JPH0879008A - Gm-c filter - Google Patents

Abstract

PURPOSE: To provide the filter to keep a low current consumption characteristic with low distortion. CONSTITUTION: Captions 12,13 are Gm amplifiers with a relation to a cut-off frequency, a caption 14 is a Gm amplifier relating to the Q, a caption 11 is a Gm amplifier relating to a gain of the filter. Captions 17, 18 are amplifiers controlling a common mode output signal level of the Gm amplifiers 12, 13. A caption 2 is a PLL circuit controlling the Gm of the Gm amplifiers 12, 13. A caption 3 is a PLL circuit controlling the Gm of the amplifiers 11, 14. Since the cut-off frequency and the Q/gain are controlled independently, even when a ratio of the Gm of the Gm amplifier deciding the cut-off frequency to the Gm of the Gm amplifier deciding the Q is increased, the size ratio of transistors(TRs) in the Gm amplifiers is selected independently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Gm-C filter having good linear performance, capable of low current consumption operation, and excellent filter accuracy.

[0002]

2. Description of the Related Art In recent years, a Gm-C filter has attracted attention as a filter that can be integrated into an LSI and can operate at high speed, and that makes use of the characteristics of a time-continuous system. The Gm-C filter includes a Gm amplifier, a capacitor, and a circuit for controlling the Gm value.

As conventionally known, since the Gm-C filter is a continuous-time filter, it can be operated at high speed, and the switched capacitor filter (SC
Since it does not have the folding effect as in F), it has been drawing attention in recent years. However, due to the linearity problem inherent to the Gm amplifier, no practical level has been known. Therefore, in order to solve this problem, the applicant of the present invention has disclosed in FIG.
The Gm-C filter shown in is proposed.

The Gm-C filter shown in FIG. 10 is a biquadratic type filter having good linear performance and capable of low current consumption operation. Here, 80 is a Gm amplifier that controls the gain of the filter, 81 and 82 are Gm amplifiers that control the cutoff frequency of the filter, 83 is a Gm amplifier that controls the Q value of the filter, and 84 is an in-phase output signal of the Gm amplifier 81. An amplifier for controlling the level, 8
Reference numeral 5 is an amplifier for controlling the in-phase output signal levels of the Gm amplifiers 80, 82 and 83. A bias circuit 86 generates a common-mode signal level, which is usually a PLL circuit.

FIG. 2 shows a detailed circuit configuration within a broken line shown in FIG. In FIG. 2, M ₁ and M ₂ are input MOSFETs. M ₃ and M ₄ are MOSFETs for in-phase level control. M ₅ and M ₆ are cascode MOSFETs for increasing the DC gain. As described above, the first Gm amplifier Gm ₁ is composed of six Ms M _{1 to} M _6.
It is composed of an OSFET.

Further, CMC in FIG. 2 is an in-phase level control amplifier, B ₁₁ is a reference voltage input terminal for determining the in-phase level, and B ₁₂ is a MOSFET M ₃ for controlling the in-phase level.
An input terminal of M ₄ and B ₁₃ are bias terminals of the cascode MOSFET. Further, Gm ₂ is a second Gm amplifier, and its circuit configuration is the same as Gm ₁ .

Next, the operation of the circuit shown in FIG. 2 will be described. First, when the input voltage V _in is applied to the input terminals I _n1 and I _n2 as an initial value, it is assumed that the output level thereof matches the voltage value V _B11 applied to the bias terminal B ₁₁ . Here, consider a case where the common-mode voltage applied to the input terminals I _n1 and I _n2 rises.

At this time, the output level tends to fall below V _B11 by the inverter operation. Further, the voltage shown in the following expression (1) is output from the output terminal B _{12 of} the common-mode level control amplifier CMC.

[0009]

[Equation 1]

As a result, the voltage at the terminal B ₁₂ falls below the initial value level, and the output voltage tries to rise again. When the gain of the control amplifier CMC is sufficiently higher than the gain of Gm ₁ , the output level (Vout11
+ Vout12) / 2 is always equal to the V _B11. As a result, the Gm value of Gm ₂ which is the Gm amplifier of the next stage is the voltage V applied to the third input terminal of the control amplifier CMC.
It will be controlled by _B11 .

That is, in the circuit shown in FIG.
The in-phase output level of the Gm amplifier indicated by m ₁ coincides with B 11. The Gm value of this Gm amplifier is

[0012]

[Number 2] _{Gm = 2K (V IN -V TH} ) ... a (2). Here, K is a constant including the size ratio of the transistors, V _IN is an input signal applied to the gate terminals of M ₁ and M ₂ , and V _TH is the threshold voltage of the MOSFET.

According to the above equation (2), the Gm of the Gm amplifier is
Since the m value is determined by the input signal level, the Gm value can be controlled by controlling the in-phase input level. Therefore, by controlling the in-phase output signal level of the amplifier indicated by Gm ₁ in FIG. 2 by the amplifier CMC, Gm ₂
It is possible to control the Gm value of. As a result, the Gm value of Gm ₂ becomes linear with respect to the input signal as shown in the equation (2), which means that the linear performance is excellent.

[0014]

However, in FIG. 10 showing a conventional filter circuit, the Q value of the filter is the ratio of the geometric mean of the Gm values of the Gm amplifiers 81 and 82 to the Gm value of the Gm amplifier 83 (√ (Gm ₂ , Gm ₃ ): Gm
₄ ), so when designing a filter with a high Q value, the geometric mean of the Gm values of the Gm amplifiers 81 and 82 √ (G
It is necessary to increase the ratio of m ₂ , Gm ₃ ) to the Gm value (Gm ₄ ) of the Gm amplifier 83.

For example, when designing the Q value of the filter to be 20, if the Gm values of the Gm amplifiers 81 and 82 are the same (Gm ₂ = Gm ₃ ), Gm ₃ : Gm ₄ =
It will be 20: 1. When the size W / L of the MOSFET is calculated so as to maintain this ratio, the input MOS of the Gm amplifier 82 is calculated.
FET size is W / L = 40μ / 6μ, Gm amplifier 83
The input MOSFET size is W / L = 2μ / 6μ (W: MOSFET channel width, L: MOSFET channel length).

At this time, in order to maintain the relative accuracy of the size, G
20 m / A = 2 μ / 6 μ may be used as the m-amp 82, but a small device size of W / L = 2 μ / 6 μ is not a preferable size in terms of manufacturing technology. On the other hand, in order to improve the relative accuracy, W /
L = 20 μ / 60 μ may be used as the unit size. However, when such a large size is used, the channel length L of the MOSFET becomes large, so that the high speed filter is caused by the parasitic pole generated depending on the channel length L. The performance will be compromised when

Here, the parasitic pole = μC _OX / L ² . Here, μ represents the mobility of carriers, C _OX represents the gate capacitance value, and L represents the channel length of the MOSFET.

As described above, since the Gm value of the Gm amplifier which determines the cutoff frequency, the filter gain value and the Q value of the filter all depends on the in-phase input voltage level of the Gm amplifier, the relative accuracy of the above W / L. It is an important factor to improve the filter design.

Therefore, in view of the above points, the first object of the present invention is to control the Gm value of a Gm amplifier that defines the cutoff frequency in the Gm value control system of the Gm amplifier that defines the Q value or gain of the filter. An object of the present invention is to provide a Gm-C filter configured to achieve desired electrical characteristics without being restricted by the device size by making the Gm-C filter completely independent of the system.

A second object of the present invention is to provide a Gm-C filter which can accurately set the Q value, the gain of the filter and the cutoff frequency while having low distortion and low current consumption. is there.

[0021]

In order to achieve such an object, the present invention provides a Gm amplifier including Gm amplifiers for controlling the gain, cutoff frequency and Q value of a filter.
-A C filter that controls the cutoff frequency of the filter, controls the Gm value of the Gm amplifier according to the in-phase input signal value of the Gm amplifier, and controls the gain and / or the Q value of the filter. Regarding the Gm amplifier, the Gm value is controlled by the bias value supplied from the bias setting means provided separately (see FIG. 1).

Here, the Gm amplifier for controlling the cutoff frequency connects a differential input type MOSFET pair and a MOSFET pair having a gate input end for common mode level control in series, and a pair of differential output ends. It is preferable to supply the output current from (see FIG. 2).

In addition, a voltage corresponding to the difference between the output voltage of the Gm amplifier controlling the cutoff frequency and a predetermined voltage is applied to the common mode level control terminal of the Gm amplifier controlling the cutoff frequency. It is preferable to have means (see FIG. 1).

G for controlling the cutoff frequency
The m-amplifier includes a first Gm amplifier in which a differential input type MOSFET pair and a MOSFET pair having a gate input end for in-phase level control are connected in series and which supplies an output current from a pair of differential output ends. A second Gm amplifier connected to the pair of differential output terminals and three input terminals, and a pair of differential output terminals of the first Gm amplifier for the first and second input terminals. And a third input end connected to a bias voltage source for controlling the Gm value of the second Gm amplifier, and the gate input for the common mode level control included in the first Gm amplifier. By providing an in-phase level control amplifier having an output end for applying a control voltage to the end, the Gm value in the second Gm amplifier is
It is also possible to control by the bias voltage applied to the third input terminal of the common-mode level control amplifier (see FIG. 2).

Further, the Gm amplifier for controlling the gain and / or Q value is a differential input type MOSFET pair,
A current source MOSFET connected to the sources of the differential input type MOSFET pair may be provided, and a predetermined bias voltage may be applied to the control gate of the current source MOSFET. (See Figure 3). Alternatively, the Gm amplifier that controls the gain and / or the Q value is a differential input type M
OSFET pair, current source MOSFET connected to the source side of the differential input type MOSFET pair, and the differential input type M
Load MOSF connected to the drain side of the OSFET pair
It is preferable to have an ET pair and to make the current value of the current source MOSFET and the current value of the load MOSFET pair match (see FIG. 3).

Further, the differential input MOSFET of the Gm amplifier for controlling the cutoff frequency according to claim 2
It is preferable that the polarities of the differential input MOSFETs of the Gm amplifier for controlling the gain and / or the Q value in claim 5 are opposite to each other (FIG. 2, FIG.
(See FIG. 3).

[0027]

According to the above configuration of the present invention, the Gm amplifier having the characteristics of low distortion and low current consumption is used as the Gm amplifier for defining the cutoff frequency of the filter.
Gm that defines the gain and / or Q value of the filter
As for the amplifier, it is possible to use a Gm amplifier that can set the Gm value irrespective of the common-mode input voltage. Therefore, select the optimum device in terms of response speed and relative accuracy as the device used for each Gm amplifier. Will be possible.

[0028]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a block configuration of a Gm-C filter to which the present invention is applied. In the figure, 11 to 14 are Gm amplifiers. G shown as 12, 13
The m-amplifier is a Gm-amplifier related to the cutoff frequency and uses the circuit shown in FIG. Reference numeral 14 is a Gm amplifier related to the Q value, and 11 is a Gm amplifier related to the gain of the filter. Reference numerals 15 and 16 are capacitors, and 17 and 18 are amplifiers for controlling the in-phase output signal levels of the Gm amplifiers 12 and 13.

Reference numeral 2 is a PLL circuit for controlling the Gm value of the Gm amplifiers 12 and 13. The Gm amplifier having the same circuit configuration as the Gm amplifiers 12 and 13 is used for this PLL circuit. 3 is a PL for controlling the Gm value of the Gm amplifiers 11 and 14.
The Gm amplifiers 11 and 1 are L circuits.
A Gm amplifier having the same circuit configuration as that of No. 4 is used. The circuit used for the Gm amplifiers 11 and 14 is shown in FIG.
Shown in

As described above, the Gm amplifiers 12 and 13 use the circuit shown in FIG. Therefore, as described above, there are advantages that the linear performance is excellent and the current consumption can be suppressed. However, in order to control the Gm value, it is necessary to control the common mode input voltage level of the Gm amplifier by the amplifier.

On the other hand, as the Gm amplifier for controlling the Q value or the gain, a Gm amplifier different from the configuration of FIG. 2 is used.
Specifically, the Gm value control of these Gm amplifiers adopts a circuit configuration irrelevant to the in-phase input voltage level. By doing so, the cutoff frequency and the Q value / gain can be controlled independently, so that even if the ratio of the Gm values of the Gm amplifier that determines the cutoff frequency and the Gm amplifier that determines the Q value becomes large, The advantage is that the size ratio of the transistors can be set independently. In other words, the fact that the transistor sizes can be set independently of each other eliminates the need to use a large number of unit transistors of small size, which is not preferable in terms of relative accuracy, in order to ensure relative accuracy, and also reduces the number of unit transistors. The advantage also occurs.

FIG. 3 shows an example of a circuit used for the Gm amplifiers 11 and 14 in FIG. In FIG. 3, reference numeral 31 is a current source MOSFET for controlling the Gm value, and the bias voltage generated from the PLL circuit 3 is used as its bias voltage. Further, 32 and 33 are input MOSFET pairs, which are input MOSFs of the Gm amplifiers 12 and 13 in FIG.
The polarity is opposite to that of the ET pair. Because G in Fig. 1
The polarity of the input MOSFETs of the m amplifiers 12 and 13 is NMO.
In the case of S, the signal operating point in FIG. 1 is offset to the V _SS side, and when the Gm amplifier shown in FIG. 3 is used in FIG. _The signal range can be extended to the _SS side.

34 and 35 are current source MOSFETs for supplying a bias current value such that the output level can realize a preferable output voltage level in the circuit of FIG.
Is. That is, the current flowing through the MOSFET 31 and M
It is more preferable to set the sum of the currents flowing through the OSFETs 32 and 33 to be the same, because the output current value becomes zero when there is no signal and does not affect other circuits. On the other hand, when the currents do not match, the output current is constantly generated from the Gm amplifier 14 in the circuit of FIG. 1, for example, and the in-phase output signal level of the Gm amplifier 13 is set to a certain value in order to set the same-phase output signal level. The amplifier 18 for controlling the output signal level is overloaded, and in some cases delicate frequency control may not be possible.

For this purpose, the MOSFET 3 shown in FIG.
A bias voltage may be applied to the transistors 4 and 35 by the bias circuit 38 including the MOSFETs 36 and 37 so that half the current of the MOSFET 31 flows.

FIG. 4 is a specific circuit diagram of the PLL circuit 2 shown in FIG. This PLL circuit is a bias generation circuit for controlling the Gm values of the Gm amplifiers 12 and 13 in FIG. In FIG. 4, 41 to 44 are Gm amplifiers, and particularly Gm amplifiers 42 and 43 are
It is necessary to use a circuit having the same configuration as the m amplifiers 12 and 13. The reason for this will be clarified in the following explanation of the operation of the PLL circuit. Because of the simplicity of the circuit, the Gm amplifier 4
The same Gm amplifier is used for 1 to 44.

Further, 45 and 46 in FIG.
Reference numerals 48 and 48 are amplifiers for controlling the in-phase output signal level used for the same reason as 17 and 18 in FIG.

Gm-C having these elements 41 to 48
When the input terminal is 40 and the output terminal is 40a, the filter has a low-pass filter characteristic, and at the same time has a phase shift of 0 ° in the low range and a phase shift of 180 ° in the high range as shown in FIG. , And has a phase characteristic that the phase shift becomes 90 ° at the cutoff frequency. That is, when the frequency of the input signal matches the cutoff frequency, the filter input signal and the filter output signal pass through the comparators 49 and 50, and further pass through the exclusive OR circuit EXOR which functions as a phase comparator. The frequency of the output signal is twice that of the input signal, and the periods of high-level logic and low-level logic are equal (so-called duty ratio is 50%). At this time, the DC output level of the integrator does not change even if it passes through the integrator (INT) that functions as a low pass filter, and the phase locked state can be realized.

If the cutoff frequency of the filter composed of 41 to 48 is smaller than the set value, the phase delay becomes larger than the designed value, as can be seen from FIG. As a result, in the output of the phase comparator EXOR, the high level logic period becomes shorter than the low level logic period, and the integrator output level is lowered. In a device that generates a bias voltage when the output level drops,
The Gm value of all Gm amplifiers is increased.
In particular, the Gm values of the Gm amplifiers 42 and 43 determine the cutoff frequency of the filter composed of 41 to 48, and the cutoff frequency also increases as the Gm value increases. Then, the integrator output level shifts in the direction in which the cutoff frequency of the filter becomes equal to the design value, and finally the duty ratio of the output signal of the phase comparator EXOR becomes 5
When it reaches 0% (that is, when the cutoff frequency of the filter becomes equal to the designed value), the integrator output settles at a constant level.

Even when the cutoff frequency of the filter is higher than the designed value, the same operation is performed, and finally the cutoff frequency of the filter becomes equal to the designed value, and the integrator output falls to a constant level. Gm amplifier 4
Since the Gm values of 1 and 44 are independent of the cutoff frequency, it is not necessary to have the same circuit configuration as the Gm amplifiers 42 and 43. However, from the viewpoint of facilitating the circuit design, it is preferable to use the same Gm amplifier.

FIG. 7 is a specific circuit diagram of the PLL circuit 3 shown in FIG. 1, and controls the Gm value in the Gm amplifiers 11 and 14 in FIG. In FIG. 7, 51 to 54 are G
This is an m-amplifier, and in particular, the circuits of the Gm amplifiers 52 and 53 need to use circuits having the same configuration as the Gm amplifiers 11 and 14 in FIG. The reason for this will be clarified in the following explanation of the operation of the PLL circuit. Gm because of the simplicity of the circuit
The same Gm amplifier circuit is used for the amplifiers 51 to 54. Also, 55 and 56 are capacitors.

The Gm-C filter having the elements 51 to 56 has a low pass filter characteristic when the input terminal is 57 and the output terminal is 57a, and at the same time, the phase shift is 0 in the low band as shown in FIG. °, phase shift is 18 at high frequencies
90 ° phase shift at 0 ° and cutoff frequency
It has the following phase characteristics. That is, when the frequency of the input signal matches the cutoff frequency, the filter input signal and the filter output signal are the comparators 59 and 6 respectively.
After passing 0, and further passing through the exclusive OR circuit EXOR which functions as a phase comparator, the output signal has twice the frequency of the input signal and the periods of the high level logic and the low level logic are equal (so-called). Duty ratio 50%
Will be). At this time, the DC output level of the integrator does not change even when passing through the integrator INT that also functions as a low-pass filter, and the phase locked state can be realized.

If the cutoff frequency of the filter composed of 51 to 56 is smaller than the designed value, the phase delay becomes larger than the designed value, as can be seen from FIG. As a result, in the output of the phase comparator EXOR, the high level logic period becomes shorter than the low level logic period, and the integrator output level is lowered. In a device that generates a bias voltage when the output level drops,
The Gm value of all Gm amplifiers is increased.
In particular, the Gm values of the Gm amplifiers 52 and 53 determine the cutoff frequency of the filter composed of 51 to 56, and the cutoff frequency also increases as the Gm value increases. Therefore, the integrator output level shifts in the direction in which the cutoff frequency of the filter becomes equal to the design value, and finally the duty ratio of the output signal of the phase comparator EXOR becomes 5
When it reaches 0% (that is, when the cutoff frequency of the filter becomes equal to the designed value), the integrator output settles at a constant level.

Even when the cutoff frequency of the filter is higher than the design value, the same operation is performed, and finally the cutoff frequency of the filter becomes equal to the design value, and the integrator output falls to a constant level. The Gm amplifier 5
Since the Gm values of 1 and 54 are independent of the cutoff frequency, it is not necessary to have the same circuit configuration as the Gm amplifiers 52 and 53. However, from the viewpoint of facilitating the circuit design, it is preferable to use the same Gm amplifier.

In the circuit of FIG. 1, the circuit of the Gm amplifier 11 that controls the gain of the filter and the Gm amplifier 14 that controls the Q value have the same circuit configuration.
If the Gm value of the m-amplifier 11 has a value closer to the Gm value of the Gm amplifiers 12 and 13 that controls the cutoff frequency than the Gm value for the Q value, the same circuit as the Gm amplifiers 12 and 13 is used. It is more preferable to use.

In the circuit of FIG. 1, the Gm amplifier 1
1 and 14, the Gm amplifier of FIG. 2 which is excellent in terms of low current consumption is not used, but in the case of a filter with a high Q value, the Gm value of the Q value control amplifier is originally The Gm value of the cut-off frequency control Gm amplifier is as small as 1 / Q times, and the current consumption is also small at the same time. Therefore, even if the circuit of FIG. Not only that, but as a result,
Low current consumption characteristics are inherited.

Further, as the Gm amplifiers 11 and 14, any circuit other than the circuit shown in FIG. 3, for example, a circuit shown in FIG. 8 can be used as long as the Gm value can be determined without depending on the in-phase input signal level. It may be configured. In the circuit of FIG. 8, 61 and 62 are input MOSFETs, 63 to 6
6 is a MOSFET for a constant current source, and 67 and 68 are MOSFETs that determine the Gm value.

FIG. 9 shows another embodiment of the present invention. This embodiment is a 6th-order bandpass filter (BPF) of the reprog type. In the figure, 101 to 113
Is a Gm amplifier. Of these, Gm amplifiers 101 to 106 determine the cutoff frequency of the filter, and the circuit of FIG. 2 is used. Reference numerals 107 to 112 denote Gm amplifiers that determine the Q value of the filter, for example, the circuit of FIG. 3 is used. 1
Reference numeral 13 is a Gm amplifier that determines the gain of the filter, and for example, the circuit of FIG. 3 is used. 114 to 119 are capacity, 12
0 to 125 are amplifiers that control the in-phase output signal levels of the Gm amplifiers 101 to 106. 126 is a Gm amplifier 1
This is a PLL circuit that controls the Gm value of 01 to 106.
A circuit as shown in FIG. 4 can be used for the LL circuit. The Gm amplifiers 42 and 43 in FIG.
The same circuit configuration as 1 to 106 is used.

127 is the Gm of the Gm amplifiers 107-113.
This is a PLL circuit that controls a value, and as the PLL circuit, a circuit as shown in FIG. 7 can be used. The Gm amplifiers 52 and 53 in FIG. 7 have the same circuit configuration as the Gm amplifiers 107 to 113.

In each of the embodiments shown in FIGS. 1 and 9, each signal line has a single line output (single signal), but in order to improve the distortion characteristic and the S / N characteristic, it is shown in FIG. It is also possible to perform such a fully differential output. Also in this case, the same effect as the above-described embodiment can be obtained.

[0051]

As described above, according to the present invention, the Gm of the Gm amplifier that defines the Q value or gain of the filter is determined.
For the m-value control system, G that defines the cutoff frequency
Since it is configured to be completely independent of the Gm value control system of the m-amplifier, it is possible to maintain low distortion and low current consumption characteristics, and to impair the above characteristics even when the gain and / or Q value is increased. It is possible to improve the accuracy of the gain and / or the Q value and the cut-off frequency without the need.

[Brief description of drawings]

FIG. 1 is a block diagram showing a Gm-C filter according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific example of the Gm amplifier shown in FIG.

FIG. 3 is a circuit diagram showing a specific example of the Gm amplifier shown in FIG.

4 is a block diagram showing an example of a PLL circuit 2 shown in FIG.

FIG. 5 is a diagram showing phase characteristics when the filter in the PLL circuit is ideal.

FIG. 6 is a diagram showing a phase characteristic when the filter in the PLL circuit deviates from an ideal case to a low frequency.

7 is a block diagram showing an example of a PLL circuit 3 shown in FIG.

8 is a circuit diagram showing a specific example of the Gm amplifier shown in FIG.

FIG. 9 is a block diagram showing a Gm-C filter according to a second embodiment of the present invention.

FIG. 10 is a block diagram showing a conventionally known Gm-C filter.

[Explanation of symbols]

 2,3 PLL circuit 11 Gm amplifier for gain control 12,13 Gm amplifier for cutoff frequency control 14 Gm amplifier for Q value control 15,16 Capacitance 17,18 Amplifier for common mode output signal level control

Claims (7)

[Claims] 1. A filter gain, cutoff frequency,
Gm-C with each Gm amplifier for controlling Q value respectively
A Gm amplifier that controls a cutoff frequency of the filter, controls the Gm value by the in-phase input signal value of the Gm amplifier, and controls the gain and / or the Q value of the filter.
The Gm-C filter is characterized in that the Gm value of the amplifier is controlled by the bias value supplied from the bias setting means provided separately. 2. The Gm amplifier for controlling the cutoff frequency according to claim 1, wherein a differential input type MOSFET pair and a MOSFET pair having a gate input terminal for common mode level control are connected in series, A Gm-C filter characterized by supplying an output current from a differential output terminal. 3. The Gm amplifier according to claim 1, further comprising: a voltage corresponding to a difference between an output voltage of the Gm amplifier that controls the cutoff frequency and a predetermined voltage of a Gm amplifier that controls the cutoff frequency. A Gm-C filter comprising means for applying to a common mode level control terminal. 4. The Gm amplifier for controlling the cutoff frequency according to claim 1, wherein a differential input type MOSFET pair and a MOSFET pair having a gate input end for common mode level control are connected in series,
A first Gm amplifier that supplies an output current from a pair of differential output terminals, a second Gm amplifier that is connected to the pair of differential output terminals, and three input terminals. The second input terminal is connected to a pair of differential output terminals of the first Gm amplifier, and the third input terminal is connected to a bias voltage source for controlling the Gm value of the second Gm amplifier. And a common-mode level control amplifier having an output terminal for applying a control voltage to the common-mode level control gate input terminal included in the first Gm amplifier. A Gm-C filter, the value of which is controlled by a bias voltage applied to a third input terminal of the common-mode level control amplifier. 5. The Gm amplifier for controlling the gain and / or Q value according to claim 1, wherein a differential input type MOSFET pair and the differential input type MOSFET are provided.
A Gm-C filter comprising: a current source MOSFET connected to the source side of the T pair; and applying a predetermined bias voltage to the control gate of the current source MOSFET. 6. The Gm amplifier for controlling the gain and / or Q value according to claim 1, wherein a differential input type MOSFET pair and the differential input type MOSFET are provided.
A current source MOSFET connected to the source side of the T pair and a load MOSFET pair connected to the drain side of the differential input type MOSFET pair, and the current value of the current source MOSFET and the current of the load MOSFET pair. A Gm-C filter characterized by matching a value. 7. The polarities of the differential input MOSFET of the Gm amplifier for controlling the cutoff frequency according to claim 2 and the differential input MOSFET of the Gm amplifier for controlling the gain and / or Q value according to claim 5. The Gm-C filter is characterized in that