FILTER ARRANGEMENT

Abstract: Between the inverting output (5) and the non-­ inverting input (2) of a fully balanced amplifier (1) a first junction capacitor (C₁) is arranged and between the non-inverting output (4) and the inverting input (3) a second junction capacitor (C₂) is arranged. Further, a first resistor (R₁) is arranged between a first input terminal (6) and the non-inverting input (2) and a second resistor (R₂) is arranged between a second input terminal (7) and the inverting input (3). A first current source (I₁) is connected to the non-inverting input (2) and a second current source (I₂) is connected to the inverting input (3). The current sources (I₁, I₂) produce the reverse voltages for the junction capacitors (C₁, C₂) across the resistors (R₁, R₂) to define the capacitance values of these capacitors.

Description: The invention relates to a filter arrangement comprising a balanced amplifier having an inverting input, a non-inverting input, an inverting output and a non-inverting output, at least a first input circuit arranged between a first input terminal and the inverting input, at least a second input circuit arranged between a second input terminal and the non-inverting input, a first feedback circuit arranged between the non-inverting output and the inverting input, and a second feedback circuit arranged between the inverting output and the non-inverting input, which first and second input circuit each comprise a substantially identical first circuit element, which first and second feedback circuits each comprise a substantially identical second circuit element, one of said two circuit elements being a capacitor and the other circuit element being a resistor and the magnitude of at least one of said two circuit elements being adjustable. Such a filter arrangement may be employed in, for example filters, oscillators and resonators. Such a filter arrangement is known from United States Patent Specification US-A- 4,509,019. In said arrangement one circuit element is constituted by a capacitor having a fixed capacitance and the other circuit element is constituted by a resistor formed by a MOS transistor whose resistance value can be adjusted by means of a control voltage applied to its gate electrode. By means of this control voltage the RC-time constant of the resistor and the capacitor can be adjusted to the desired value. The capacitors may be arranged in the feedback circuits and the resistors in the input circuits or the capacitors may be arranged in the input circuits and the resistors in the feedback circuits. In the first case the filter circuit constitutes an integrator or low-pass filter and in the second case it constitutes a differentiator or high-pass filter. A balanced input voltage is applied to the first and the second input terminal of the filter arrangement and a balanced output voltage is available on the inverting and the non-inverting output of the amplifier. This output voltage is substantially free of even-order harmonic distortion, which is inter alia caused by non-linearities of the MOS resistors. This is because these harmonic components cancel one another as a result of the symmetrical construction of the arrangement and the balanced drive. A drawback of this known arrangement is that it can be realised only by means of a semiconductor process enabling MOS transistors to be fabricated. The arrangement cannot be realised by means of a purely bipolar process. Therefore, it is the object of the invention to provide a balanced filter arrangement with a variable RC-time constant which can also be fabricated by means of a bipolar process. According to the invention an arrangement of the type defined in the opening paragraph is characterized in that one element is constituted by a junction capacitor and in that a bias-current source is connected to the inverting input and to the non-inverting input for adjusting the capacitance of the junction capacitor connected to the relevant input. In the filter arrangement in accordance with the invention use is made of junction capacitors whose capacitance can be varied to adjust the time constant of the arrangement. In a bipolar process such junction capacitors are formed by a reverse-biassed semiconductor junction (PN-junction), the capacitance depending on the voltage across this junction. In the filter arrangement in accordance with the invention this capacitance can be adjusted by means of a bias-current source which produces the bias voltage for the junction capacitor across resistors connected to the relevant input. It has been found that distortions in the arrangement produced as a result of non-linearities of the junction capacitors are compensated for in the same way as those caused by the MOS resistors in the known arrangement. It is to be noted that the invention is not limited to a filter arrangement which can be fabricated exclusively by means of a bipolar process. The filter arrangement in accordance with the invention may also be fabricated by means of processes employing both bipolar and MOS circuit elements, such as BIMOS and BICMOS processes. This enables the junction capacitors to be fabricated in bipolar technology and the resistors to be constructed for example as MOS resistors. Am embodiment of a filter arrangement in accordance with the invention may be characterized in that the first circuit element is a resistor and the second circuit element is a junction capacitor. The arrangement then constitutes an integrator. Generally, the arrangement will be constructed as such an integrator. However, in principle it is also possible to construct the arrangement as a differentiator, the resistors then being arranged in the feedback circuits and the junction capacitors in the input circuits. A suitable embodiment of the filter arrangement in accordance with the invention may be characterized in that the amplifier comprises two transistors of a first conductivity type which are arranged as a differential pair and which have their bases coupled to the non-inverting input and the inverting input of the amplifier and their collectors to the inverting output and the non-inverting output of the amplifier, between which outputs a load circuit is arranged, which load circuit comprises a first and a second current-source transistor of a second conductivity type, whose collectors are connected to the non-inverting output and the inverting output, whose bases are connected to a common control input which is coupled to the junction point between two load resistors which are arranged between the non-inverting output and the inverting output of the amplifier. This amplifier of simple construction supplies a balanced output signal current which is large relative to the balanced input signal, so that the amplifier has a satisfactory signal-to-noise ratio. The load circuit ensures that the direct voltage on the outputs is accurately defined, so that the output signal remains fully balanced. The resistors, which are arranged in the input circuits of the filter arrangement when their arrangement is constructed as an integrator, may have comparatively large values. It is known that such resistors when integrated give rise to an undesired phase shift of the signal owing to parasitic capacitances between these resistors and the underlying layer, for example the substrate of the integrated circuit. As a result of the parasitic capacitances the resistors behave as distributed RC networks. In practice the filter arrangement will be used in circuits comprising a plurality of such coupled filter arrangements. In that case the undesired effects of parasitic capacitances can be avoided in that the filter arrangement in accordance with a further embodiment of the invention is characterized in that the input terminals of a second filter arrangement are connected to the outputs of the first filter arrangement and in that a resistor arranged in an input circuit of the second filter arrangement and the load resistor which forms part of the first filter arrangement and which is connected to said resistor are integrated in one semiconductor body respectively as a diffused resistor and a buried resistor arranged in parallel therewith. The parasitic capacitances are now present between the diffused resistor and the buried resistor. Across these two resistors the same signal voltage appears, so that no signal voltage is present across the parasitic capacitances. As a result of this, the parasitic capacitances of the resistor cannot adversely affect the operation of the arrangement. In yet another embodiment the undesired effects of the parasitic capacitances can be compensated for if the filter arrangement is characterized in that a compensation capacitor is arranged in parallel with the resistors in the input circuits or the feedback circuits. This embodiment may be characterized further in that a compensation resistor is arranged in series with the compensation capacitor. This compensation may be employed both in the case of diffused bipolar resistors and in the case of MOS resistors. In the case of MOS transistors this compensation mainly compensates for the parasitic gate-substrate capacitance. Embodiments of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which Fig. 1 shows some basic diagrams of a filter arrangement in accordance with the invention, Fig. 2 shows an amplifier for use in the filter arrangements shown in Fig. 1, Fig. 3a shows two coupled filter arrangements in accordance with the invention, Figs. 3b to 3c show some examples of a part of the circuit arrangement shown in Fig. 3a. Fig. 1a shows a first basic diagram of a filter arrangement in accordance with the invention constructed as an integrator. The filter arrangement comprises a balanced amplifier 1 which is arranged between a positive and a negative power-supply terminal and which comprises a non-inverting input 2, an inverting input 3, a non-inverting output 4 and an inverting output 5. A first input circuit comprising a resistor R₁ = R is arranged between the first input terminal 6 and the non-inverting input 2 and a second input circuit comprising a resistor R₂ identical to the resistor R₁ is arranged between a second input terminal 7 and the inverting input 3. A first feedback circuit comprising a first junction capacitor C₁ is arranged between the inverting output 5 and the non-inverting input 2 and a second feedback circuit comprising a junction capacitor C₂ identical to the first junction capacitor is arranged between the non-inverting output 4 and the inverting input 3. Further, a first current source I₁ = I is arranged between the non-inverting input 2 and the power-supply terminal and a second current source I₂ identical to the first current source is arranged between the inverting input 3 and the negative power-supply terminal. As is known per se , the junction capacitors C₁ and C₂ are constituted by reverse-biassed semiconductor junctions (PN-junctions), the capacitance depending on inter alia the reverse voltage across the junction. This reverse voltage is defined by the current sources I₁ and I₂. It is assumed that the direct voltages on the inputs 6 and 7 and the outputs 4 and 5 are equal to each other. If the operation of the current source is ignored, the non-inverting input 2 and the inverting input 3 are also at the same direct voltage. In that case no direct voltage is present across the junction capacitors C₁ and C₂. However, across the resistor R₁ the current source I₁ produces a voltage which is equal to V c = IR. This voltage then also appears across the capacitor C₁ and also determines its capacitance. In the same way the current source I₂ defines the voltage across the capacitor C₂. By constructing the current sources I₁ and I₂ as variable-current sources the RC-time constant of the arrangement can be adjusted accurately. The manner in which such an adjustment is effected falls beyond the scope of the present Application and is therefore not described herein. For this, reference is made to the article "Continuous-Time MOS-FET-C Filters in VLSI" in IEEE Jnl. of Solid-State Circuits, Vol. SC-21, No. 1, Febr. '86, pp. 15-29, in which a number of such adjustments are described. The junction capacitors C₁ and C₂ exhibit non-linearities, causing distortion of the output signal. As a result of the symmetrical construction of the arrangement and the balanced drive of this arrangement these distortions are found to cancel one another in the arrangement. Thus, the range in which the balanced output signal increases as a linear function of the balanced input voltage is extended. Fig. 1b shows a second basic diagram of a filter arrangement in accordance with the invention constructed as an integrator, identical parts bearing the same reference numerals as in Fig. 1a. In the arrangement shown in Fig. 1b the cathodes of junction capacitors C₁ and C₂ are connected to the non-inverting input 2 and the inverting input 3 of the amplifier 1 instead of the anodes, as in the arrangement of Fig. 1a. The current sources I₁ and I₂ are now connected to the positive power-supply terminal instead of to the negative power-supply terminal. Across the resistors R₁ and R₂ the current sources I₁ and I₂ produce a voltage which is an amount V c = IR higher than the voltage on the inputs 6 and 7. Consequently, this voltage also appears across the junction capacitors C₁ and C₂ and thus also determines the capacitance values of these capacitors. Further, the arrangement is identical to that shown in Fig. 1a. Fig. 1c shows a third basic diagram of a filter arrangement in accordance with the invention constructed as an integrator, identical parts again bearing the same reference numerals as in Fig. 1a. In addition to a first and a second input circuit the arrangement further comprises a third input circuit arranged between a third input terminal 8 and the non-inverting input 2 and comprising a resistor R₆ and a fourth input circuit arranged between a fourth input terminal 9 and the inverting input 3 and comprising a resistor R₇ identical to the resistor R₆. A second balanced input signal +v i2 , -v i2 is applied to the input terminals 8 and 9. The first feedback circuit between the inverting output 5 and the non-inverting input 2 comprises the parallel arrangement of a junction capacitor C₁ and a resistor R₈ and the second feedback circuit between the non-inverting output 4 and the inverting input 3 comprises an identical parallel arrangement of a junction capacitor C₂ and a resistor R₉. The reverse voltage across the capacitor C₁, which also determines its capacitance, is equal to the voltage produced across the parallel arrangement of the resistors R₁, R₆ and R₈ by the current source I₁. Similarly, the reverse voltage across the capacitor is equal to the voltage produced across the parallel arrangement of the resistors R₂, R₇ and R₉ by the current source I₂. As is shown in Fig. 1c a large number of input circuits may be connected to the non-inverting input 2 and the inverting input 3. In addition, the outputs of a number of such arrangements may be connected directly to these inputs. In all these cases the reverse voltages across the junction capacitors are determined by the voltage produced across the resistors connected to the relevant input of the amplifier by the relevant current source. It is to be noted that, in principle, the arrangements shown in Fig. 1 may be constructed as differentiators, in which case the junction capacitors must be arranged in the input circuits and the resistors in the feedback circuits. In these cases the voltage produced across the resistors by the current sources forms the bias voltage for the junction capacitors, defining their capacitances. Fig. 2 shows a suitable amplifier 1 for use in the filter arrangements of Fig. 1. The amplifier 1 comprises a differential pair of transistors T₁ and T₂ whose bases constitute the non-inverting input 2 and the inverting input 3, whose collectors are connected to the positive power-supply terminal, and whose emitters are connected to the bases of two transistors T₅ and T₆ which are arranged as a differential pair whose collectors are cross-coupled to the bases and whose emitters are connected to a current source I₃ via a common resistor R₃. Further, the amplifier comprises two transistors T₃ and T₄ arranged as a differential pair, whose bases are connected to the emitters of the transistors T₁ and T₂ and whose emitters are connected to the current source I₃. The collectors of the transistors T₄ and T₃ constitute the non-inverting output 4 and the inverting output 5 of the amplifier. Between the outputs 4 and 5 a load circuit is arranged, comprising a PNP transistor T₇ which has its emitter connected to the positive power-supply terminal, its collector to the output 5, and its base to the base of a PNP transistor T₈, whose emitter is connected to the positive power-supply terminal and whose collector is connected to the output 4. The common base of the transistors T₇ and T₈ is connected to junction point between two identical resistors R₄ and R₅ via a diode D₁, which resistors are arranged between the outputs 4 and 5. Instead of a diode, it is also possible to use a transistor whose base is at a reference voltage, or an operational amplifier having one input connected to the junction point between the resistors R₄ and R₅ and its other input to a reference-voltage source. It is to be noted that this amplifier is the subject of a Patent Application EP-A- 0 270 156, published 08.06.88 and filed simultaneously with the present Application. In this respect it is observed only that the balanced input voltage applied to the bases 2 and 3 is converted into a balanced current by the differential pair T₁, T₂, the differential pair T₅, T₆ providing an increased slope of the arrangement. The differential pair T₃, T₄ provides additional current gain, which is defined by the value of the resistor R₃, which determines the ratio between the bias currents of the differential pair T₃, T₄ and the differential pair T₅, T₆. The load circuit maintains the direct voltage on the outputs 4 and 5 accurately constant. The transistors T₇ and T₈ compensate for the bias currents of the transistors T₃ and T₄, so that only the signal currents appear on the outputs 4 and 5. The resistors R₄ and R₅ have comparatively high resistance values, so that only a very small fraction of the signal current flows through these resistors and substantially the entire signal current is effectively available on the outputs 4 and 5. Since the signal current on the outputs 4 and 5 remains accurately balanced, no signal voltage will appear on the junction point between the resistors R₄ and R₅. Therefore, this junction point constitutes a virtual signal-earth. By way of illustration Fig. 3a shows two coupled filter arrangements in accordance with the invention. Identical parts bear the same reference numerals as in Fig. 1, the parts of the second filter arrangement being primed. The input terminals 6′ and 7′ of the second filter arrangement are connected to the output terminals 5 and 4 of the first filter arrangement. It is to be noted that alternatively the inputs of the second filter arrangement may be cross-coupled to the outputs of the first filter arrangement. If the amplifiers are constructed as shown in Fig. 2, regardless of how the entire filter arrangement is constructed, an input resistor and a load resistor are at any rate connected to each output. In fig. 3b these resistors are shown separately. The load resistors R₅ and R₄ of the first filter arrangement have large resistance values relative to the input resistors R₁′ and R₂' of the second filter arrangement, so that substantially the entire output signal current flows through the resistors R₁′ and R₂′. In order to obtain comparatively large RC-time constants, it is necessary to use comparatively large resistors R₁′ and R₂′. If these resistors are constructed as diffused bipolar resistors, they will behave as distributed RC networks as a result of the parasitic capacitances present between the resistors and the substrate. At higher frequencies this gives rise to a phase shift of the signal, which adversely effects the high-frequency characteristics of the filter arrangement. The influence of these parasitic capacitances can be reduced substantially if each of the resistors R₁′ and R₂′ is constructed as a diffused resistor and each of the resistors R₄ and R₅ is constructed as a buried resistor arranged in parallel therewith. As already stated, the junction point between the resistors R₄ and R₅ constitutes a virtual signal-earth. The signal voltage on the non-inverting input 2′ and the inverting input 3′ is very small, so that these inputs may also be regarded as virtual signal-earths. Therefore, substantially the same signal voltage appears across the resistors R₁′ and R₅ and across the resistors R₂′ and R₄. As a result of this, there is hardly any signal voltage across the parasitic capacitances between the resistors R₁′ and R₅ and between the resistors R₂′ and R₄, so that these capacitances do not give rise to a phase shift of the signal. The parasitic capacitances between the buried resistor and the substrate hardly affect the behaviour of the arrangement. Fig. 3c is a diagrammatic sectional view of the integrated circuit for the set of resistors R₄ and R₂′. On a P-type substrate a P-SUB an N-type epitaxial layer N-EPI is deposited. In this layer a P-type diffused or implanted resistor SPB is formed with low-impedance end portions SP, constituting the resistor R₂′. Over this resistor an insulating layer ISO is arranged, in which metallised apertures MET1 and MET2 are formed at the location of the resistor end portions SP. One terminal MET1 is connected to the output 4 of the first filter arrangement and the other terminal MET 2 is connected to the input 2′ of the second filter arrangement. In the island beneath the resistor SPB a partly buried layer BN is formed. The high-impedance region N-EPI between the two portions of the buried layer BN constitutes the resistor R₄. In the insulating layer ISO two metallised apertures MET3 and MET4 are formed, which are each connected to one terminal of the resistor R₄ by means of a deep N-type diffusion DN and a portion of the buried layer BN. The terminal MET3 is connected to the output 4 of the first filter arrangement and the terminal MET 4 is connected to the junction point between the resistors R₄ and R₅. If the filter resistors are constructed as diffused resistors the filter arrangement can be fabricated completely by means of a bipolar process. However, alternatively the filter arrangement may be fabricated in a BIMOS or BICMOS process. These are processes in which both bipolar transistors and MOS transistors can be fabricated. In the present example the junction capacities are constructed as bipolar PN junctions, whilst the resistors may then be constructed as MOS transistors. Such MOS transistors also exhibit parasitic capacitances which adversely affect the behaviour of the filter arrangement. The channel-gate capacitance is the most important of these parasitic capacitances. This parasitic capacitance causes a MOS transistor to behave as a distributed RC-network in the same way as a diffused resistor, causing the signal to be phase-shifted. This phase shift can be compensated for by providing a compensation capacitor C p between the source and the drain of the transistor, as is shown in Fig. 3b. It is not necessary to arrange a compensation capacitor across the load resistors R₄ and R₅, because these resistors are not included in the signal path of the arrangement. For an optimum compensation the compensation capacitor has a value which is substantially equal to one sixth of the channel-gate capacitance. The MOS resistors may have fixed resistance values, but it is alternatively possible to make their resistance values adjustable by means of a control voltage applied to the gate of the MOS transistor. The RC-time constant of such a filter arrangement can then be adjusted to the desired value both by adjusting the capacitance values of the junction capacitors and by adjusting the resistance values of the MOS-resistors. Preferably, the gates of the load resistors R₄ and R₅ are then connected to the gates of the resistors R₁′ and R₂′. When the control voltage varies the ratio between the resistance values R₁′, R₂′ and R₄, R₅ is maintained, so that the ratio between the signal currents through these resistors also remains constant. If the resistors are diffused resistors, the parasitic capacitances between these resistors and the substrate can be compensated for by arranging a compensation capacitor C p across the resistor. This compensation capacitor C p provides first-order compensation. As is shown in Fig. 3e, second-order compensation can be obtained by arranging a resistor R p in series with the capacitor C p . Suitably, this resistor R p has a resistance value which is substantially equal to 0.7 × the resistance value of the resistor to be compensated.

Claims: A filter arrangement comprising a balanced amplifier having an inverting input (3), a non-inverting input (2), an inverting output (5) and a non-inverting output (4), at least a first input circuit arranged between a first input terminal (7) and the inverting input (3), at least a second input circuit arranged between a second input terminal (6) and the non-inverting input (2), a first feedback circuit arranged between the non-inverting output and the inverting input, and a second feedback circuit arranged between the inverting output and the non-inverting input, which first and second input circuit each comprise a substantially identical first circuit element, which first and second feedback circuits each comprise a substantially identical second circuit element, one of said two circuit elements being a capacitor (C₁, C₂) and the other circuit element being a resistor (R₁, R₂) and the magnitude of at least one of said two circuit elements being adjustable, characterized in that one element is constituted by a junction capacitor (C₁, C₂) and in that a bias-current source (I₁, I₂) is connected to the inverting input (3) and to the non-inverting input (2) for adjusting the capacitance of the junction capacitor (C₁, C₂) connected to the relevant input. A filter arrangement as claimed in Claim 1, characterized in that the first circuit element is a resistor (R₁, R₂) and the second circuit element is a junction capacitor (C₁, C₂). A filter arrangement as claimed in Claim 3, characterized in that the first circuit element is a junction capacitor (C₁, C₂) and the second circuit element is a resistor (R₁, R₂). A filter arrangement as claimed in Claim 1, 2 or 3, characterized in that the amplifier comprises two transistors (T₃, T₄) of a first conductivity type which are arranged as a differential pair and which have their bases coupled to the non-inverting input (2) and the inverting input (3) of the amplifier and their collectors to the inverting output (5) and the non-inverting output (4) of the amplifier, between which outputs a load circuit is arranged, which load circuit comprises a first and a second current-source transistor (T₇, T₈) of a second conductivity type, whose collectors are connected to the non-inverting output (4) and the inverting output (5), whose bases are connected to a common control input which is coupled to the junction point between two load resistors (R₄, R₅) which are arranged between the non-inverting output and the inverting output. A filter arrangement as claimed in Claim 4, where appendant to Claim 2, characterized in that the input terminals (6', 7') of a second filter arrangement are connected to the outputs of the first filter arrangement and in that a resistor (R'₁, R'₂) arranged in an input circuit of the second filter arrangement and the load resistor (R₄, R₅) which forms part of the first filter arrangement and which is connected to said resistor are integrated in one semiconductor body respectively as a diffused resistor and a buried resistor arranged in parallel therewith. A filter arrangement as claimed in Claim 1, 2, 3 or 4, characterized in that a compensation capacitor (C p ) is arranged in parallel with the resistors in the input circuits or feedback circuits. A filter arrangement as claimed in Claim 6, characterized in that the compensation resistor (R p ) is arranged in series with the compensation capacitor. A filter arrangement as claimed in Claim 6 or 7, characterized in that the resistors are diffused or implanted resistors. A filter arrangement as claimed in Claim 6 or 7, characterized in that resistors are MOS resistors.

Inventors: VOORMAN, JOHANNES OTTO
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H03H11
IPC codes: H03H11, H03F3