JPH06318843A - Gm-c filter - Google Patents

Abstract

PURPOSE:To avoid production of leakage by providing the Gm-C filter with a linear characteristic having a small input voltage dependency to an in-phase level control cascode amplifier thereby setting an input of an in-phase signal within an appropriate range having a small error. CONSTITUTION:When an input voltage Vin is applied to input terminals In1, In2 as an initial value, it is supposed that its output level is equaled to a voltage VB11 applied to a vias terminal B11. In the case of the in-phase voltage applied to the input terminals In1, In2 to be increasing, the output level is going to be decreased more than the voltage VB11. Moreover, a voltage expressed in equation I is outputted from an output terminal B12 of an in-phase level control amplifier CMC. Thus, the voltage at the output terminal B12 is decreased more than the initial value level and the output voltage is going to increase again. When the gain of the control amplifier CMC is sufficiently higher than the gain Gm1, an output level (Vout11+Vout12)/2 is always equal to the voltage VB11. As a result, the Gm2 of the Gm amplifier of a next stage is controlled by the voltage VB11 applied to a 3rd input terminal of the control amplifier CMC.

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 and capable of low current consumption operation.

[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.

FIG. 10 shows a conventionally known Gm-C.
The structure of a filter is shown. The Gm values of the illustrated Gm amplifiers (transconductance amplifiers) Gm ₁ to Gm ₄ are
When the Gm amplifier as shown in FIG. 7 is used, the following (1)
As shown in the equation, it varies depending on the input voltage V _in .

[0004]

[Equation 1]

[0005]

[Number 2] K = (W / L) · (μC OX / 2) ... (2) where, W: MOSFET of channel width.

L: MOSFET channel length.

Μ: Mobility of carrier.

C _OX : Unit capacitance of the gate

I: Source / drain current flowing in one input MOSFET.

[0010]

[Problems to be Solved by the Invention] However, (1)
Since the Gm value shown in the equation changes depending on the input voltage V _in, when a filter is configured by using this Gm amplifier, there is a drawback that the frequency characteristic varies depending on the input voltage.

As a Gm amplifier that does not require a bias voltage, the circuit configuration shown in FIG. 8 is known. Gm value of the amplifier shown in this figure, as shown in the following equation (3), never varies depending on the input voltage V _in (IE
EE 1991, CICC, 9.2.1, M.I. SNEL
(GROVE)

[0012]

[Number 3] _{Gm (V in) = 2K (} V cm -V th) ... (3) here, V _cm: a common voltage, when the differential input voltage V _d is zero, the voltage between the gate and source Equivalent to.

V _th : Threshold voltage of MOSFETs M ₁ and M ₂ .

When a filter is constructed by using the Gm amplifier shown in FIG. 8, the circuit is as shown in FIG.

However, as shown in equation (3),
Since the Gm value of this Gm amplifier is determined by (V _cm −V _th ), the Gm value is uniquely determined by the output operating point level of the preceding Gm amplifier.

Therefore, for controlling the Gm value, there is known a method of controlling the input operating point by using the resistors R ₁ and R ₂ as shown in FIG. 9, but the resistors are used. Due to, leakage occurs in the integrator formed by the Gm amplifier and the electrostatic capacitance in the previous stage (more specifically, the charge of the capacitor is discharged through the resistor).
There is a drawback that the frequency characteristics are deteriorated.

Therefore, an object of the present invention is to provide a Gm-C filter which has little dependence on the input voltage and which does not require a resistor to be connected to the output side of the amplifier.

[0018]

In order to achieve such an object, a Gm-C filter according to the present invention is a differential input type MO.
A SFET pair and a MOSFET pair having a gate input end for common-mode level control are connected in series, and are connected to a first Gm amplifier that supplies an output current from a pair of differential output ends, and the pair of differential output ends. Has a second Gm amplifier and three input terminals, and the first and second input terminals are connected to a pair of differential output terminals of the first Gm amplifier, and
Is connected to a bias voltage source for controlling the Gm value of the second Gm amplifier, and a control voltage is applied to the common-mode level control gate input terminal included in the first Gm amplifier. And an in-phase level control amplifier having an output terminal for

Here, the common-mode level control amplifier is
It is of a folded cascode type in which an input section and an output section are connected in parallel, the polarity of the input MOSFET of the input section is opposite to the polarity of the input MOSFET pair of the first Gm amplifier, and the output section. Load MOSFET
The polarity of the first Gm amplifier is for the in-phase level control MO
It has the same polarity as the SFET. Further, the (Vgs-Vth) value of the input MOSFET of the common-mode level control amplifier is constant, and the current value of the loading MOSFET is proportional to the current value flowing through the differential input type MOSFET pair of the first Gm amplifier. Is preferred.

[0020]

According to the above configuration of the present invention, the Gm value in the second Gm amplifier is controlled by the bias voltage applied to the third input terminal of the common mode level control amplifier.

[0021]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows a Gm amplifier included in a Gm-C filter to which the present invention is applied and its control circuit. In the figure, M ₁ and M ₂ are input MOSFETs. M
₃ and M ₄ are MOSFETs for controlling the common mode level. M
₅ , M ₆ are cascode MOSFETs for increasing the DC gain. Thus, the first Gm amplifier Gm
₁ is composed of six MOSFET shown in M ₁ ~M _6.

CMC is an in-phase level control amplifier,
B ₁₁ is a reference voltage input terminal for determining the common mode level, B ₁₁
Reference numeral ₁₂ is an input terminal of the MOSFETs M ₃ and M ₄ controlling the common mode level, and B ₁₃ is a bias terminal of the cascode MOSFET. Furthermore, Gm ₂ is the second Gm amplifier,
The circuit configuration is the same as Gm ₁ .

Next, the operation of the circuit shown in FIG. 1 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, the output level thereof agrees with the voltage value V _B11 applied to the bias terminal B ₁₁ . I assume. 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 (4) is output from the output terminal B _{12 of} the common-mode level control amplifier CMC.

[0027]

[Equation 4]

As a result, the voltage of 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 (V _out11
+ V _out12 ) / 2 is always equal to V _B11 .

As a result, the Gm value of Gm ₂ which is the Gm amplifier of the next stage is controlled by the voltage V _B11 applied to the third input terminal of the control amplifier CMC.

FIG. 2 shows an equivalent circuit for calculating the common mode gain of the Gm amplifier Gm ₁ . Where A ₁ is G
Gain value from the input terminal of m ₁ to the output terminal (M ₁ , M
₂ corresponds to ₂ ), A ₂ indicates a gain value (corresponding to M ₃ and M ₄ ) from the input terminal to the output terminal of the load transistor of Gm ₁ , and A ₃ indicates a gain value of the common-mode level control amplifier CMC.

The following formulas (5) and (6) are obtained by deriving the voltage formulas for the terminals N ₁ and N ₂ based on FIG.

[0032]

[Equation 5] V _out = -A ₁ · V _in −A ₂ · V _B12 (5)

[0033]

[Equation 6] V _B12 = A ₃ · V _out (6) When V _out is obtained from the above equations (5) and (6), the following equation (7) is obtained.

[0034]

[Equation 7]

Generally, the above A ₁ and A ₂ are almost the same value, so if the value of A ₃ is sufficiently large, V _out becomes small,
As a result, the output common-mode level can always be maintained at the same level regardless of the input signal, and the Gm value of the Gm ₂ of the next-stage Gm amplifier can be determined.

FIG. 3 shows Gm- which includes the circuit shown in FIG.
It is a circuit configuration example of a C filter. In this figure, the part surrounded by the broken line corresponds to the circuit of FIG.

In the circuit of the Gm-C filter shown in FIG. 3, the output terminals of the _three Gm amplifiers Gm ₁ , Gm ₃ and Gm ₄ are connected to the V _out terminal.

The common-mode level control amplifier CMC2 for determining the output operating point for the V _out terminal can be shared as shown in the figure. Here, BG is a bias generator.

In the embodiment described above,
Although NMOS was used as the input MOSFET,
May be used, or the Gm amplifier may have a folded cascode configuration.

FIG. 4 shows an example of an in-phase level control amplifier CMC used in an embodiment of the present invention.
In the figure, M41 to M44 are input MOSFETs. M51 and M52 are load MOSFETs.
M45 to M48 are current source MOSFETs. M4
9, M50 are cascode MOSFETs. Also,
B41 and B42 are terminals for applying a bias voltage for determining the bias current, and B43 is a cascode MOSFE.
This is a terminal for applying a bias voltage to T. IN1 +
IN2 + is a non-inverting input terminal, IN3- is an inverting input terminal, and OUT is an output terminal.

In the circuit shown in FIG. 1, since the output signal is centered around V _th + V _on , the output signal is V
_{Although it depends on} the magnitude of the _th value, it operates near V _ss .

Therefore, as an in-phase level control amplifier,
When the polarity of the input MOSFET of the Gm amplifier is NMOS as shown in FIG. 4, there is an advantage that a signal in the vicinity of V _ss can be surely received by using a PMOS of opposite polarity.

Further, the folded structure causes the output to be substantially the same as the operating point level used in the PMOS current mirror circuit, and provides an extremely appropriate level for the gate input of the MOSFET for controlling the common mode level of the Gm amplifier. be able to. As a result, the offset voltage can be reduced, and the DC gain is improved by the cascode structure to reduce the common-mode error. Therefore, the circuit shown in FIG. 4 is an extremely optimum control for the Gm amplifier of this embodiment. It can be said to be a circuit.

Regarding the input voltage range of the common-mode level control amplifier, it is necessary to reliably receive the output signal of the Gm amplifier. Therefore, V _gs -V _th (hereinafter referred to as V _on ) of the input MOSFET should be set sufficiently large. There must be. From a practical point of view, the _Von value fluctuates greatly according to the usual design method due to the influence of the process-dependent power supply voltage and temperature. That is, if the V _on value is made to be at least 1 V, the maximum value becomes around 3 V, and there is a problem that normal operation is no longer possible and the current consumption value becomes very large.

On the other hand, the load M of the common-mode level control amplifier
The current value flowing in the OSFET is the input MOS of the Gm amplifier.
It is preferably proportional to the value of the current flowing through the FET. If not proportional, in-phase level control load MOSF
Since the common mode control MOSFETs of the ET and Gm amplifiers form a current mirror circuit, an offset error occurs in the common mode level control amplifier. Then, this offset error induces an output operating point error of the Gm amplifier, and eventually causes the Gm value of the next-stage Gm amplifier to deviate from the originally required Gm value.

In order to solve such a problem, in this common mode level control amplifier, V _{on of the} input MOSFET is turned _on.
The value is constant, and the current value flowing in the load MOSFET is set to be proportional to the current flowing in the input MOSFET of the Gm amplifier. By doing so, an offset error does not occur with respect to all process fluctuations and environmental fluctuations, and a sufficient common-mode input signal level can be secured.

FIG. 5 shows a bias circuit which does not cause an offset error in the circuit shown in FIG. 4 and ensures a sufficient in-phase input signal level. Here, the same figure (A)
Is a bias circuit for generating a bias voltage applied to the bias terminal B41, and FIG. 6B is a bias circuit for generating a bias voltage applied to the bias terminal B42. Further, in the same figure (A), I _gm is the input MO of the Gm amplifier.
A current source that flows a current proportional to the current flowing in the SFET, I
_VON is a current value for setting the _Von value of the input MOSFET of the common-mode control amplifier constant.

Next, the operation when the bias circuit shown in FIG. 5 is used as the bias circuit of FIG. 4 will be described.

The current flowing through M45 and M46 in FIG. 4 is as shown in FIG.
Since it has a current mirror relationship with the circuit of (B), it is proportional to I _VON . Therefore, the current flowing through M41 to M44 is also proportional to I _VON .

The current flowing through M47 and M48 is shown in FIG.
Since the circuit (A) and the current mirror have a relationship, they are proportional to ( _IVON + _Igm ). M51, M52
The current flowing in M4 is calculated from the current flowing in M47 and M48.
5, it is a value obtained by subtracting the current flowing through M46. Therefore, if the current mirror ratios relating to the voltages applied to the bias voltage applying terminals B41, B42 are the same, M45, M4
The current flowing through 6 is proportional to I _gm .

That is, by using the bias circuit shown in FIG. 5, the load MOS of the common-mode level control amplifier is
The current value flowing in the FETs M51, M52 is made proportional to the current value flowing in the input MOSFETs M1, M2 (see FIG. 1) of the Gm amplifier, and the input M of the common-mode level control amplifier.
A current such that V _on becomes constant can be applied to the OSFETs M21 to M24.

FIG. 6 shows a detailed circuit configuration of the current source ( _IVON ) used in FIGS. 5A and 5B. MO here
The SFET M63 can correspond to M45 and M46 in FIG. In FIG. 6, the current is narrowed down so that (V _gs −V _th ) of M62 becomes almost zero. M64
The current flowing through is equal to the current source current value I _{0 due} to the action of the current mirror circuit, and the bias voltage applying terminal B6
The applied voltage of 2 is lower than the point A by (I ₀ × R). Accordingly, the gate voltage of M63 is V _dd- (I ₀ × R),
The V _on value is (I ₀ × R).

When such a bias circuit is used, as shown in FIG.
The V _on of the MOSFET M45, M46 shown in (I ₀
XR), and similarly MOSFET M shown in FIG.
The desired operation is achieved in proportion to V _{on of} 41 to M44 in proportion to (I ₀ × R).

In addition, the current source (I
_gm ) directly from the I _gm signal generation circuit or via a current mirror circuit even when the characteristics of the filter circuit are controlled by the automatic tuning circuit or when the filter characteristics are controlled by an external signal. Can be generated.

[0055]

As described above, according to the present invention, a Gm-C filter having a linear characteristic with little input voltage dependency is provided, and an in-phase control amplifier is used to determine the input operating point of the Gm amplifier. Therefore, it is possible to provide a Gm-C filter having no leakage and excellent performance.

Further, by using a cascode type amplifier for the common mode level control amplifier of the present invention, it is possible to have an appropriate common mode signal input range with a small error and to generate an output of an appropriate level. Can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a part of a Gm-C filter according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram showing the operation of FIG.

FIG. 3 is a circuit diagram showing a Gm-C filter according to an embodiment of the present invention.

FIG. 4 is a circuit diagram showing an example of an in-phase level control amplifier according to an embodiment of the present invention.

FIG. 5 is a diagram showing an example of a bias circuit used in an in-phase level control amplifier according to an embodiment of the present invention.

FIG. 6 is a diagram showing an example of a bias circuit used in an in-phase level control amplifier according to an embodiment of the present invention.

FIG. 7 is a circuit diagram showing an example of a conventionally known Gm amplifier.

FIG. 8 is a circuit diagram showing an example of a conventionally known Gm amplifier.

FIG. 9 is a circuit configuration diagram for controlling a Gm value of a Gm amplifier.

FIG. 10 is a circuit diagram showing an example of a conventionally known Gm-C filter.

[Explanation of symbols]

Gm ₁ , Gm ₂ Gm amplifier (transconductance) CMC common mode level control amplifier

Claims (3)

[Claims] 1. A first Gm amplifier in which a differential input type MOSFET pair and a MOSFET pair having a gate input end for common-mode 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. The third input terminal is 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 is included in the first Gm amplifier. And a common-mode level control amplifier having an output end for applying a control voltage to the end.
-C filter. 2. The in-phase level control amplifier according to claim 1, which is of a folded cascode type in which an input section and an output section are connected in parallel, and an input MOS of the input section is provided.
The polarity of the FET is the input MOSFET of the first Gm amplifier.
A Gm-C filter having a polarity opposite to that of the T pair, and the polarity of the load MOSFET of the output section being the same as the polarity of the common-mode level control MOSFET of the first Gm amplifier. 3. The differential input type amplifier according to claim 2, wherein the input MOSFET of the common mode level control amplifier has a constant (Vgs-Vth) value and the load MOSFET has a current value of the first Gm amplifier. A Gm-C filter characterized by being proportional to a current value flowing through a MOSFET pair.