DE2806921C2 - Filter circuit with a third degree transfer function - Google Patents

Description

The invention relates to a filter circuit according to the preamble of claim!

A number of circuit principles have already become known for setting up active filter circuits. Among other things, it is known to use coilless active filter circuits with the aid of so-called operational amplifiers to realize. In the meantime, a circuit of this type has also become known that it allows a third degree transfer function to be implemented. In detail, this circuit is in the Magazine "IEEE Transaction on Circuits and Systems", VoL CAS-22, No. 4, April 1975, pages 329 to 334, and this circuit also has the Property that a pole of attenuation can be generated at finite frequencies.

This known circuit also uses an operational amplifier in conjunction with resistors and capacitors used As sie shows, however, it needs a size. «Number of capacitors, what for the construction in integrated circuit technology is to be regarded as annoying. A further complicating factor is that the known circuit is not unconditionally grade 3 with regard to its transfer function, d. h, at Variation of the switching elements changes the degree of switching.

From the journal »Scientific reports AEG-Teiefunken 43, (1970) 1, pages 46 to 62 is a general method for the synthesis of active RC filters known, in which the principle of forward and backward coupling is applied. A filter circuit with an eticr transfer function is from this publication any integer, i.e. also implicitly third degree, which consists of an operational amplifier and arranged between its output and its two (inverting and non-inverting) inputs There are resistors and capacitors, and the one continuous line at reference potential has known. The sound system for both inputs of the operational amplifier can be seen in Figure 2. the in this figure, not shown, is at reference potential Hegende line, as in Figure 3 of this publication can be seen, each connected to the two RC-t-ports 1 and 2 and, as is common practice, also serves as a reference potential for the input and output signal of the filter.

The invention is based on the object of providing an AC filter circuit of the type mentioned in the introduction specify, in which, on the one hand, the transfer function remains unconditionally of degree 3 and, on the other hand, the Number of capacitors can be kept as low as possible.

Based on the filtering circuit mentioned in the introduction this object is according to the invention according to the characterizing features of claim I. solved.

Advantageous refinements are given in the subclaims.

In accordance with the requirements mentioned in the introduction, the task to be solved is to create a circuit, in particular to specify a low-pass circuit which has the following transfer function.

= T ₀

ρ + a

In the above equation, p = jcu means the complex frequency, T ₀ a constant factor for setting the basic attenuation or basic gain, λ is its pole parts on the real axis of the complex frequency plane, ω _ρ the pole frequency and Q _p the pole quality of a complex pole pair, and ω _ζ is the frequency of the damping pole; the transfer function IXp) is also a measure of the voltage ratio of the output voltage Ui to the input voltage U \. ■

In the following, the circuit is explained in detail with reference to the drawing

The drawing shows the filter input labeled with the reference numerals 2 and 1 and the filter output labeled with the reference numerals 3 and 1, with ί simultaneously indicating a continuous line at a reference potential, for example Niassepotential, at the filter input the voltage U \, at the filter output the voltage Ui. From the filter input 2, a resistor Gc leads to the non-inverting input 5 of an operational amplifier 4. From the input 5, a first parallel circuit of a resistor Gd and a capacitor Ca leads to the continuous line 1, while a second parallel circuit of a resistor Gb and a capacitor Cb leads to the output 3 of the operational amplifier performs As indicated in the diagram immediately, the output of the operational amplifier at the same time the filter output -3, which is particularly advantageous in circuit-technical point of view and with regard to an integrated construction manner Further, the output of the operational amplifier 4 via a capacitor Ci with its inverting input 6, which in turn is connected to the continuous line 1 via the resistor Gi. The output 3 of the operational amplifier 4 is also connected via a resistor Gi to a circuit node 7, from which on the one hand a resistor G \ to non-inverse animal input 6 and on the other hand a resistor G is connected to the filter input 2. At the circuit node 7 there is also a parallel connection of a resistor Gs and a capacitor Cu, the second connection point of which leads directly to the continuous line 1

If it is important to achieve switching elements that are advantageous in terms of production technology, three additional resistors can be introduced into the circuit described above, which are shown in dashed lines in the drawing and provided with the reference numerals Gt, Gi, and Gi . The resistor Gs is in the input branch, the resistor Gt in the output branch and the resistor Gj bridges the circuit from filter input 2 to filter output 3. Two of these additional resistors can have the value infinite, that is, only one or it can only two additional resistors have to be connected.

A favorable dimensioning of the circuit is given in particular if the relationship (Gc + Gd) / f> i Ca = Gb / Cb = G / C \ is observed. In this case, the switching element values can be calculated when designing the filter by solving linear and quadratic equations. Above, G, Gb, Gc, Gd or C _A , Cr Q denote the conductance values or the capacitance values of the correspondingly assigned resistors or capacitors.

The following consideration also applies to the connection of the resistors G ₅ to G _{7 shown in dashed lines.}

To cover the area required by a filter when implemented using a layer technology, such as thin-film technology To keep it low, the impedance level that is always freely available must be optimally selected. About that addition is mostly by applying the? r- ^ transformation a reduction of the resistance sum possible with a constant total capacitance. Well is mostly a such a resistor JT link is not a priori available. But it can often be done by adding resistors that increase the transfer function of the filter do not change, a jr-member will be generated. Such resistors can be installed parallel to the filter input (at usual supply from an ideal voltage source) or filter output, bridging from the input to the output as well as placed parallel to the operating surgeon will.

summary
Active filter circuit

The invention relates to an active filter circuit in which a transfer function of the third degree is to be implemented with the smallest possible number of resistors and capacitors and only one operational amplifier. According to the invention this is a resistor (Gq) to the non-inverting input (5) connected in an operational amplifier (4) from the filter inlet (2) Here, although parallel circuits of one resistor (Gd or Gb), and one capacitor (Ca or Cb ) , which lead to a continuous line (1) or to the output (3) of the operational amplifier (4), which is connected to its inverting input (6) via a capacitor (C ₂ ). There is a resistor (G4) there continuous line (1). The output (3) is connected via a resistor-capacitor circuit (Gi, G% or G or G3, G) to the inverting input (6) or to the filter input (2) or to the continuous line (1 ) tied together. The circuit can in particular be built up using layer technology and is suitable as a selection means in communication systems (FIG. 1).

1 sheet of drawings

Claims (4)

Patent claims: 1. Active filter circuit with a 3rd degree transfer function, in the case of the filter input one toilet network each to the inverting and non-inverting input of one Operational amplifier is switched, and at the output of the operational amplifier, which is simultaneously represents the filter output, each galvanic 'see connections to the two ÄC networks exist, and the one common for the two AC networks and the filter input and the filter output, has a continuous line at reference potential, characterized in that that the /? C networks are designed as follows: a) a first resistor (Gc ) leads from the filter input (2) to the non-inverting input (5) of the operational amplifier (4), b) from the non-inverting input (5) of the operational amplifier (4) leads a parallel circuit of a second resistor (Gd) and a first capacitor (C _A ) to the continuous line (1),. c) from the non-inverting input (5) of the operational amplifier (4), a parallel connection of a third resistor (Gb) and a second capacitor (Cb) leads to the output (3) of the operational amplifier (4), d) a third capacitor (Ci ) leads from this output (3) to the inverting input (6) of the operational amplifier (*), e) from this inverting input (6) a fourth resistor (G _A ) is connected to the continuous line (1), f) A fifth resistor (G ₂ ) leads from the output (3) of the operational amplifier (4) to a circuit node (7) which is connected to the inverting input (6) of the operational amplifier (4) via a sixth resistor (G \) , g) the circuit node (7) is connected to the filter input (2) via a seventh resistor (G) and to the line (1) at reference potential via a parallel circuit of an eighth resistor (Gj) and a fourth capacitor (C \) . 2. Filter according to claim 1, characterized in that three additional resistors are provided, of which the first resistor (Gs) is connected from the filter input (2) and the second resistor (Gs) from the filter output (3) to the continuous line (1) , and that the filter input (2) and filter output (3) are connected to one another via the third resistor (Gi). 3. Filter according to claim 2, characterized in that one or two of the additional resistors (GuG ₆ , G7) assumes the value infinity. 4. Filter according to one of the preceding claims, characterized by a dimensioning according to following relationship in this case, G, Gb, Gc, G _d and C * Cb, Q denote the conductance values or the capacitance values of the associated resistors or capacitors.