JPH04223604A - Offset adjusting circuit for voltage follower - Google Patents

Abstract

PURPOSE:To attain the increase of the variable range of an output and the reduction of a gain error as for a voltage follower, and to prevent the deterioration of the reality of the voltage follower. CONSTITUTION:The voltage follower is constituted of an operation amplifier U1, and a constant current circuit 1 having operation amplifiers U2 and U3, and resistances R0, R6-R9, and RL. Then, currents in proportion to a reference voltage set by the constant current circuit 1 run through a feedback line connecting the output terminal of the voltage follower with the reversed input terminal of the voltage follower. At that time, the relation between an input voltage Vin and an output voltage Vout is indicated by the following expression: Vout= Vin+RO.IL=Vin+(RO/RL).Vrefo by establishing the following expression: R6=R7=R8=R9. Therefore, the currents IL are changed by adjusting the value of the Vref, and the offset is adjusted by changing the RO.IL =(RO/ RL).Vref.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an offset adjustment circuit for an operational amplifier, and more particularly to an offset adjustment circuit when an operational amplifier is used as a voltage follower.

0002

2. Description of the Related Art Conventionally, there has been a circuit shown in FIG. 2, for example, as a circuit for adjusting the offset of a non-inverting amplifier circuit using an operational amplifier. In Figure 2, U is an operational amplifier, R1~
R 3 and Rf are resistors, and VR is a variable resistor.
In such a non-inverting amplifier circuit, the output variable range Va
is given by the following equation. Va = ±V・{R 2/(R 1+R 2)}・
{Rf /(Rs +Rf)}


...(1) Rs = R
2+R 3 In this formula, the same symbol as the symbol indicating resistance is also used for the resistance value. The same applies to the figures below. When the operational amplifier becomes a voltage follower, the resistance value Rf =
Therefore, the variable range Va = 0 from equation (1). For this reason, the non-inverting amplifier circuit in Figure 2 is
If you use a di-follower, you won't be able to adjust the offset.

An example of a circuit capable of adjusting the offset of a voltage follower is shown in FIG. Figure 3
Components that are the same as those in FIG. 2 are given the same reference numerals. The same applies to the figures below. In FIG. 3, R 4 and R 5 are resistors. In this circuit, the output variable range Va is given by the following equation. Va = ±V・{R 5/(R 4+R 5)}

...(2) Also, the amplification factor ANF is given by the following equation. ANF=1+(R5/R4)

(3) By increasing R4, the gain of the voltage follower can be brought closer to 1 according to equation (3), but on the other hand, the variable range Va becomes smaller than from equation (2). As a result, the circuit shown in FIG. 3 cannot simultaneously widen the output variable range and reduce gain errors. In addition, in a voltage follower, the feedback path from the output terminal to the inverting input terminal is originally insulated from other parts, but in the circuit of Figure 3, the feedback path is connected to a potential of ±V through a resistor. Since the voltage follower is connected to a point, linearity deteriorates due to the nature of the non-inverting amplifier circuit of the voltage follower. The conventional example shown in FIGS. 2 and 3 is described in "Design of OP Amplifier Circuit", written by Okamura, published by CQ Publishing, p. 38.

0004

[Problems to be Solved by the Invention] The present invention has been made to solve these problems, and it is an object of the present invention to widen the variable range of output for a voltage follower.
It is possible to reduce gain error while also reducing voltage.
The purpose of the present invention is to realize an offset adjustment circuit for a voltage follower that does not impair the linearity of the follower.

[0005]

[Means for Solving the Problems] The present invention provides a voltage converter that uses an operational amplifier to configure a non-inverting amplifier circuit with a gain of 1.
In a circuit that adjusts the offset of the voltage follower, the circuit is connected to the inverting input terminal of the operational amplifier, has an infinite input resistance value, and inverts a current proportional to the reference voltage set to itself to the output terminal of the voltage follower. This is a voltage follower offset adjustment circuit characterized by comprising a constant current circuit that flows through a feedback path connecting input terminals.

[0006]

According to the present invention, the offset is adjusted by changing the current flowing through the feedback path of the voltage follower in accordance with the reference voltage applied to the constant current circuit and changing the potential of the inverting input terminal.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below with reference to the drawings. Figure 1
1 is a configuration diagram of an embodiment of the present invention. In Figure 1, U
1 is an operational amplifier that constitutes a voltage follower, R 0
is a resistor connected to the voltage follower. 1 is a constant current circuit which is a feature of the present invention. In the constant current circuit 1, U2 and U3 are operational amplifiers, R0, R
6 to R 9,RL are resistances.

The operation of such a constant current circuit will be explained. The potentials V 1 to V 4 of each part in FIG. 1 are as shown in the following equation. V 1={R 6/(R 6+R 7)}・V 3
V2={R8/(R8+R9)}・(V
4-Vref )+Vref Here, R 6=R
By setting 7=R 8=R 9, V 1=
V 3/2

...(4) V2=(V4+Vref)/
2
...(5). Here, from the nature of the imaginary short of the operational amplifier, V 1 = V
2, so from equations (4) and (5), it becomes. Therefore, the current IL flowing through the resistor RL is given by the following equation. As a result, the input voltage Vin and the output voltage Vout
The relationship is as follows. Vout=Vin+R0・IL
=Vin+(R0/RL)・Vref
…(
6) As shown in equation (6), by adjusting the value of Vref, the value of current IL changes, and R 0・IL =
Adjust the offset by changing (R 0/RL )・Vref.

[0009]

[Effects] According to the present invention, the following effects can be obtained. (2) The circuit according to the present application is obtained by replacing the broken line portion of the circuit in FIG. 3 with the current detection circuit in FIG. 1. In the circuit of FIG. 3, the gain of the voltage follower is given by 1+(R 5 /R 4). In the circuit according to the present application, since the input resistance of the current detection circuit is infinite, R 4 with a gain of 1 + (R 5 / R 4)
becomes infinite. As a result, according to the circuit according to the present invention, the gain of the voltage follower can be asymptotically approached to 1, and the gain error can be reduced. (2) In the conventional example shown in FIG. 3, the variable range of the output is determined by the resistance value of the resistor connected to the inverting input terminal of the voltage follower. In contrast, in the circuit according to the present invention, offset adjustment is performed by controlling the current flowing through the feedback path of the voltage follower using a constant current circuit. Since a constant current circuit outputs a current proportional to a given reference voltage, offset adjustment can be performed over a wide range by setting the reference voltage to various values. ■Since the input resistance of the constant current circuit connected to the voltage follower's feedback path is infinite, it is equivalent to the feedback path being isolated from other parts. As a result, even if a constant current circuit is connected, the linearity of the voltage follower will not be impaired.

[Brief explanation of the drawing]

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

FIG. 2 is a diagram showing a conventional example of an offset adjustment circuit.

FIG. 3 is a diagram showing a conventional example of an offset adjustment circuit.

[Explanation of symbols]

U1, U2, U3 operational amplifier

Claims (1)

[Claims] Claim 1: In a circuit for adjusting the offset of a voltage follower configured as a non-inverting amplifier circuit with a gain of 1 using an operational amplifier, the circuit is connected to an inverting input terminal of the operational amplifier and has an infinite input resistance value. An offset adjustment circuit for a voltage follower, comprising: a constant current circuit that causes a current proportional to a reference voltage set therein to flow through a feedback path connecting an output terminal and an inverting input terminal of the voltage follower.