JP3108551B2 - Filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter for realizing a filter characteristic using a conductance amplifier, and more particularly to a filter circuit for passing only a signal in a low frequency band.

[0002]

2. Description of the Related Art FIG.
There were the following. This filter circuit mainly includes conductance amplifiers G ₁ and G ₂ , capacitors C ₁ and C ₂
And buffer amplifiers BUF ₁ and BUF ₂ . In this case, the input terminal transconductance amplifier G ₁ positive phase input terminal IN + inputs an external signal IN
, The negative-phase input terminal IN− is connected to an external output terminal OUT for outputting a signal to the outside, and the output terminal is grounded via a capacitor C _1, and the buffer amplifier BUF ₁ has a gain of _1. Is connected to the input terminal of
In addition, the positive phase input terminal IN of the transconductance amplifier G ₂
+ Is connected to the output terminal of the buffer amplifier BUF _1,
The negative-phase input terminal IN− is connected to the external output terminal OUT,
The output terminal is grounded via a capacitor C _2,
Gain is connected to one input terminal of the buffer amplifier BUF _2. The output terminal of the buffer amplifier BUF ₂ is also connected to the external output terminal OUT.

Here, buffer amplifiers BUF ₁ , BU
Since F ₂ is the gain, respectively 1, substantially, the output of the transconductance amplifier G ₁ is the input for transconductance amplifier G _2, the output of the transconductance amplifier G ₂ appears at the external output terminal OUT. Therefore, the conductances of the conductance amplifiers G ₁ and G ₂ are gm ₁ and gm, respectively.
If m ₂ , the transfer function of this filter circuit is expressed by the following equation.

[0004]

(Equation 1) Here, V _IN : voltage of the input terminal IN V _OUT : voltage of the external output terminal OUT C ₁ : capacitance of the capacitor C ₁ C ₂ : capacitance of the capacitor C ₂ .

On the other hand, a general expression of a transfer function of a second-order low-pass filter is expressed by the following expression.

[0006]

(Equation 2) Therefore, it can be seen that the filter circuit shown in FIG. 3 is a secondary low-pass filter. Here, ω ₀ = {(gm ₁ · gm ₂ ) / (C ₁ · C ₂ )｝ ^1/2 (3) Q = {(gm ₁ / gm ₂ ) (C ₂ / C ₁ )} ^{1 / 2} … (4)

[0007]

As is well known, a differential output amplifier has a positive output terminal and a negative output terminal, and a voltage V as a differential component from the positive output terminal with respect to a ground level.
_1, noise components such as power supply ripple, i.e., if the voltage V ₁ + V _N overlapped with the noise component V _N having various frequency components are outputted, from the negative output terminal -
V ₁ + V _N is output. Of these, the noise component V _N is referred to as in-phase component.

The input terminal IN of the above-described filter circuit is
One terminal output of the differential output type amplifier A _1, ie, V
If the application of the ₁ + V _N, among the noise component V _N, the is higher component frequency than the cut-off frequency f _c is removed, a lower component frequency than the cut-off frequency f _c is not removed, the external It appears at the output terminal OUT.

Thus, the conventional filter circuit shown in FIG. 4 has a problem that the common mode noise component contained in the output of the differential output type amplifier cannot be removed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a filter circuit capable of removing a common mode noise component with a high common mode rejection ratio (CMRR).

[0011]

According to the present invention, there is provided a conductance amplifier having an output terminal directly or indirectly connected to an external output terminal, one end connected to the output terminal of the conductance amplifier, and the other end grounded. One end is connected to the first input terminal and the other end is a conductance
A first resistor connected to the positive input terminal of the amplifier, a second resistor connected at one end to the positive input terminal of the conductance amplifier and the other end grounded, and a second input connected to the second input terminal; A third resistor connected to the terminal and the other end connected to the negative-phase input terminal of the conductance amplifier, and one end connected to the negative-phase input terminal of the first conductance amplifier and the other end connected to the external output terminal And a fourth resistor connected to
The resistance ratio of the first and second resistors is equal to the resistance ratio of the third and fourth resistors.

According to another aspect of the present invention, a first conductance amplifier, a first capacitor having one end connected to the output terminal of the conductance amplifier and the other end grounded, and an output terminal directly or indirectly connected to an external terminal. Connected to the output terminal,
A second conductance amplifier having a positive-phase input terminal connected directly or indirectly to an output terminal of the first conductance amplifier, a negative-phase input terminal connected to an external output terminal, and one end connected to the second conductance amplifier. A second capacitor connected to the output terminal of the amplifier and having the other end grounded; a second capacitor connected to one end to the first input terminal and the other end connected to the positive-phase input terminal of the first conductance amplifier; 1 resistor, one end is connected to the positive-phase input terminal of the first conductance amplifier, the other end is grounded, the other end is connected to the second input terminal, and the other end is connected. Conductance
A third resistor connected to the negative-phase input terminal of the amplifier, a fourth resistor having one end connected to the negative-phase input terminal of the first conductance amplifier, and the other end connected to the external output terminal; And the resistance value ratio of the first and second resistors is made equal to the resistance value ratio of the third and fourth resistors.

[0013]

According to the present invention, a positive-phase input terminal of a conductance amplifier to which an input signal is applied is connected to a first input terminal via a first resistor, and a ground point is connected via a second resistor. On the other hand, the negative-phase input terminal of the conductance amplifier is connected to the second input terminal via the third resistor, and connected to the external output terminal via the fourth resistor. The resistance ratio of the first and second resistors is made equal to the resistance ratio of the third and fourth resistors, and one of the signals having the in-phase signal component is supplied to the first input terminal and the other is supplied to the second input terminal. Since the differential signal component is transmitted to the output terminal because it is applied to the input terminal, the common mode signal component does not appear at the output terminal, thereby removing the common mode signal component with a high common mode rejection ratio. be able to.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. FIG. 1 is a circuit diagram showing a configuration of an embodiment of the present invention. In the drawing, components denoted by the same reference numerals as those in FIG. 4 indicate the same components.

It has input terminals IN ₁ and IN ₂ . Among them, one end of the resistor R ₁ is connected to the input terminal IN _1. The other end of the resistor R ₁ is connected to the positive phase input terminal of the transconductance amplifier G ₁ IN +. One end of the resistor R ₂ is connected to the positive phase input terminal of the transconductance amplifier G ₁ IN +. The other end of the resistor R ₂ is grounded. On the other hand, the input terminal IN ₂
One end of the resistor R ₃ is connected to. Reverse phase input terminal IN of the other end of the resistor R ₃ is transconductance amplifier G ₁
Connected to-. Also, conductance amplifier G
One end of the resistor R ₄ is connected to the _first inverting input terminal IN-. The other end of the resistor R ₄ is connected to the external output terminal OUT. Incidentally, it omitted the structure description because it is just as an output terminal of the transconductance amplifier G ₁ to the external output terminal OUT and FIG.

The operation of the embodiment constructed as described above will be described below.

[0017] First, as an example of adding a common-mode signal components to the input terminals IN ₁ and IN _2, by applying a signal appearing at the positive output terminal of the differential output type amplifier A ₁ to the input terminal IN _1, the input terminal IN ₂ applying a signal appearing at the negative output terminal of the differential output type amplifier a _1. At this time, a voltage V _OUT expressed by the following equation appears at the external output terminal OUT.

[0018]

(Equation 3) However, R _1: a resistor R ₁ of the resistance value R _2: resistor R ₂ of the resistance value R _3: the resistance value of the resistor R ₃ R _4: is the resistance of the resistor R _4.

Here, as the resistors R _{1 to} R ₄ , those which satisfy the following equation between the resistance values are used.

R ₁ / R ₂ = R ₃ / R ₄ (6) By using the relation of the equation (6), the equation (5) can be transformed into the following equation.

[0021]

(Equation 4) Here, for the sake of clarity, the input differential signal component V _IN / 2 to the terminal IN ₁ is applied, the transfer function when the input terminal IN ₂ is a differential signal component -V _IN / 2 is applied following It is expressed by an equation.

[0022]

(Equation 5) However A: it is the R ₂ / R _1.

The general equation of the transfer function of the second-order low-pass filter is given by equation (2). As can be seen by comparing equation (2) and equation (8), the filter circuit of FIG. Is a second-order low-pass filter with gain A and ω ₀ and Q given by:

Ω ₀ = [{(gm ₁ · gm ₂ ) / (C ₁ · C ₂ )} {(R ₁ / (R ₁ + R ₂ )}] ^1/2 (9) Q = [(gm ₁ / Gm ₂ ) (C ₂ / C ₁ ) {R ₁ / (R ₁ + R ₂ )}] ^1/2 (10) Next, in-phase signal components V _N are applied to the input terminals IN ₁ and IN ₂ , respectively. In this case, as is apparent from equation (7), V
_OUT = 0.

[0025] As a result, the signal V _IN / 2 + V _N appearing in the positive output terminal of the differential output type amplifier is applied to the input terminal IN _1, appeared to the negative output terminal of the differential output type amplifier to the input terminal IN ₂ Assuming that the signal −V _IN / 2 + V _N is applied, the differential signal component V _IN is transmitted to the external output terminal OUT according to the equation (8), but the in-phase component V _N does not appear at the external output terminal OUT. . Therefore, the filter circuit shown in FIG.
It can be said that the filter is a secondary low-pass filter having a high common mode rejection ratio (CMRR).

As a special case, R ₁ / R ₂ = R
If the condition of ₃ / R ₄ = 1 is satisfied, then A = 1, and the transfer function can be expressed in a simple form as in the following equation.

[0027]

(Equation 6) At this time, ω ₀ and Q are as follows.

Ω ₀ = {(gm ₁ · gm ₂ ) / (2C ₁ · C ₂ )} ^1/2 (12) Q = {(gm ₁ / gm ₂ ) (C ₂ / C ₁ ) / 2} ^1/2 (13) In the above embodiment, the buffer amplifiers BUF ₁ , B
UF ₂ is used, but conductance amplifier G ₂
The can remove this buffer amplifier BUF ₁ when the input impedance is sufficiently high, furthermore, the resistance value of the resistor R ₄ is sufficiently large, the input of the load circuit connected to the external output terminal OUT impedance when sufficiently high, it is possible to also eliminate a buffer amplifier BUF _2.

In the above embodiment, the input terminal IN
₁ to apply a signal appearing at the positive output terminal of the differential output type amplifier, but by applying a signal appearing on the negative output terminal of the differential output type amplifier to the input terminal IN _2, the input terminal IN _1, IN ₂ signal input is not limited to the output signal of the differential output amplifier, the input terminal iN ₁ of the one signal including an in-phase component from each other, and respectively input to the other signal input terminal iN _2, the in-phase signal component It goes without saying that it can be used to remove it.

Furthermore, in the above embodiment, transconductance amplifier G ₂ of the negative-phase input terminal IN- an external output terminal OU
Connected to T, the inverting input terminal of the voltage appearing at the external output terminal OUT as it transconductance amplifier G ₂ IN-
Although addition is to, conventional means for changing the value of Q, i.e., the output terminal voltage to attenuate be added signal attenuation means for applying to the inverting input terminal of the transconductance amplifier G _2, and described above A similar operation can be performed.

FIG. 2 shows a specific embodiment of the present invention, in which transistors Q ₁ , Q ₂ , Q ₆ , Q ₇ , Q ₈ , Q ₉
And diodes Q ₃ , Q ₄ , Q ₅ constitute a conductance amplifier G ₁ , and transistors Q ₁₁ , Q ₁₂ ,
Q ₁₆ , Q ₁₇ , Q ₁₈ , Q ₁₉ and diodes Q ₁₃ , Q ₁₄ , Q
Conductance amplifier G ₂ is constituted by _15. In addition, the transistor Q ₁₀ is the buffer amplifier BUF
As _1, transistor Q ₂₀ is the buffer amplifier BUF
Used as ₂ .

Of these, the conductance amplifier G ₁ ,
Since G ₂ is constituted is exactly identical, the transconductance amplifier G ₁ will be only explained the outline of the operation.

Here, the transistors Q ₁ and Q ₂ form a differential amplifier circuit in which the emitters are connected to each other by a resistor, and the diodes Q ₃ and Q ₄ are loads of each transistor. The transistors Q ₆ and Q ₇ form a differential amplifier circuit in which the emitters are connected to each other, and the transistors Q ₈ and Q ₉ inserted in the collector circuits of these transistors are connected to each other by connecting the bases to each other. Mirror load.

Now, when the signal level applied to the input terminal IN ₁ increases, the current flowing through the transistor Q ₁ increases, and conversely, the current flowing through the transistor Q ₂ decreases. Accordingly, the collector voltage of the transistor Q ₁ is decreased, the collector voltage of the transistor Q ₂ is increased. Also, along with this, the current flowing through the transistor Q ₆ is increased, the current flowing through the transistor Q ₇ is reduced. However, if increasing the current flowing through the transistor Q ₆ is also increased current at the same rate as it flows through the transistor Q _9. As a result, the current flowing through the transistor Q _9, a difference of the current flowing through the transistor Q ₇ flows to the capacitor C _1, the voltage generated in the capacitor C ₁ is Buffer
It increases the base current of the transistor Q ₁₀ as an amplifier.

[0035] On the contrary, with decreasing applied signal level to the input terminal IN _1, a completely opposite operation to that described above, reduces the base current of the transistor Q _10.

As a result, the filter circuit described with reference to FIG. 1 can be realized easily and reliably.

Although the embodiment described with reference to FIGS. 1 and 2 relates to a secondary low-pass filter, the input terminal IN ₁ and the conductance amplifier are connected via a _first resistor R ₁ . Connect to the positive-phase input terminal and
While the positive input terminal and the ground point are connected via a second resistor, the second input terminal and the negative input terminal of the conductance amplifier are connected via a third resistor. The negative-phase input terminal and the external output terminal are connected via the fourth resistor, and the resistance ratio between the first and second resistors and the resistance ratio between the third and fourth resistors are determined. The technique of equalizing and removing the in-phase signal component can be applied to a first-order low-pass filter.

FIG. 3 is a circuit diagram showing an embodiment of this primary low-pass filter. Elements denoted by the same reference numerals as those in FIG. 1 indicate the same elements, and the conductance amplifier G _{2 in} FIG. , Capacitor C ₂ and buffer amplifier B
The UF ₂ is removed, which are connected directly to the output terminal of the buffer amplifier to the external output terminal OUT. In this case, since there is only one conductance amplifier, one buffer amplifier, and one capacitor,
The amplifier G ₁ and G, BU a buffer amplifier BUF ₁
F and the capacitor C ₁ as C.

[0039] In FIG. 3, voltages V ₁ to the input terminal IN ₁
However, when the voltage generated at the external output terminal OUT is V _OUT when the voltage V ₂ is applied to the input terminal IN ₂ , the following relationship is established.

[0040]

(Equation 7) However R _1: the resistance value of the resistor R ₁ R _2: resistor R ₂ of the resistance value R _3: Resistors values of R ₃ R _4: the resistance value of the resistor R ₄ gm: conductance of the transconductance amplifier G C : Capacitance of the capacitor C.

Here, if there is a relation of R ₁ / R ₂ = R ₃ / R ₄ , the equation (14) can be rewritten as the following equation.

[0042]

(Equation 8) Then, the differential signal component V _IN / 2 is applied to the input terminal IN _1, Table transfer function T (s) in the following formula in the case of differential output components -V _IN / 2 is applied to the input terminal IN ₂ Is done.

[0043]

(Equation 9) However A: it is the R ₂ / R _1.

In general, a general expression of a transfer function of a first-order low-pass filter is expressed by the following expression.

[0045]

(Equation 10) Thus, the circuit shown in FIG. 4 has a gain of A and ω ₀ = ｛R
It can be seen that the filter is a first-order low-pass filter of ₁ / (R ₁ + R ₂ )｝ · gm / C.

Next, when the in-phase signal component V _N is applied to the input terminals IN ₁ and IN ₂ , V _OUT = 0 as is apparent from the equation (13).

As a result, V _IN / 2 + V is applied to the input terminal IN _1.
Assuming that _N is applied and −V _IN / 2 + V _N is input to the input terminal IN ₂ , the differential signal V _IN is transmitted to the external output terminal OUT according to the equation (13), but the in-phase component V _N is It does not appear at the external output terminal OUT. Therefore, the filter circuit shown in FIG. 3 is also a filter having a high common mode rejection ratio.

In this embodiment, the buffer amplifier B
While using UF, resistor R ₄ of the resistance value is sufficiently large, shows sufficient input impedance of the load circuit connected to the external output terminal OUT, and the buffer amplifier B
UF can be removed.

In this embodiment, there is no conventional means for changing the value of Q, that is, no signal attenuating means for attenuating the output terminal voltage and applying it to the negative-phase input terminal of the conductance amplifier G. Even if this signal attenuating means is added, the same operation as described above can be performed.

By the way, since specific configurations of the conductance amplifier G and the buffer amplifier G of the present embodiment can be easily conceived from FIG. 2, the description thereof will be omitted.

[0051]

As apparent from the above description, according to the present invention, the differential signal component is transmitted to the output terminal.
The in-phase signal component does not appear at the output terminal, which has the effect of removing the in-phase signal component with a high in-phase rejection ratio.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing a specific configuration of one embodiment of the present invention.

FIG. 3 is a circuit diagram showing a configuration of another embodiment of the present invention.

FIG. 4 is a circuit diagram showing a configuration of a conventional filter circuit.

[Explanation of symbols]

IN _1, IN ₂ input terminal OUT external output terminal _{R 1, R 2, R 3} , R 4 resistors G _1, G _2, G transconductance amplifier BUF _1, BUF _2, BUF buffer amplifier C _1, C _2, C capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hiroshi Mitani 3-3-9 Shimbashi, Minato-ku, Tokyo Inside Toshiba AV EE Co., Ltd. (56) References JP-A-61-96816 (JP, A) JP-A-58-221514 (JP, A) JP-A-58-5015 (JP, A) JP-A-1-246912 (JP, A) JP-A-2-46433 (JP, U) JP-A-5 -85124 (JP, U) US Patent 4,150,433 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 11/04

Claims (2)

(57) [Claims] 1. A conductance amplifier having an output terminal directly or indirectly connected to an external output terminal, a capacitor having one end connected to the output terminal of the conductance amplifier and the other end grounded, and one end connected to a ground terminal. 1, a first resistor connected to the input terminal of the conductance amplifier and the other end connected to the positive input terminal of the conductance amplifier, and one end connected to the positive input terminal of the conductance amplifier and the other end grounded. A second resistor, one end of which is connected to the second input terminal, the other end of which is connected to the negative-phase input terminal of the conductance amplifier, and one end of which is the first conductance. A fourth resistor connected to the negative-phase input terminal of the amplifier and the other end connected to the external output terminal, wherein the resistance value ratio of the first and second resistors and the third and 4
A filter circuit in which the resistance value ratio of the resistors is equal. 2. A first conductance amplifier, a first capacitor having one end connected to an output terminal of the conductance amplifier and the other end grounded, and an output terminal directly or indirectly connected to an external output terminal. A second conductance amplifier having a positive-phase input terminal connected directly or indirectly to an output terminal of the first conductance amplifier, and a negative-phase input terminal connected to the external output terminal; A second capacitor connected to the output terminal of the second conductance amplifier, the other end of which is grounded; and one end connected to the first input terminal, and the other end connected to the positive phase of the first conductance amplifier. A first resistor connected to the input terminal, a second resistor having one end connected to the positive-phase input terminal of the first conductance amplifier, and the other end grounded;
A third resistor having one end connected to the second input terminal and the other end connected to the negative-phase input terminal of the conductance amplifier, and one end connected to the negative-phase input terminal of the first conductance amplifier; A fourth resistor having the other end connected to the external output terminal, wherein a resistance value ratio between the first and second resistors and a resistance value ratio between the third and fourth resistors are provided. Filter circuit with equal.