Active filter

Details Active filter Active filter

1987-12-04

1988-10-18

Mullins, James B.

Amplifiers

Signal feedback

Phase shift means in loop path

328167, 330294, 330302, H03F 134

Patent

active

047790563

ABSTRACT:
A filter comprises an amplifier circuit arrangement having a signal input (1) and a signal output (8) and a coupling (36) between a point (37) on a signal path (39) to the signal input and a point (38) on a signal path from the signal output. The amplifier circuit arrangement comprises an inverting amplifier (6) to the input (5) of which the signal input (1) is connected via a series impedance (7), e.g. a resistor. A feedback impedance (9), e.g. a capacitor, connects the amplifier output (8) to its input so that this input constitutes a virtual ground. The phase relationship between the input and output signals of such an arrangement changes from inverting to non-inverting at frequencies at which the gain of the amplifier drops to below unity because of forward feed through the feedback impedance (9), thereby upsetting the phase relationship between the signals passing through the arrangement and those passing through the coupling. Accordingly, a further impedance (10), e.g. a capacitance, is provided between the common point ( 11) of the series and feedback impedances and ground. At frequencies at which the amplifier gain is high, this further impedance has virtually no effect because of the virtual ground existing at the common point (11). However, at frequencies at which the amplifier gain drops to unity or below, the virtual ground effect is lost and the further impedance forms a potential divider with the series impedance (7), reducing the proportion of the input signal which can take the direct forward path through the feedback impedance.

REFERENCES:
Jacobs et al., "Design Techniques for MOS Switched Capacitor Ladder Filters", IEEE Transactions on Circuits and Systems, vol. CAS-25, No. 12, Dec. 1978, pp. 1014-1021.

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