Operational filter building block

• US 6,271,719 B1
• Filed: 09/02/1998
• Issued: 08/07/2001
• Est. Priority Date: 09/02/1998
• Status: Expired due to Term

• Current Assignee: Analog Devices International Unlimited Company (Analog Devices, Inc.)
• Original Assignee: Linear Technology Corporation (Analog Devices, Inc.)
• Inventors: Sevastopoulos, Nello George
• Primary Examiner(s): LE, DINH THANH
• Application Number:
  US09/145,536
• Time in Patent Office: 1,070 Days
• Field of Search: 327/552, 327/558, 327/559, 327/336, 327/557, 327/553, 330/107, 330/109
• US Class Current: 327/552
• CPC Class Codes: H03H 11/04 Frequency selective two-port networks

Abstract

A 3-terminal operational filter circuit is presented that can be used to construct various types of active filters. The filter circuit can be configured to provide 2nd order low pass and band-pass frequency responses by coupling three resistors to the three filter terminals. Similarly, the filter circuit can be configured to provide 2nd order band-pass and high pass frequency responses by coupling two resistors and a capacitor to the three filter terminals. Furthermore, a plurality of filter circuits can be cascaded to construct various types of higher order filters. The filter circuit can be manufactured to operate within a selected range of center frequencies by selecting particular values for the internal filter capacitance and resistance. Users can then select a particular center frequency, quality factor, and gain of the filter circuit by selecting particular values for the circuit elements to be coupled to the three terminals.

Claims

1. A 3-terminal operational filter circuit comprising:

an input terminal for receiving an input signal from an input signal source;

a first output terminal that provides a first frequency response in response to a first condition, said first condition comprising a resistor coupled in series between said input terminal and said input signal source, said first output terminal providing a second frequency response in response to a second condition, said second condition comprising a capacitor coupled in series between said input terminal and said input signal source;

a second output terminal that provides said second frequency response in response to said first condition and that provides a third frequency response in response to said second condition;

an operational amplifier having an inverting input, a non-inverting input, and an amplifier output, said inverting input coupled to said input terminal, said non-inverting input coupled to ground, and said amplifier output coupled to said second output terminal;

a capacitor coupled in series between said inverting input and said amplifier output; and

a non-inverting integrator having an integrator input and an integrator output, said integrator input coupled to said amplifier output, and said integrator output coupled to said first output terminal;

wherein;

said first, second, and third frequency responses are each selected from the group consisting of low pass, band-pass, and high pass frequency responses.

2. The filter circuit of claim 1 wherein said non-inverting integrator comprises:

an inverter having an inverter input and an inverter output, said inverter input being said integrator input;

a resistor coupled in series between said inverter output and a node;

an operational amplifier having an inverting input, a non-inverting input, and an amplifier output, said inverting input coupled to said node, said non-inverting input coupled to ground, and said amplifier output being said integrator output; and

a capacitor coupled between said inverting input of said amplifier of said integrator and said integrator output.

3. The filter circuit of claim 2 wherein said filter circuit has a selectable range of center frequencies based on values selected for said resistor and said capacitor of said non-inverting integrator and said capacitor coupled in series between said inverting input and said amplifier output, said filter circuit tunable to a center frequency within said range in accordance with a value of a resistor coupled in series between said first output terminal and said input terminal.

4. The filter circuit of claim 1 wherein said filter circuit is an integrated circuit.

5. The filter circuit of claim 1 further comprising:

a first resistor coupled between said first output terminal and said input terminal;

a second resistor coupled between said input terminal and said second output terminal; and

a third resistor coupled between said input terminal and an input signal source;

wherein

said first output terminal provides a 2nd order low pass frequency response and said second output terminal provides a 2nd order band-pass frequency response.

6. The filter circuit of claim 5 wherein said filter circuit has a selectable center frequency based on a value of said first resistor.

7. The filter circuit of claim 5 wherein said filter circuit has a selectable center frequency, quality factor, low pass gain, and band-pass gain based on values of said first, second, and third resistors.

8. The filter circuit of claim 1 further comprising:

a first resistor coupled between said first output terminal and said input terminal;

a second resistor coupled between said input terminal and said second output terminal; and

a capacitor coupled between said input terminal and an input signal source;

wherein

said first output terminal provides a 2nd order band-pass frequency response and said second output terminal provides a 2nd order high pass frequency response.

9. The filter circuit of claim 8 wherein said filter circuit has a selectable center frequency based on a value of said first resistor.

10. The filter circuit of claim 8 wherein said filter circuit has a selectable center frequency, quality factor, band-pass gain, and high pass gain based on values of said first and second resistors and said capacitor coupled between said input terminal and an input signal source.

11. A configurable operational filter comprising:

a first voltage input terminal for coupling to a positive voltage supply;

a second voltage input terminal for coupling to a negative voltage supply;

a ground terminal for coupling to ground; and

a plurality of 3-terminal operational filter circuits each coupled to said first and second voltage input terminals and to said ground terminal, each one of said plurality of filter circuits comprising;

an input terminal for receiving an input signal from an input signal source,

a first output terminal that provides a first frequency response in response to a first condition, said first condition comprising a resistor coupled in series between said input terminal and said input signal source, said first output terminal providing a second frequency response in response to a second condition, said second condition comprising a capacitor coupled in series between said input terminal and said input signal source,

a second output terminal that provides said second frequency response in response to said first condition and that provides a third frequency response in response to said second condition,

an operational amplifier circuit having an inverting input, a non-inverting input, and an amplifier output, said inverting input coupled to said input terminal, said non-inverting input coupled to ground, and said amplifier output coupled to said second output terminal,

a capacitor coupled in series between said inverting input and said amplifier output, and

a non-inverting integrator having an integrator input and an integrator output, said integrator input coupled to said amplifier output, and said integrator output coupled to said first output terminal;

wherein;

said first, second, and third frequency responses are each selected from the group consisting of low pass, band-pass, and high pass frequency responses.

12. The configurable filter of claim 11 further comprising a bias generator coupled to said first and second voltage terminals and to said plurality of filter circuits for biasing said filter circuits.

13. The configurable filter of claim 11 wherein said non-inverting integrator comprises:

an inverter having an inverter input and an inverter output, said inverter input being said integrator input;

a resistor coupled in series between said inverter output and a node;

an operational amplifier having an inverting input, a non-inverting input, and an amplifier output, said inverting input coupled to said node, said non-inverting input coupled to ground, and said amplifier output being said integrator output; and

a capacitor coupled between said inverting input of said amplifier of said integrator and said integrator output.

14. The configurable filter of claim 13 wherein at least one of said plurality of filter circuits has a selectable range of center frequencies based on values selected for said resistor and said capacitor of said non-inverting integrator and said capacitor coupled in series between said inverting input and said amplifier output, said at least one of said plurality of filter circuits tunable to a center frequency within said range in accordance with a value of a resistor coupled in series between said first output terminal and said input terminal of said at least one filter circuit.

15. The configurable filter of claim 11 wherein said configurable filter is an integrated circuit.

16. The configurable filter of claim 11 further comprising:

a shutdown terminal; and

shutdown circuitry coupled to said shutdown terminal, to said first and second input voltage terminals, and to said plurality of filter circuits;

wherein

said shutdown circuitry disables said plurality of filter circuits when said shutdown terminal receives a shutdown signal.

17. The configurable filter of claim 11 further comprising:

a first resistor coupled between said first output terminal of a first filter circuit of said plurality of filter circuits and said input terminal of said first filter circuit;

a second resistor coupled between said input terminal of said first filter circuit and said second output terminal of said first filter circuit;

a third resistor coupled between said input terminal of said first filter circuit and an input signal source;

a fourth resistor coupled between said first output terminal of said first filter circuit and said input terminal of a second filter circuit of said plurality of filter circuits;

a fifth resistor coupled between said first output terminal of said second filter circuit and said input terminal of said second filter circuit; and

a sixth resistor coupled between said input terminal of said second filter circuit and said second output terminal of said second filter circuit;

wherein

said first output terminal of said second filter circuit provides a 4th order low pass frequency response.

18. The configurable filter of claim 11 further comprising:

a first resistor coupled between said first output terminal of a first filter circuit of said plurality of filter circuits and said input terminal of said first filter circuit;

a second resistor coupled between said input terminal of said first filter circuit and said second output terminal of said first filter circuit;

a first capacitor coupled between said input terminal of said first filter circuit and an input signal source;

a second capacitor coupled between said second output terminal of said first filter circuit and said input terminal of a second filter circuit of said plurality of filter circuits;

a third resistor coupled between said first output terminal of said second filter circuit and said input terminal of said second filter circuit;

a fourth resistor coupled between said second output terminal of said second filter circuit and said input terminal of said second filter circuit;

a third capacitor coupled between said second output terminal of said second filter circuit and said input terminal of a third filter circuit of said plurality of filter circuits;

a fifth resistor coupled between said first output terminal of said third filter circuit and said input terminal of said third filter circuit;

a sixth resistor coupled between said second output terminal of said third filter circuit and said input terminal of said third filter circuit;

a fourth capacitor coupled between said second output terminal of said third filter circuit and said input terminal of a fourth filter circuit of said plurality of filter circuits;

a seventh resistor coupled between said first output terminal of said fourth filter circuit and said input terminal of said fourth filter circuit; and

an eighth resistor coupled between said second output terminal of said fourth filter circuit and said input terminal of said fourth filter circuit;

wherein

said second output terminal of said fourth filter circuit provides an 8th order high pass frequency response.

19. A method of constructing a 3-terminal filter circuit to electrically filter an input signal from an input signal source, said filter circuit comprising an input terminal, a first output terminal, and a second output terminal, said method comprising:

coupling an inverting input of an operational amplifier to said input terminal;

coupling an output of said amplifier to said first output terminal;

coupling a capacitor in series between said inverting input and said amplifier output;

coupling an input of a non-inverting integrator to said amplifier output; and

coupling an output of said integrator to said second output terminal;

wherein said filter circuit provides;

a first frequency response at said first output terminal and a second frequency response at said second output terminal in response to a first condition, said first condition comprising a resistor coupled in series between said input terminal and said input signal source;

a third frequency response at said first output terminal and said first frequency response at said second output terminal in response to a second condition, said second condition comprising a capacitor coupled in series between said input terminal and said input signal source; and

wherein

said first, second, and third frequency responses are each selected from the group consisting of low pass, band-pass, and high pass frequency responses.

20. The method of claim 19 further comprising:

coupling a resistor in series between said input and first output terminals;

coupling a resistor in series between said input and second output terminals; and

coupling a resistor in series between an input signal source and said input terminal;

wherein;

said band-pass response is provided at said first output terminal and said low pass response is provided at said second output terminal.

21. The method of claim 20 further comprising replacing said resistor coupled in series between an input signal source and said input terminal with a capacitor, wherein said high pass response is provided at said first output terminal and said band-pass response is provided at said second output.

22. The method of claim 19 further comprising:

coupling a resistor in series between said input and first output terminals;

coupling a resistor in series between said input and second output terminals; and

coupling a capacitor in series between an input signal source and said input terminal;

wherein;

said high pass response is provided at said first output terminal and said band-pass response is provided at said second output terminal.

23. A method of configuring a 3-terminal filter circuit to provide one of first and second sets of filter responses, said first set of filter responses including low pass and band-pass responses and said second set of responses including high pass and band-pass responses, said filter circuit comprising an input terminal, a first output terminal, a second output terminal, an operational amplifier coupled between said input terminal and said second output terminal, and an integrator coupled between said second output terminal and said first output terminal, said method comprising:

coupling a first resistor in series between said input terminal and said first output terminal;

coupling a second resistor in series between said input terminal and said second output terminal; and

coupling one of a third resistor and a capacitor in series between an input signal source and said input terminal, wherein coupling said third resistor provides said first set of filter responses and coupling said capacitor provides said second set of filter responses.

24. A method of electrically filtering an input signal from an input signal source with a filter having an input terminal first output terminal and a second output terminal, said method comprising:

filtering said input signal to provide a low pass frequency response at the first output terminal and a band-pass frequency response at the second output terminal in response to a resistor coupled in series between said input terminal and said input signal source; and

filtering said input signal to provide said band-pass frequency response at said first output terminal and a high pass frequency response at said second output terminal in response to a capacitor coupled in series between said input terminal and said input signal source.