JP5008125B2 - Filter circuit and semiconductor integrated circuit device - Google Patents

Description

  The present invention relates to a filter circuit for limiting the frequency band of a differential signal and a semiconductor integrated circuit device equipped with the filter circuit, and more particularly to a Gm-C filter combining a Gm cell for performing voltage-current conversion and a capacitor, and the Gm thereof. The present invention relates to a semiconductor integrated circuit device equipped with a -C filter.

  In recent large-screen TV (television) sets, there is a strong demand for higher image quality as the screen becomes larger. For this reason, even in the conventional processing of analog video input signals of standard image quality, signal processing is performed by a digital circuit that can improve image quality at a high level and does not deteriorate image quality due to signal processing. In this case, the input analog video signal is once converted into a digital signal and then subjected to signal processing by a digital circuit. An LSI (Large Scale Integrated Circuit) that performs such signal processing is realized. It consists of an analog filter that receives an analog video signal, an analog / digital converter (hereinafter abbreviated as an AD converter) that receives the analog filter output and converts it into a digital signal, and receives a digital signal from the AD converter. A digital signal processing circuit that performs signal processing is included. The input analog video signal is not completely removed from the signal source side of the analog video signal, and other digital LSIs used around the signal processing LSI in the device. There may be superimposition of clock noise due to interference. Therefore, the analog filter used here removes these clock noises by frequency separation, and prevents image quality deterioration such as beat disturbance caused by interference between the clock noise and the sampling clock of the AD converter.

  This analog filter is often composed of an active filter that has a high degree of design freedom and can easily obtain a desired frequency characteristic. As the circuit system, it is necessary to select one that can realize a wideband filter so that an analog video signal can pass sufficiently. As one of such circuit systems, there is a Gm-C filter that forms a filter by combining a broadband voltage-current converter called a Gm cell and a capacitor, and is used as a filter for an analog video signal.

  Also, this analog filter is separated from the digital circuit as much as possible in terms of circuit and layout so as not to be affected by switching noise caused by a digital circuit made on the same LSI chip. . At least the power supply wiring of the analog filter and the power supply wiring of the digital circuit are separated. However, in an LSI chip, a reverse-biased PN junction is used for element isolation on the LSI chip. In order to maintain this reverse bias, the substrate substrate of the LSI chip must be connected to the ground of both the digital circuit and the analog filter. Therefore, an intrusion path of switching noise from the digital circuit ground to the analog filter ground via the substrate substrate is inevitably created. Therefore, the analog filter needs to be in a circuit form that is not easily affected by the switching noise via the ground. Therefore, a differential Gm-C filter that transmits a signal from an input to an output as a differential signal is used as an analog filter.

  A circuit diagram of the Gm-C filter disclosed in Patent Document 1 is shown in FIG. As shown in FIG. 1, this Gm-C filter includes Gm cells 91 to 94, a capacitor 97 (capacitance C1), and a capacitor 98 (capacitance C2), and receives a differential input signal. A node 11 and an inverting input node 12, a non-inverting output node 13 for outputting a differential output signal, and an inverting output node 14 are provided. The Gm cells 91 to 94 have conductances gm1 to gm4, respectively. Each of the Gm cells 91 to 94 converts the input voltage received at the non-inverting input node I + and the inverting input node I− into a current corresponding to the conductance, and outputs the current to the non-inverting output node O + and the inverting output node O−.

  A differential input signal is applied to the input nodes 11 and 12 of the Gm-C filter. That is, the input signal + ei is input to the non-inverting input node 11 and the input signal −ei is input to the inverting input node 12. The non-inverting input node 11 is connected to the non-inverting input node I + of the Gm cell 91, and the inverting input node 12 is connected to the inverting input node I- of the Gm cell 91. The Gm cell 91 converts the differential voltage between the differential signals input to the non-inverting input node I + and the inverting input node I− into a current corresponding to the conductance gm1 of the Gm cell 91, and outputs the differential output signal to the non-inverting output. Output from node O + and inverted output node O-.

  The differential signals output from the non-inverting output node 13 and the inverting output node 14 of the Gm-C filter are inverted and input to the Gm cell 92 and the Gm cell 94. That is, the non-inverting output node 13 is connected to the inverting input node I− of the Gm cell 92 and the inverting input node I− of the Gm cell 94, and the inverting output node 14 is connected to the non-inverting input node I + and Gm cell of the Gm cell 92. It is connected to 94 non-inverting input node I +.

  The Gm cell 92 converts the differential voltage between the differential signals input to the non-inverting input node I + and the inverting input node I− into a current corresponding to the conductance gm2, and converts the differential output signal to the non-inverting output node O + and the inverting output. Output from node O-. The non-inverting output node O + of the Gm cell 92 and the non-inverting output node O + of the Gm cell 91 are connected in common, and the synthesized signal is input to the non-inverting input node I + of the Gm cell 93. The inverting output node O− of the Gm cell 92 and the inverting output node O− of the Gm cell 91 are connected in common, and the synthesized signal is input to the inverting input node I− of the Gm cell 93. That is, a feedback path is formed that is input from the output nodes 13 and 14 of the Gm-C filter to the Gm cell 93 via the Gm cell 92.

  A capacitor 97 is connected between the non-inverting output node O + and the inverting output node O− of the Gm cells 91 and 92, and the combined current of the output current of the Gm cell 91 and the output current of the Gm cell 92 is a capacitor. It flows to 97. The combined current value is a current value obtained by subtracting a current value obtained by converting the output voltage of the Gm-C filter from the current value obtained by converting the input voltage of the Gm-C filter by the Gm cell 91. This current is converted into a voltage by the impedance of the capacitor 97 and input to the Gm cell 93.

  The Gm cell 93 converts the differential voltage between the differential signals input to the non-inverting input node I + and the inverting input node I− into a current corresponding to the conductance gm3, and converts the differential output signal to the non-inverting output node O + and the inverting output. Output from node O- to capacitor 98. The Gm cell 94 converts the difference voltage between the output signals -eo and + eo of the Gm-C filter inputted to the non-inverting input node I + and the inverting input node I- into a current corresponding to the conductance gm4, and the non-inverting output node O +. And output from the inverted output node O-. The non-inverting output node O + of the Gm cell 94 and the non-inverting output node O + of the Gm cell 93 are connected in common. The inverting output node O− of the Gm cell 94 and the inverting output node O− of the Gm cell 93 are connected in common. That is, the output nodes O + and O− of the Gm cell 94 are connected to the input nodes I− and I + of the Gm cell 94 to form a feedback path. Therefore, the currents output from the non-inverting output node O + and the inverting output node O− of the Gm cells 93 and 94 are combined and output to the capacitor 98.

A current obtained by combining the output current of the Gm cell 93 and the output current of the Gm cell 94 flows through the capacitor 98. The current is obtained by subtracting the current obtained by converting the output voltage of the Gm-C filter to which the Gm cell 94 is input according to the conductance gm4 from the current obtained by converting the input voltage by the Gm cell 93 according to the conductance gm3. Become. This current is converted into a voltage by the impedance of the capacitor 98 and output from the non-inverting output node 13 and the inverting output node 14 of the Gm-C filter. Therefore, the following equation (1) is established, assuming that the output voltage between the non-inverted output 3 and the inverted output 4 is equal to the voltage of the capacitor 98.

If this is rearranged, the following equation (2) is obtained.

  The left side of equation (2) is the ratio between the output voltage eo and the input voltage ei of the Gm-C filter, and indicates the voltage gain of the Gm-C filter. The right side of equation (2) is a quadratic transfer function with a denominator of s quadratic function and a numerator constant, and shows a second order LPF (Low Pass Filter) characteristic in the frequency domain. That is, the Gm-C filter shown in FIG. 1 is a second-order LPF.

  Patent Document 1 also discloses a Gm-C filter different from the above. As shown in FIG. 2, the Gm-C filter includes a Gm cell 91, a Gm cell 92, and a capacitor 97 (capacitance C1), and a non-inverting input node 11 that receives a differential input signal and an inverting input. The node 12 includes a non-inverting output node 13 for outputting a differential output signal and an inverting output node 14. The Gm cells 91 and 92 have conductances gm1 and gm2, respectively. Each of the Gm cells 91 and 92 converts the input voltage received at the non-inverting input node I + and the inverting input node I− into a current corresponding to the conductance and outputs the current to the non-inverting output node O + and the inverting output node O−. To do.

The Gm cell 91 converts input signals + ei and −ei applied to the non-inverting input node I + and the inverting input node I− through the non-inverting input node 11 and the inverting input node 12 of the Gm-C filter, respectively, into currents. , Output from the non-inverting output node O + and the inverting output node O− to the capacitor 97. The non-inverting output node O + and the inverting output node O− of the Gm cell 92 are also connected to the capacitor 97. The Gm cell 92 forms a feedback path for converting the output signals -eo and + eo of the Gm-C filter received at the non-inverting input node I + and the inverting input node I- into currents and outputting them to the capacitor 97. A current obtained by combining the output current of the Gm cell 91 and the output current of the Gm cell 92 flows through the capacitor 97. The value is a current value obtained by subtracting a current obtained by converting the output voltage of the Gm-C filter by the Gm cell 92 from a current obtained by converting the input voltage of the Gm-C filter by the Gm cell 91. This current is converted into a voltage by the impedance of the capacitor 97 and becomes an output voltage generated between the non-inverting output node 13 and the inverting output node 14 of the Gm-C filter. Therefore, the following equation (3) is established, assuming that both are equal.

When this is arranged, the following equation (4) is obtained.

  The left side of the equation (4) is the ratio between the output voltage eo and the input voltage ei of the Gm-C filter and indicates the voltage gain of the Gm-C filter. The right side of the equation (4) is a first-order transfer function in which the denominator is a linear function of S and the numerator is a constant, and shows a first-order LPF characteristic in the frequency domain. That is, the Gm-C filter shown in FIG. 2 is a first-order LPF.

  The two Gm-C filters described above both convert the input voltage 2 · ei of the filter and the output voltage 2 · eo of the filter into currents separately in the Gm cell 91 and the Gm cell 92, and then combine them. It is a circuit format that applies feedback. Therefore, the conductances when the input voltage of the filter and the output voltage of the filter are converted to current are not always the same. Therefore, the gain of the Gm-C filter is transmitted as gm1 / gm2, which is the ratio of the conductance gm1 of the Gm cell 91 and the conductance gm2 of the Gm cell 92, as shown in the above equations (2) and (4). It takes the form of the whole function. That is, the voltage gain of the filter varies due to variations in conductance between the Gm cell 91 and the Gm cell 92.

JP 2002-314374 A

  The present invention provides a Gm-C filter that is not affected by variations in conductance of Gm cells and that suppresses variations in the voltage gain of the filter.

  Hereinafter, means for solving the problem will be described using the numbers and symbols used in [Best Mode for Carrying Out the Invention]. These numbers and symbols are added to clarify the correspondence between the description of [Claims] and [Best Mode for Carrying Out the Invention]. However, these numbers and symbols should not be used for the interpretation of the technical scope of the invention described in [Claims].

  In an aspect of the present invention, the filter circuit includes a first circuit (21), a second circuit (22), and a third circuit (17, 18, 19, 23, 24, 33, 34). The first circuit (21) uses a first input signal (+ ei), which is one of differential input signals, and a first output signal (+ eo), which is one of differential output signals, as differential signals. The voltage difference (ei−eo) between the first input signal (+ ei) and the first output signal (+ eo) is converted into a current according to a predetermined conductance, and a differential signal converted into a current is output. . The second circuit (22) differentially differentiates a second input signal (−ei) that is the other signal of the differential input signals and a second output signal (−eo) that is the other signal of the differential output signals. A differential signal that is input as a signal, converts a voltage difference (ei-eo) between the second output signal (-eo) and the second input signal (-ei) into a current according to a predetermined conductance, and is converted into a current. Output a signal. The third circuit receives a combined differential signal obtained by synthesizing the differential signal output from the first circuit (21) and the differential signal output from the second circuit (22), and outputs a differential output signal (+ eo). , -Eo).

  In another aspect of the present invention, the filter circuit includes a first input node (11) and a second input node (12) to which a differential input signal is input, and a first output node (to which a differential output signal is output). 13) and a second output node (14), a first Gm cell (21), a second Gm cell (22), and a circuit block. The first Gm cell (21) includes a non-inverting input node (I +) connected to the first input node (11), an inverting input node (I−) connected to the first output node (13), and a differential A non-inverting output node (O +) for outputting a signal and an inverting output node (O−). The second Gm cell (22) includes a non-inverting input node (I +) connected to the second output node (14), an inverting input node (I−) connected to the second input node (12), and a differential A non-inverting output node (O +) for outputting a signal and an inverting output node (O−). The circuit block includes a third input node, a fourth input node, a third output node, a fourth output node, and a capacitive element. The third input node is connected to the non-inverting output node of the first Gm cell (21) and the non-inverting output node of the second Gm cell (22). The fourth input node is connected to the inverted output node of the first Gm cell (21) and the inverted output node of the second Gm cell (22). The third output node is connected to the first output node, and the fourth output node is connected to the second output node.

  As described above, the Gm-C filter of the present invention takes the voltage difference between the input voltage and the output voltage of the filter and then converts it into a current in the Gm cell. In a conventional Gm-C filter that takes a difference after converting an input voltage and an output voltage into a current in separate Gm cells, the variation in conductance of the Gm cell affects the variation in the voltage gain of the filter. On the other hand, in the Gm-C filter of the present invention, even when the conductance of the Gm cell varies, the voltage gain of the filter is not affected and does not vary.

  According to the present invention, it is possible to provide a Gm-C filter that is not affected by variations in conductance of Gm cells and that suppresses variations in voltage gain.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

(First embodiment)
FIG. 3 is a circuit diagram of the Gm-C filter according to the first embodiment.

  As shown in FIG. 3, the Gm-C filter according to the first embodiment includes Gm cells 21 to 24 and capacitors 17 and 18, and includes a non-inverting input node 11 that receives a differential input signal, and An inverting input node 12, a non-inverting output node 13 for outputting a differential output signal, and an inverting output node 14 are provided. The Gm cells 21 to 24 have conductances gm1 to gm4, respectively, and the capacitors 17 and 18 have capacitances C1 and C2, respectively. The Gm cells 21 to 24 convert the input voltages received at the non-inverting input node I + and the inverting input node I− into currents according to their conductances, to the non-inverting output node O + and the inverting output node O−. Output.

  The non-inverting input node I + of the Gm cell 21 is connected to the non-inverting input node 11 of the Gm-C filter, and the inverting input node I- of the Gm cell 21 is connected to the non-inverting output node 13 of the Gm-C filter. . The non-inverting input node I + of the Gm cell 22 is connected to the inverting output node 14 of the Gm-C filter, and the inverting input node I- of the Gm cell 22 is connected to the inverting input node 12 of the Gm-C filter. The non-inverting output node O + of the Gm cell 21 and the Gm cell 22 is connected in common and connected to the non-inverting input node I + of the Gm cell 23, and the inverting output node O− of the Gm cell 21 and Gm cell 22 is connected in common. Connected to the inverting input node I− of the cell 23. A capacitor 17 is connected between the non-inverting output node O + and the inverting output node O− of the Gm cell 21 and the Gm cell 22 that are commonly connected.

  A capacitor 18 is connected between the non-inverting output node 13 and the inverting output node 14 of the Gm-C filter. Further, the non-inverting output node 13 of the Gm-C filter is connected to the inverting input node I− of the Gm cell 24, and the inverting output node 14 of the Gm-C filter is connected to the non-inverting input node I + of the Gm cell 24. The The non-inverting output node O + of the Gm cell 24 is connected in common with the non-inverting output node O + of the Gm cell 23, and is further connected to one electrode of the capacitor 18 and the non-inverting output node 13 of the Gm-C filter. The inverting output node O− of the Gm cell 24 is connected in common with the inverting output node O− of the Gm cell 23, and is further connected to the other electrode of the capacitor 18 and the inverting output node 14 of the Gm-C filter.

  Next, the operation of the Gm-C filter according to the first embodiment will be described. The Gm cell 21 receives the signal + ei applied to the non-inverting input node 11 of the Gm-C filter at the non-inverting input node I + and also inverting the signal + eo output from the non-inverting output node 13 of the Gm-C filter. Node I- receives. The Gm cell 21 converts the differential voltage between the signal + ei and the signal + eo into a current corresponding to the conductance gm1, and outputs a differential output signal from the non-inverted output node O + and the inverted output node O−.

  The Gm cell 22 receives the signal -eo output to the inverting output node 14 of the Gm-C filter at the non-inverting input node I + and also inverting the signal -ei applied to the inverting input node 12 of the Gm-C filter. Node I- receives. The Gm cell 22 converts the difference voltage between the signal -eo and the signal -ei into a current corresponding to the conductance gm2, and outputs a differential output signal from the non-inverted output node O + and the inverted output node O-.

  Therefore, the capacitor 17 is driven by a differential signal obtained by synthesizing the differential output signals output from the Gm cell 21 and the Gm cell 22. That is, a current obtained by combining the output current output from the Gm cell 21 and the output current output from the Gm cell 22 flows through the capacitor 17. The current value is the sum of the current values converted by the Gm cell 21 and the Gm cell 22 from the difference between the input voltage and the output voltage of the Gm-C filter. This current is converted into a voltage by the impedance of the capacitor 17 and applied to the non-inverting input node I + and the inverting input node I− of the Gm cell 23.

The Gm cell 23 converts a voltage applied between the non-inverting input node I + and the inverting input node I− into a current, and outputs the current to the capacitor 18 from the non-inverting output node O + and the inverting output node O−. The non-inverting output node O + and the inverting output node O− of the Gm cell 24 are also connected to the capacitor 18. The Gm cell 24 converts the output signal −eo and the output signal + eo of the Gm-C filter received at the non-inverting input node I + and the inverting input node I− into a current and outputs the current to the capacitor 18. Therefore, the Gm cell 24 forms a feedback path. A current obtained by combining the output current of the Gm cell 23 and the output current of the Gm cell 24 flows through the capacitor 18. The current value is the current obtained by converting the voltage between the output nodes 13 and 14 of the Gm-C filter by the Gm cell 24 from the current obtained by converting the voltage across the capacitor 18 applied to the Gm cell 23. Subtracted value. This current is converted into a voltage by the impedance of the capacitor 18, and becomes an output of the Gm-C filter, that is, an output voltage generated between the non-inverting output node 13 and the inverting output node 14. Therefore, the following equation (5) is established, assuming that both are equal.

By arranging this, the following equation (6) is obtained.

  The left side of the equation (6) is a ratio between the output voltage eo and the input voltage ei of the Gm-C filter and represents the voltage gain of the Gm-C filter. The right side of the equation (6) is a quadratic transfer function with a denominator S quadratic function and a numerator constant, and shows quadratic LPF characteristics in the frequency domain. That is, the Gm-C filter shown in FIG. 3 is a second-order LPF.

  In the equation (6), the coefficient applied to the entire right side is 1, and is constant without depending on the conductances gm1 to gm4 of the Gm cells 21 to 24. That is, even if the conductances gm1 to gm4 of the Gm cells 21 to 24 vary, the voltage gain of the Gm-C filter does not vary and is always constant.

  In the conventional Gm-C filter, the input differential signal is converted into a current by the Gm cell 91, and the output differential signal is converted into a current by the Gm cell 92, and the difference is obtained. In the Gm-C filter according to the first embodiment, the connection is changed. The non-inverting input node I + of the Gm cell 21 is connected to the non-inverting input node 11 of the Gm-C filter, and the inverting input node I- is connected to the non-inverting output node 13 of the Gm-C filter. The non-inverting input node I + of the Gm cell 22 is connected to the inverting output node 14 of the Gm-C filter, and the inverting input node I- is connected to the inverting input node 12 of the Gm-C filter.

  The order of the transfer function of the Gm-C filter according to the present embodiment is the same as that of the conventional Gm-C filter. However, by connecting as described above, the coefficient applied to the entire transfer function can be eliminated. The coefficient expressed by the conductance ratio of the Gm cell indicates the voltage gain, and means that the voltage gain varies due to the influence of the variation in the conductance of the Gm cell. That is, by the above-described connection, it is possible to eliminate the variation in voltage gain due to the variation in conductance of the Gm cell that has occurred in the conventional Gm-C filter.

  Further, an even-order LPF having an order of 4th or higher can be realized by subordinate connection of LPFs of a second-order transfer function. That is, the even-order LPF transfer function is factored into a product of the second-order transfer function. An even-order LPF is realized by cascading LPFs that realize each of the decomposed second-order transfer functions. The Gm-C filter described above can be used as the second-order LPF. That is, an even-order fourth-order or higher-order LPF with no voltage gain variation can be realized by cascading two or more Gm-C filters according to this embodiment.

(Second Embodiment)
FIG. 4 shows a circuit diagram of the Gm-C filter according to the second embodiment. The Gm-C filter according to the second embodiment has a shape in which the Gm cells 23 and 24 and the capacitor 18 of the Gm-C filter according to the first embodiment are removed. Accordingly, portions corresponding to those of the Gm-C filter according to the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  The non-inverting input node I + of the Gm cell 21 is connected to the non-inverting input node 11 of the Gm-C filter, and the inverting input node I- is connected to the non-inverting output node 13 of the Gm-C filter. The non-inverting input node I + of the Gm cell 22 is connected to the inverting output node 14 of the Gm-C filter, and the inverting input node I- is connected to the inverting input node 12 of the Gm-C filter. The non-inverting output node O + of the Gm cell 21 and the non-inverting output node O + of the Gm cell 22 are commonly connected to the non-inverting output node 13 of the Gm-C filter, and the inverting output node O- of the Gm cell 21 and the Gm cell are connected. The inverting output node O− of 22 is commonly connected to the inverting output node 14 of the Gm-C filter. Capacitor 17 (capacitance C1) is connected between non-inverting output node 13 and inverting output node 14 of the Gm-C filter.

Next, the operation of this Gm-C filter will be described. The Gm cell 21 receives the signal + ei applied to the non-inverting input node 11 of the Gm-C filter at the non-inverting input node I +, and receives the signal + eo output to the non-inverting output node 13 at the inverting input node I−. . The Gm cell 21 converts the voltage difference between the signal + ei and the signal + eo into a current, and outputs the current to the capacitor 17 from the non-inverting output node O + and the inverting output node O−. The Gm cell 22 receives the signal -eo output to the inverting output node 14 of the Gm-C filter at the non-inverting input node I + and the signal -ei applied to the inverting input node 12 to the inverting input node I-. . The Gm cell 22 converts the voltage difference between the signal -eo and the signal -ei into a current, and outputs the current to the capacitor 17 from the non-inverting output node O + and the inverting output node O-. Therefore, a current obtained by combining the output current of the Gm cell 21 and the output current of the Gm cell 22 flows through the capacitor 17. The value of the combined current is the sum of the current converted by the Gm cell 22 and the current converted by the Gm cell 22 of the difference voltage between the input voltage of the Gm-C filter 2 and the output voltage of the Gm-C filter 2. This current is converted into a voltage by the impedance of the capacitor 17 and becomes an output voltage output from the non-inverting output node 13 and the inverting output node 14 of the Gm-C filter. Therefore, the following equation holds.

If this is rearranged, the following equation (8) is obtained.

  The left side of the equation (8) is the ratio between the output voltage eo and the input voltage ei of the Gm-C filter and indicates the voltage gain of the Gm-C filter. The right side of equation (8) is a linear transfer function with a denominator of a linear function of S and a numerator of a constant. Therefore, this transfer function exhibits a first-order LPF characteristic in the frequency domain. That is, the Gm-C filter shown in FIG. 4 is a first-order LPF.

  In the equation (8), the coefficient applied to the entire right side is 1, and is constant without depending on the conductances gm1 and gm2 of the Gm cells 21 and 22. That is, even if the conductances of the Gm cells 21 and 22 vary, the voltage gain of the Gm-C filter does not vary and is always constant.

  An odd-order LPF having an order of 3 or more can be realized by a cascade connection of LPFs of the first-order and second-order transfer functions. That is, the transfer function of the odd-order LPF is factored into the product of the first-order transfer function and several second-order transfer functions. An odd-order LPF can be realized by cascade-connecting LPFs that realize the decomposed first-order and second-order transfer functions. The Gm-C filter shown in the first embodiment can be used as the second-order LPF, and the Gm-C filter shown in the second embodiment can be used as the first-order LPF. That is, by combining the Gm-C filter shown in the present embodiment and the Gm-C filter shown in the first embodiment, an odd-order third-order or higher-order LPF having no variation in voltage gain is formed. be able to.

(Third embodiment)
FIG. 5 shows a circuit diagram of a Gm-C filter according to the third embodiment. Portions corresponding to the Gm-C filters according to the first and second embodiments are denoted by the same reference numerals, and description thereof is omitted.

  As shown in FIG. 5, the Gm-C filter includes the capacitor 17 of the Gm-C filter according to the second embodiment shown in FIG. 4, the Gm cells 33 and 34, and the capacitor 19 (capacitance C <b> 2). The circuit is replaced with a circuit block including The non-inverting input node I + of the Gm cell 33 and the inverting output node O− of the Gm cell 34 are connected to the non-inverting output node O + of the Gm cell 21 and the Gm cell 22 and the non-inverting output node 13 of the Gm-C filter. The The inverting input node I− of the Gm cell 33 and the non-inverting output node O + of the Gm cell 34 are connected to the inverting output node O− of the Gm cell 21 and the Gm cell 22 and the inverting output node 14 of the Gm-C filter. . The non-inverting output node O + and the inverting output node O− of the Gm cell 33 are connected to the non-inverting input node I + and the inverting input node I− of the Gm cell 34, respectively. The capacitor 19 is connected between the non-inverting output node O + and the inverting output node O− of the Gm cell 33.

  Next, the operation of this Gm-C filter will be described. The operations of the Gm cells 21 and 22 are the same as those in the second embodiment. First, the operation of the circuit block (Gm cells 33 and 34, capacitor 19) replaced with the capacitor 17 will be described.

The output voltage + eo output to the non-inverting output node 13 of the Gm-C filter and the output voltage -eo output to the inverting output node 14 are converted into a current according to the conductance gm3 of the Gm cell 33, and the capacitor 19 Is output to (capacitance C2). The output current of the Gm cell 33 is converted into a voltage by the impedance of the capacitor 19 and input to the Gm cell 34. The voltage input to the Gm cell 34 is converted into a current according to the conductance gm4 of the Gm cell 34 and output from the Gm cell 34. Here, the current output from the Gm cell 34 is expressed by the following equation (9).

In the circuit block including the Gm cell 33, the Gm cell 34, and the capacitor 19, a current represented by the above formula flows with respect to a voltage of 2 · eo. Therefore, this circuit block can be regarded as a two-terminal element having an impedance represented by the following equation (10).

Here, in the Gm-C filter according to the third embodiment, the capacitor 17 of the Gm-C filter according to the second embodiment is replaced with a circuit block including Gm cells 33 and 34 and a capacitor 19. It is a thing. Therefore, the following equation (11) is established in which the impedance of the capacitor 17 (capacitance C1) represented by the equation (7) described in the second embodiment is replaced by the equation (10).

When this is arranged, the following expression (12) is obtained.

  The left side of the equation (12) is the ratio between the output voltage eo and the input voltage ei of the Gm-C filter and indicates the voltage gain of the Gm-C filter. In the right side of the equation (12), the denominator is a linear function of S, and the numerator is a transfer function of S. Therefore, this transfer function exhibits a first-order HPF (High Pass Filter) characteristic in the frequency domain. That is, the Gm-C filter shown in FIG. 5 is a first-order HPF.

  In the equation (12), the coefficient applied to the entire right side is 1, and is constant without depending on the conductances gm1, gm2, gm3, and gm4 of the Gm cells 21, 22, 33, and 34. That is, the Gm-C filter according to the third embodiment has HPF characteristics, and even if there are variations in the conductances gm1 to gm4 of the Gm cells 21, 22, 33, and 34, the voltage gain is affected. Always constant.

(Fourth embodiment)
FIG. 6 shows a circuit diagram of a Gm-C filter according to the fourth embodiment. Portions corresponding to the Gm-C filters according to the first to fourth embodiments are denoted by the same reference numerals, and description thereof is omitted.

  As shown in FIG. 6, in the Gm-C filter, a capacitor 17 (capacitance C1) is added to the Gm-C filter of the third embodiment shown in FIG. That is, the capacitor 17 is connected in parallel to a circuit block including the Gm cell 33, the Gm cell 34, and the capacitor 19 (capacitance C2).

  Next, the operation of this Gm-C filter will be described. The Gm-C filter of the fourth embodiment has the same connection as the third embodiment because the connection is the same except for the Gm-C filter and the capacitor 17 in the third embodiment. A description of the operation of certain Gm cells 21, 22, 33, 34 and capacitor 19 is omitted.

Since the capacitor 17 is connected in parallel to the circuit block including the Gm cells 33 and 34 and the capacitor 19, the total impedance of these is the impedance shown in the equation (10) and the impedance of the capacitor 17 (capacitance C1). Is expressed by the following equation.

This synthetic impedance corresponds to the equation (10) described in the third embodiment, and the following equation is obtained by substituting the above equation (13) into the corresponding part of the equation (11).

If this is rearranged, the following equation (15) is obtained.

  The left side of equation (15) is the ratio between the output voltage eo and the input voltage ei of the Gm-C filter, and indicates the voltage gain of the Gm-C filter. The right side of the equation (15) is a transfer function whose denominator is a quadratic function of S and whose numerator is S, and shows a BPF (Band Pass Filter) characteristic in the frequency domain. That is, the Gm-C filter according to the present embodiment shown in FIG. 6 is a BPF.

  In the above equation (15), the coefficient applied to the entire right side is 1, and is constant without depending on the conductances gm1 to gm4 of the Gm cells 21, 22, 33, and 34. That is, the Gm-C filter according to the fourth embodiment has a BPF characteristic, and even if the conductances gm1 to gm4 of the Gm cells 21, 22, 33, and 34 vary, the voltage gain is affected. Always constant.

  As described above, according to the present invention, the Gm-C filter includes the first Gm cell that converts the difference voltage between the non-inverting input and the non-inverting output into a current and outputs the current, and between the inverting input and the inverting output. The second Gm cell that converts the current difference voltage into a current and outputs the current is output, and the outputs of the first Gm cell and the second Gm cell are connected in common. As a result, the voltage gain of the Gm-C filter can be kept constant regardless of the conductance of the first Gm cell and the second Gm cell. Therefore, it is possible to eliminate variations in the voltage gain of the Gm-C filter caused by variations in the conductance of Gm cells in the differential Gm-C filter.

  The present invention is applicable to a filter having any pass characteristic of LPF, BPF, and HPF. In the embodiment of the present invention, in order to simplify the description, the connection destination of the non-inverting input node I + and the inverting input node I− and the non-inverting output node O + and the inverting output node O− of each Gm cell. However, even if the connection is changed so that the non-inverting input node I + and the inverting input node I− and the non-inverting output node O + and the inverting output node O− of each Gm cell are simultaneously replaced, the differential input Since the polarity of the differential output current output with respect to the voltage is not reversed, it goes without saying that each embodiment operates as it is.

It is a circuit diagram which shows the structure of the conventional Gm-C filter. It is a circuit diagram which shows the other structure of the conventional Gm-C filter. It is a circuit diagram which shows the structure of the Gm-C filter which concerns on the 1st Embodiment of this invention. It is a circuit diagram which shows the structure of the Gm-C filter which concerns on the 2nd Embodiment of this invention. It is a circuit diagram which shows the structure of the Gm-C filter which concerns on the 3rd Embodiment of this invention. It is a circuit diagram which shows the structure of the Gm-C filter which concerns on the 4th Embodiment of this invention.

Explanation of symbols

11 Non-inverting input node 12 Inverting input node 13 Non-inverting output node 14 Inverting output nodes 17 to 19 Capacitors 21 to 24, 33 and 34 Gm cells 91 to 94 Gm cells 97 and 98 Capacitors

Claims (16)

A first input signal that is one of the differential input signals and a first output signal that is one of the differential output signals are input as differential signals, and the first input signal and the first output signal are input. A first circuit that converts a voltage difference between the first and second currents into a current according to a predetermined conductance and outputs a differential signal converted into a current;
A second input signal that is the other signal of the differential input signal and a second output signal that is the other signal of the differential output signal are input as differential signals, and the second output signal and the second signal are input. A second circuit for converting a voltage difference from an input signal into a current according to a predetermined conductance and outputting a differential signal converted into a current;
A third circuit for inputting a synthesized differential signal obtained by synthesizing the differential signal output from the first circuit and the differential signal output from the second circuit, and outputting the differential output signal; Filter circuit. The third circuit includes:
A first capacitive element connected between input nodes for inputting the combined differential signal;
A second capacitive element connected between output nodes that output the differential output signal;
A fourth circuit that inputs a voltage generated by the first capacitive element, converts the voltage into a current according to a predetermined conductance, and outputs a differential signal converted into a current to the output node;
The filter circuit according to claim 1, further comprising: a fifth circuit that inputs the differential output signal, converts the current into a current according to a predetermined conductance, and outputs the differential signal converted into the current to the output node. The filter circuit according to claim 2, wherein the first circuit, the second circuit, the fourth circuit, and the fifth circuit include Gm cells. The third circuit includes:
A first capacitor connected between input nodes for inputting the combined differential signal;
The filter circuit according to claim 1, wherein the input node is connected to an output node that outputs the differential output signal. The third circuit includes:
An input node for inputting the combined differential signal and an output node for outputting the differential output signal are respectively connected.
A sixth circuit that inputs a differential signal applied to the input node, converts the current into a current according to a predetermined conductance, and outputs the differential signal;
A second capacitive element that converts a differential signal output from the sixth circuit into a voltage;
7. A seventh circuit that inputs a voltage converted into a voltage by the second capacitive element and outputs a differential signal having a reverse phase converted into a current according to a predetermined conductance to the output node. Filter circuit. The filter circuit according to claim 5, wherein the third circuit further includes a third capacitance element connected between the input nodes. The filter circuit according to claim 5, wherein the first circuit, the second circuit, and the sixth circuit and the seventh circuit include a Gm cell.   A filter circuit formed by cascading a combination of the filter circuits according to claim 1.   9. A semiconductor integrated circuit device on which the filter circuit according to claim 1 is mounted. A first input node and a second input node to which a differential input signal is input;
A first output node and a second output node from which a differential output signal is output;
A first Gm cell comprising: a non-inverting input node connected to the first input node; an inverting input node connected to the first output node; a non-inverting output node that outputs a differential signal; and an inverting output node; ,
A second Gm cell comprising: a non-inverting input node connected to the second output node; an inverting input node connected to the second input node; a non-inverting output node that outputs a differential signal; and an inverting output node; ,
A third input node connected to the non-inverting output node of the first Gm cell and the non-inverting output node of the second Gm cell; and an inverting output node of the first Gm cell and an inverting output node of the second Gm cell. A circuit block comprising: a fourth input node connected to the first output node; a third output node connected to the first output node; a fourth output node connected to the second output node; and a capacitive element. circuit. The circuit block is
A first capacitive element connected between the third input node and the fourth input node;
A second capacitive element connected between the third output node and the fourth output node;
A non-inverting input node connected to the third input node, an inverting input node connected to the fourth input node, a non-inverting output node connected to the third output node, and a fourth output node A third Gm cell comprising an inverting output node connected;
A non-inverting input node connected to the fourth output node; an inverting input node connected to the third output node; a non-inverting output node connected to the third output node; and a fourth output node The filter circuit according to claim 10, further comprising: a fourth Gm cell including an inverting output node connected thereto. The circuit block is
A first capacitive element connected between the third input node and the fourth input node;
The filter circuit according to claim 10, wherein the third output node is connected to the third input node, and the fourth output node is connected to the fourth input node. The circuit block is
A fifth Gm cell comprising a non-inverting input node connected to the third input node, an inverting input node connected to the fourth input node, a non-inverting output node for outputting a differential signal, and an inverting output node; ,
A third capacitive element connected between a non-inverting output node and an inverting output node of the fifth Gm cell;
A non-inverting input node connected to the non-inverting output node of the fifth Gm cell; an inverting input node connected to the inverting output node of the fifth Gm cell; and a non-inverting output node connected to the fourth output node; A sixth Gm cell including an inverting output node connected to the third output node, wherein the third input node and the third output node are connected, and the fourth input node and the fourth output The filter circuit according to claim 10, wherein the filter circuit is connected to the node. The filter circuit according to claim 13, wherein the circuit block further includes a fourth capacitance element connected between the third input node and the fourth input node.   A filter circuit formed by cascading the filter circuits according to any one of claims 10 to 14.   16. A semiconductor integrated circuit device on which the filter circuit according to claim 10 is mounted.

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