JPH0614505Y2 - Noise removal circuit for active filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a noise elimination circuit for an active filter using an active circuit such as an operational amplifier.

(B) Conventional technology As described in "Operational Op-amp Battle Technology" pages 157 to 182 issued by "Seibundo Shinkosha" on October 10, 1980, an active filter using an operational amplifier is provided. Are known. The active filter is frequently used because it can easily filter a signal in a low frequency region, and in general, it is often used in a second or higher order configuration.

(C) Problems to be solved by the invention When an operational amplifier is used as an active filter, a power supply and a reference power supply are required, but when a plurality of operational amplifiers are used, they are commonly used. However, when noise is generated from the power source or the reference power source, the noise generated from the active filters of each stage are all in the same phase. Therefore, as the number of stages increases, the level of the noise increases and cannot be ignored. There was a problem of reaching.

(D) Means for Solving the Problems The present invention has been made in view of the above points, and is characterized in that an inverting circuit for noise cancellation is inserted between stages of a plurality of active filters connected in series. To do.

(E) Function According to the present invention, since the inverting circuit for noise removal is inserted between the stages of multiple active filters that use the same power supply and ground with the same circuit configuration, the active filter in the previous stage is used. The generated noise will be canceled by the active filter in the subsequent stage.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention. (1) is an input terminal to which an input signal is applied, (2) is an operational amplifier ( 3 )
A first active filter, (4) is the first active filter
An inverting circuit for inverting the output signal of the active filter (2), and (5) a second active filter to which the output signal of the inverting circuit (4) is applied and which has the same circuit configuration as the first active filter (2) , And (6) are power supplies for supplying a power supply voltage (+ V _CC ) equal to the first and second active filters (2) and (5).

Next, the operation will be described. First active filter
Since (2) has an operational amplifier ( 3 ) as shown in the figure, it generates noise from itself. The noise may be mixed from the power source (6) or ground. The generated noise is applied to the inverting circuit (4) from the output end of the first active filter (2), inverted, and then applied to the second active filter (5). Since the second active filter (5) has the same circuit configuration as the first active filter (2), the noise generated from itself is the same as the noise generated from the first active filter (2). Therefore, from the inverting circuit (4) to the second active filter
The noise applied to (5) and the noise generated from the second active filter (5) itself are the second active filter (5).
Will be canceled out in the second active filter
No noise is generated at the output terminal (7) of (5).

Therefore, the input signal from the input terminal (1) is
After frequency selection by active filters (2) and (5),
It is led to the output terminal (7) without including noise.

FIG. 2 is a circuit diagram showing another embodiment of FIG. 1, in which (8) is an inverting amplifier circuit, and (9) is the same circuit configuration as the first and second active filters (2) and (5). Is a third active filter having In FIG. 2, the same circuit elements as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. FIG. 2 shows the case of a third-order filter. In the case of a multiple-order filter, the inverting amplifier circuit (8) is arranged according to the number of active filters arranged in the subsequent stage of the inverting amplifier circuit (8). The gain of may be adjusted. That is, in FIG. 2, since two active filters are arranged in the latter stage of the inverting amplifier circuit (8) and one active filter is arranged in the preceding stage thereof, the gain of the inverting amplifier circuit (8) is increased. Is set to double. By doing so, the noise generated at the output terminal (7) can be reduced to zero as a result.

(G) Effect of the Invention As described above, according to the present invention, with the same circuit configuration,
Since the inverting circuit is inserted between the stages of the active filter that uses a common power source or the like, noise can be canceled by the active filter at the stage subsequent to the inverting circuit. Therefore, it is possible to improve the filter characteristic of the active filter of a plurality of orders.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing another embodiment of the present invention. (2) …… First active filter, (4) …… Inversion circuit,
(5) …… Second active filter.

Claims (1)

[Scope of utility model registration request] 1. A plurality of active filters having a common power supply or ground and connected in series, and a series connection between the nth stage (n is a positive integer) active filter and the (n + 1) th stage active filter. And adjusting the gain of the inverting amplifier such that the level of noise generated in the active filter of the (n + 1) th stage and thereafter and the level of noise output from the inverting amplifier are equal to each other. Noise filter circuit for active filter.