JPS6359210A - Rc active filter - Google Patents

Abstract

PURPOSE:To improve the efficiency of circuit integration by connecting a 1st MOS resistor and a 1st resistor in series and connecting the 2nd resistor in series with 2nd MOS resistor. CONSTITUTION:The resistors 16, 17 are connected and a series connection resistor comprising a MOS resistor and a diffusion resistor is used as the resistor being the component of the RC active filter. Let the resistance of the resistor 16 be R1, the resistance of the MOS resistor 12 be R2, then a voltage across the MOS resistor 12 is a multiple of R2/(R1+R2) in comparison with the constitution by the MOS resistor 12 only. That is, the distortion by the MOS resistor 12 is decreased. This is similar as to the MOS resistor 15.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an RC active filter, and particularly to an RC active filter in a semiconductor integrated circuit.

〔overview〕

The present invention provides a Sallen-Key type RC active filter that includes an operational amplifier, a resistor, and a capacitor forming a positive feedback loop, in which the resistor is composed of a parallel connection circuit of a P-channel MOS transistor and an N-channel MOS transistor. M.O.
By using a series connection circuit of an S resistor and a diffused resistor, distortion can be reduced, the area can be reduced, and the efficiency of integration can be increased.

[Conventional technology]

Conventionally, as an RC active filter for band limiting in a semiconductor integrated circuit, there is, for example, a Sallen-Key type second-order low-pass filter shown in FIG. In this conventional example, one end is connected to a resistor 2 with one end connected to the input terminal 1 and the other end connected to the connection point 3, and a resistor 4 whose one end is connected to the connection point 3 and the other end is connected to the connection point 6. a capacitor 5 connected to point 6 and the other end connected to ground potential, an operational amplifier 8 whose positive phase input is connected to connection point 6 and whose negative phase input and output are connected to output terminal 9, and one end connected to connection point 3.
and a capacitor 7 whose other end is connected to the output terminal 9. As the resistors 2 and 4, diffused resistors are used.

FIG. 3 is a circuit diagram showing another example of a conventional RC active filter. This conventional example uses MOS resistors 12 and 15 instead of diffused resistors as resistors 2 and 4 in the circuit shown in FIG.

Here, the MOS resistor 12 consists of a parallel connection circuit of an N-channel MOS transistor 10 and a P-channel MOS transistor 11, and the MOS resistor 15 is an N-channel MOS).
It consists of a circuit connected in parallel with a transistor 13 and a P-channel MOS transistor 14.

[Problem that the invention seeks to solve]

In the conventional RC active filter described above, a diffused resistor or MOS resistor is used as a resistor, but when a diffused resistor is used as a resistor, a high resistance is required, such as when the cut-off frequency is low. However, the area occupied by the resistor increases in proportion to the resistance value, resulting in a reduction in integration efficiency.

On the other hand, this drawback can be overcome by using a MOS resistor;
There is a drawback that the signal is distorted because it has drain voltage dependence.

Next, the reason will be explained.

The relational expression between the voltage and current of the MOS transistor is input terminal 1.
And the voltages at the connection point 3 are assumed to be ■ and v2, a positive power supply voltage VD is applied to the gate of the N-channel MOS transistor 10, and a negative power supply voltage -■ is applied to the gate voltage of the P-channel MOS transistor 11.

is applied, the current I flowing between input terminal 1 and connection point 3 is 3 α Lp [γ7 ((V, +V, -2φ, ,) 3/Z(V
2+V11. , 2φFn) 3/2 )+r, ((-V
, +VllF-2φ,p)3/2(V2+V,, 2
φrp)””) ) −+1) However, μh Cox is the gate width w1 of the N-channel MOSI transistor 10.
, gate length Lll, threshold voltage v77, gate width Wp1 of P-channel MOS) transistor 11, gate length, threshold voltage ■TP, 1 αP
Lp v7. %=Vtp=Vy. Cox is the capacitance of the semiconductor gate oxide film, and μ7, ■, φ1, and γ7 are N, respectively.
channel MOS) carrier mobility of transistor 1o,
With substrate voltage, Fermi level, and bias effect constant, μp,
Vlp, φF2, γ2 are the carrier mobility of the P-channel MOS transistor 11, the substrate voltage, the Fermi level,
is the bias effect constant.

In equation (1), the first term has a proportional relationship with V, -V2, and current, but the second term remains as a factor of distortion. This second term has lv+vzl dependence, and the larger lv+-21, the larger the absolute value of the second term and the larger the distortion.

SUMMARY OF THE INVENTION An object of the present invention is to provide an RC active filter that can reduce distortion, reduce area, and increase integration efficiency by eliminating the above-mentioned drawbacks.

[Means for solving problems]

The present invention provides a first N-channel MOS transistor whose gate is connected to a positive power supply, whose drain is connected to an input terminal, and whose source is connected to a first connection point, and whose gate is connected to a negative power supply and whose source is connected to the input terminal. A first MOS resistor consisting of a first P-channel MOS (first P-channel MOS) transistor whose gate is connected to the positive power supply and whose drain is connected to the first connection point and whose source is connected to the first connection point. a second N-channel MOS transistor connected to the second connection point, a second PMOS transistor having a gate connected to a negative power supply, a source connected to the first connection point, and a drain connected to the second connection point;
A second MOS resistor consisting of a channel MOS transistor, an operational amplifier whose positive phase input is connected to the second connection point, and whose negative phase input and output are connected to the output terminal, and one end of which is connected to the second connection point. and a second capacitor having one end connected to the first connection point and the other end connected to the output terminal.
The active filter is characterized in that a first resistor is connected in series with the first MOS resistor, and a second resistor is connected in series with the second MOS resistor.

Further, in the present invention, it is preferable that the first and second resistors are diffused resistors.

[Effect]

Since the resistor consists of a series-connected circuit of a resistor and a MOS resistor, the voltage applied across the MOS resistor is divided according to the respective resistance values, so it is smaller than when using only a conventional string of MOS resistors. , the distortion also decreases accordingly.

On the other hand, when a diffused resistor is used as a resistor, the area is MO
Since the S resistor has a higher resistance value than the diffused resistor, the diffused resistor can be reduced to a value that corresponds to the degree of distortion improvement mentioned above, and the area as a resistor is smaller than the conventional diffused resistor alone. can.

Therefore, it is possible to reduce distortion and area, thereby increasing integration efficiency.

〔Example〕

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

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

In this embodiment, one end is connected to the input terminal and the other end is connected to the connection point 18.
The first resistor 16 connected to and the gate connected to the positive power supply ■. a first N-channel MOS transistor 1 having a drain connected to a connection point 18 and a source connected to a first connection point 3;
0, the gate is connected to the negative power supply ■, and the source is connected to the connection point 18
A first P-channel MOS transistor 11 whose drain is connected to the connection point 3 and whose gate is connected to the positive power supply ■. a second N-channel MOS) transistor 13 connected to
A second P-channel MO whose gate is connected to the negative power supply V3, whose source is connected to connection point 3, and whose drain is connected to connection point 19.
SI-transistor 14, a second resistor 17 with one end connected to connection point 19 and the other end connected to connection point 6, a positive phase input connected to connection point 6, and a negative phase input and output connected to output terminal 9. A first capacitor 5 has one end connected to the connection point 6 and the other end connected to ground potential, and a second capacitor 5 has one end connected to the connection point 3 and the other end connected to the output terminal 9. It includes a capacitor 7.

Here, an N-channel MOS) transistor 10 and a P-channel MOS) transistor 11 constitute a MOS resistor 12, and an N-channel MOS) transistor 13 and a P-channel MOS transistor 13 constitute a MOS resistor 12.
(OS) transistor 14 constitutes a MOS resistor 15.

Further, resistors 16 and 17 are diffused resistors.

The feature of the present invention is that the resistors 16 and 17 are connected in FIG. 1, and a series-connected resistor of a MOS resistor and a diffused resistor is used as the resistor constituting the RC active filter.

Next, the operation of this embodiment will be explained.

In this embodiment, the gate of the N-channel MOSI transistor 10 is connected to the positive power supply (2). Since the gate of the P-channel MOS transistor 11 is connected to the negative power supply V, both the N-channel MoS transistor 10 and the P-channel MOS transistor 11 are always on.
It is used as an S resistor. Let the voltages of input terminal 1, connection point 18, and connection point 3 be v, Sv, and v2, respectively, and let the resistance value of resistor 16 be R, and the resistance value of MOS resistor 12 be R2, then R1+R2 −・−・−・-(2), and the voltage across the MOS resistor 12 is R compared to the case where the MOS resistor 12 is used as the conventional example shown in FIG.

R1+R.

As a result of doubling, it becomes smaller. That is, the distortion caused by the MOS resistor 12 is reduced. This also applies to the MOS resistor 15. Furthermore, since the area of the MOS resistor is smaller than that of the diffused resistor, the area is about I RI + R2 smaller than that of the conventional example shown in FIG. 2, which is composed of only diffused resistors.

〔Effect of the invention〕

As described above, the present invention has the effect of reducing distortion and reducing area by using a resistor in which a diffused resistor and a MOS resistor are connected in series, thereby increasing the efficiency of integration.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. FIGS. 2 and 3 are circuit diagrams showing conventional examples. 1...Input terminal, 2.4.16.17...Resistor, 3
．． 6.1B, 19... Connection point, 5.7... Capacitor, 8... Operational amplifier, 9... Output terminal, 10.1
3...N channel MOS transistor, 11.14.
...P channel MOS) transistor, 12.15...
MOS resistance, v8...Positive power supply, ■,...Negative power supply.

Claims (2)

[Claims] (1) A first N-channel MOS transistor (10) whose gate is connected to the positive power supply (V_D), whose drain is connected to the input terminal (1), and whose source is connected to the first connection point (3); A first MOS resistor (12) consisting of a first P-channel MOS transistor (11) connected to a power supply (V_S), a source connected to the input terminal, and a drain connected to the first connection point; A second N-channel MOS transistor (13) is connected to the positive power supply, its drain is connected to the first connection point, and its source is connected to the second connection point (6), and the gate is connected to the negative power supply, and the source is connected to the a second P-channel MOS transistor (14) connected to the first connection point and having its drain connected to the second connection point;
The positive phase input is connected to the second connection point, and the negative phase input and output are connected to the output terminal (
9), a first capacitor (5) with one end connected to the second connection point and the other end connected to ground potential, and one end connected to the first connection point. and a second capacitor (7) whose other end is connected to the output terminal, a first resistor (16) is connected in series with the first MOS resistor, and the second MOS resistor is connected in series with the second MOS resistor. A second resistor (17
) is connected to the RC active filter. (2) The RC active filter according to claim (1), wherein the first and second resistors are diffused resistors.