Fail-safe low-pass filtering circuit

• US 4,368,440 A
• Filed: 10/23/1980
• Issued: 01/11/1983
• Est. Priority Date: 10/23/1980
• Status: Expired due to Term

• Current Assignee: Union Switch & Signal, Inc. (Hitachi, Ltd.)
• Original Assignee: American Standard Inc. (Trane Technologies plc)
• Inventors: Darrow, John O. G.
• Primary Examiner(s): Mullins, James B.
• Application Number:
  US06/199,779
• Time in Patent Office: 810 Days
• Field of Search: 330/302, 330/303, 330/305, 307/271, 307/445, 307/446, 307/570, 307/571, 307/580, 307/582, 307/584, 307/241, 333/172, 333/174, 333/167
• US Class Current: 333/172
• CPC Class Codes: H03H 11/1291 Current or voltage controlled filters

Abstract

A fail-safe active low-pass filter circuit including a plurality of transistor gates having their inputs coupled to a source of a.c. signals, one of a plurality of field-effect transistor switches coupled to an associated one of the plurality of transistor gates, and a variable R-C network having a variable upper a.c. signal passing level which is determined by which one of the associated transistor gates and field-effect switches is rendered conductive by application of d.c. supply voltage.

Claims

1. A vital circuit arrangement comprising, a source of a.c. signals having a variable frequency, a plurality of electronic gates having their inputs coupled to said source of a.c. signals, one of a plurality of electronic switches coupled to the output of an associated one of said plurality of electronic gates, and a variable low-pass filter network having an upper signal frequency limit which is determined by which of said associate electronic gates and switches is rendered conductive by supplying a d.c. voltage to said associated gates and switches.

2. The vital circuit arrangement as defined in claim 1, wherein said plurality of electronic gates each include an emitter-follower amplifier which provides a high immunity to ripple voltage which may appear on said d.c. voltage.

3. The vital circuit arrangement as defined in claim 1, wherein said plurality of electronic switches each include a field-effect transistor which ensures that said low-pass filter network is not unduly loaded.

4. The vital circuit arrangement as defined in claim 1, wherein said plurality of electronic gates each include a PNP transistor.

5. The vital circuit arrangement as defined in claim 1, wherein said variable low-pass filter network includes selected ones of a plurality of resistors and a capacitor.

6. The vital circuit arrangement as defined in claim 1, wherein said variable low-pass filter network includes a series R-C circuit.

7. The vital circuit arrangement as defined in claim 1, wherein said variable low-pass filter network includes one of a plurality of series resistors and a four-terminal capacitor.

8. The vital circuit arrangement as defined in claim 1, wherein said plurality of electronic gates each include a unity gain amplifying circuit.

9. The vital circuit arrangement as defined in claim 1, wherein said plurality of electronic gates each includes a transistor having a base electrode, a collector electrode and an emitter electrode, a voltage divider including a first and a second resistor connected between said d.c. voltage and a reference point, a third resistor connecting the base electrode to the junction point of said first and second resistors, a capacitor connecting the junction point of said first and second resistors to said source of a.c. signals, a fourth resistor connecting said emitter electrode to said reference point, and said collector electrode connected to said d.c. voltage.

10. The vital circuit arrangement as defined in claim 9 wherein said plurality of electronic switches each includes a field-effect transistor having a gate electrode, a source electrode and a drain electrode, said source electrode connected to said emitter electrode, a fifth resistor connecting said gate electrode to said d.c. voltage, and said drain electrode connected by a sixth resistor to a filtering capacitor.