JPS606134B2 - RC active bundle direct filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an RC active band reject filter.

FIG. 1 shows a generally known band reject filter using a Wien bridge, where the amplifier is a differential amplifier and the B section is a band pass filter.

The output S of the band pass filter B section at the tuning point with respect to the input S of the band reject filter is S. I become a turtle.

Here, in order to minimize the output of the amplifier at the tuning point, it is necessary that S, - S2 = O, that is, S2 = Clearance = R - Ice2. However, as shown in FIG. 2, the output S of the band-pass filter B section exhibits a wide characteristic that becomes S at the resonant frequency P o. Therefore, the output of the amplifier also exhibits a wide range of characteristics as shown in FIG. Now, if we consider using this kind of band reject filter as a distortion meter, it must be flat above the second high frequency. Resistance to A, “R.

, Rf, good removal characteristics shown in FIG. 2 and 3 are obtained. The relationship between input and output in FIG. 1 is shown in the first equation.

8: R reed Rf Here, A, is the amplification factor of amplifier A, are the amplification factors of amplifier A2.

In addition, to explain how to derive the '1' formula, from Figure 1, if we set j■ = P, = S (Ki - 3 PCR + Teimai C stem R Otoju.) A2 Here, S = {Sin 1 (OUT )8}A
, OUT=sin (ki-3 points) A, A2-(ki-3 points)
A secret (om) where Q = PCR Toshi.

AIA2Q ・90ku30〃AIA2)Q Then, the formula (1) is found.

The harmonic distortion in this case is generated by amplifier A in FIG.

Harmonic distortion in amplifiers occurs because the amplification elements are nonlinear. This can be reduced by applying a large amount of negative feedback. Distortion can be reduced in proportion to the amount of negative feedback. Furthermore, distortion can be reduced by using a linear portion of the operating region, that is, a small region. But the first
Amplifier A in the figure has no negative feedback at the tuning point,
As the frequency shifts, negative feedback is applied.

As a result, the gain increases with respect to the fundamental wave, the amplitude of vibration increases, and the amount of negative feedback decreases, resulting in an increase in distortion. Further, since negative feedback is provided from the output of the amplifier A to the amplifier A, it is difficult to apply a large amount of negative feedback. The limit of the amount of negative feedback is determined by the decrease in gain at the frequency where the phase circuit becomes 1800, so if the number of increasing stages is large, the phase rotation will increase and the amount of negative feedback will also decrease. Therefore, it becomes impossible to realize a removal circuit with a low distortion rate. This problem is extremely important when used in an actual distortion meter. For example, if an attempt is made to achieve a fundamental wave removal performance of 10N, even if the bridge is perfectly balanced, the tuning must be within 5/1,000,000, and manual tuning becomes impossible. Therefore, in order to widen the band, the circuits shown in FIG. 1 would be connected in cascade, but this becomes impossible due to the problem of phase rotation mentioned above. The present invention eliminates these drawbacks, and realizes a band reject filter that approximately doubles the required gain of amplifier A and has low distortion and good rejection characteristics.

FIG. 3 shows an example of this, in which the amplifier A receives a signal S, whose phase is shifted by 1800 with respect to the input signal Sin.
The amplifier A2 constitutes a summing amplifier together with filter elements B and Z2. Here, the filter elements Z, Z are composed of a resistor R and a capacitor C, and the filter element Z is composed of a series circuit thereof, and the filter element Z is composed of a parallel circuit thereof.

Since B, Z at the tuning point is Z2i, in order to make the output of amplifier A2 at the tuning point maximum 4, S, =
Must be $2.

Therefore, the amplification factor of amplifier A only needs to be twice. In this case as well, harmonic distortion is generated by amplifier A, but since A does not have negative feedback from L as in the conventional case, A can independently apply the maximum negative feedback, thereby minimizing the generation of distortion. and the distortion generated in amplifier A is
There are advantages that are reduced by bandpass filters. In the circuit shown in FIG. 3, the frequency characteristic of the output of the amplifier 2 when there is no positive feedback circuit Rf is as shown in FIG. 41.

Considering the case where the circuit in Figure 1 is used in a distortion meter of the same machine, if positive feedback is applied by Rf to suppress the attenuation within the error with CRi2, good rejection characteristics as shown in Figure 4 2 can be obtained. Obtainable. The input-to-output relationship of the circuit shown in FIG. 3 can be expressed as in the second equation.

8=R;ki;(A2): amplification factor of amplifier A2;
Formula 1 will be further explained.

In FIG. 3, if A is an ideal operational amplifier with infinite gain, then s. F\Sheep 8・OUT-R2・sin・Toru. ...Side '3'0M = e31A2 (e3 is A2
signal at the input of S. = Three Poisons 328・e3A2-Shizuki...If the input impedance of '41A2 is infinite, then Z
．． The current 1 flowing through Z2 is equal to the current 12 flowing through Z2.

J Agriculture (s'e3): (R ten J woman) 2 (e3-s'n) J
DemiG-w=(・S in 10J)2G3-s,n) {・10(
3-Three Innkeepers 28A2) J; Left R + Ko Furote 2} e3: {ku・ten J; House) 2-R2R. If it is 2, then the following formula is derived.

The circuit in Figure 1 and the circuit in Figure 3 of the same aircraft must be tuned within 2/100 directions in order to obtain the fundamental wave removal performance of 10%, but similarly, the circuits can be connected in cascade. Since this is also positive feedback, there is no increase in distortion no matter how many stages are used, so stable positive feedback can be applied, and a good removal circuit can be constructed.

[Brief explanation of drawings]

Figure 1 is a wiring diagram of a conventional RC active band reject filter; Figure 2 is its characteristic diagram; Figure 3 is a wiring diagram of an RC active band reject filter according to an embodiment of the present invention; Figure 4 is its characteristic diagram. A,,... amplifier, Z,, Z... filter element. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. A first amplifier for shifting the phase of the input signal by 180°, a second amplifier, a first filter element consisting of a series connection or parallel connection of a resistor and a capacitor, and a first filter element that opposes the connection of the first filter element. a second filter element and a positive feedback circuit consisting of a resistor and a capacitor connected in parallel or in series; the output of the first amplifier is connected to the first filter element, and the input signal is connected to the second filter element. An RC active band reject filter characterized in that the output of a second amplifier to which the outputs of both filter elements are added is positively fed back to the first amplifier.