JPS62208702A - Voltage-current conversion circuit - Google Patents

Abstract

PURPOSE:To output the sum of input voltages as a current with a simple circuit and to improve the characteristic in reducing the power supply voltage by using a current mirror as a feedback circuit of an operational amplifier and connecting a resistor between an input terminal and an inverting input of the operational amplifier. CONSTITUTION:Denoting the area ratio of transistors (TR) 3,2 as 1:m when the hFE of TRs constituting a current mirror circuit 15 is sufficiently high, the output current I0 is expressed as I0=m.Iia=(m/R).Via, which indicates that an output current proportional to the input voltage Via is obtained. When n-set of input terminals 8a-8n are provided, the resistance value of corresponding resistors 4a-4n is R and input voltages Via-Vin are impressed respectively, then the total sum of the input currents is Ii=(Via+... +Vin)/R. The sum current flows to the current mirror circuit 15, the output current I0=m.(Via+... +Vin)/R is obtained, which is an output current proportional to the sum of the input voltages.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the configuration of a voltage-current conversion circuit using an operational amplifier.

[Conventional technology]

Figure 2 shows a conventional voltage-current conversion circuit.
1 is an operational amplifier, 2 is an NPN) transistor (hereinafter referred to as NPN)
Tr), 4 is a voltage-current conversion resistor, 6 is a power supply (voltage value Vcc) terminal, 7 is a ground terminal, 8 is an input terminal which is also a non-inverting input terminal of the operational amplifier 1, 9 is a reference voltage terminal, 10 is an output terminal, 11 is an output terminal of operational amplifier 1, 12 is an inverting input terminal of operational amplifier 1, and NPN
It is connected to the emitter of Tr2 and also to the reference voltage terminal via resistor 4. Further, 13 indicates a current flowing into the reference voltage terminal (1 current value ■.''), and 14 indicates an output current (current value ■.).

Next, the operation will be explained. Consider a case where an input voltage Vi is input to the input terminal 8 as seen from the reference voltage terminal 9. Here, feedback is applied to the inverting input terminal and non-inverting input terminal of the operational amplifier 1 so that an imaginary short circuit occurs.
The potential of the inverting input terminal 12 is Vi when viewed from the reference voltage terminal 9.
So, if the resistance 4 value is R, this current will be 13
The value H01 is ■. '=(Vi)/R. Here the current is 1
The three values 101 are the base current value of NPN Tr2 and the output current 14 value ■. h of NPN Tr2
Considering that F is sufficiently high, IO#I0° and To
#(V i)/R, and voltage-current conversion is performed.

Note that in this circuit, the voltage Vl of the output terminal 11 of the operational amplifier
When the reference voltage of terminal 9 is Vs, l is V,,=V s
+V i +V,,2.

In addition, when converting voltage to current by adding multiple input voltages, a voltage adding circuit should be provided before the voltage/current conversion circuit shown in Figure 2, or multiple circuits in Figure 2 should be arranged in parallel. , the output 10 is connected after each voltage-current conversion, and the respective currents are added.

[Problem that the invention seeks to solve]

Since the conventional voltage-current conversion circuit is configured as described above, the output voltage (1) of the operational amplifier is determined by adding the input voltages Vi and (2) to the reference voltage Vs.
There was a drawback that the output characteristics were poor when the lX voltage Vcc decreased.

Also, if you want to convert the sum of multiple input voltages into a current and output it, add an adder before the voltage-current conversion circuit, or arrange multiple voltage-current conversion circuits in parallel and convert each input voltage into a current. There was a problem in that the circuit became large-scale because the current was added after conversion.

This invention was made in order to solve the above-mentioned problems, and it is possible to improve the characteristics when the power supply voltage decreases, and also to create a voltage and current system that can output the sum of input voltages as a current using a simple circuit. The purpose is to obtain a conversion circuit.

[Means for solving problems]

The voltage-current conversion circuit according to the present invention uses a current mirror circuit as a feedback circuit of an operational amplifier, and connects a resistor between an input terminal and an inverting terminal of the operational amplifier.

[Effect]

In this invention, since a current mirror circuit is used as the feedback circuit of the operational amplifier, the feedback circuit outputs a value obtained by dividing the voltage applied between the input terminal and the non-inverting or inverting input terminal by the input resistance. It stably converts into current, and functions so that the output of the operational amplifier becomes significantly negative with respect to the reference potential.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows a voltage-current conversion circuit according to an embodiment of the present invention,
In the figure, 1 is an operational amplifier, and 2 and 3 are NPN transistors forming a current mirror circuit.

4a to 4n are input resistors, 5 is a resistor for offset correction due to the input current of the operational amplifier connected to the reference voltage terminal 9, 6 is a power supply terminal (voltage value Vcc), 7 is a ground terminal, and 8a to 8n are input terminals , 9 is a reference voltage terminal, 10 is an output terminal, 11 is an output terminal of an operational amplifier, 12 is an inverting terminal, 13a to 13n are input currents (current value Ia-In), 1
4 is an output current (current value ■.), and 15 is a current mirror circuit with a bearing.

Next, the operation will be explained.

In FIG. 1, consider the case where there is only one input terminal and one input resistor, that is, the case where the input terminal is 8a and the input resistor is only 4a. When the input voltage Via is applied to the input terminal 8a with reference to the voltage Vs of the reference voltage terminal 9, the potential of the inverting input terminal 12 of the operational amplifier 1 is imaginary equal to that of the non-inverting input terminal.
Since it is short-circuited and the voltage is almost v3, the input resistor 4a is set to R.
Then, the current 13a that flows is Iiam(Via
)/R. This current is current mirror circuit 1
Assuming that the hrE of the transistor forming the current mirror circuit flowing into the transistor 3 of No. 5 is sufficiently high,
If the area ratio of transistors 3 and 2 is 1:m, the output current I0 is lo-m-lLa-(m/R) ・Via
As a result, an output current proportional to the input voltage is obtained. At this time, the voltage vll of the output terminal 11 of the operational amplifier is Vll
-V 3 Vmt3, which has a sufficiently wide dynamic range with respect to the voltage reduction of the power supply voltage Vcc.

Next, there are n input terminals 8a to 8n, and the resistance values of the corresponding resistors 4a to 4n are all R, and each Vi
Consider the case where input voltages a to Vin are applied.

The total input current is ll-1ia+...+l1n
= (Via+...+Vin)/R,
Since this flows into the current mirror circuit 15, 1. xm
- It is expressed as (Via+...+Vin)/R, and it can be seen that by adding the input voltages, an output current proportional to the voltage can be obtained.

In the above embodiment, an NPN type transistor is used as the transistor of the current mirror circuit, and an output current sinking type is described. However, a PNP type current mirror circuit may be used as a sinking type, and the same as in the above embodiment. It has the effect of

Further, in the above embodiment, the non-inverting terminal 9 of the operational amplifier is used as a reference voltage, but the input terminal 8 may be used as a reference voltage and the non-inverting terminal 9 may be used as an input terminal with high input impedance. The same effects as in the embodiment are achieved.

Furthermore, in the above embodiment, the n input resistors have the same resistance value, but each input terminal has a different resistance value, and the gain may be changed for each, or a resistor may be inserted into the emitter of the current mirror transistor. The gain may be changed, and the same effect of improving voltage reduction characteristics as in the above embodiment can be achieved.

〔Effect of the invention〕

As described above, according to the voltage-current conversion circuit according to the present invention, a current mirror circuit is used as a feedback circuit of an operational amplifier, and a resistor is connected between the input terminal and the inverting or non-inverting input terminal of the operational amplifier. This has the effect that the voltage reduction characteristics of the power supply voltage are improved and that an output current proportional to the sum of a plurality of input voltages can be easily obtained.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional voltage-current conversion circuit. In the figure, 1 is an operational amplifier, 15 is a current mirror circuit, 12 is an inverting terminal, 4a to 4n are resistors, and 83 to 8n are input terminals. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] (1) In a voltage-current conversion circuit using an operational amplifier, there is a current mirror circuit connected between the output terminal and the inverting input terminal of the operational amplifier, and a current mirror circuit connected between the inverting or non-inverting input terminal of the operational amplifier and one or more main voltage currents. It is equipped with input resistors of the same number and the same resistance value as the input terminals that connect the input terminals of the conversion circuit, and the voltage between the input terminal and the non-inverting or inverting input terminal or the sum of the voltages is set at the input resistance value. A voltage-current conversion circuit characterized in that a current proportional to the divided value is taken out from the current mirror circuit.