Filter circuit having a biquadratic transfer function

• US 4,253,069 A
• Filed: 04/02/1979
• Issued: 02/24/1981
• Est. Priority Date: 03/31/1978
• Status: Expired due to Term

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Nossek, Josef
• Primary Examiner(s): Mullins, James B.
• Application Number:
  US06/026,356
• Time in Patent Office: 694 Days
• Field of Search: 330/107, 330/109, 330/294
• US Class Current: 330/107
• CPC Class Codes: H03H 11/126 using a single operational amplifier H03H11/1204 takes precedence; parallel-T filters H03H11/1295
• CPC Class Codes: H03J 5/0218 Discontinuous tuning using an electrical variable impedance element, e.g. a voltage variable reactive diode, by selecting the corresponding analogue value between a set of preset values

Abstract

An active filter circuit has a biquadratic transfer function and is constructed without coils with the smallest possible number of passive components, particularly capacitors, and comprises a single operational amplifier. A first resistor is connected from the filter input to the non-inverting input of an operational amplifier and a second resistor connects the same input of the operational amplifier to a reference potential. A third resistor is connected between the output of the operational amplifier and the non-inverting input. A first capacitor is connected from the output of the operational amplifier to the inverting input and the same input is connected, by way of a fourth resistor to the reference potential for representation of a low-pass transfer function and a fifth resistor is connected between the input of the filter and the inverting input of the operational amplifier for representing a high-pass transfer function. In addition, a sixth resistor connects the inverting input of the operational amplifier to a junction which is connected to the output of the filter by way of a seventh resistor and to the reference potential by way of an eighth resistor. A second capacitor also connects the filter input to the aforementioned junction. The reference potential is connected to a line extending through the filter and forming one terminal of a pair of input terminals and one terminal of a pair of output terminals. The circuit can be constructed, in particular, in layer technology and is suited as a selection device, particularly in the low-frequency range in communications systems.

Claims

1. In an active RC filter with a biquadratic transfer function and of the type which includes an input having first and second input terminals, an output having first and second output terminals, an operational amplifier having an inverting input, a non-inverting input, and an output connected to said first output terminal, and a line connected to said second input terminal and said second output terminal and connected to a reference potential, a first resistor connected between the non-inverting input and the first input terminal, a second resistor connected between the non-inverting input and the line, a first capacitor connected between the inverting input and the first output terminal, a third resistor connected between the inverting input and a junction, a second capacitor connected between the junction and the first input terminal, and a fourth resistor connected between the junction and the line, the improvement therein comprising:

means for providing a low-pass transfer function, including a fifth resistor connected between the first output terminal and the non-inverting input, and a sixth resistor connected between the inverting input and the line.

2. The improvement of claim 1, and further comprising:

for increasing the primary amplification, a third capacitor connected in parallel with said sixth resistor.

3. The improvement of claim 1, and further comprising:

for reducing the primary amplification, a third capacitor connected in parallel with the fourth resistor.

4. In an active RC filter with a biquadratic transfer function and of the type which includes an input having first and second input terminals, an output having first and second output terminals, an operational amplifier having an inverting input, a non-inverting input, and an output connected to said first output terminal, and a line connected to said second input terminal and said second output terminal and connected to a reference potential, a first resistor connected between the non-inverting input and the first input terminal, a second resistor connected between the inverting input and the line, a first capacitor connected between the inverting input and the first output terminal, a third resistor connected between the inverting input and a junction, a second capacitor connected between the junction and the first input terminal, and a fourth resistor connected between the junction and the line, the improvement therein comprising:

means for providing a high-pass transfer function, including a fifth resistor connected between the first output terminal and the non-inverting input and a sixth resistor connected between the inverting input and the first input terminal.

5. The improvement of claim 4, and further comprising:

for reducing the primary amplification, a third capacitor connected in parallel with the fourth resistor.

6. In an active RC filter with a biquadratic transfer function and of the type which includes an input having first and second input terminals, an output having first and second output terminals, an operational amplifier having an inverting input, a non-inverting input, and an output connected to said first output terminal, and a line connected to said second input terminal and said second output terminal and connected to a reference potential, a first resistor connected between the non-inverting input and the first input terminal, a second resistor connected between the inverting input and the line, a first capacitor connected between the inverting input and the first output terminal, a third resistor connected between the inverting input and a junction, a second capacitor connected between the junction and the first input terminal, and a fourth resistor connected between the junction and the line, the improvement therein comprising:

means for providing a low-pass transfer function, including a fifth resistor connected between the first output terminal and the non-inverting input, and a sixth resistor connected between the non-inverting input and the line,

a seventh resistor connected between the first and second input terminals, and

a eighth resistor connected between the first and second output terminals and a ninth resistor connected between the first output terminal and the first input terminal.

7. In an active RC filter with a biquadratic transfer function and of the type which includes an input having first and second input terminals, an output having first and second output terminals, an operational amplifier having an inverting input, a non-inverting input, and an output connected to said first output terminal, and a line connected to said second input terminal and said second output terminal and connected to a reference potential, a first resistor connected between the non-inverting input and the first input terminal, a second resistor connected between the non-inverting input and the line, a first capacitor connected between the inverting input and the first output terminal, a third resistor connected between the inverting input and a junction, a second capacitor connected between the junction and the first input terminal, and a fourth resistor connected between the junction and the line, the improvement therein comprising:

means for providing a high-pass transfer function, including a fifth resistor connected between the first output terminal and the non-inverting input and a sixth resistor connected between the inverting input and the first input terminal,

a seventh resistor connected between the first and second input terminals,

a eighth resistor connected between the first and second output terminals, and

an ninth resistor conected between the first input terminal and the first output terminal.