JPH0662632U - Active filter - Google Patents

Abstract

(57) [Abstract] [Purpose] An active filter of a state variable type composed of an OP amplifier realizes a high Q and excellent frequency characteristics. [Structure] An OP amplifier 11 to which an input signal vi is added, and an O
An OP amplifier 12 to which an output signal vh of the P amplifier 11 is given,
A feedback resistor R ₈ for returning the output signal vb of the OP amplifier 12 to the OP amplifier 11 and an OP amplifier 13 provided with the output signal vb are provided, and the output signals of the OP amplifiers 11, 12, 13 are respectively set to a predetermined frequency. In the active filter configured to have the pass characteristic, the non-inverting input terminal of the OP amplifier 11 and the OP
Both inverting input terminals of the amplifiers 12 and 13 are grounded respectively, the input signal vi is applied to the inverting input terminal of the OP amplifier 11, and the output signal vb of the OP amplifier 12 is output by the feedback resistor R ₈ .
Is fed back to this inverting input terminal.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an active filter, and more particularly to a state variable type active filter including an OP amplifier.

[0002]

[Prior art]

 FIG. 3 is a circuit diagram of an example of a conventional active filter. The active filter 30 shown in the figure is an example of a state variable type active filter including an OP amplifier.

The active filter 30 includes an adder / subtractor circuit including an OP amplifier 31 and the like, an integrating circuit including an OP amplifier 32 and the like, and an integrating circuit including an OP amplifier 33 and the like. A resistor R ₇ is connected between the non-inverting input terminal of the OP amplifier 31 and the ground.

From each circuit, terminals 34, 35, and 36 have basic output signals vh, vb, and high-pass, band-pass, and low-pass for input signal vi coming into input terminal 37. vl is output, and these are functions of variables x / s ⁱ (i = 0, 1, 2). Further, by adding / subtracting each of these output signals by an adder / subtractor circuit not shown in the figure, a desired band elimination characteristic can be obtained.

In the active filter 30 of FIG. 3, assuming that the values of the resistors R _{1 to} R ₆ are R, the transfer function of the low-pass output signal obtained at the terminal 36 is

[0006]

[Equation 1]

It is represented by

Further, the sharpness Q of resonance at the cutoff frequency fo is

[0009]

[Equation 2]

Then, the angular frequency ωo that gives the cutoff frequency fo = ωo / 2π is

[0011]

[Equation 3]

[0012]

By the way, when the active filter 30 of FIG. 3 is configured on the same semiconductor chip including the resistors R _{1 to} R ₇ and the capacitors C ₁ and C ₂ , the values of the resistors and the capacitors are determined by the process. It varies according to the conditions. Therefore, variations in R / R ₇ and denominators of √ (C ₁ / C ₂ ) and numerator in the equation (2) are canceled out, and Q does not fluctuate.

However, since the resistance variation and the capacitor variation are independent of each other, the value of the denominator in the equation (3) varies widely, which causes variation in the cutoff frequency fo.

Therefore, as shown in FIG. 4, another example of a state variable type active filter including an OP amplifier has been proposed.

The active filter 40 shown in FIG. 1 is generally composed of an adder / subtractor circuit including an OP amplifier 41, an integrating circuit including an OP amplifier 42, and an integrating circuit including an OP amplifier 43. Has become. A resistor R ₇ is connected between the non-inverting input terminal of the OP amplifier 41 and the ground.

The OP amplifiers 42 and 43 are variable-conductance OP amplifiers of the same configuration with their non-inverting input terminals grounded. The output signals from the respective circuits are the same as those in the active filter 30. These signals are vh, vb, and vl for passing and low-passing, respectively.

In the active filter 40 shown in FIG. 4, the transfer function of the low-pass output signal obtained at the terminal 45 with respect to the input signal v i coming into the input terminal 44 is obtained by the resistors R _{1 to} R ₃ and R ₆ . If the value is R and the input conductance seen from the inverting input terminals of the OP amplifiers 42 and 43 is gm,

[0019]

[Equation 4]

It is represented by

Further, the sharpness Q of resonance at the cutoff frequency fo is

[0022]

[Equation 5]

Then, the angular frequency ωo giving the cutoff frequency fo = ωo / 2π is

[0024]

[Equation 6]

Are respectively represented by

By the way, as described above, the OP amplifiers 42 and 43 are variable conductance type OP amplifiers having the same configuration, and have the configuration shown in FIG. In the figure, 51 is an inverting input terminal, 52 is a non-inverting input terminal, 53 is an output terminal, and the non-inverting input terminal 52 is grounded and used in FIG.

Therefore, the input signal vin coming into the inverting input terminal 51 arrives at the base of the transistor Q ₂ via the transistor Q ₁ and the resistor R _51, and is connected to the collector of the transistor Q ₂ by a constant current load. Taken out from J ₁ .

The input conductance gm is changed by changing the current value of the current source J _{2 that} drives the differential pair transistors Q ₂ and Q ₃ with a constant current.

[0029]

However, the conventional active filters shown in FIGS. 3 and 4 have a drawback that it is difficult to realize a high Q and the filter characteristics deteriorate in the video signal band. It was

That is, in order to improve the capacitance ratio of the capacitors formed on the semiconductor chip, C ₁ and C ₂ (or C ₃ and C ₄ ) may have the same shape and the same capacitance value, but C ₁ = C ₂ (or C ₃ = C ₄ ), from equation (2) (or equation (5))

[0031]

[Equation 7]

It becomes

Therefore, for example, in order to obtain a filter characteristic of Q = 15, the resistance ratio (R / R ₇ ) Must be 44. In order to realize this resistance ratio, the chip area must be extremely large, which is practically difficult.

Further, since the non-inverting input terminals of the OP amplifiers 31 and 41 forming the adder / subtractor circuit are not grounded, when the input signal vi is near the video signal band, the common-mode rejection ratio is lowered and the filter characteristic is reduced. to degrade.

Further, the conventional active filter shown in FIG. 4 has a drawback that the cutoff frequency does not vary but the frequency characteristic deteriorates.

That is, the transistor Q ₂ forming the OP amplifier 42 (43) has the collector-base junction capacitance C _bc between the collector and base as shown in FIG. 5, and the input signal vin from the inverting input terminal 51 is Has a structure in which it enters the base and is output from the collector, so that there is a problem that the frequency characteristic of the signal obtained at the output terminal 53 is deteriorated by the Miller effect.

This means that when the load impedance seen from the output terminal 53 is R _L , the input capacitance seen from the base of the transistor Q ₂ increases by the mirror capacitance C _M = (1 + gm _RL ) C _bc (8). It is well known that this is for narrowing the 3 dB bandwidth. Although not shown in FIG. 5, other transistors also have a collector-base junction capacitance _Cbc .

Therefore, an object of the present invention is to provide an active filter that solves the above problems.

[0039]

[Means for Solving the Problems]

 The above problem can be solved by configuring as follows.

That is, the first OP amplifier to which the input signal is applied, the second OP amplifier to which the output signal of the first OP amplifier is applied, and the output signal of the second OP amplifier are set to the first OP amplifier. It comprises a feedback means for feeding back to the OP amplifier, and a third OP amplifier to which the output signal of the second OP amplifier is given, and outputs the output signals of the first, second and third OP amplifiers respectively. In an active filter configured to have a predetermined frequency pass characteristic, a non-inverting input terminal of the first OP amplifier and both inverting input terminals of the second and third OP amplifiers are grounded, and the input signal is This is solved by applying the signal to the inverting input terminal of the OP amplifier and returning the output signal of the second OP amplifier to the inverting input terminal by the feedback means.

[0041]

[Action]

 According to the above configuration in which the inverting input terminal of the first OP amplifier is grounded and the input signal is applied to the inverting input terminal, the common-mode rejection ratio of the first OP amplifier does not deteriorate in the high frequency range. Acts like.

According to the above configuration in which both inverting input terminals of the second and third OP amplifiers are grounded, the output signals of the second and third OP amplifiers are affected by the Miller effect and have a frequency difference. It acts so that the characteristics do not deteriorate.

[0043]

【Example】

 FIG. 1 is a circuit diagram of an embodiment of the present invention, showing a state variable type active filter composed of an OP amplifier.

The active filter 10 shown in FIG. 1 is generally composed of an adder / subtractor circuit including an OP amplifier 11, an integrating circuit including an OP amplifier 12, and an integrating circuit including an OP amplifier 13. Has become.

The OP amplifier 11 has the same configuration as the OP amplifiers 31 and 41 described above, but the input signal vi input to the input terminal 14 is input to the inverting input terminal via the resistor R ₁ , and the non-inverting input terminal is input. Is grounded.

Further, the OP amplifiers 12 and 13 are variable conductance type OP amplifiers having the same configuration as the OP amplifiers 42 and 43, respectively, and the output signals from the respective circuits are the active filters 30 and Similar to 40, the signals are high-pass, band-pass, and low-pass signals vh, vb, and vl.

However, while the OP amplifiers 42 and 43 are configured such that the non-inverting input terminals are grounded and the input signal is input to the inverting input terminals, the OP amplifiers 12 and 13 are configured to have the inverting input terminals. Is grounded and the input signal comes in to the non-inverting input terminal.

FIG. 2 is a circuit diagram specifically showing the configuration of the OP amplifiers 12 and 13. In the figure, the same components as those in FIG. 2 are denoted by the same reference numerals, and each transistor has a collector-base junction capacitance C _bc between the collector and base.

As described above, the inverting input terminal 51 is grounded as shown, and the input signal vin is input to the non-inverting input terminal 52. Then, the input signal vin arrives at the base of the transistor Q ₃ forming a differential pair with the transistor Q ₂ via the transistor Q ₄ and the resistor R ₅₂ , and a signal corresponding to the voltage between both bases of the differential pair transistor is generated. , A constant current load J ₁ connected to the collector of the transistor Q ₂ .

With this configuration, the frequency characteristics of the signal obtained at the output terminal 53 are not deteriorated by the mirror effect, and good frequency characteristics can be obtained.

By the way, in the active filter 10 shown in FIG. 1, the transfer function of the low-pass output obtained at the terminal 15 with respect to the input signal vi input to the input terminal 14 has the values of the resistors R _{1 to} R ₃ as R , C is the value of the capacitors C ₃ and C ₄ , and gm is the input conductance viewed from the non-inverting input terminals of the OP amplifiers 12 and 13.

[0052]

[Equation 8]

It is represented by

The sharpness Q of resonance at the cutoff frequency fo is the ratio of the value of the feedback resistor R _{8 to} the resistance value R of each resistor, that is,

[0055]

[Equation 9]

Then, the angular frequency ωo giving the cutoff frequency fo = ωo / 2π is

[0057]

[Equation 10]

Are respectively represented by

Therefore, for example, if the resistance ratio (R / R ₈ ) is 15, a filter characteristic of Q = 15 can be obtained, and it can be easily formed on a semiconductor chip without increasing the chip area unlike the conventional active filter. can do.

Further, the OP amplifier 12 is not affected by the Miller effect because the input signal is supplied to the non-inverting input terminal as described above, and at the same time, the output signal is output from the buffer 16 and the feedback resistor R ₈ as shown in FIG. It can be configured to feed back to the inverting input terminal of the OP amplifier 11 via the, and the non-inverting input terminal of the OP amplifier 11 forming the adder / subtractor circuit can be grounded.

Therefore, unlike the conventional active filter, the common-mode rejection ratio does not decrease with respect to the input signal having the frequency close to the video signal band, and the filter characteristic excellent in the frequency characteristic can be obtained.

As described above, according to the active filter of the present embodiment, even when formed on a semiconductor chip, it is possible to easily realize a high Q without variation in ωo, that is, the cutoff frequency, and to reduce the influence of the Miller effect. Good frequency characteristics can be obtained without being affected. Therefore, it is extremely effective in achieving high Q and constant current consumption in a high frequency region such as a video signal band.

[0063]

[Effect of device]

 As described above, according to the present invention, the common mode rejection ratio of the first OP amplifier does not deteriorate in the high frequency range, and the output signals of the second and third OP amplifiers are influenced by the mirror effect. Since the frequency characteristics are not deteriorated in response to this, the output signals having good predetermined frequency pass characteristics can be obtained from the first to third OP amplifiers in the high frequency range such as the video signal band, and the semiconductor Even if it is formed on a chip, it has the feature that a high Q can be easily realized without variation in the cutoff frequency that gives the prescribed frequency passage characteristics.

[Brief description of drawings]

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

FIG. 2 is a specific circuit diagram of a main part of FIG.

FIG. 3 is a circuit diagram of an example of a conventional active filter.

FIG. 4 is a circuit diagram of another example of a conventional active filter.

5 is a specific circuit diagram of a main part of FIG.

[Explanation of symbols]

10, 30, 40 active filter 11～13,31～33,41～43 OP amp R ₁ to R ₈ resistor C ₁ -C ₄ capacitors

Claims (1)

[Scope of utility model registration request] 1. A first OP amplifier to which an input signal is applied, and a second OP amplifier to which an output signal of the first OP amplifier is applied.
An amplifier, feedback means for returning an output signal of the second OP amplifier to the first OP amplifier, and a third OP to which an output signal of the second OP amplifier is given
And a non-inverting input terminal of the first OP amplifier, wherein the output signals of the first, second, and third OP amplifiers have predetermined frequency pass characteristics. Both the inverting input terminals of the second and third OP amplifiers are grounded respectively, and the input signal is applied to the inverting input terminal of the first OP amplifier, and the feedback means causes the feedback circuit of the second OP amplifier to operate. An active filter configured to feed back an output signal to the inverting input terminal.