1. Field of the Invention
The present invention relates to a bias circuit, and a gm-C filter circuit and a semiconductor integrated circuit each including the same.
2. Description of the Related Art
A filter circuit for executing signal processing is extensively used in electronic apparatuses such as a wireless communication apparatus and an optical disc device. Miniaturization and speed-up are strongly demanded for such an electronic apparatus. Along with this strong request, the miniaturization and the speed-up are also demanded for the filter circuit.
A gm-C filter circuit composed of Operational Transconductance Amplifiers (OTAs) and capacitors is known as a filter circuit which is provided in order to realize the miniaturization and the speed-up.
For the purpose of suppressing a change in value of a transconductance gm caused by a change in temperature or change in power source voltage, as shown in
In the figure, when a ratio of a current supplied from a current source I10 to the NMOS transistor Q13 to a current supplied from the current source I10 to an NMOS transistor Q14 is k:1, a transconductance gm1 of the NMOS transistor Q13 can be expressed by Expression (1):
Where W1 is a channel width of the NMOS transistor Q13, L1 is a channel length of the NMOS transistor Q13, W2 is a channel width of the NMOS transistor Q14, and L2 is a channel length of the NMOS transistor Q14.
It is understood from Expression (1) that there is obtained a value of the transconductance gm1 which is independent of a manufacture dispersion of threshold independent of a manufacture dispersion of threshold voltages Vth of the NMOS transistors Q13, and changes in temperature and power source voltage.
However, the bias circuit 100 shown in
In a bias circuit 200 shown in
Therefore, a ratio, k, of a drain-to-source current of the PMOS transistor Q11 to a drain-to-source current of the PMOS transistor Q12 decreases with a rise in temperature. As a result, as shown in
It is also expected that a PMOS transistor is cascode-connected between the PMOS transistor Q11 and the NMOS transistor Q13, and a PMOS transistor is cascode-connected between the PMOS transistor Q12 and the NMOS transistor Q14. In this case, however, the phenomenon described above is merely lightened, and the situation stops short of solving the problem.
A bias circuit including a first voltage-to-current converting circuit, a resistor Rext, a second voltage-to-current converting circuit, a current-to-voltage converting circuit, and a feedback section, as shown in
With the circuit described in Patent Document 1, the transconductance gm of a transistor M3 is expressed by a product of a reciprocal number of a resistance value of the resistor Rext, and (V2/V1), and thus can be precisely set.
However, the circuit described in Patent Document 1 involves a problem that at least two buffers need to be provided and so forth, and thus a mounting area largely increases.
A present embodiment has been made in order to solve the problems described above, and it is therefore desirable to provide a bias circuit which is capable of suppressing an influence caused by a change in temperature, and a manufacture dispersion of threshold voltages Vth, and a gm-C filter circuit and a semiconductor integrated circuit each including the same.
In order to attain the desire described above, according to an embodiment of the present invention, there is provided a bias circuit including: a first PMOS transistor forming a first current source; a second PMOS transistor composing a current mirror circuit of the first PMOS transistor and forming a second current source; a first NMOS transistor having a drain to which a current is supplied from the first current source; a second NMOS transistor composing a current mirror circuit together with the first NMOS transistor, and having a drain to which a current is supplied from the second current source; and a resistor connected between a source of the second NMOS transistor and a ground; in which a resistance component for gm adjustment is connected between a source of the first NMOS transistor and the ground.
According to another embodiment of the present invention, there is provided a gm-C filter circuit including: a gm-C filter portion; and a bias circuit configured to output a bias voltage to the gm-C filter portion; the bias circuit including: a first PMOS transistor forming a first current source; a second PMOS transistor composing a current mirror circuit of the first PMOS transistor and forming a second current source; a first NMOS transistor having a drain to which a current is supplied from the first current source; a second NMOS transistor composing a current mirror circuit together with the first NMOS transistor, and having a drain to which a current is supplied from the second current source; and a resistance connected between a source of the second NMOS transistor and a ground; in which a resistor component for gm adjustment is connected between a source of the first NMOS transistor and the ground, and the gm-C filter portion receives as its input a bias voltage at the drain of the first NMOS transistor.
According to still another embodiment of the present invention, there is provided a semiconductor integrated circuit including: a gm-C filter portion; and a bias circuit configured to output a bias voltage to the gm-C filter portion; the bias circuit including: a first PMOS transistor forming a first current source; a second PMOS transistor composing a current mirror circuit of the first PMOS transistor and forming a second current source; a first NMOS transistor having a drain to which a current is supplied from the first current source; a second NMOS transistor composing a current mirror circuit together with the first NMOS transistor, and having a drain to which a current is supplied from the second current source; and a resistor connected between a source of the second NMOS transistor and a ground; in which a connection terminal is provided for connection of a resistance component for gm adjustment between a source of the first NMOS transistor and the ground.
As set forth hereinabove, according to the present invention, it is possible to provide the bias circuit which is capable of suppressing the influence of the change in temperature and the manufacture dispersion, and the gm-C filter circuit and the semiconductor integrated circuit each including the same. Therefore, for example, the highly precise gm-C filter circuit which is capable of setting a desired cut-off frequency can be provided with a smaller mounting area than that in an existing art. In addition, since it is unnecessary to provide a differential amplifier used to compose a general operational amplifier, the operation is also possible at a lower voltage (for example, a power source voltage of 1 V or less) than that in the differential amplifier.
A gm-C filter circuit according to an embodiment of the present invention is formed within a semiconductor integrated circuit (IC). Also, the gm-C filter circuit is highly precise filter circuit which can suppress an influence of a change in temperature and a manufacture dispersion, and thus can set a desired cut-off frequency. Thus, miniaturization and speed-up are realized for the gm-C filter circuit.
The gm-C filter circuit has a gm-C filter portion composed of Operational Transconductance Amplifiers (OTAs) and capacitors.
In addition, the gm-C filter circuit is provided with a bias circuit for maintaining a transconductance gm of each of the OTAs in the gm-C filter portion.
The bias circuit includes a first PMOS transistor forming a first current source, and a second PMOS transistor composing a current mirror circuit of the first PMOS transistor and forming a second current source.
In addition, the bias circuit includes a first NMOS transistor having a drain terminal to which a current is supplied from a first current source, and a second NMOS transistor composing a current mirror circuit together with the first NMOS transistor, and having a drain terminal to which a current is supplied from a second current source.
Also, the bias circuit includes a resistor connected between a source terminal of the first NMOS transistor and the ground.
Moreover, the bias circuit includes a resistance component for gm adjustment which is connected between a source terminal of the second NMOS transistor and the ground.
By providing the resistance component for gm adjustment in such a manner, it is possible to provide the bias circuit for which the influence of the change in temperature and the manufacture dispersion is suppressed.
Therefore, for example, by using the bias circuit, the highly precise gm-C filter circuit can be provided with a smaller mounting area than that in the existing art. In addition, since it is unnecessary to provide a differential amplifier used to configure a general operational amplifier, the operation is also possible at a lower voltage (for example, a power source voltage of 1 V or less) than that in the general operational amplifier.
Hereinafter, a bias circuit according to an embodiment of the present invention, and a gm-C filter circuit including the same will be concretely described.
1. gm-C Filter Circuit
Concrete configurations of the bias filter and the gm-C filter circuit including the same will be described in detail hereinafter with reference to the accompanying drawings.
Firstly, the concrete configuration of the gm-C filter circuit will be described with reference to
As shown in
The gm-C filter portion 10 is a filter circuit which can set a desired cut-off frequency, and is a composed of an OTA1 to an OTA4, and capacitors C1 and C2. It is noted that transconductances of the OTA1, the OTA2, the OTA3, and the OTA4 are represented by gm1, gm3, gm2, and gm1, respectively.
An input signal Vin is inputted across input terminals (+, −) of the OTA1, and is amplified by the OTA1 to be outputted through output terminals (+, −) of the OTA1. The output (+, −) from the OTA1 is inputted across input terminals (+, −) of the OTA2, and is amplified by the OTA2 to be outputted in the form of an output signal Vout through output terminals (+, −) of the OTA2.
The output signal Vout outputted through the output terminals (+, −) of the OTA2 is applied across the capacitor C2, and is then inputted across input terminals (+, −) of the OTA3, input terminals (−, +) of the OTA4, and output terminals (+, −) of the OTA3. In addition, the output terminals (+, −) of the OTA4 are connected between the opposite terminals of the capacitor C1, and between the input terminals (+, −) of the OTA2.
By configuring the gm-C filter circuit 1 in such a manner, the gm-C filter circuit 1 becomes a second-order low pass filter (LPS) having the following characteristics expressed by. Expressions (2) to (4):
where ω0 is a cut-off frequency of the gm-C filter circuit 1, and Q is a quality factor of the gm-C filter circuit 1.
As shown in
In the existing bias circuit, however, the change in power source voltage, the dispersion of the threshold voltage Vth, and the like result in a change in output bias voltage Vb. In order to cope with this situation, in the bias circuit 11 of the embodiment, the influence of the change in temperature, and the manufacture dispersion of the threshold voltages Vth is suppressed.
Next, the configuration and characteristics of the bias circuit 11 described above will be concretely described.
As shown in
The first current source Ia is a current source for supplying a current having a current value KI, and is composed of a first PMOS transistor Q1. Each of a source terminal and a back gate terminal of the first PMOS transistor Q1 is connected to a power source VDD.
The second current source Ib is a current source for supplying a current having a current value I, and is composed of a second PMOS transistor Q2. In this case, each of a gate terminal and a drain terminal of the second PMOS transistor Q2 is connected to a gate terminal of the first PMOS transistor Q1, thereby composing a current mirror circuit. In addition, each of a source terminal and a back gate terminal of the second PMOS transistor Q2 is connected to the power source VDD.
In the bias circuit 11, currents are supplied from the first current source Ia and the second current source Ib to drain terminals of first and second NMOS transistors Q3 and Q4, respectively. That is to say, the drain terminal of the first PMOS transistor Q1 as the first current source Ia, and a drain terminal of the first NMOS transistor Q3 are connected to each other. In addition, the drain terminal of the second PMOS transistor Q2 as the second current source Ib, and a drain terminal of the second NMOS transistor Q4 are connected to each other. Also, a drain current of the first NMOS transistor Q3 is supplied from the first current source Ia, and a drain current of the second NMOS transistor Q4 is supplied from the second current source Ib.
A gate terminal of the second NMOS transistor Q4 is connected to each of a gate terminal and the drain terminal of the first NMOS transistor Q3. Thus, the first and second NMOS transistors Q3 and Q4 compose a current mirror circuit together with each other.
In addition, a resistor R is connected between the source terminal of the second NMOS transistor Q4 and the ground GND.
With the configuration described above, the bias circuit 11 operates so that a product of a mobility (μn) of an electron in the first NMOS transistor Q3, and a current value of a drain-to-source current of the first NMOS transistor Q3 becomes constant, thereby outputting the bias voltage Vb.
However, the ratio, k, of the drain-to-source current of the first PMOS transistor Q1 to the drain-to-source current of the second PMOS transistor Q2 decreases with a rise in temperature. As a result, the transconductance gm1 of the first NMOS transistor Q3 monotonically decreases with the rise in temperature.
In order to cope with this situation, in the bias circuit 11 of the embodiment, the source terminal of the first NMOS transistor Q3 is connected to the ground GND through a resistance component Zr instead of connecting the source terminal of the first NMOS transistor Q3 to the ground GND as with the existing bias circuit.
It is noted that the resistance component Zr may be composed of either a resistor or an NMOS transistor which operates in a triode region.
Here, when a composite transconductance between the third transistor Q3 and the resistance component Zr connected in series with each other is gmt, the composite transconductance gmt can be expressed by Expression (5):
where gm1 is a transconductance of the third transistor Q3.
Therefore, the resistance component Zr is set so that a decreasing rate of the transconductance gm1 following the rise in temperature, and a decreasing rate of {1+(gm1×Zr)} become equal to each other. As a result, as shown in
As described above, in the bias circuit 11, the resistance component Zr is provided, so that a voltage Vr is developed across the source terminal of the first NMOS transistor Q3, and the ground GND. As a result, the decrease in transconductance gm1 caused by the rise in temperature, or the like is offset with the voltage Vr thus developed.
In addition, when the back gate terminal of the first NMOS transistor Q3 is connected to the ground GND, a source-to-back gate voltage Vsb is developed across the source terminal and back gate terminal of the first NMOS transistor Q3, and thus the threshold voltage Vth changes. At this time, however, the provision of the resistance component Zr makes it possible to suppress the temperature dependency of the composite transconductance gmt, and thus the resistance component Zr becomes an effective section configured to suppress the temperature dependency of the composite transconductance gmt.
It should be noted that either a channel width or channel length of each of the first and second PMOS transistors Q1 and Q2, and the first and second NMOS transistors Q3 and Q4 can be given a variable range, and thus the composite transconductance gmt can be given an adjustment range.
In addition, since there is generally the dispersion of at least 10% in the resistance values of the resistors formed within the semiconductor integrated circuit (IC), it is preferable to provide the resistor R outside the semiconductor integrated circuit.
As has been described, in the bias circuit 11 of the embodiment, the resistance component Zr is provided between the source terminal of the first NMOS transistor Q3, and the ground GND, thereby suppressing the temperature dependency of the composite transconductance gmt in the bias circuit 11.
Therefore, the highly precise gm-C filter circuit which can set the desired cut-off frequency can be provided with the smaller mounting area than that in the existing art.
In addition, since it is unnecessary to provide the differential amplifier used to configure the general operational amplifier, the operation is also possible at the lower voltage (for example, the power source voltage of 1 V or less) than that in the general operational amplifier.
Next, a change of the bias circuit of the embodiment will be described.
The bias circuit 21 shown in
In addition to the resistance component Zr, the second resistance component Zs is provided in such a manner, thereby making it possible to further suppress the temperature dependency of the transconductance component gmt.
The semiconductor integrated circuit according to an embodiment of the present invention includes the gm-C filter portion 10, and the bias circuit 11 for outputting the bias voltage to the gm-C filter portion 10. In this case, the bias circuit 11 includes the first PMOS transistor Q1 forming the first current source Ia, the second PMOS transistor Q2 composing the current mirror of the first PMOS transistor Q1 and forming the second current source Ib, the first NMOS transistor Q3 having the drain terminal to which the current is supplied from the first current source Ia, the second NMOS transistor Q4 composing the current mirror circuit together with the first NMOS transistor Q3, and having the drain terminal to which the current is supplied from the second current source Ib, and the resistor R connected between the source terminal of the second NMOS transistor Q4 and the ground GND. In this case, a connection terminal is provided for connection of the resistance component Zr for gm adjustment between the source terminal of the first NMOS transistor Q3 and the ground GND.
Although the embodiments of the present invention has been concretely described so far, the present invention is by no means limited to the embodiments described above, and thus various changes based on the technical idea of the present invention can be made.
The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-012589 filed in the Japan Patent Office on Jan. 23, 2009, the entire content of which is hereby incorporated by reference.
Number | Date | Country | Kind |
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2009-012589 | Jan 2009 | JP | national |