Filter circuit

Abstract: A fully differential circuit in which all inputs and outputs of transconductors Gm1+, Gm1-, Gm2+, Gm2- and fixed gain amplifier GA are fully differential signals. The number of feedback loop elements is made odd by the addition of a fixed gain amplifier with a gain of 1 which is not used in conventional filter architecture, each element of the configuration consequently inverts the common voltage, and since the feedback loop has negative feedback with respect to the common voltage, the operating point can be determined without a need for a dedicated DC feedback circuit.

Description: The present invention relates to a filter circuit used as a basic circuit when processing analog signals in a MOS integrated circuit (IC). In recent years, due to the increase in digital devices and advances in digital signal processing technology, CMOS integrators applied for digital signal processing account for a large portion of the semiconductor market. However, since video and audio signals have analog input/output, they can be more easily processed by analog processing. Even when video and audio signals are digitally processed, analog circuits are required for A/D and D/A conversion and filtering carried out before or after conversion and in a clock-generating oscillator and such like. Conventionally, bipolar technology has been regarded as suitable for analog circuits, whereas CMOS technology is regarded as unsuitable except for some circuits such as analog switches, sample holders or such like. However, bipolar and BiCMOS processing are rather expensive, and in view of the strong demand for a 1-chip CMOS structure achieved by digital/analog consolidation, there has been an increase in development of circuits for processing analog signals with CMOS circuitry. Analog signal processing features an important function known as an "active filter" which has a high frequency of use and exerts considerable influence on total performance. Conventionally, discrete-time processing filters such as switched capacity filters (SCF) or sample data filters have been the main active filters used in CMOS analog technology. While these filters have the advantage of high precision, since frequencies are fixed precisely according to a clock, and have low sensitivity to capacitor variations originating during manufacturing, these filters have the following disadvantages: 1. continuous-time filters are required before and after due to the existence of "aliasing"; 2. since a frequency band several times greater than frequency used for operational amplifier and sample-and-hold (S/H) circuit is required, the filter cannot be used for frequencies higher than the video band. 3. circuit scale is large and therefore not economical. Therefore, it is not possible to produce a simple and inexpensive filter which can be used at high frequencies. This problem cannot easily be resolved since it derives directly from the fact that the filter is a discrete-time filter. Recently, attempts are under way to develop a high-performance continuous-time CMOS filter. The most popular continuous-time filter is a "biquad circuit", consisting of two integrators each comprising a transconductor (Gm) circuit and a capacitor, in which a second-order filter is multistage-connected in order to obtain desired filtering properties. In a bipolar technology, transconductor (voltage-current conversion characteristic) is linearized its characteristic using a resistor and a "gain cell structure of transistors". However, when the above method is used in CMOS, since element Gm (transconductance) is small, many elements of enormous size are required, which is extremely uneconomical. Transconductance properties are therefore obtained by performing voltage-to-current conversion using a differential pair-transistor direct-coupled to the source. However, in a MOS transistor, gate-source voltage versus drain current is a square relationship and the integration property of capacitor current versus voltage is linear, a single output of the integrator has second-order distortion. It is therefore necessary to cancel second-order distortion using a fully differential input and output signals. In such fully differential processing, since the output DC voltage cannot usually be determined, a bias circuit must be added to determine the DC operating point. Generally, therefore, output DC voltage is detected and DC feedback technique is applied to the bias of the transconductor input. This method is called DC feedback (or common-mode feedback or such like). Fig. 6 shows a conventional example of this type of fully differential biquadratic circuit provided with DC feedback. This circuit is made up of integrators in two stages, each stage consisting of two transconductors sharing a common output terminal and a capacitor connected to said output terminal. Lower transconductors Gm1+ and Gm2+ correspond to + input in a single configuration; upper transconductors Gm1- and Gm2- correspond to - input (feedback input) in a single configuration. Two-stage integrators are cascade-connected as above to form a low-pass filter or such like. The output DC voltage is controlled to a predetermined voltage by monitoring the output of DC feedbacks 1 and 2 at each integrator and controlling the bias current of the output terminal in each stage. In addition to an LPF, other filter types such as BPF and HPF can be created by modifying the signal input position and output signal extraction position. Fig. 7 shows a concrete example of integrators in the stages forming the filter in Fig. 6 realized using CMOS circuitry. A pair of source-coupled differential transconductors are used to produce transconductance. The differential circuit comprising M1, M2 and I1 corresponds to transconductor Gm1+ or Gm2+; the differential circuit comprising M3, M4 and I2 corresponds to transconductor Gm1- or Gm2-. The current outputs are summed at the output point. Output is differentially extracted by biasing with two current supplies I3, 14 which join at GND. With respect to differential input bias currents I1 and I2, these current supplies I3, I4 must be precisely (I1 + I2) / 2 respectively. If this relationship is evenly slightly disrupted, since the DC impedance of the output terminal of each stage is extremely high, the unbalance between the upper and lower current supplies causes the output DC voltage to be greatly disrupted, leading to instability. "DC feedback" is a circuit designed to counter this problem, ensuring stability by fixing output DC potential at a certain voltage. One of the resistor terminals is connected to the output terminal in Fig. 7. The other terminal is now connected each other, and is compared by the operational amplifier with the intended voltage Vref. When all output signal are fully differential and the two resistors are equal values, the DC potential of the output signal can be extracted from the center point and compared with Vref. When the DC potential is higher than Vref, the current from the current supply is increased and the common voltage of the output is lowered. Conversely, when the DC potential is lower than Vref, the current from the current supply is decreased, raising the common voltage of the output. Thus current is controlled so that the common voltage of the output signal equals Vref. The circuit is biased so that the DC voltages of differential outputs in each stage of the filter equal Vref, but the number of circuit elements is likely to be increased. Not only does the operational amplifier itself require a considerable number of elements, but a buffer circuit and such like must be provided as shown in the diagram in order to prevent the resistors which detect center point voltage from influencing the high-impedance integrator output terminals. Moreover, a DC feedback circuit is required for each integrator, thereby taking up a large portion of the overall area of the filter. In the circuit shown in Fig. 7, for instance, the essential portion of the integrator within the dotted line on the left side comprises 4 MOS transistors, 4 current supplies and 1 capacitor, thus requiring approximately 10 - 15 elements. By contrast, a DC feedback circuit for setting bias has as many as 20 - 30 elements, thereby taking up two thirds of the area. Since the filter is formed simply by assembling these elements, it follows that the DC feedback circuit takes up roughly two thirds of the total filter area. This need for a DC feedback has resulted in increased costs for fully differential filters and has been an obstacle for producing an inexpensive filter. Steyaert M et al.: "A 10.7 MHz CMOS OTA-R-C bandpass filter with 68dB dynamic range and on-chip automatic tuning", IEEE International Solid State Circuits Conference, New York, vol. 35, 1 February 1992, pages 66-67 and 245 discloses a filter circuit comprising a first circuit block having a first differential voltage input, a second circuit block having a second differential voltage and a differential voltage output, both circuit blocks are connected within a loop. A capacitor is connected to the differential current output. The circuit blocks of this document are ordinary differential amplifiers, which reject the common-mode voltage input to the input ends. An object of the present invention is to provide a fully differential filter circuit which does not require DC feedback circuit. According to the present invention the above object is achieved by an filter circuit according to claim 1. The dependent claims are related to further advantageous aspects of the invention. One aspect of the present invention is to provide a filter circuit comprising a first circuit block, having a first differential voltage input and a differential current output, and a common voltage of the first differential voltage input being inverted at the output, and a second circuit block, having a second differential voltage input and a differential voltage output, and a common voltage of the second differential voltage input being inverted at the output, wherein the first and second circuit blocks form one of building elements on a feedback loop, wherein a total number of the building elements formed with the first and second circuit blocks is odd including one and wherein a capacitor is connected to the differential current output. In the filter circuit, a plurality of at least the transconductor and the capacitor and the current supply are used and a given combination of these is mutually connected. The building element is not limited to the first circuit block and the second circuit block. Typically, the transconductor is referred to as Operational Transconductance Amplifier (OTA). In the specification, the term "input" means "a pair of input terminals" and the term "output" means "a pair of output terminals." According to the first aspect of the filter circuit, the first circuit block is a transconductor which has a certain transconductance for the first differential voltage input and has high negative gain for the common voltage of the first differential voltage input, and the second circuit block is an amplifier which has a certain gain for the second differential voltage input and has a negative gain at the same level as the certain gain for the common voltage of the second differential voltage input. Further according to the above filter circuit, the transconductor has a pair of first field effect transistors, connects a pair of sources of the first transistors to a common fixed-voltage terminal; a pair of gates of the first transistors are used as input terminals of the transconductor; a pair of.drains of the first transistors are connected to a current source and are used as output terminals; and the amplifier has two pair of second field effect transistors having a pair of input side second transistors and a pair of output side second transistors; a pair of sources of the input side second transistors are connected to the common fixed-voltage terminal; a pair of gate of the input side second transistors are used as input terminals; a pair of drains of the input side second transistors are respectively connected to a pair of sources of the output side second transistors; a pair of gates and a pair of drains of the output side second transistors are connected to at least one common fixed voltage terminal. Still further according to the above filter circuit, a filter frequency property is proportionally adjusted by controlling all of the current sources with their mutual current ratio being constant. Second aspect of the present invention is to provide a filter circuit comprising a transconductor, having a first differential voltage input and a differential current output, having a certain transconductance for the first differential voltage input and having a high negative gain for a common voltage of the first differential voltage input, an amplifier, having a second differential voltage input and a differential voltage output, having a certain gain for the second differential voltage input, and having a negative gain at the same level as the certain gain for the common voltage of the second differential voltage input, a current source connected to the first differential voltage output of the transconductor, and a capacitor connected to the first different voltage output of the transconductor, wherein the transconductor and the amplifier forms a feedback loop that makes a round of a pair of nodes, input and output terminals of the transconductor or the amplifier and the pair of nodes, and wherein a total number of the transconductor and the amplifier which pass through the feedback loop is odd including one. According to the second aspect of the filter circuit, one transconductor and even number of the amplifiers exist in the feedback loop. Further according to the second aspect of the filter circuit, two transconductors and odd number of the amplifiers exist in the feedback loop. Alternatively, three capacitors may be connected to the two transconductors; one of the three capacitors may be connected to the differential current output of one of the transconductors and two of the three capacitors may be connected between the fixed voltage terminal and the differential current output of the transconductors. Third aspect of the present invention is to provide a filter circuit comprising first, second, third and fourth transconductors, each having a differential voltage input and a differential current output, each having a certain transconductance for the differential voltage input and each having a high negative gain for a common voltage of the differential voltage input, and an amplifier, having a differential voltage input and a differential voltage output, having a certain gain for the differential voltage input and having a negative gain at the same level as the certain gain for the common voltage of the differential voltage input, wherein the differential current output of the first transconductor is connected to the differential current output of the second transconductor via a first pair of nodes; the nodes are connected the differential voltage input of the amplifier, and each node is connected to a capacitor and a current source, wherein the differential current output of the third transconductor is connected to the differential current output of the fourth transconductor via a second pair of nodes; each node is connected to a capacitor and a current source, and the nodes are connected to the differential voltage inputs of the second and the fourth transconductors with being inverted the differential current outputs of the third and the fourth transconductors, wherein the differential voltage output of the amplifier is connected to the differential voltage input of the third transconductor, and wherein the differential voltage input of the first transconductor is used as a filter input. Alternatively, the differential voltage input of the amplifier may be connected to the differential voltage input of the fourth transconductor, the differential voltage output of the amplifier may be connected to the differential input voltage of the second transconductor, and the differential current output of the first transconductor may be connected to the differential input voltage of the third transconductor. Fourth aspect of the present invention is to provide a filter circuit comprising first, second and third transconductors, each having a differential voltage input and a differential current output, each having a certain transconductance for the differential voltage input and each having a high negative gain for a common voltage of the differential voltage input, and an amplifier, having a differential voltage input and a differential voltage output, having a certain gain for the differential voltage input and having a negative gain at the same level as the certain gain for the common voltage of the differential voltage input, wherein the differential current output of the first transconductor is connected to a pair of capacitors and a first pair of current sources via a first pair of nodes, the nodes are connected to the differential voltage input of the amplifier, wherein the differential current output of the second transconductor is connected to the differential current output of the third transconductor via a second pair of nodes; the nodes are connected to the differential voltage inputs of the first and the third transconductors with being inverted the differential current outputs of the second and third transconductors, a capacitor is connected between the second pair of nodes, and a second pair of current sources is connected to the nodes, wherein the differential voltage output of the amplifier is connected to the differential voltage input of the second transconductor, and wherein the pair of capacitors is used as a filter input. Alternatively, the differential voltage input of the amplifier may be connected to the differential voltage input of the third transconductor, the differential voltage output of the amplifier may be connected to the differential voltage input of the first transconductor, and the differential current output of the first transconductor may be connected to the differential voltage input of the second transconductor. Fifth aspect of the present invention is to provide a filter circuit comprising first, second and third transconductors, each having a differential voltage input and a differential current output, each having a certain transconductance for the differential voltage input and each having a high negative gain for a common voltage of the differential voltage input, and an amplifier, having a differential voltage input and a differential voltage output, having a certain gain for the differential voltage input and having a negative gain at the same level as the certain gain for the common voltage of the differential voltage input, wherein the differential current output of the first transconductor is connected to a capacitor and a first pair of current sources via a first pair of nodes, the nodes are connected to the differential voltage input of the amplifier, wherein the differential current output of the second transconductor is connected to the differential current output of the third transconductor via a second pair of nodes; the nodes are connected to the differential voltage inputs of the first and the third transconductors with being inverted the differential current outputs of the second and third transconductors, a pair of capacitor and a second pair of current sources is connected to the nodes, wherein the differential voltage output of the amplifier is connected to the differential voltage input of the second transconductor, and wherein the pair of capacitors is used as a filter input. Alternatively, the differential voltage input of the amplifier may be connected to the differential voltage input of the third transconductor, the differential voltage output of the amplifier may be connected to the differential voltage input of the first transconductor, and the differential current output of the first transconductor may be connected to the differential voltage input of the second transconductor. Sixth aspect of the present invention is to provide a high-order filter circuit comprising a combination of the above-described filter circuits. Fig. 1 is a diagram depicting a circuit configuration explaining a first embodiment of the present invention; Fig. 2 is a diagram depicting a transconductor used in a filter circuit of the present invention; Fig. 3 is a diagram depicting a fixed gain amplifier used in a filter circuit of the present invention; Fig. 4 is a diagram depicting a circuit configuration explaining a second embodiment of the present invention; Fig. 5 is a diagram depicting a circuit configuration explaining a third embodiment of the present invention; Fig. 6 is a diagram depicting a circuit configuration explaining a conventional second-order biquad; and Fig. 7 is a diagram depicting a circuit explaining the configuration of the fully differential filter in Fig. 6. The preferred embodiments of the present invention will next be explained with reference to the diagrams. Fig. 1 shows a circuit configuration explaining a first embodiment of the present invention. In the first embodiment, a low-pass filter (LPF) consists of two pairs of transconductors Gm1+ (first), Gm1- (second) and Gm2+ (third), Gm2- (fourth), a gain amplifier GA and integrating capacitors. Input Vin connects to the positive and negative inputs of transconductor Gm1+. Positive output of transconductor Gm1+ connects to the positive output of transconductor Gm1- and the positive input of fixed gain amplifier GA which has a gain coefficient K of 1 and connects to the reference potential node via integrator capacitor C1 and current supply Il which are connected in parallel. The negative output of transconductor Gm1+ connects to the negative output of transconductor Gm1- and the negative input of fixed gain amplifier GA and connects to the reference potential node via integrator capacitor C1 and current supply 12 which are connected in parallel. Positive output of fixed gain amplifier GA connects to the positive input of transconductor Gm2+. The negative output of fixed gain amplifier GA connects to the negative input of transconductor Gm2+. Positive output of transconductor Gm2+ connects to the positive output of transconductor Gm2- and connects to the reference potential node via current supply I3. Negative output of transconductor Gm2+ connects to the negative output of transconductor Gm2- and connects to the reference potential node via current supply I4. Positive and negative outputs of transconductor Gm2+ connect to both terminals of integrator capacitor C2 and to output Vout. The positive output of transconductor Gm2+ connects to the negative inputs of both transconductors Gm1- and Gm2-. The negative output of transconductor Gm2+ connects to the positive inputs of both transconductors Gm1- and Gm2-. The positive and negative inputs of the transconductors refer to the differential voltage input, and the positive and negative outputs of the transconductors refer to the differential current output. The positive and negative inputs of the amplifier refer to the differential voltage input, and the positive and negative outputs refer to the differential voltage output. The features of this configuration are firstly that the circuit is a fully differential one, in which all inputs and outputs of transconductors Gm1+, Gm1-, Gm2+ and Gm2- and fixed gain amplifier GA are fully differential signals, and secondly that the number of these building elements on the feedback loop is 3. The number of elements is increased to an odd number by inserting a fixed gain amplifier GA with a gain of 1 which does not usually feature in conventional filter topology. Consequently, since each element of the circuit inverts the common voltage as explained below, this feedback loop has negative feedback for the common voltage; the operating point can therefore be determined without the need for a dedicated DC feedback (common mode feedback) circuit. Each stage consists of two transconductors and an integrating capacitor connected to the output terminals thereof. Transconductors Gm1+ and Gm2+ correspond to + input in a single configuration; transconductors Gm1- and Gm2- correspond to - input (feedback input) in a single configuration. The gm value of each transconductor is as follows: Gm1+, Gm1- : gm1 Gm2+, Gm2- : gm2 The transmission coefficient between input and output in the circuit is determined by: Vo VI = (gm1gm2) / (C1C2) S + {(gm2) / (C2)} S+ (gm1gm2) / (C1C2) As is clear from this equation, the circuit of the present embodiment exhibits a second-order LPF effect since the zero point of the transmission coefficient exists in duplicate at infinite frequency. Each of the transconductors Gm1+, Gm1-, Gm2+ and Gm2- in Fig. 1 has the configuration shown in Fig. 2. In other words, each transconductor Gm1+, Gm1-, Gm2+ and Gm2- comprises source-coupled differential MOS transistors M1 and M2 whose source is directly connected to power supply, and inputs a differential signal of the previous stage to a gate. The drain terminals of these two transconductors are connected each other with the same polarity and integrating capacitors are connected to the positive and negative output terminals which then function as differential output terminals. Both these terminals are biased by the current supply. In order to investigate the operation of this circuit, the voltage-to-current conversion properties of differential MOS transistors M1 and M2 are measured. It is assumed here that all input signals are fully differential signals, and it is presumed that both transistors have identical properties and are biased in the saturation mode region. Ignoring output resistance and substrate body effects, the drain current of the two transistors can be defined as follows: M1 : I1 = (k1 / 2) (VGS1 - Vth1) 2 M2 : I2 = (k1 / 2) (VGS2 - Vth1) 2 In the above, k = µCoxW/L = µ (εox/tox) W/L, µ = electron mobility of the device, εox = permittivity of the gate oxide film, tox = gate oxide film thickness, wafer W is gate width, L is gate length. When output is the difference in the drain currents of differential MOS transistors M1 and M2, based on (1) and (2), output I1 - I2 can be determined by: I1 - I2 = (k1/2) (VGS1 + VGS2 - 2Vth1) (VGS1 - VGS2) = (k1/2) (VGS1 + VGS2 - 2Vth1) Vin When Vb denotes input signal DC voltage corresponding to the power supply voltage, the input signal is fully differential, and since the source terminals of differential MOS transistors M1 and M2 are fixedly connected to the power supply, VGS1 + VGS2 = 2Vb. Therefore: I1 - I2 = k1 (VB - Vth1) Vin Thus transconductance Gm from differential input voltage to differential output voltage can be defined as: Gm = Iout / Vin = (I1 - I2) / Vin = k1 (VB - Vth1) Equation (4) has no term dependent on input signal Vin nor term "Vin 2 " which can cause second-order distortion. If the pair elements are matched completely (M1 = M2), Gm is ideally determined by the W/L ratio of the elements. Therefore, as long as output is extracted differentially, transconductance is fixed and is not dependent on input. Furthermore, deviations in Vth1 can be adjusted by controlling the input bias voltage Vb. Moreover, equation (4) shows that the Gm value can be actively controlled in accordance with Vb. A transconductor as above which is source-grounded and fully differential solves the problem of second-order distortion liable to occur with a CMOS analog circuit and produces highly linear transconductance. Furthermore, since current supply need not be placed at the differential source connection point, it is possible to used only the voltage portion of the current supply as power voltage, thereby giving the advantage of voltage reduction. Fig.2 shows an example in which PMOS was used in a differential transistor, but exactly the same integrator circuit can be formed when NMOS is used in a differential transistor with the power and the GND reversed, and the same effects can be obtained. Fig. 3 shows an actual circuit in which the fixed gain amplifier GA in Fig. 1 is set at K =1. In other words, MOS transistors M3 and M4 are provided as loads for differential MOS transistors M1 and M2 which are the same transistor pair as the transconductor of Fig. 2. The previous stage differential signal is input to gates of MOS transistors M1 and M2. Here, when voltage-to-current conversion properties of differential MOS transistors M1 and M2 are measured under the same conditions as in the calculation above, both M1 and M2 were found to have the same operating properties as was determined in equations (1) and (2) respectively. Similarly, operating properties (drain voltages) of M3 and M4 can be defined by the following equations: M3 : I1 = (k2 / 2) (VGS3 - Vth2) 2 M4 : I2 = (k2 / 2) (VGS4 - Vth2) 2 By solving equations (1), (2), (5) and (6) simultaneously, the gain from differential input voltage to differential output voltage can be determined by: Vout / Vin = (VGS3 - VGS4) / (VGS1 - VGS2) = / (k1 / k2) Equation (7) shows that when the pair of elements are perfectly matched (M1 = M2, M3 = M4), gain can ideally be determined simply from the geometric factor W/L of the elements; in principle, differential signals produce an amplifier having fixed gain with no distortion. Alternatively, for common signals, the amplifier is source-ground and has gain roughly equal to the gain for a differential signal but is phase-inverted. For differential signals, connection to the next stage determine a phase polarity of the amplifier to output negative-phase, but for a common signal, the amplifier is always phase inverted. In the circuit shown in Fig. 1, a common signal is inverted after this amplifier. The source of the transconductor shown in Fig. 2 is directly grounded. Since the source is not connected to the current supply for biasing, the common voltage of an input signal cannot be removed. In fact, utilizing this feature eliminates the need for a dedicated DC feedback circuit as shown in the filter circuit in Fig. 1. A differential circuit comprising MOS transistors M1 and M2 has high gain between input and output with respect to common voltages because the sources of M1 and M2 are fixed by the power supply. In other words, if input gate voltages both increase, output drain voltages will greatly decrease. Conversely, if input gate voltages both decrease, output drain voltages greatly increase. Moreover, the fixed gain amplifier shown in Fig. 3 has inverse gain with respect to common voltages as already explained. The transmission of common-voltages in the Fig. 1 circuit is measured. This circuit has a feedback loop returning along the path: Gm2+ ~ Gm1- ~ 1 ~ Gm2+. When we consider the transmission of common voltages along this loop it is clear that, since each constituent of the configuration inverts a total of three times before returning, the loop has negative feedback. In other words, the output voltage of each constituent of the configuration stabilizes to a voltage determined according to the current value of the bias current supply on the GND side. For instance, when the shapes of the MOS transistors forming each transconductor are all equal and the GND side bias current values biasing the transconductor are all equal, the output voltages of each stage will all be equal. I1 and I2 have half the current value of each current supply, these voltages are equal to the VGS value determined by entering these-current values into equations (1) and (2). However, this convergence voltage must be predicted beforehand and a common voltage close to that convergence voltage must be given as input Vin. Since each field effect transistor self-biases to a current value determined with the drain side current supply, there is no need to provide a dedicated DC feedback circuit. Therefore, the number of elements in the filter circuit of the present embodiment can be greatly reduced to obtain a low-cost CMOS filter. The above example described a filter circuit consisting of two stages, but by adjusting the number of fixed gain amplifiers on the feedback loop, common voltage transmission with negative feedback can be realized in a filter circuit with an increased number of stages. Therefore, by combining the transconductor shown in Fig. 2 with the fixed gain amplifier in Fig. 3, a low-cost, small-scale filter having given properties can be produced with no need for a dedicated DC feedback circuit. This type of common feedback loop is susceptible to voltage oscillation. Since the "Gm2+ ~ Gm1- ~ 1 ~ Gm2+" loop in Fig. 1 has two high-gain stages Gm2+ and Gm1-, phase compensation is essential to prevent oscillation resulting from high-frequency phase delay. In Fig. 1, this is achieved by dividing Gm1 output terminal capacitor C1 into 2 portions and connecting each portion to a corresponding GND. In this way, while 2C1 has a capacitive load with respect to common voltage at the output terminal of transconductor Gm1, there is no capacitive load with respect to common voltage at the output terminal of transconductor Gm2. Therefore, phase compensation in which Gm1 and 2C1 are used to create 1 low-frequency pole can be provided to the common feedback loop. Since the capacitor defines filter characteristics and also provides phase compensation for the common feedback loop, this method has the advantage of being highly economical. Furthermore, when the current values of all current supplies which bias the output terminal of each stage are proportioned, the VGS of the MOS transistors forming each transconductor are changed at the same time. This signifies a change in VB in equation (4) and enables the Gm of each transconductor to be proportionally controlled as explained above. As a consequence, filter characteristics can be shifted on the frequency axis, which is particularly effective for adjusting time constant variation caused by variation in IC manufacturing processes. Next, a second embodiment of the present invention will next be explained with reference to the circuit configuration shown in Fig. 4. The second embodiment differs form the first depicted in Fig. 1 in that transconductor Gm1+ is removed, capacitors C1 and C2, which were connected to the reference voltage point, are now connected to input Vin, and transconductors Gm1- (first), Gm2+ (second) and Gm2- (third) are remained. In this case, it can be easily surmised that there is no DC transmission from input to output and the filter becomes a band-pass filter (BPF). As in the first embodiment, since the common transmission loop is exactly the same as that in Fig. 1, stable self-bias is similarly produced by means of a common voltage feedback loop and there is no need for a dedicated DC feedback circuit. However, in the second embodiment, in comparison with Fig. 1, current supplies I1 and I2 for the portion from which transconductor Gm1+ has been removed may acceptably have low current values. Fig. 5 shows a circuit configuration of a third embodiment of the present invention. The third embodiment differs from the embodiment shown in Fig. 4 in that input is connected to output Vout via capacitors C2 and C2 and both terminals of capacitor C1 are connected between the input terminals of fixed gain amplifier GA. Transconductors Gm-1 (first), Gm2+ (second) and Gm2- (third) are remained. When a signal is input in this configuration, since the zero point of the transmission coefficient exists in duplicate at zero frequency, the circuit exhibits the effects of a high-pass filter (HPF). Since the common transmission loop is exactly the same as that in Fig. 4, stable self-bias is similarly produced by means of a common voltage feedback loop and there is no need for a dedicated DC feedback circuit. According to the present invention as described in embodiments 1 ~ 3, it is possible to create a given high-order filter circuit by cascade-connecting the main properties of three types of filter circuits or alternative types of the filter circuits according to the present invention. This further utilizes the advantage of the small-scale and low-cost filter provided by the present invention.

Claims: A filter circuit comprising: a transconductor (Gm1,Gm2) having a pair of differential voltage input ends and a pair of differential current output ends, the transconductor comprising a pair of first field effect transistors (M1,M2) having their gates connected to the differential voltage input ends of the transconductor and having their drains connected to a pair of current sources and to the differential current output ends of the transconductor, while their sources are commonly connected to a power supply, wherein the transconductor converts a first differential-mode input voltage into a differential-mode output current, while a common mode input voltage is outputted with its phase being inverted; an amplifier (GA) having a pair of differential voltage input ends and a pair of differential voltage output ends, the amplifier comprising two pairs of second field effect transistors, one pair of which are input side transistors having their gates connected to the differential voltage input ends of the amplifier and having their sources commonly connected to the power supply, while their drains are connected to the differential voltage output ends of the amplifier and to the sources of the other pair of second field effect transistors which have their gates commonly connected to a common fixed voltage source and their drains commonly connected to a reference potential, wherein the amplifier converts a second differential-mode input voltage into a differential-mode output voltage, while a common-mode input voltage is outputted with its phase being inverted; the transconductor and the amplifier form one building element in a feedback loop, wherein a total number of such building elements formed with the transconductor and the amplifier is odd including one, and wherein a capacitor is connected to the differential current output ends of the transconductor. A filter circuit as claimed in claim 1, wherein the frequency characteristic of the filter circuit is proportionally shifted on a frequency axis by controlling all the current sources connected to the drains of the pair of first field effect transistors, while their mutual current ratio remains constant. A filter circuit as claimed in claim 1, wherein the feedback loop is constituted by a single transconductor and an even number of amplifiers. A filter circuit as claimed in claim 1, wherein the feedback loop is constituted by a first transconductor (Gm1), a second transconductor (Gm2) and an odd number of amplifiers. A filter circuit as claimed in claim 4, wherein the differential current output ends of the second transconductor are connected to a first capacitor element (C2) as the capacitor there between, while the differential current output ends of the first transconductor are connected to a pair of capacitors having their other end connected to the reference potential. A filter circuit according to claim 1, wherein the feedback loop is constituted by the single amplifier, a first transconductor having a first pair of differential voltage input ends and a first pair of differential current output ends, and a second transconductor having a second pair of differential voltage input ends and a second pair of differential current output ends, the filter circuit further comprising: an input side transconductor having a pair of input ends and a pair of output ends, wherein the input side transconductor converts an input differential-mode voltage into an output differential-mode current, while an input common-mode voltage is outputted with its phase being inverted; and a third transconductor having a third pair of differential voltage input ends and a third pair of differential current output ends, wherein the third transconductor converts a third differential-mode input voltage into a third differential-mode output current, while a third common-mode input voltage is outputted with its phase being inverted, wherein the pair of output ends of the input side transconductor is connected to the first pair of differential current output ends of the first transconductor at a first pair of nodes, the first pair of nodes being further connected to the pair of differential voltage input ends of the amplifier and a pair of current sources, wherein the pair of differential voltage output ends of the amplifier is connected to the second pair of differential voltage input ends of the second transconductor, wherein the second pair of differential current output ends of the second transconductor is connected to the third pair of differential current output ends of the third transconductor at a second pair of nodes, the second pair of nodes being further connected to a first capacitor (C2) as the capacitor there between, a second pair of current sources, the first pair of differential voltage input ends of the first transconductor with inverted polarity, and the third pair of differential voltage input ends of the third transconductor with inverted polarity, and wherein the pair of input ends of the input side transconductor is used as an input of the filter circuit. A filter circuit according to claim 1, wherein the feedback loop is constituted by the single amplifier, a first transconductor having a first pair of differential voltage input ends and a first pair of differential current output ends, and a second transconductor having a second pair of differential voltage input ends and a second pair of differential current output ends, the filter circuit further comprising: an input side transconductor having a pair of input ends and a pair of output ends, wherein the input side transconductor converts an input differential-mode voltage into an output differential-mode current, while an input common-mode voltage is outputted with its phase being inverted; and a third transconductor having a third pair of differential voltage input ends and a third pair of differential current output ends, wherein the third transconductor converts a third differential-mode input voltage into a third differential-mode output current, while a third common-mode input voltage is outputted with its phase being inverted, wherein the pair of output ends of the input side transconductor is connected to the first pair of differential current output ends of the first transconductor at a first pair of nodes, the first pair of nodes being further connected to the second pair of differential voltage input ends of the second transconductor and a pair of current sources, wherein the second pair of differential current output ends of the second transconductor is connected to the third pair of differential current output ends of the third transconductor at a second pair of nodes, the second pair of nodes being further connected to a first capacitor element (C2) as the capacitor there between, a second pair of current sources, the third pair of differential voltage input ends of the third transconductor with inverted polarity, and the pair of differential voltage input ends of the amplifier with inverted polarity, wherein the pair of differential voltage output ends of the amplifier is connected to the first pair of differential voltage input ends of the first transconductor, and wherein the pair of input ends of the input side transconductor is used as an input of the filter circuit. A filter circuit according to claim 1, wherein the feedback loop is constituted by the single amplifier, a first transconductor having a first pair of differential voltage input ends and a first pair of differential current output ends, and a second transconductor having a second pair of differential voltage input ends and a second pair of differential current output ends, the filter circuit further comprising: a third transconductor having a third pair of differential voltage input ends and a third pair of differential current output ends, wherein the third transconductor converts a third differential-mode input voltage into a third differential-mode output current, while a third common-mode input voltage is outputted with its phase being inverted, wherein the first pair of differential current output ends of the first transconductor is connected to the pair of differential voltage input ends of the amplifier at a first pair of nodes, the first pair of nodes being further connected to a pair of current sources and a pair of capacitors (C1 C1), wherein the pair of differential voltage output ends of the amplifier are connected to the second pair of differential voltage input ends of the second transconductor, wherein the second pair of differential current output ends of the second transconductor is connected to the third pair of differential current output ends of the third transconductor at a second pair of nodes, the second pair of nodes being further connected to a first capacitor (C2) as the capacitor there between, a second pair of current sources, the third pair of differential voltage input ends of the third transconductor with inverted polarity, and the first pair of differential voltage input ends of the first transconductor with inverted polarity, and wherein the pair of capacitors' ends that is not connected to the first pair of nodes is used as an input of the filter circuit. A filter circuit according to claim 1, wherein the feedback loop is constituted by the single amplifier, a first transconductor having a first pair of differential voltage input ends and a first pair of differential current output ends, and a second transconductor having a second pair of differential voltage input ends and a second pair of differential current output ends, the filter circuit further comprising: a third transconductor having a third pair of differential voltage input ends and a third pair of differential current output ends, wherein the third transconductor converts a third differential-mode input voltage into a third differential-mode output current, while a third common-mode input voltage is outputted with its phase being inverted, wherein the first pair of differential current output ends of the first transconductor is connected to the second pair of differential voltage input ends of the second transconductor at a first pair of nodes, the first pair of nodes being further connected to a pair of capacitors (C1, C1) as the capacitor and a pair of current sources, wherein the second pair of differential current output ends of the second transconductor is connected to the third pair of differential current output ends of the third transconductor at a second pair of nodes, the second pair of nodes being further connected to a first capacitor (C2) as the capacitor there between, a second pair of current sources, the third pair of differential voltage input ends of the third transconductor with inverted polarity, and the pair of differential voltage input ends of the amplifier with inverted polarity, wherein the pair of differential voltage output ends of the amplifier is connected to the first pair of differential voltage input ends of the first transconductor, and wherein the pair of capacitors ends that is not connected to the first pair of nodes is used as an input of the filter circuit. A filter circuit according to claim 1, wherein the feedback loop is constituted by the single amplifier, a first transconductor having a first pair of differential voltage input ends and a first pair of differential current output ends, and a second transconductor having a second pair of differential voltage input ends and a second pair of differential current output ends, the filter circuit further comprising: a third transconductor having a third pair of differential voltage input ends and a third pair of differential current output ends, wherein the third transconductor converts a third differential-mode input voltage into a third differential-mode output current, while a third common-mode input voltage is outputted with its phase being inverted, wherein the first pair of differential current output ends of the first transconductor is connected to the pair of differential voltage input ends of the amplifier at a first pair of nodes, the first pair of nodes being further connected to a pair of current sources and a first capacitor (C1) as the capacitor there between, wherein the pair of differential voltage output ends of the amplifier is connected to the second pair of differential voltage input ends of the second transconductor, wherein the second pair of differential current output ends of the second transconductor is connected to the third pair of differential current output ends of the third transconductor at a second pair of nodes, the second pair of nodes being further connected to a pair of capacitors (C2, C2) as the capacitor, a second pair of current sources, the third pair of differential voltage input ends of the third transconductor with inverted polarity, and the first pair of differential voltage input ends of the first transconductor with inverted polarity, and wherein the pair of capacitors' ends that is not connected to the second pair of nodes is used as an input of the filter circuit. A filter circuit according to claim 1, wherein the feedback loop is constituted by the single amplifier, a first transconductor having a first pair of differential voltage input ends and a first pair of differential current output ends, and a second transconductor having a second pair of differential voltage input ends and a second pair of differential current output ends, the filter circuit further comprising: a third transconductor having a third pair of differential voltage input ends and a third pair of differential current output ends, wherein the third transconductor converts a third differential-mode input voltage into a third differential-mode output current, while a third common-mode input voltage is outputted with its phase being inverted, wherein the first pair of differential current output ends of the first transconductor is connected to the second pair of differential voltage input ends of the second transconductor at a first pair of nodes, the first pair of nodes being further connected to a first capacitor (C1) as the capacitor there between and a pair of current sources, wherein the second pair of differential current output ends of the second transconductor is connected to the third pair of differential current output ends of the third transconductor at a second pair of nodes, the second pair of nodes being further connected to a pair of capacitors (C2, C2) as the capacitor, a second pair of current sources, the third pair of differential voltage input ends of the third transconductor with inverted polarity, and the pair of differential voltage input ends of the amplifier with inverted polarity, wherein the pair of differential voltage output ends of the amplifier is connected to the first pair of differential voltage input ends of the first transconductor, and wherein the pair of capacitors' ends that is not connected to the second pair of nodes is used as an input of the filter circuit.

Inventors: YAMAMOTO TAKESHI, YAMAMOTO, TAKESHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: H03H11
IPC codes: H03H11