DE2806921B1 - Filter circuit with a third degree transfer function - Google Patents

Description

In the above equation, p = j is the complex frequency, T0 a constant factor for setting the basic damping or basic gain, os is a pole on the real axis of the complex frequency plane, O> p the pole frequency and Qp the pole quality of a complex pole pair, and æz is the frequency of the damping pole; the transfer function T (p) is also a measure for that Voltage ratio of the output voltage U2 to the input voltage Ul.

The following is based on the drawing

Circuit explained in detail.

In the drawing it can be seen that with the reference numerals 2 and 1 designated filter input, and the filter output designated with the reference numerals 3 and 1, with 1 at the same time a continuous, on a reference potential, for example Ground potential, horizontal line is identified. The is at the filter inlet Voltage U1, at the filter output the voltage U2. A resistor leads from the filter inlet 2 Gc to the non-inverting input 5 of an operational amplifier 4. From the input 5 from leads a first parallel connection of a resistor Go and a capacitor CA to the continuous line 1, while a second parallel connection of a Resistor CB and a capacitor CB leads to output 3 of the operational amplifier. As can be seen directly in the circuit diagram, the output of the operational amplifier represents at the same time the filter output 3 represents what in terms of circuitry and in terms of an integrated structure is particularly advantageous. Furthermore is the exit of the operational amplifier 4 via a capacitor C2 with its inverting Input 6 connected, which in turn via the resistor G4 with the continuous line 1 is connected. The output 3 of the operational amplifier 4 is still on a Resistor G2 connected to a circuit node 7, from which on the one hand a Resistor G1 to the non-inverting input 6 and on the other hand a resistor G is switched to filter input 2. At the circuit node 7 there is also a parallel connection from a resistor G3 and a capacitor C1, the second connection point of which is directly leads to continuous line 1.

When it comes down to it, switching elements that are inexpensive to manufacture To achieve this, three additional resistors can be added to the circuit described above are introduced, shown in dashed lines in the drawing and with the reference numerals Cs G6 and G7 are provided. The resistor G5 is in the input cross branch, the resistor G6 in the output branch and the resistance G? bridges the circuit from the filter input 2 to the filter output 3. Two of these additional resistors can den Assume infinite value, i.e. H. So, there may only be one, or there may be only two additional resistors have to be connected.

A favorable dimensioning of the circuit is given, in particular, if the relationship (Gc + GD) / CA = GB / CB = G / C1 is observed. In this case, leave Namely, the switching element values in the filter design change by solving linear ones and a quadratic equation. In the foregoing, G, GB mean. Gc, CD resp. CA, CB, C, the conductance values or the capacitance values of the correspondingly assigned Resistors or capacitors.

For the connection of the dashed resistors C5 The following consideration still applies to G7.

In order to reduce the area required by a filter when using a layer technique, such as B. to keep low in thin film technology, the always freely available impedance level must be optimally chosen. In addition, it is mostly by applying the z-T transformation a reduction of the total resistance is possible with a constant total capacitance. In most cases, such a resistance 3s link is not available from the start. It but can often be done by adding resistors that increase the transfer function of the Do not change filters, an n-term will be generated. Such resistances can be parallel to the filter input (with the usual supply from an ideal voltage source) or

Filter output, bridging from input to output as well as parallel to the operational amplifier.

Summary Active Filter Circuit The invention relates to to an active filter circuit with a third degree transfer function with as few resistors and capacitors as possible and only one Operational amplifier is to be realized. According to the invention, this is from the filter inlet (2) a resistor (Gc) to the non-inverting input (5) of an operational amplifier (4) connected Here, parallel connections each consisting of a resistor (GD resp. GB) and one capacitor (CA or Cg) each connected to form a continuous Lead (1) or to the output (3) of the operational amplifier (4), the over a capacitor (C2) is connected to its inverting input (6) there there is a resistor (G4) to the continuous line (1). The exit (3) is over a resistor-capacitor circuit (G2, Gt or G or G3, C1) with the inverting Input (6) or

connected to the filter inlet (2) or to the continuous line (1). The circuit can in particular be built up using layering technology and is suitable as selection means in communication systems (Fig. 1).

Claims (4)

Claims: 1. Filter circuit with a transfer function third degree coming from an operational amplifier and between its output and its input consists of resistors and capacitors, and the one has a continuous line at reference potential, thereby marked e i c h n e t that from the filter input (2) a resistor (Gc) on the non-inverting Input (5) of the operational amplifier (4) leads from which the parallel connection from a resistor (GD) and a capacitor (CA) to the continuous line (1) and a further parallel connection of a resistor (Gs) and a capacitor (CB) to the output (3) of the operational amplifier (4) that leads from this output (3) a capacitor (C2) to the inverting input (6) of the operational amplifier (4) is connected, from which a resistor (G4) to the continuous line (1) leads, and that a resistor continues from the output (3) of the operational amplifier (4) (G2) leads to a circuit node (7), which via a resistor (Gl) with the inverting input (6) of the operational amplifier (4), via a resistor (G) with the filter input (2) and a resistor connected in parallel (G3) and a capacitor (C) is connected to the continuous line (1). 2. Filter according to claim 1, characterized in that three additional Resistors are provided, of which the first resistor (G5) from the filter input (2) and the second resistor (G6) from the filter output (3) to the continuous line (1) is switched, and that filter input (2) and filter output (3) via the third Resistance (G7) are interconnected. 3. Filter according to claim 2, characterized in that one or two of the additional resistors (, G6, G7) take on the value infinite. 4. Filter according to one of the preceding claims, characterized by dimensioning according to the following relationship Gc + GD = GB = G CA CB Cl G, Ga Gc, GD or CA, C & C1 are the conductance values or the capacitance values of the associated Resistors or capacitors. The invention relates to a filter circuit with a transfer function third degree coming from an operational amplifier and between its output and its input consists of resistors and capacitors applied, and the one Continuous line at reference potential has To set up active filter circuits a number of circuit principles have already become known. It is known, inter alia, to use coilless active filter circuits to be realized with the help of so-called operational amplifiers. It is in the meantime also a circuit of this type has become known that it allows a transfer function to realize third degree. This circuit is detailed 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 of being a Damping pole can be generated at finite frequencies. An operational amplifier is also used in this known circuit used in conjunction with resistors and capacitors. As it turns out, needs However, they have a larger number of capacitors, which is essential for building in integrated Circuit technology is to be regarded as disturbing. To make matters worse, the well-known Switching is not unconditionally free from the degree in terms of its transfer function 3 is, i.e. That is, when the switching elements vary, the degree of switching changes. The invention is based on the object of an RC filter circuit of the type mentioned in the introduction, in which, on the one hand, the transfer function remains unconditional of degree 3 and on the other hand the number of capacitors as possible can be kept low. Based on a filter circuit with a transfer function third degree, made up of resistors, capacitors and an operational amplifier exists, and which has a continuous line at reference potential This object is achieved according to the invention in that a resistor from the filter input leads to the non-inverting input of the operational amplifier from which the parallel connection of a resistor and a capacitor for continuous Line and another parallel connection of a resistor and a capacitor leads to the output of the operational amplifier, and that from this output a capacitor is connected to the inverting input of the operating amplifier, from which a resistor leads to the continuous line that continues from the output of the operational amplifier a resistor leads to a circuit node, which is connected to the inverting input of the operational amplifier via a resistor to the filter input and via a parallel connection of a resistor and a capacitor with the continuous line is connected. Advantageous refinements are given in the subclaims. In accordance with the requirements mentioned in the introduction, the problem to be solved is to specify a circuit, in particular a low-pass circuit, which has the following transfer function.