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Bandpass Filter Used as an Oscillator

IP.com Disclosure Number: IPCOM000074443D
Original Publication Date: 1971-Apr-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Morlec, E: AUTHOR

Abstract

The gain of the so-called multiloop band-pass active filter is independent of the grounded resistor, while the central frequency depends on the value of this resistor. Consequently, if an oscillator is constructed by placing this filter in the feedback loop of a transistor circuit, any variation of the grounded resistor will vary the central frequency without modifying the conditions of oscillation. The oscillator can, therefore, be tuned very easily.

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Bandpass Filter Used as an Oscillator

The gain of the so-called multiloop band-pass active filter is independent of the grounded resistor, while the central frequency depends on the value of this resistor. Consequently, if an oscillator is constructed by placing this filter in the feedback loop of a transistor circuit, any variation of the grounded resistor will vary the central frequency without modifying the conditions of oscillation. The oscillator can, therefore, be tuned very easily.

Bandpass filter BPF is placed in the feedback loop of a transistor circuit T to provide oscillation at the central frequency Fo of the BPF. The coupling circuit between the transistor circuit T and BPF includes two capacitors C for removal of DC components and antiparallel mounted diodes D for limiting the feedback gain.

The central frequency Fo = Omega o/2 Pi and the gain at this frequency can be derived from the filter transfer function by using the conventional relations. This transfer function H(P) = E2/E1, where p = j Omega is derived from the basic equations on the currents at nodes A and B. In practical design, the openloop gain K of the operational amplifier is very high and therefore V(B) approx. 0.

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Equations (2) and (3) show that Omega o can be varied through R2 without any variation on Ao, and, therefore, a multifrequency oscillator can be made by using a variable R2.

Another advantage of this circuit is that voltage E3 is 90 degrees out of phase as co...