JPS6226918A - Active filter - Google Patents

Abstract

PURPOSE:To obtain a RC active filter with differential input constitution having a circuit simpler than the conventional one by using impedance elements acting similarly in common for both terminals of a differential input amplifier among impedance elements provided at both terminals of the differential input amplifier in a RC active filter of differential input constitution. CONSTITUTION:Protective capacitors connected respectively to an input voltage E1 and a common mode voltage Ec act similarly in a conventional circuit, then they are used in common as a protection capacitor Cs'. The elements acting similarly are used in common among the impedance elements provided at both terminals of the RC active filter of differential input constitution. Thus, the number of the impedance elements is decreased to form the RC active filter of the differential input constitution with simple circuit constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to an active filter as a signal filter using a differential input type amplifier.

[Prior art and its problems]

Generally, an RC active filter is known as a signal filter in a low frequency region. This RC active filter is realized by replacing the LRC passive network with an RC active network, and is specifically composed of an active element such as an amplifier and a feedback circuit consisting of a combination of a resistor R and a capacitor C. .

Signal filters are broadly classified into low-pass types, high-pass types, band-pass types, and band-elimination types. Therefore, RC active filters can also be roughly classified into the four types mentioned above. The basic configuration of a conventional low-pass RC active filter is shown in FIG. The ILc active filter shown in FIG. 2 includes an amplifier A, and a protection resistor R11, an input resistor R1, and an input capacitor Ci at the negative input terminal of the amplifier A.
The circuit has a circuit configuration in which a circuit consisting of a feedback resistor Rf and a feedback capacitor Cf is connected, and the positive input terminal is directly grounded. Input voltage E and output voltage E in this circuit
.

The gain of is given by the following equation. Regarding this formula, ■
Since it is described in pages 251 to 253 of the Analog IC Application Handbook published by Kogyo Kenkyukai on September 10, 1976, the explanation of the calculation method will be omitted.

By the way, in general, in a signal amplification circuit, it is necessary to consider common mode signal removal characteristics. In this case, it is necessary to pay attention not only to the common-mode signal rejection characteristics of the amplifier itself, but also to the signal input circuit section. However, according to the circuit configuration shown in FIG. 2, the common-mode component is likely to be superimposed on the signal from the signal source as a signal component, and therefore the common-mode signal rejection ratio (CMRR) characteristic of the entire circuit is not good. There is.

In order to solve these problems, it has been considered to input signals differentially using a differential input type amplifier. FIG. 3 shows the basic structure of an active filter having such a structure. In Figure 3,
Bl is the input voltage, Eo is the common mode voltage, R2 is the output voltage, R
8 indicates a protection resistor, and Rf indicates a feedback resistor. In this circuit configuration, the output voltage E2 is expressed by the following equation.

As is clear from equation (2) above, in the differential input amplifier A shown in FIG. FIG. 4 shows the configuration of a conventional RC active filter using such a differential input type amplifier.

In the figure, the same components as in FIG. 2 are indicated by the same symbols, but the points that are different from FIG. This is the point where a circuit consisting of a feedback resistor section and a feedback capacitor Cf is connected. Note that the gain of this circuit is the same as in equation (1).

However, such an ltC active filter has a disadvantage in that it has a large number of circuit components because equal impedance elements are configured at both poles of a differential input type amplifier in order to improve common-mode signal rejection characteristics.

[Purpose of the invention]

The present invention aims to eliminate the above-mentioned drawbacks and provide a differential input configuration (C active filter) with a simpler circuit configuration.

[Key points of the invention]

The main point of the present invention is that among the impedance elements provided at both poles of a differential input type amplifier of an RC active filter having a differential input configuration, elements having the same function are shared between both poles of the differential input type amplifier. , which attempts to reduce the number of circuit components.

[Embodiments of the invention]

FIG. 1 is a circuit diagram showing an embodiment of an RC active filter according to the present invention. The same components as in FIG. 4 are given the same reference numerals, but the difference from the circuit in FIG. 4 is that the circuit in FIG. Since the protective capacitor C8 has the same function, they are used in common as the protective capacitor C8. The gain of the circuit shown in FIG. 1 is expressed by the following equation.

Now, if we assume that the circuit shown in Figure 1 has the same low-pass filter characteristics as the circuit shown in Figure 2, then equation (1) and (
4) As is clear from comparing the formulas, C3=2CI ・
・・・・・・・・・・・・・・・(5) may be used. Therefore, the protective capacitor C8 is
It can be done with 1/2 of the capacity of .

In addition to the above description of the preferred embodiments of the present invention, the present invention is not limited to the low-pass type C active filter using the multiple feedback method described in the embodiments, and can of course be applied to various RC active filters. It can be widely applied as a measurement signal input circuit. It goes without saying that various other design changes can be made without departing from the spirit of the invention.

〔Effect of the invention〕

In the present invention, among the impedance elements at both poles of the tC active filter with a differential input configuration, elements having the same function are shared, so the number of impedance elements is reduced and the differential input with a simple circuit configuration is achieved. It becomes the RC active filter of the configuration. In addition, in order to suppress the deterioration of the CMRR characteristics due to the difference in impedance between the elements on both poles of a differential input amplifier, the circuit components must have an accuracy of 1% or less, but are combined into one element. Since it is shared by both poles of the input type positive spreader, this single element has the effect that high accuracy with respect to the CMRR characteristic is not required.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the structure of an embodiment of an I/C active filter according to the present invention, and FIGS. 2 to 4 are circuit diagrams showing the structure of conventional active filters. A,...Differential input type amplifier, El...Input voltage,"
2... Output voltage 1 (...: common mode voltage, ■ also...
・Protection resistance, R4, ... input resistance, Rf ... feedback resistance, C,, C, ... protection capacitor 1 q Rf

Claims (1)

[Claims] 1) Using a differential input type amplifier, a voltage component in phase with the input signal source and the sum of the voltage component and the input signal are input to the differential input type amplifier through equivalent impedance circuit networks. The active filter is characterized in that, among the impedance elements in the impedance network, elements having the same function are shared by both poles of the differential input type amplifier.