GB2227387A

GB2227387A - Filter circuit arrangement

Info

Publication number: GB2227387A
Application number: GB8928749A
Authority: GB
Inventors: Colin Leslie Perry
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1988-12-21
Filing date: 1989-12-20
Publication date: 1990-07-25
Anticipated expiration: 2009-12-20
Also published as: GB8928749D0; GB8829820D0; GB2227387B

Abstract

An active-ladder filter circuit arrangement comprises a plurality of resistance-capacitance integrator circuits (1A, 1B, 1C) in cascade between an input (2) and an output (3). Each integrator circuit comprises a respective transistor which is controllable via a control input (10A, 10B, 10C) to vary the effective resistance of a resistance component of the relevant integrator circuit, which component connects inputs (6A, 6B, 6C, 7A, 7B, 7C) of the relevant integrator circuit to the capacitance component thereof. The filter characteristic is thus controllable. In order to reduce intermodulation products produced by out of pass-band signals the arrangement input (2) is connected to the capacitance component of the first integrator circuit via a passive resistor which can have a more linear voltage-current characteristic than a resistance which is variable by controlling a transistor. When the above arrangement is constructed as an integrated circuit, a set of alternative passive resistors of slightly different value connected to alternative inputs may be provided and a selected resistor in conjunction with the control input (10A, 10B, 10C) contributes to an optimum filter characteristic. <IMAGE>

Description

DESCRIPTION FILTER CIRCUIT ARRANGEMENT This invention relates to an active-ladder filter circuit arrangement comprising a plurality of resistance-capacitance integrator circuits in cascade between an input and an output together with a respective feedback path from the output of each integrator circuit except the first to an input of that integrator circuit which precedes it in the cascade combination, each integrator circuit comprising a respective transistor which is controllable to vary the effective resistance of a resistance component of the corresponding circuit.

Arrangements of this general kind are known from, for example, the article "Fully Integrated RC Filters in MOS Technology" by M. Banu and Y. Tsividis in IEEE Journal of Solid-state Circuits Volume SC18 No. 6 (December 1983) pages 644-651, the article "Continuous Time MOSFET-C Filters in VLSI" by Banu et al. in Volume SC21 No. 1 (February 1986) of the same Journal - see in particular Figure 6 on page 19 and Figure 9 on page 21, and in US Patent Specification 4509019. Variation of the effective resistances of resistance components of the various integrator circuits by controlling the said transistors results in variation of the filter characteristic, which can be an advantage particularly when the arrangement is constructed in integrated circuit form, as it enables the effects of production spreads to be compensated for by a suitable choice of control signal.The said resistance components may be constituted, for example, by the source-drain resistances of field-effect transistors, these transistors then being the aforesaid transistors, as in the quoted publications or, as another example, by the transconductances of transconductance amplifiers of which the said respective transistors form part. In either case the voltage-current characteristics of the said effective resistances tend to be non-linear, which can be a disadvantage as it is liable to result, for example, in the generation of undesired intermodulation products by the filter. It is an object of.the invention to mitigate this disadvantage.

According to the invention an arrangement as specified in the first paragraph is characterised in that a further resistance component of the first integrator circuit comprises a passive resistor which connects the arrangement input to the capacitance component of said first integrator circuit.

It has now been recognized that intermodulation products occurring at the outputs of the known arrangements derive from both input signals lying within the filter pass-band and input signals lying outside this band. The latter are attenuated as they pass through the cascaded integrator circuits so that intermodulation products derived therefrom are generated mainly at the input of the arrangement and can be significantly reduced by connecting the arrangement input to the capacitance component of the first integrator circuit by means of a passive resistor (which can be constructed to have a substantially linear voltage-current characteristic) rather than via a resistance component of controllable effective resistance.This of course has the disadvantage that, if the value of this passive resistor has to be adjusted, this will have to be done by, for example, substituting one such resistor for another, but in many applications this disadvantage will be outweighed by the improved intermodulation performance obtainable.

Each said effective resistance may be constituted by the transresistance of a respective trans conductance amplifier having its output connected to the capacitance component of the relevant integrator circuit, said transconductance amplifier comprising a differential pair of transistors the standing current in which is supplied in operation by the relevant said respective transistor.

If the arrangement is integrated on a semiconductor substrate a set of alternative said passive resistors may be provided connected to respective alternative inputs of the arrangement.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing in which Figure 1 is the circuit diagram of an active-ladder filter circuit arrangement, and Figure 2 shows a possible practical construction for circuits shown in symbolic form in Figure 1.

The arrangement shown in Figure 1 comprises three resistance-capacitance integrator circuits 1A, 1B and 1C respectively in cascade between an input 2 and an output 3, the output 4A of circuit 1A being connected to an inverting input 7B1 of circuit 1B and the output 4B of circuit 1B being connected to a non-inverting input 6C of circuit 1C. The output 4B of circuit 1B is, moreover, connected via a feedback path 5 to a non-inverting input 6A of circuit 1A, and the output 4C of circuit 1C is connected to a further inverting input 7B2 of circuit 1B via a feedback path 8.In addition the output 4A of circuit 1A is connected to an inverting input 7A thereof and the output 4C of circuit 1C is connected to an inverting input 7C thereof. The arrangement input 2 is connected to a further inverting input 9 of circuit 1A. The arrangement simulates in known manner a third-order low-pass LC ladder filter (see, for example, the article "The leapfrog or active-ladder synthesis" by F. Girling and E. Good in Wireless World, July 1970, pages 341-345). The effective resistances of resistance components of the circuits 1A, 1B and 1C, and hence the filter characteristic, can be varied by varying control signals applied to respective control signal inputs 10A, 10B and lOC of these circuits.These control signals in fact control respective transistors included in the various integrator circuits, for example in a manner which will be described with reference to Figure 2, and thereby control resistance components which connect the inputs 6 and 7 of each circuit 1 to the capacitance component thereof.

Figure 2 shows a possible construction for the resistance-capacitance integrator circuits 1A and 1C of Figure 1 in more detail. If the construction shown in Figure 2 is used a resistance component of each circuit 1A and 1C is in the form of a trans conductance amplifier comprising a differential pair of transistors 11 and 12 the commoned emitters of which are fed from the output of a current mirror comprising transistors 13 and 14 and the difference between the collector currents of which is fed to a node 15 by means of a current mirror comprising transistors 16 and 17. The bases of the transistors 11 and 12 constitute the non-inverting and inverting inputs 6 and 7 respectively of the circuit 1A or 1C (c.f.Figure 1) and the collector/base of the diode-connected transistor 14 constitutes the control signal input 10. Varying the value of a control current applied to input 10 varies the collector current of transistor 3 and hence the transconductance of the transconductance amplifier, i.e. the effective resistance thereof. A further resistance component of the circuit 1A (only) of Figure 1 is constituted by a passive resistor 18 (shown in dashed lines) which connects the further inverting input 9 of the circuit 1A to the node 15.

The node 15 constitutes one terminal of the capacitance component of the circuit 1A or 1C which is formed in known manner by an inverting operational amplifier 19 having a feedback capacitor 20 connected between its output and its inverting input.

The input of amplifier 19 is connected to the node 15 and its output is connected to the output 4 of the circuit 1A or 1C.

The circuit 1B of Figure 1 may be constructed basically as shown in full lines in Figure 2 but with the addition of a duplicate of the arrangement of transistors 11,12 and 13, the base-emitter paths of the transistor 13 and its duplicate being connected in parallel, the collectors of transistor 12 and its duplicate being connected together and the collectors of transistor 11 and its duplicate being connected together. If this is the case the bases of the transistor 11 and its duplicate should be connected to a reference potential point, the bases of the transistor 12 and its duplicate then constituting the inputs 7B1 and 7B2 respectively.

If the circuits 1A-1C of Figure 1 are controlled via their inputs 10 in step with each other, and the constructions described with reference to Figure 2 are used for each, only one transistor 14 need in fact be provided, its collector/base being connected to the base of the respective transistor(s) 13 included in each circuit 1 and its emitter being connected to the emitter thereof, so that all the transistors 13 are controlled by a common control signal.

If the arrangement of Figure 1 is constructed in integrated circuit form the passive resistor 18 may be provided either on or off-chip. If it is provided on-chip duplicates thereof of slightly different values may also be provided on-chip, each being connected between the node 15 and a respective input 9, so that the arrangement input 2 can be connected to whichever of the resulting several inputs 9, in conjunction with the control signal applied to the inputs 10, gives the optimum filter characteristic.

The controllable resistance components of the circuits 1 of Figure 1 are not, of course, necessarily constituted by transconductance amplifiers. They may, as an alternative, be constituted for example by controllable field-effect transistors as in the publications quoted in the preamble. However, in all cases the arrangement input 2 should be connected to the capacitance component of the circuit 1A by means of a passive resistor.

Obviously, the integrator circuits 1 of Figure 1 may be of the balanced type, if desired. (Examples of balanced integrator circuits are shown in Figures 9(a) and 9(b) of the aforementioned article by Banu et al). If this is the case then the two, balanced, input terminals of the filter circuit arrangement which will then be present will be connected via individual passive resistors to respective ones of the two, balanced, capacitance components of the integrator circuit 1A which will then be present.

Of course the particular arrangement shown in Figure 1 is only one of many possibilities; the invention is applicable to integrator-employing low-pass, band-pass and high-pass active-ladder filter arrangements of any order, which arrangements may be constructed in known manner to give zeros in their characteristics as well as poles, if desired. Band-pass active-ladder filter arrangements employing integrator circuits, albeit of a type employing switched capacitors as their resistance elements are discussed, for example, on page 1020 of an article entitled "Design Techniques for MOS Switched Capacitor Ladder Filters" by G.M. Jacobs et al. in IEEE Transactions on Circuits and Systems, Vol. CAS-25 No. 12, December 1978 at pages 1014-1021.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of circuit arrangements and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

CLAIM(S)

1. An active-ladder filter circuit arrangement comprising a plurality of resistance-capacitance integrator circuits in cascade between an input and an output together with a respective feedback path from the output of each integrator circuit except the first to an input of that integrator circuit which precedes it in the cascade combination, each integrator circuit comprising a respective transistor which is controllable to vary the effective resistance of a resistance component of the corresponding circuit, characterised in that a further resistance component of the first integrator circuit comprises a passive resistor which connects the arrangement input to the capacitance component of said first integrator circuit.

2. An arrangement as claimed in Claim 1, wherein each said effective resistance is constituted by the transresistance of a respective transconductance amplifier having its output connected to the capacitance component of the relevant integrator circuit, said trans conductance amplifier comprising a differential pair of transistors the standing current in which is supplied in operation by the relevant said respective transistor.

3. An arrangement as claimed in Claim 1 or Claim 2, which is integrated on a semiconductor substrate, a set of alternative said passive resistors being provided connected to respective alternative inputs of the arrangement.

4. An active-ladder filter circuit arrangement substantially as described herein with reference to the drawing.