plurality of circuits.

Furthermore, the present invention uses a switched capacitor, and uses control input and voltage control. There is an advantage that voltage-current conversion can be realized without loss of accuracy even when the reference potentials of the H.211 current sources are different.

Incidentally, if only the former advantage is to be obtained, it is also possible to connect E, , E2 in each embodiment and delete S2 (connect one end of C to a common potential).

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of the present invention, FIG. 2 shows a second embodiment showing a specific configuration of a voltage controlled current source, FIG. 3 shows a conventional current generating circuit, and FIG. 4 shows a conventional current generating circuit. is a conventional current generating circuit in which the voltage controlled current source and the control signal have different reference potentials. CS...Switched capacitor capacitor C5...
・Capacitor E,...Reference potential E2 of voltage controlled current source...Input side reference potential ■...Input terminal of current generating circuit ■,...Input reference voltage terminal I vcvs...Voltage controlled current source input terminal LVC・
... Level conversion circuit 0 ... Output terminal OP^ of voltage-controlled current source ... Operational amplifier Rt, R- ... Resistor Sl+ Sz ... Switch SC ... Swiss capacitor T1 ... Transistor vcvs... Voltage controlled current source ■,... Control voltage

Claims (1)

[Claims] 1. In a current generation circuit that generates a current proportional to an input voltage using a voltage-controlled current source, a capacitor is connected in parallel to the input of the voltage-controlled current source, and the input voltage is applied to the capacitor via a switched capacitor. A current generating circuit characterized by: