Fail-safe electronic band-pass filter

Claim 2

A fail-safe electronic filter for passing a signal having a preselected frequEncy comprising, an amplifier having an input and an output circuit, a feedback path including a twin-T network connected from said output circuit to said input circuit of said amplifier, said twin-T network is imperfectly nulled to provide regeneration at said preselected frequency and to provide degeneration at all other frequencies from said output circuit to said input circuit of said amplifier so that an output signal is only available at said output circuit of said amplifier during the presence of said preselected frequency signal at said input circuit of said amplifier and in the absence of a critical component or circuit failure in the fail-safe electronic filter.

Claim 3

A fail-safe electronic filter as defined in claim 1, wherein the gain of the feedback path is less than unity.

Claim 4

A fail-safe electronic filter as defined in claim 1, wherein said feedback path includes a cascaded emitter-follower amplifier.

Claim 5

A fail-safe electronic filter as defined in claim 1, wherein said amplifier includes a transistor connected in a common-emitter configuration.

Claim 6

A fail-safe electronic filter as defined in claim 4, wherein said twin-T network operates as the collector load impedance for said transistor amplifier.

Claim 7

A fail-safe electronic filter as defined in claim 4, wherein a constant current source supplies d.c. collector current to said transistor amplifier.

Claim 8

A fail-safe electronic filter as defined in claim 1, wherein said twin-T network provides substantially a 180* phase shift only for said preselected frequency signal.

Claim 9

A fail-safe electronic filter as defined in claim 1, wherein said amplifier includes an NPN transistor.

Claim 10

A fail-safe electronic filter as defined in claim 1, wherein said twin-T network is an unbalanced symmetrical resistance-capacitance circuit.

Claim 11

A fail-safe active band-pass filter circuit comprising, an amplifier circuit including an input circuit and an output circuit and having a gain greater than unity, a feedback circuit connected between said output circuit and said input circuit of said amplifier circuit having a loop gain of less than unity, said feedback circuit including a parallel-T network, said parallel-T network providing positive feedback at a center frequency from said output circuit to said input circuit of said amplifier circuit so that an output signal is produced at said output circuit of said amplifier circuit only when an input signal having said center frequency is applied to said input circuit of said amplifier circuit and no critical component or circuit failure is present in the fail-safe active band-pass filter circuit.

Claim 12

A fail-safe active band-pass filter circuit comprising, an amplifier circuit including an input circuit and an output circuit and having a gain greater than unity, a feedback circuit connected between said output circuit and said input circuit of said amplifier circuit having a loop gain of less than unity, said feedback circuit including a parallel-T network, said parallel-T network providing positive feedback at a center frequency from said output circuit to said input circuit of said amplifier circuit so that an output signal is produced at said output circuit of said amplifier circuit only when an input signal having said center frequency is applied to said input circuit of said amplifier circuit and no critical component or circuit failure is present in the fail-safe active band-pass filter circuit.

Claim 13

A fail-safe active band-pass filter circuit as defined in claim 10, wherein said amplifier circuit includes a transistor connected in a common-emitter configuration.

Claim 14

A fail-safe active band-pass filter circuit as defined in claim 11, wherein said feedback circuit extends from the collector electrode to the base electrode of said transistor.

Claim 15

A fail-safe active band-pass filter circuit as defined in claim 10, wherein said feedback circuit includes an amplifying means having a gain less than unity.

Claim 16

A fail-safe active band-pass filter circuit as defined in claim 13, wherein said amplifying means includes a plurality of transistors connected in Darlington configuration.

Claim 17

A fail-safe active band-pass filter circuit as defined in claim 10, wherein said parallel-T network includes a plurality of resistors and capacitors which invert the phase of the center frequency signal.

Claim 18

A fail-safe active band-pass filter circuit as defined in claim 11, wherein a constant current source including a PNP transistor supplies d.c. collector current to said transistor amplifier circuit.

Claim 19

A fail-safe active band-pass filter circuit as defined in claim 10, wherein a high impedance element is coupled to the input of said circuit.