JPS635611A - Active filter - Google Patents

Abstract

PURPOSE:To obtain an inexpensive low noise active filter without using an expensive low noise operational amplifier by providing a band limit circuit at the post-stage, reducing noise generated by an operational amplifier itself used at the pre-stage. CONSTITUTION:The active filter consists of a voltage amplifier circuit 17 comprising operational amplifiers 1, 2, resistive elements 3-9, capacitive elements 10-12 and circuit components 1, 3, 4 and a low pass filter circuit 18 comprising circuit elements 2, 5-12 in common use for a sweep oscillator or the like. Suppose that the low pass filter 18 has the gain/frequency characteristic shown in figure (a) 21, the noise generated in the operational amplifiers 1, 2 has a characteristic shown in 22, 23 in figure (c), the sum of noise generated from the two operational amplifiers is shown in characteristic 24. Thus, the noise generated in the amplifier of the pre-stage is subject to band limit because of passing through the low pass filter 18 and the noise is reduced at a high frequency region.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to reducing the noise of an active filter used in a loop filter such as a control circuit 1 phase locked circuit.

[Conventional technology]

Figure 3 shows, for example, a circuit used in a conventional phase-locked circuit.
It is an electric circuit diagram showing a loop filter, and 1 and 2 in the figure
is an operational amplifier, 3 to 9 are resistive elements, and 10 is an operational amplifier.
~12 is a capacitive element, 13 is an input terminal (voltage value ei), 1
4 is an output terminal (positive value go). Further, 15 is an active low-pass three-channel waveform circuit constituted by the above-mentioned circuit elements 1, 3 to 5, and 10, and 16 is the above-mentioned circuit elements 2.6 to 5.
This is a sweep oscillator/amplifier configured with 9°11.12.

Next, the operation will be explained. In FIG. 3, the input terminal 13 voltage e1 is band-limited by the active low-pass single wave amplifier circuit 15, and then predetermined amplification is performed by the sweep oscillator/amplifier 16, and an output voltage e0 is output.

In addition, when this circuit is used in a phase-locked loop, if synchronization is lost, the sweep oscillator/amplifier 16 functions as an oscillation circuit, and when the negative phase-locked loop is locked, it stops oscillating and the amplifier functions as

[Problem that the invention seeks to solve]

Therefore, the noise generated by the operational amplifier 2, which constitutes the sweep oscillator/amplifier in the subsequent stage, is not band-limited, which causes the problem that noise in the high frequency region is not limited. In order to solve this problem, it is necessary to use a low-noise operational amplifier 2 used in the subsequent stage, and it cannot be solved without using a very expensive low-noise operational amplifier. There were problems such as.

This invention was made to solve the above-mentioned problems, and without using an expensive low-noise operational amplifier.
The purpose of this study is to obtain an active filter with low noise in the high frequency region.

[Means for solving problems]

The active filter according to the present invention has a configuration of (amplifier) + (low-pass f-wave generator circuit), with a low-pass f-wave generator in the final stage and an amplifier in the preceding stage, and is different from a conventional loop filter. is the arrangement order reversed.

[Effect]

In this invention, since the low-pass filter circuit is placed at the final stage, the noise generated by the amplifier at the previous stage is also band-limited along with the input voltage, and noise in the high frequency region is reduced.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows an active filter according to an embodiment of the present invention, in which 1 and 2 are operational amplifiers, 3 to 9 are resistive elements, 10 to 12 are capacitive elements, and 2 is a low-pass filter that also serves as a sweep oscillator. This is a three-wave circuit. Further, 13 is an input terminal (positive value e1), and 16 is an output terminal (voltage value eo).

Next, the effects will be explained.

Assuming that the low-pass 5F wave filter 18 in FIG. 1 has the gain/frequency characteristics shown by 21 in FIG. 2(a), the noise generated in the operational amplifiers 1 and 2 in FIG. (C)-22
．． The characteristic is shown by 23, and the total noise generated from the two operational amplifiers is the characteristic shown by 24 in FIG. 2(C). Here, the point to note is that the front stage amplifier 17 in FIG.
The noise generated by the low-pass two-wave filter 1! in Figure 1 is ? In order to pass through the radio frequency band, the noise is reduced even in the high frequency range.

This can be clearly seen by comparing it with a conventional circuit. In the conventional circuit, as shown in FIG. Therefore, the noise generated by the operational amplifier shown at 1°2 in Figure 3 has the characteristics shown at 25.26 in Figure 2 ('b), and the total noise generated from the two differential amplifiers is The characteristic is shown by 27 in FIG. 2(b). In this manner, in the conventional circuit, the amplifier 16 shown in FIG. 3 is placed in the latter stage, and its output noise is not subjected to band limitation, so that the noise output becomes large in the high frequency region. Therefore, as can be seen by comparing Figure 2 (bl, (C)), in the frequency range lower than f, the output noise level does not change, but in the frequency range higher than f8, the output noise level is
It can be clearly seen that the reduction is more remarkable in this example. In contrast, in the conventional circuit shown in FIG. 3, in order to achieve a similar reduction in noise, an operational amplifier with a very low noise level must be used in the subsequent stage.

In addition, in the above embodiment, the filter is applied to a loop filter of a phase-locked oscillator circuit, but an active filter used in an oven loop for boat fishing may also be used, and the same effect as in the above embodiment can be obtained. It is something.

Further, in the above embodiment, the operational amplifier circuit has two stages, but it may have three or more stages.
The same effects as in the above embodiment are achieved.

[Effects of the Invention] As described above, according to the present invention, in a filter composed of a plurality of operational amplifiers, the band-limiting circuit is configured in the final stage, so that the operational amplifier used in the previous stage is The noise generated by the filter itself can be reduced, and an inexpensive, low-noise active filter can be obtained without using an expensive low-noise operational amplifier.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing an active filter according to an embodiment of the present invention, FIG. 2 is a diagram showing a specific characteristic curve showing the effects of the present invention, and FIG. 2(a) is a diagram showing a specific characteristic curve of a loop filter. A diagram showing the frequency characteristics of the gain of a low-pass f-wave device,
FIG. 2('b) is a diagram showing the frequency characteristics of the output noise level of a conventional loop filter, and FIG. 2(C) is a diagram showing the frequency characteristics of the output noise level of an active filter according to an embodiment of the present invention. be. Further, FIG. 3 is an electrical circuit diagram showing a conventional loop filter. In the figure, 17 is a voltage amplifier (operational amplifier circuit before the final stage), 18 is a low-pass filter that also serves as a sweep starter (
1 & final stage operational amplifier circuit), 1゜2 is operational amplifier, 3
9 are resistive elements, and 10 to 12 are capacitive elements. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (2)

[Claims] (1) In an active filter composed of multiple operational amplifiers and their peripheral circuits, each operational amplifier circuit in the stage before the final stage has a sufficiently wider band than the operational amplifier circuit in the final stage, and An active filter characterized in that the operational amplifier circuit has a band that substantially determines band characteristics as an active filter. (2) The active filter according to claim 1, wherein the final stage operational amplifier circuit is a low-pass filter circuit.