Amplifiers – Signal feedback – Phase shift means in loop path
Reexamination Certificate
1999-08-16
2001-07-03
Shingleton, Michael B. (Department: 2817)
Amplifiers
Signal feedback
Phase shift means in loop path
C330S109000, C330S069000, C330S303000
Reexamination Certificate
active
06255905
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a An active electronic filter circuit, comprising
at least one filter input,
at least one amplifier element having at least one amplifier input and for at least one of the filter inputs and at least one of the amplifier inputs a network which defines the filter characteristic and which comprises an input impedance arrangement for connecting the filter input to the amplifier input.
2. Description of Related Art
From U.S. Pat. No. 4,509,019 a tunable high-pass or low-pass filter is known, which filter includes a push-pull amplifier having electronically controllable elements in either the two input branches or the two feedback branches (or both in the input branches and the feedback branches). Each of the other branches includes a reactance element. This arrangement serves to attain a low distortion, which allows a simple integration on a semiconductor body using a non-linear element, for example a MOS transistor, as voltage-controlled element.
It has been found that this known filter circuit has the drawback that the time constant attainable with such a filter circuit is only adjustable over a comparatively small control range because the resistance values of the MOS transistors used as controllable elements can be varied only over a comparatively small control range.
From U.S. Pat. No. 4,780,690 a filter arrangement is known whose time constant should be adjustable over a comparatively large range. This filter arrangement comprises a push-pull amplifier having an inverting input, a non-inverting input, an inverting output, a non-inverting output, a first input terminal and a second input terminal, a first feedback circuit including a first capacitor and arranged between the non-inverting output and the inverting input, and a second feedback circuit including a second capacitor substantially identical to the first capacitor and arranged between the inverting output and the non-inverting input. This known filter arrangement further comprises a transconductance circuit with variable transconductance, which has two inputs connected to the input terminals of the filter arrangement, as well as two outputs respectively connected to the inverting input and the non-inverting input of the amplifier. The transconductance circuit is a voltage-controlled current source which converts a balanced input voltage into a balanced output current. The transconductance dictates the proportionality factor between the output current and the input voltage. Generally, this transconductance can be varied over a comparatively large range, as a result of which the time constant of the known filter arrangement should also be variable over a comparatively large range.
The transconductance circuit of the filter arrangement in accordance with U.S. Pat. No. 4,780,690 includes several active elements which each have a control input and a first and a second input terminal. The control inputs of at least two of said active elements are connected to the input terminals of said filter arrangement. The first output terminals of said at least two active elements are connected directly to one another and the second output terminals of these two active elements are connected to said inverting amplifier input and said non-inverting amplifier input, respectively.
The filter arrangement known from U.S. Pat. No. 4,780,690 further comprises a load circuit for loading the transconductance circuit. This load circuit comprises a first and a second current source transistor whose interconnected base electrodes are connected to a node between two resistors via a diode, which resistors are arranged between the outputs of the transconductance circuit.
From U.S. Pat. No. 4,780,690 it is further known to form a transconductance circuit by means of MOS transistors. This known transconductance circuit is also assumed to have a comparatively large linear voltage range.
BRIEF SUMMARY OF THE INVENTION
it is an object of the invention to provide an active electronic filter arrangement including an input impedance arrangement having a high impedance, which arrangement can be fabricated on a semiconductor body with a high precision and on a small semiconductor area.
According to the invention, this object is achieved in an active electronic filter circuit of the type defined in the opening paragraph in that the input impedance arrangement comprises at least one T network having at least three impedance branches, of which a first impedance branch is connected to the filter input, a second impedance branch is connected to the amplifier input and a third impedance branch is connected to a circuit point for draining a current from the filter input.
The implementation of the input impedance arrangement by means of a T network results in an impedance branch via which a part of a signal current applied to the filter input or the filter inputs is passed around the associated amplifier input or the associated amplifier inputs. As a result of this, said part of the signal current is no longer available in the network which defines the filter characteristic and, as a consequence, the effective impedance acting at the filter input or the filter inputs increases accordingly. The circuit point for draining this current from the filter input or the filter inputs can be a common ground terminal of the circuit.
In an advantageous embodiment of the invention an active electronic filter circuit, having two filter inputs and one amplifier element having two amplifier inputs, is designed in such a manner that the input impedance arrangement comprises at least one T network having a first, a second, a third and a fourth impedance branch respectively connected to the first one and the second one of the filter inputs and the first and the second one of the amplifier inputs, and the first and the second impedance branch, on the one hand, are connected to the second and the fourth impedance branch and, on the other hand, via a fifth impedance branch.
Preferably, an active filter circuit of this type takes the form of a push-pull amplifier element. The part of the signal current which bypasses the amplifier inputs is routed from one of the filter inputs to the other filter input via the fifth impedance branch, as a result of which it is not necessary to have a separate circuit point to which this current is drained.
Instead, one of the filter inputs serves as such a circuit point and in this embodiment the input impedance arrangement of the electronic filter circuit can preferentially be floating.
In a further embodiment of the active electronic filter circuit in accordance with the invention the input impedance arrangement comprises at least two T networks in cascade. Thus, the impedance value of the input impedance arrangement can be increased considerably by means of a minimal number of circuit elements.
In another embodiment of the invention the T network is or the T networks are symmetrical with respect to the filter inputs and/or the amplifier inputs. This symmetrical arrangement is particularly suitable in push-pull circuits.
In another variant of invention the impedance branches are essentially formed by means of ohmic elements; in particular, at least some of the impedance branches may include controllable essentially ohmic elements. Preferably, the controllable essentially ohmic elements are arranged symmetrically within each T network. The filter characteristic of the active electronic filter circuit in accordance with the invention can be defined advantageously and can be adjusted precisely by means of controllable essentially ohmic elements, which are preferably formed by means of MOS transistors. When MOS transistors are used this can be achieved in a manner known per se by means of control voltages.
In a preferred embodiment of an active electronic filter circuit in accordance with the invention the network which determines the filter characteristic takes the form of an integrating stage, in conjunction with the amplifier element. In such an integrating stage the input impe
Shingleton Michael B.
U.S. Philips Corporation
Vodopia John
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