Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
1999-05-13
2001-02-06
Nuton, My-Trang (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
Reexamination Certificate
active
06184747
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to filters, and more particularly, to continuous differential filters that use gyrators or the like to simulate one or more inductance values. Most electrical systems include at least some form of an electrical filter such as a low pass, a high pass, or a bandpass filter. These filters are typically implemented using a combination of discrete components such as resistors, inductors and/or capacitors. In some technologies, such as integrated circuit and some printed circuit board technologies, inductors cannot readily be produced. To overcome this limitation, gyrators have been developed. Gyrators simulate an inductance using, for example, only active elements such as transistors and a capacitance load. Gyrators thus help eliminate the need for conventional physical inductors (e.g., coils).
Gyrators often have an input impedance that is proportional to the load admittance. Therefore, when a gyrator is loaded with a capacitance, the input impedance behaves like an inductance. Some prior art gyrator circuits are described in, for example, U.S. Pat. No. 3,643,183 to Geffe, U.S. Pat. No. 3,715,693 to Fletcher et al., U.S. Pat. No. 3,758,885 to Voorman et al., and U.S. Pat. No. 4,812,785 to Pauker.
In integrated circuit technologies, the load capacitance used by the gyrator is typically formed using a gate oxide type capacitor. Gate oxide capacitors include a gate oxide layer cladded by the substrate layer and the polysilicon gate layer. The capacitance value of a gate oxide capacitor is primarily dictated by the area of the polysilicon gate region. Even though the gate oxide layer is relatively thin, the amount of capacitance that can be generated per unit area is relatively small. Therefore, to generate an adequate capacitance value for many filter applications, the area of the gate oxide capacitor must be relatively large, which can significantly reduce the overall density, reliability and yield of the integrated circuit (IC).
In many integrated circuit processes, the gate oxide layer may be susceptible to pin holing, wherein one or more pinhole defects in the gate oxide effectively short the substrate to the polysilicon gate layer. The probability of having a pin hole in any given circuit is typically dependent on the total gate oxide area in the circuit. Thus, when large gate oxide capacitors are used, the chance of having one or more pin holes in the circuit increases, and the overall yield of the circuit decreases. Therefore, it would be desirable to produce a gyrator based filter circuit that minimizes the total area of the gate oxide capacitors.
Gyrator filters are also often only adapted to accept and filter single ended input signals. For some applications, it would be desirable to provide a gyrator based filter that is adapted to accept and filter differential input signals. Differential input signals typically provide an improved signal-to-noise ratio relative to a single ended input signal, and can increase the dynamic range of the circuit. This is particularly important for low power applications where the peak-to-peak signal level is relatively small, and when the gyrator circuit is integrated on a single IC along with other circuits that generate substantial substrate and power supply noise. Therefore, it would also be desirable to provide a gyrator based filter that is adapted for receiving and filtering a differential input signal.
SUMMARY OF THE INVENTION
The present invention overcomes many of the disadvantages of the prior art by providing a gyrator based filter that is adapted to receive and filter a differential input signal. The differential gyrator filter preferably includes a first gyrator connected to the positive input signal of the differential input signal and a second gyrator connected to the negative input signal of the differential input signal. A load capacitor is connected between the load terminals of the first gyrator and second gyrator, which minimizes the total load capacitance required for the gyrator based filter. This implementation is believed to increase the overall density, reliability, yield, signal-to-noise ratio and dynamic range of the gyrator based filter and related circuitry.
In one illustrative embodiment, the differential gyrator based filter includes a positive filter input terminal and a negative filter input terminal for receiving the positive input signal and the negative input signal, respectively, of the differential input signal. The positive filter input terminal is coupled to a first gyrator and the negative filter input terminal is coupled to a second gyrator. A load capacitor is then connected between the load terminal of the first gyrator and the load terminal of the second gyrator. By connecting the load capacitor between gyrators, rather than providing a separate capacitor from each gyrator to ground, the overall capacitor area is reduced. This may reduce the overall die area, increase the overall yield, reduce the manufacturing and test costs associated with each device, and provide a number of other advantages.
The first gyrator may include a first differential amplifier and a second differential amplifier. Likewise, the second gyrator may include a first differential amplifier and a second differential amplifier. The first differential amplifier and the second differential amplifier of each gyrator preferably has a positive input terminal, a negative input terminal, a positive output terminal and a negative output terminal.
The positive input terminal of the first differential amplifier of the first gyrator and the negative output terminal of the second differential amplifier of the first gyrator are preferably coupled to the input terminal of the first gyrator. The negative input terminal of the first differential amplifier of the first gyrator is preferably coupled to the positive output terminal of the second differential amplifier of the first gyrator. Likewise, the positive output terminal of the first differential amplifier of the first gyrator is preferably coupled to the positive input terminal of the second differential amplifier of the first gyrator. Finally, the negative output terminal of the first differential amplifier of the first gyrator is preferably coupled to the negative input terminal of the second differential amplifier of the first gyrator. The second gyrator is preferably similarly constructed.
Because the first and second gyrators are formed using two fully differential amplifiers, each gyrator may have two separate load terminals. To balance the load on each of the load terminals, the load capacitance preferably includes matched first and second capacitor loads, each having a first terminal and a second terminal. The positive output terminal of the first differential amplifier of the first gyrator and the positive input terminal of the second differential amplifier of the first gyrator are preferably coupled to the first terminal of the first capacitor. The negative output terminal of the first differential amplifier of the first gyrator and the negative input terminal of the second differential amplifier of the first gyrator are preferably coupled to the first terminal of the second capacitor. Likewise, the positive output terminal of the first differential amplifier of the second gyrator and the positive input terminal of the second differential amplifier of the second gyrator are preferably coupled to the second terminal of the first capacitor. Finally, the negative output terminal of the first differential amplifier of the second gyrator and the negative input terminal of the second differential amplifier of the second gyrator are preferably coupled to the second terminal of the second capacitor.
It is contemplated that the above differential gyrator circuit may be used in conjunction with other impedance elements such as capacitors and resistors to form a desired filter. In one illustrative embodiment, a first filter capacitor may be connected in parallel with the first gyrator to form a parallel LC network. Likewise, a second fil
Helgeson Michael A.
Maile Keith R.
Honeywell International , Inc.
Nuton My-Trang
Shudy, Jr. John G.
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