JP2005295252A - Variable resistance circuit, filter circuit and variable gain amplifier circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable resistance circuit that is suitable for driving at a low power supply voltage. <P>SOLUTION: This variable resistance circuit comprises first and second resistance elements connected in series between an input terminal and an output terminal; a third resistance element, one end of which is connected between the first and second resistance elements; and a transistor with a source grounded, the drain of which is connected to the other end of the third resistance element; changes the impedance of the transistor by a control signal to be applied to the gate of the transistor and controls a resistance value between the input terminal and the output terminal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a variable resistance circuit, a filter circuit using the variable resistance circuit, and a variable gain amplifier circuit.

  The variable resistance circuit capable of making the resistance value variable is used, for example, in a tuning circuit that makes the cutoff frequency of the active filter variable, and a variable gain amplifier that switches the gain according to the ratio of the resistance.

As such a variable resistance circuit, a method is known in which the on-resistance between the drain and source of a field effect transistor in a linear region is used as a variable resistance, and the resistance value is changed by controlling the gate voltage of the transistor. (Refer patent document 1 etc.).
U.S. Pat. No. 4,710,726

  In a circuit using the on-resistance of a field effect transistor itself as a variable resistor as in Patent Document 1, the drain and source of the field effect transistor are connected in series to the signal path. If the power supply voltage is lowered, the gate-source voltage of the field effect transistor may not be sufficiently large.

  If the voltage between the gate and the source becomes too low, the field effect transistor is not turned on and cannot operate as a variable resistor.

  Therefore, an object of the present invention is to provide a variable resistance circuit that can operate even under low power supply voltage conditions.

  The first aspect of the present invention includes: first and second resistance elements connected in series between an input terminal and an output terminal; and a third one end connected between the first and second resistance elements. And a common-source transistor having a drain connected to the other end of the third resistor element; the impedance of the transistor is changed by a control signal applied to the gate of the transistor, and the input terminal And a resistance value between the output terminals is controlled.

  In this aspect, the drain of the transistor does not need to be directly connected to the first and second resistance elements, and may be connected via the third resistance element.

  Another aspect of the present invention includes: first and second resistance elements connected in series between an input terminal and an output terminal; and a drain connected between the first and second resistance elements, A source-grounded transistor connected to a gate; a variable current source controls a resistance value between the input terminal and the output terminal by changing a current applied as a control signal to the drain of the transistor This is a variable resistance circuit.

  Also in this aspect, the drain of the transistor does not need to be directly connected to the first and second resistance elements, and may be connected via the third resistance element. In this case, the current of the variable current source as the control signal may be supplied to the transistor via the third resistance element, or may be directly supplied to the drain.

  According to the present invention, since the transistor for changing the resistance value between the input and output terminals is not connected in series between the input and output terminals, the variable resistance function can be stably exhibited even under a low power supply voltage. A resistance circuit can be provided.

  The variable resistance circuit according to the present invention can be used for a filter circuit and a variable gain amplifier.

  For example, a filter circuit including an operational amplifier; the above-described variable resistance circuit connected to the input side of the operational amplifier; and a capacitive load provided between the output and input of the operational amplifier.

  A variable gain amplifier circuit comprising: an operational amplifier; the above-described variable resistance circuit connected to an input side of the operational amplifier; and the above-described variable resistance circuit provided between an output and an input of the operational amplifier. It is.

  Thus, according to the present invention, a variable resistance circuit suitable for low voltage driving, a filter circuit using the variable resistance circuit, and a variable gain amplifier circuit can be obtained.

  An embodiment of the present invention will be described.

  FIG. 1 is a circuit diagram showing a variable resistance circuit according to an embodiment of the present invention.

  Constant resistors R1 and R2 (resistance values R1 and R2) connected between the signal input / output terminals V1 and V2, an impedance element Z1 connected between the two constant resistors R1 and R2, and the impedance element Z1 It is constituted by a transistor N1 having a common source.

  The constant resistors R1 and R2 are grounded via an impedance element Z1 and a transistor N1 that are connected in series.

  In the circuit configured as described above, the impedance Z (impedance value Z) formed by the impedance element Z1 and the transistor N1 can be changed by a voltage applied to the gate terminal of the transistor N1.

  When star delta conversion is performed using the variable impedance Z and the constant resistances R1 and R2, an equivalent impedance connected between the input / output terminals V1 and V2 is expressed by the following equation. The equivalent impedance circuit is the circuit shown in FIG.

  A variable resistor R12 (resistance value R12) is connected in series between the input / output terminals V1 and V2, and the V1 side of the variable resistor R12 is grounded via the variable resistor R1g (resistance value R1g), and the variable resistor R12 is connected to the V2 side. The circuit grounded via the variable resistor R2g (resistance value R2g) is an equivalent impedance circuit.

R1g = (R1 * R2 + R2 * Z + Z * R1) R2 ^-1
= (Z * R1) R2 ^-1 + R1 + Z
R2g = (R1 * R2 + R2 * Z + Z * R1) R1 ^-1
= (R2 * Z) R1 ^-1 + R2 + Z
R12 = (R1 * R2 + R2 * Z + Z * R1) Z ⁻¹
= (R1 * R2) Z ^-1 + R1 + R2
If the impedances of the input / output terminals V1 and V2 are sufficiently lower than the equivalent resistances R1g and R2g, the influence of the resistances R1g and R2g is minute. Therefore, the impedance between the input and output terminals can be substantially regarded as the impedance R12.

  Thus, if the impedance of the input / output terminals V1 and V2 is sufficiently low, the impedance Z is changed by the voltage applied to the gate terminal of the transistor N1, thereby connecting the impedance between the input and output terminals V1 and V2. Can be changed.

  Further, in such a circuit configuration, since the transistor N1 itself is grounded at the source, it is possible to configure so that the voltage applied to the gate terminal can be changed from the threshold voltage of the transistor to the power supply voltage or less. There is no fear that the variable resistor will not operate depending on the voltage of the input / output terminals.

  Next, FIG. 2 shows a circuit diagram of another embodiment of the present invention.

  In this circuit configuration, a portion corresponding to the variable impedance Z is configured by a diode-connected transistor N1. The output impedance of the transistor N1 is determined by its voltage / current conversion gain gm. Since the gm of the transistor is determined by the current that flows, the impedance can be changed by changing the current supplied to the diode-connected transistor. The equivalent variable resistance value at this time is as follows. The equivalent impedance circuit is the same as in FIG.

R1g = (R1 * R2 + R2 * (1 / gm) + (1 / gm) * R1) R2 ^-1
= R1 (gm * R2) ^-1 + R1 + (1 / gm)
R2g = (R1 * R2 + R2 * (1 / gm) + (1 / gm) * R1) R1 ⁻¹
= R2 * (gm * R1) ⁻¹ + R2 + (1 / gm)
R12 = (R1 * R2 + R2 * (1 / gm) + (1 / gm) * R1) (1 / gm) ⁻¹
= Gm * R1 * R2 + R1 + R2
If the impedances of the input / output terminals V1 and V2 are sufficiently lower than the equivalent resistances R1g and R2g, the influence of the resistances R1g and R2g is minute. Therefore, the impedance between the input and output terminals can be substantially regarded as the impedance R12.

  Therefore, by changing the current supplied to the transistor N1 and changing the voltage-current conversion gain gm, the impedance Z connected between the input and output terminals can be changed.

  Next, FIG. 3 shows a circuit diagram of another embodiment of the present invention.

  In the circuit configuration shown in FIG. 3, a constant resistor R3 and a diode-connected transistor N1 are connected in series at a portion corresponding to the variable impedance Z in FIG. The impedance of the transistor N1 can be changed by a current as in the embodiment shown in FIG. 2, but the circuit configuration shown in FIG. 3 includes a constant resistance in series. The resistance value is given by the following formula.

R1g = [R1 * R2 + R2 * (R3 + 1 / gm) + (R3 + 1 / gm) * R1] * R2 ^-1
= R1 * (R3 + 1 / gm) R2 ^-1 + R1 + (R3 + 1 / gm)
R2g = [R1 * R2 + R2 * (R3 + 1 / gm) + (R3 + 1 / gm) * R1] * R1 ^-1
= R2 * (R3 + 1 / gm) R1 ^-1 + R2 + (R3 + 1 / gm)
R12 = [R12 * R2 + R2 * (R3 + 1 / gm) + (R3 + 1 / gm) * R1] (R3 + 1 / gm) ⁻¹
= R1 * R2 * (R3 + 1 / gm) ⁻¹ + R1 + R2
Since the constant resistance R3 is connected to the transistor in series, the variable range of the variable resistance circuit can be limited by adjusting the resistance value of the constant resistance R3.

  The same operation is possible even if a current source I for supplying current to the drain of the transistor N1 in FIG. 3 is provided. FIG. 4 shows a circuit diagram thereof. Except for the current source I, the configuration is the same as in FIG.

  Compared to the configuration of FIG. 3, such a configuration is suitable for a lower power supply voltage because the voltage drop caused by the current flowing from the current source I flowing through the constant resistor R3 can be eliminated.

  FIG. 5 shows a variable resistance circuit according to another embodiment of the present invention.

  In the above-described embodiment, variable resistance is realized by changing the gate voltage of the transistor N1. Alternatively, a plurality of parallel-connected transistors can be provided, and variable resistance can be realized by turning on / off the transistors.

  In the circuit configuration shown in FIG. 5, the portion corresponding to the variable impedance Z is a constant resistor Rpx (Rp1, Rp2, Rp3,..., Rpn) and a source-grounded transistor Nx (N1, N2, N3,. Nn)) are connected in series, and a plurality (n) of sets are connected in parallel. At this time, the source-grounded transistor Nx operates as a switch, and a signal (D1, D2, D3..., Dn) input to the transistor is input with a power supply voltage or a ground voltage according to a digital signal. Configured as follows.

  The transistor is grounded at the source even when the power supply voltage is low, and the gate terminal swings to the power supply voltage or ground, so that a sufficient gate-source voltage can be applied to the transistor Nx. Even when the voltage of the output terminal is low, it can be operated as a switch.

  In this case as well, the equivalent impedance circuit as shown in FIG. 1 is obtained, but the variable impedance Z is determined by a resistor connected to the transistor Nx which is a switch in an ON state. Therefore, the equivalent variable resistance value at this time is as follows. However, Dx (x = 1... N, 1 when ON, 0 when OFF).

R1g = [R1 * R2 + R2 * Z + Z * R1] / R2
= Z * R1 / R2 + R1 + Z
R2g = [R1 * R2 + R2 * Z + Z * R1] / R1
= R2 * Z / R1 + R2 + Z
R12 = [R1 * R2 + R2 * Z + Z * R1] / Z
= R12 * R2 / Z + R1 + R2
1 / Z = D1 / Rp1 + D2 / Rp2 + D3 / Rp3 + ... + Dn / Rpn (Dx = 0 or 1)
The resistance value can be discretely varied by a digital signal applied to the transistor. The variable range can be determined by the number of parallelly connected sets in which the constant resistance Rpx and the diode-connected transistor Nx are connected in series.

  Next, an embodiment of the present invention in which the variable resistance circuit according to the present invention is applied to a filter circuit will be described with reference to FIG.

  FIG. 6 is a circuit diagram of the filter circuit according to the embodiment of the present invention. This filter circuit has a constant resistor R on the input side and an operational amplifier A1 having the same constant resistance R as a feedback resistor, and an input side of the operational amplifier A2 on the second stage. Is provided with a variable resistance circuit of the present invention (shown in FIG. 2), and a first-order low-pass filter is constituted by a complete integrator using a capacitor C provided between an output and an input. .

  As described above, when the operational amplifiers A1 and A2 are used with feedback, the terminals V1 and V2 at both ends of the variable resistance circuit have low impedance due to the feedback of the operational amplifier. Therefore, since the influence of R1g and R2g attached as the equivalent resistance described in FIG. 1 can be ignored, it is suitable for using the variable resistance of the present invention.

  The cutoff frequency fc of the second-stage low-pass filter is as follows.

fc = 1 / (2πR12 * C)
As described above, since the resistance value R12 can be changed by the current applied to the source-grounded transistor N1, the cutoff frequency of the low-pass filter can be changed by the current.

  Further, since the node of V2 is a virtual ground of the operational amplifier circuit A2, it is almost constant potential (VCM)), and the signal amplitude is almost zero. Since the node of V1 is the output terminal of the first stage constant gain amplifier circuit, the minimum value of the amplitude is the ground, and the maximum value is the power supply voltage (VDD).

  Therefore, the voltage applied to the gate of the transistor N1 whose source is grounded in the variable resistance circuit is obtained by dividing the voltage applied to both ends of the variable resistance by resistance. Therefore, when the maximum value is Vgmax and the minimum value is Vgmin, It becomes as the following formula.

Vgmax = Vcm + [R2 / (R1 + R2)] * (VDD−VCM)
Vgmin = [R1 / (R1 + R2)] * VCM
If the design is made so that the minimum value Vgmin does not fall below the threshold voltage of the transistor N1 whose source is grounded, it is possible to operate even with a low power supply voltage.

  In FIG. 6, the variable resistor circuit shown in FIG. 2 is used as the variable resistor circuit. However, the same operation is possible even if another variable resistor circuit is used.

  Next, an embodiment of the present invention in which the variable resistance circuit according to the present invention is applied to a variable gain amplifier will be described with reference to FIG.

  FIG. 7 is a circuit diagram of the variable gain amplifier circuit according to the embodiment of the present invention. This variable gain amplifier circuit includes a constant resistor R on the input side and a first-stage amplifier circuit composed of an operational amplifier A1 that also has a constant resistor R as a feedback resistor, and a second-stage amplifier circuit. The variable resistance circuit of the present invention (Rv1 (variable resistance value Rv1): shown in FIG. 2) is provided on the input side of the amplifier A2, and the variable resistance circuit of the present invention (resistance value Rv2 (variable resistance value) is also used as a feedback resistance circuit. Rv2): one shown in FIG. 2) between the output and the input.

  Also in this case, since the operational amplifiers A1 and A2 are used with feedback, the terminals V1 and V2 at both ends of the variable resistance circuit have low impedance due to the feedback of the operational amplifier. Therefore, since the influence of R1g and R2g attached as the equivalent resistance described in FIG. 1 can be ignored, it is suitable for using the variable resistance of the present invention.

  The gain (Av2) of the second stage variable gain amplifier circuit is determined by the ratio of the variable resistor Rv1 at the input terminal of the operational amplifier circuit A2 and the variable resistor Rv2 connected between the input terminal and the output terminal of the operational amplifier A2. The following equation is obtained.

Av2 = Rv2 / Rv1
The resistance values of Rv1 and Rv2 can be varied by currents I1 and I2 supplied to the common source transistor N1, respectively. By controlling I1 so that Rv1 becomes large and I2 so that Rv2 becomes small, the gain of the variable gain amplifier circuit can be made small.

  Further, the gain of the variable gain amplifier circuit can be increased by controlling I1 so that Rv1 becomes small and I2 so that Rv2 becomes large.

  Further, although the variable resistance circuit shown in FIG. 2 is used as the variable resistance circuit in FIG. 7, the same operation is possible even if another variable resistance circuit is used.

  The present invention is not limited to the above-described embodiments, and various modifications and additions of components can be made without departing from the spirit of the present invention.

FIG. 3 is a circuit diagram illustrating an embodiment of the present invention. FIG. 3 is a circuit diagram illustrating an embodiment of the present invention. FIG. 3 is a circuit diagram illustrating an embodiment of the present invention. FIG. 3 is a circuit diagram illustrating an embodiment of the present invention. FIG. 3 is a circuit diagram illustrating an embodiment of the present invention. FIG. 3 is a circuit diagram illustrating an embodiment of the present invention. FIG. 3 is a circuit diagram illustrating an embodiment of the present invention.

Explanation of symbols

N1 ... Transistor

Claims (7)

First and second resistance elements connected in series between an input terminal and an output terminal;
A third resistance element having one end connected between the first and second resistance elements;
A grounded-source transistor having a drain connected to the other end of the third resistance element;
A variable resistance circuit characterized in that a resistance value between the input terminal and the output terminal is controlled by changing an impedance of the transistor by a control signal applied to a gate of the transistor. First and second resistance elements connected in series between an input terminal and an output terminal;
A drain connected between the first and second resistance elements, and a source-grounded transistor having the drain and gate connected;
A variable resistance circuit, wherein a resistance value between the input terminal and the output terminal is controlled by changing a current applied as a control signal to the drain of the transistor by a variable current source. 3. The variable resistance circuit according to claim 2, wherein a third resistance element is interposed between the first and second resistance elements and a drain of the transistor. 4. The variable resistance circuit according to claim 3, wherein a current as a control signal from the variable current source is applied to the transistor through the third resistance element or directly to the drain of the transistor. 2. The variable resistor according to claim 1, wherein a plurality of series connections of the third resistance element and a common-source transistor having a drain connected to the other end of the third resistance element are provided in parallel. circuit. 6. A variable resistance circuit according to claim 1 connected to an input side of the operational amplifier; and a capacitive load provided between an output and an input of the operational amplifier. Filter circuit. 6. The variable resistance circuit according to claim 1 connected to an input side of the operational amplifier; and the variable resistance circuit according to claim 1 provided between an output and an input of the operational amplifier. And a variable gain amplifier circuit.

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