JP3243213B2 - Low-pass filter circuit and radio using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-pass filter circuit for use in, for example, a bipolar semiconductor integrated circuit and the like, and more particularly, to achieving a highly accurate attenuation characteristic.

[0002]

2. Description of the Related Art A filter circuit using a resistor, a capacitor and an active element does not use an inductance.
The circuit can be reduced in size and weight and integrated, and is widely used as a low-frequency pass filter of a wireless device such as a mobile phone.

[0003] Conventionally, as this kind of filter circuit,
A second-order VCVS type low-pass filter circuit, which is a second-order low-pass filter circuit, is known.

This circuit has an input terminal as shown in FIG.
40, an output terminal 41, a one-input one-output amplifier 30 with a gain of 1, a resistor 20 and a resistor 21 connected in series between the input terminal 40 and the input of the amplifier 30, and a connection of the resistor 20 and the resistor 21 A capacitor 22 is connected between the unit and the connection between the output of the amplifier 30 and the output terminal 41, and a capacitor 23 is connected between the input of the amplifier 30 and the ground.

The amplitude of the signal input from the input terminal 40 of this circuit is Vin1 [dBm], the amplitude of the signal output from the output terminal 41 is Vo1 [dBm], and the cutoff frequency is fc1.
Then, the transfer function obtained by Vo1 / Vin1, fc1,
And the frequency of the signal input from the input terminal 40 is f [H
z], the amplitude Vo1 of the signal output from the output terminal 41
Are represented by the following equations. · Vo1 / Vin1 = 1 / { (R 20 × R 21 × C 22 × C 23) S 2 + C 23 × (R 20 + R 21) S + 1} · fc1 = 1 / {2π × (R 20 × R 21 × C ₂₂ × C ₂₃ ) ^1/2 )｝ [Hz] Vo1 = Vin1−40log (f / fc1) [dBm] where S represents jω.

FIG. 7 shows the amplitude Vo1 of the output signal corresponding to the frequency of the input signal in the filter circuit.

FIG. 8 shows a conventional secondary low-pass filter circuit in which an amplifier 30 of one input and one output of the filter circuit is realized by an emitter follower circuit including a transistor and a current source. This filter circuit
Is replaced by an NPN transistor 31 and a current source 32, and further includes an input terminal 40, an output terminal 41, resistors 20, 21 and capacitors 22, 23, as in the circuit of FIG.

The second-order low-pass filter circuit operates in the same manner as the second-order low-pass filter circuit of FIG.

[0009]

However, looking at the filter characteristics of the second-order low-pass filter circuit of FIG. 8, as shown in FIG. 9, fc1 is smaller than the set value, and the frequency of the input signal is reduced. When f is made larger than fc1 and the amplitude of the output signal is attenuated at a fixed rate (-40 dB / dec), there is a disadvantage that the attenuation cannot be secured in the middle.

The second-order low-pass filter circuit is required to improve the accuracy of the cutoff frequency to be set and to keep the slope of attenuation in the attenuation region constant. The circuit cannot adequately meet this requirement.

The present inventor has investigated the cause of the appearance of a filter characteristic having a defect in the secondary low-pass filter circuit of FIG.
Based on this, a method for improving the characteristics of the secondary low-pass filter circuit in general is required.

The second-order low-pass filter circuit shown in FIG. 10 adds a resistor 24 to the filter circuit shown in FIG. 8, and the resistor 24 is connected between the emitter of the transistor 31 and the current source 32 to generate the current of the transistor 31. SPICE (Simulation Progr
am Integrated Circuit Emphasis) An emitter resistance given by a model parameter and a resistance given by the reciprocal of the conductance of the transistor 31 are connected in series, and are represented on a circuit.

Assuming that the amplitude of the signal input from the input terminal 40 of this circuit is Vin2 [dBm] and the amplitude of the signal output from the output terminal 41 is Vo2 [dBm], the transfer function obtained by Vo2 / Vin2 is as follows. Is represented by the following equation. · Vo2 / Vin2 = {(R 21 × R 24 × C 22 × C 23) S 2 + (R 24
× C ₂₂ ) S + 1｝ / {C ₂₂ × C ₂₃ × (R ₂₀ × R ₂₁ + R ₂₁ ×
R ₂₄ + R ₂₄ × R ₂₀ ) S ² + (R ₂₀ × C ₂₃ + R ₂₁ × C ₂₃ + R
₂₄ × C ₂₂ ) S + 1｝.

The transfer function determined by Vo2 / Vin2 can be expressed by two second-order low-pass filter circuits (these are referred to as a second-order low-pass filter circuit A and a second-order low-pass filter circuit B).
) Can be seen as a combination of the transfer functions. That is, it can be seen that this transfer function is formed by multiplication of the transfer function of the secondary low-pass filter circuit A and the reciprocal of the transfer function of the secondary low-pass filter circuit B.

The cutoff frequency of the transfer function of the secondary low-pass filter circuit A is fc2, and the secondary low-pass filter circuit B is
Let fc3 be the reciprocal cutoff frequency of the transfer function of
fc2 and fc3 are represented as follows.・ Fc2 = 1 / {2π (C ₂₂ × C ₂₃ × (R ₂₀ × R ₂₁ + R ₂₁ × R
₂₄ + R ₂₄ × R ₂₀ )) ^1/2 } [Hz] ・ fc3 = 1 / {2π × (R
₂₁ × R ₂₄ × C ₂₂ × C ₂₃ ) ^1/2 ｝ [Hz]

Here, the reciprocal of the transfer function of the second-order low-pass filter circuit means that when the frequency of the signal input from the input terminal 40 is lower than fc3, the amplitude of the signal output from the output terminal 41 is constant. When the frequency is higher than fc3, the amplitude of the signal output from the output terminal 41 is increased at a constant rate (+40 dB / dec) according to the height of the frequency of the signal input from the input terminal 40 with respect to fc3. Means

At this time, the frequency f [H
z], the amplitude Vo2 of the signal output from the output terminal 41 when the signal is input is represented by the following equation.

Vo2 = Vin2-40 log (f / fc2) +
40 log (f / fc3) [dBm] FIG. 11 shows this filter characteristic. Therefore, the characteristics of FIG. 9 can be explained by considering the resistance 24.

From the above, the inaccuracy of the cut-off frequency to be set and the inability to keep the slope of attenuation in the attenuation region constant were due to the low-pass filter circuit shown in FIG. It can be understood that the reason is that the transfer function was not calculated in consideration of.

The present invention has been conceived based on such considerations, and a low-pass filter circuit capable of setting a cutoff frequency with high accuracy and maintaining a constant attenuation gradient in an attenuation region. Another object of the present invention is to provide an integrated circuit incorporating the same and a wireless device using the same.

[0021]

Therefore, in the low-pass filter circuit of the present invention, the cut-off frequency is set in consideration of the resistance component of the amplifier or the emitter follower circuit, which is a component of the filter circuit, and the cut-off frequency is set. A filter circuit for canceling a filter effect resulting from a resistance component and causing a reduction in attenuation is connected in series before or after the low-pass filter circuit.

Therefore, the setting accuracy of the cutoff frequency can be improved, and the slope of attenuation in the attenuation range can be kept constant.

[0023]

DETAILED DESCRIPTION OF THE INVENTION According to a first aspect of the present invention is a low-pass filter circuit having a gain 1 amplifier having an output impedance components, this amplifier
One end of the first resistor is connected to the output side of the first resistor.
Connect the first capacitor between the terminal and the input side of the amplifier
And a connection point between the first resistor and the first capacitor and an output terminal.
Grounded through a second capacitor between the
One end of a second resistor is connected to the input side, and the other end of the second resistor
A resistor that forms a primary filter is connected directly between
Column connection, and connect a primary between this resistor and the other end of the second resistor.
Ground through the capacitor that constitutes the filter of
The first resistor is composed of the resistor and the capacitor that constitute the filter of
Of the first order filter derived from the resistance and the first capacitor
This is to cancel the effect, and the cut to be set
The accuracy of the off-frequency can be improved, and the slope of attenuation in the attenuation region can be kept constant.

[0024]

According to the second aspect of the present invention, the resistance corresponding to the output resistance of the amplifier is used as the first resistance, and the transfer function is calculated by taking this resistance into account, so that the accuracy of the cutoff frequency to be set can be improved. Can be enhanced.

According to a third aspect of the present invention, in a low-pass filter circuit having an emitter follower circuit as a component, the emitter follower circuit includes a transistor and a current source.
Between the emitter of the transistor and the current source
A first resistor is connected, and the other end of the first resistor is connected to a transistor.
A first capacitor is connected between the
Between the connection point of the resistor and the first capacitor and the output terminal.
Grounded through the capacitor 2 and the base of the transistor
Is connected to one end of a second resistor, and the other end of the second resistor is connected to an input.
Connect a resistor that constitutes a primary filter in series with the terminal
A primary filter is connected between this resistor and the other end of the second resistor.
Ground through the capacitor that constitutes the
The first resistor, the transistor,
Derived from the resistance component of the transistor and the first capacitor
The function of the first-order filter is canceled out.
Thus, the accuracy of the cutoff frequency to be set can be increased, and the slope of attenuation in the attenuation region can be kept constant.

[0027]

The invention described in claim 4 is the invention according to claim 1 or
The low-pass filter circuit described in No. 2 is incorporated in a semiconductor integrated circuit, and an integrated circuit is configured by utilizing properties suitable for integration of the filter circuit.

According to a fifth aspect of the present invention, the low-pass filter circuit according to the third aspect is built in a semiconductor integrated circuit. The integrated circuit utilizes characteristics suitable for integration of the filter circuit. Is composed.

[0030] The invention described in claim 6 is claim 4 or
The semiconductor integrated circuit described in 5 is included in a wireless device, and the size and weight of the wireless device can be reduced.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First Embodiment) As shown in FIG. 1, a second-order low-pass filter circuit according to a first embodiment of the present invention has an input terminal 40, an output terminal 41, and a gain of 1;
Amplifier 30 with 1 input, output of amplifier 30 and output terminal 41
, A capacitor 14 connected between the output terminal 41 and the input of the amplifier 30, a resistor 10 and a capacitor 13 connected in series between the input terminal 40 and the ground,
A resistor 11 connected between the connection of the capacitor 10 and the connection of the capacitor 13 and the connection of the capacitor 14 on the input side of the amplifier 30, and an output terminal
41 and a capacitor 15 connected between ground.

In this filter circuit, the amplifier 30, the input terminal 40 and the output terminal 41 are the same as those in FIG. 6, but the resistor 12 is newly connected and the primary The difference is that a filter is newly connected, and the arrangement of other circuit elements is different.

The resistance 12 corresponds to the output resistance of the amplifier 30,
By adding this resistance 12 to the calculation of the transfer function,
The accuracy of the cutoff frequency to be set can be improved.

Further, a resistor 12 is added.
Due to the different arrangement of the capacitors 14 and 15, the transfer function of the second-order low-pass filter circuit composed of the resistors 11, 12, 14, 15 and the amplifier 30 includes the resistors 12 and 14. The reciprocal of the transfer function of the first-order low-pass filter circuit is added.

The reciprocal of the transfer function of the first-order low-pass filter circuit causes a reduction in the amount of attenuation. Therefore, before the second-order low-pass filter circuit, a first-order low-pass filter circuit including a resistor 10 and a capacitor 13 is connected in series, and a first-order low-pass filter circuit derived from the resistor 12 and the capacitor 14 is provided. Negating the effect.

If the amplitude of the signal input from the input terminal 40 of this filter circuit is Vin [dBm] and the amplitude of the signal output from the output terminal 41 is Vo [dBm], Vo / Vin
Is represented by the following equation.・ Vo / Vin = {(R ₁₂ × C ₁₄ ) × S + 1} / {(R ₁₀ × R ₁₁ ×
R ₁₂ × C ₁₃ × C ₁₄ × C ₁₅ ) × S ³ + (R ₁₂ × C ₁₄ × (R ₁₀ ×
C ₁₃ + R ₁₀ × C ₁₅ + R ₁₁ × C ₁₅ ) + R ₁₀ × R ₁₂ × C ₁₃ ×
C ₁₅ ) × S ² + (R ₁₀ × C ₁₃ + R ₁₂ × C ₁₄ + R ₁₂ × C ₁₅ ) × S +
1}

At this time, R ₁₀ = n × R ₁₂ , C ₁₃ = C ₁₄ / n
Then, the transfer function determined by Vo / Vin is represented by the following equation. Here, n is a real number.・ Vo / Vin = {(R ₁₂ × C ₁₄ ) × S + 1} / {(R ₁₂ × C ₁₄ )
× S + 1) × ((R ₁₁ × R ₁₂ × C ₁₄ × C ₁₅ ) × S ² + (R ₁₂ × C
₁₄ + (1 + n) × R ₁₂ × C ₁₅ ) × S + 1) -n × R ₁₂ × C ₁₅ ×
S}

Here, the cut-off frequency to be set is fc.
If fc is lower than 1 / (2π × n × R ₁₂ × C ₁₅ ), the transfer function obtained by Vo / Vin is further expressed by the following equation.・ Vo / Vin = 1 / ((R ₁₁ × R ₁₂ × C ₁₄ × C ₁₅ ) × S ² +
(R ₁₂ × C ₁₄ + (1 + n) × R ₁₂ × C ₁₅ ) × S + 1)

In the above setting, fc and the output terminal when the frequency of the signal input from the input terminal 40 is f [Hz]
The amplitude Vo of the signal output from 41 is represented by the following equation. · Fc = 1 / (2π × (R 11 × R 12 × C 14 × C 15) 1/2) [Hz] · Vo = Vin-40log (f / fc) [dBm] However, R ₁₀ = n × R ₁₂ , C ₁₃ = C ₁₄ / n, fc <1 /
(2π × n × R ₁₂ × C ₁₅ ) [Hz]

FIG. 2 shows the filter characteristics of this filter circuit.

As described above, by calculating the transfer function by adding the resistor 12 (R ₁₂ ) corresponding to the output resistance of the amplifier 30, the accuracy of the cutoff frequency fc to be set can be improved.

Also, in order to cancel the effect of the reciprocal of the transfer function of the first-order low-pass filter circuit comprising the resistor 12 and the capacitor 14, the 1
Connect the secondary low-pass filter in series with the secondary low-pass filter, make resistor 10 equivalent to resistor 12 (R ₁₀ = R ₁₂ ), and make capacitor 13 equivalent to capacitor 14 (C ₁₃ = C ₁₄ ). Or by equalizing the product of the resistor 10 and the capacitor 13 (R ₁₀ × C ₁₃ ) with the product of the resistor 12 and the capacitor 14 (R ₁₂ × C ₁₄ ). Can be suppressed.

It should be noted that a first order for canceling the operation of the primary low-pass filter circuit derived from the resistor 12 and the capacitor 14 is provided.
The same effect can be obtained by connecting the secondary low-pass filter after the secondary low-pass filter.

(Second Embodiment) In the second embodiment,
The amplifier 30 of the filter circuit of the first embodiment (FIG. 1)
It is replaced by an emitter follower circuit composed of a transistor and a current source. As shown in FIG. 3, this second-order low-pass filter circuit includes an NPN transistor 31, a current source 32, and a resistor connected between the emitter of the transistor 31 and the current source 32, instead of a one-input one-output amplifier. 16 as well as other input terminals 40, as in the circuit of FIG.
An output terminal 41, resistors 10, 11 and capacitors 13, 14, 15 are provided.

This filter circuit differs from the first embodiment in that the amplifier 30 is replaced by an emitter follower circuit composed of a transistor 31 and a current source 32,
Is replaced with a resistor 16.

Here, the resistor 16 is obtained by subtracting a resistor 24 described below from the resistor 12. The resistor 24 is a resistor in which an emitter resistance given between the emitter of the transistor 31 and the current source 32 and given by a SPICE model parameter of the transistor 31 and a resistor given by the reciprocal of the conductance of the transistor 31 are connected in series.

The circuit of the second embodiment replaces the input of the amplifier 30 in the first embodiment with the base of the transistor 31, replaces the output of the amplifier 30 with the emitter of the transistor 31, and sets the collector of the transistor 31 to the power supply voltage. The current source 32 is connected between the emitter of the transistor 31 and the ground. Therefore, the circuit of the second embodiment is
Operates similarly to the embodiment of FIG.
When the operation of the primary low-pass filter circuit derived from the resistance component 31 and the capacitor 14 satisfies one of the following conditions (1) and (2), the primary low-pass filter composed of the resistor 10 and the capacitor 13 Canceled by a pass filter. (1) R ₁₀ = R ₁₆ + R ₂₄ and C ₁₃ = C ₁₄ (2) R ₁₀ × C ₁₃ = (R ₁₆ + R ₂₄ ) × C ₁₄

[0049] Here, R ₂₄ is the resistance value of the resistor connected to the resistance provided by the reciprocal of the conductance of the emitter resistor and the transistor 31 of the transistor 31 in series, R ₁₀ is the resistance of resistor 10, R ₁₆ is the resistance 16 the resistance value of, C ₁₃ is the capacitance value of the capacitor 13, C ₁₄ is the capacitance value of the capacitor _14.

FIG. 4 shows the filter characteristics of the circuit according to the second embodiment.

As described above, before or after the low-pass filter circuit constituted by the emitter follower circuit,
By connecting a compensation filter circuit consisting of a resistor and a capacitor in series to cancel the decrease in attenuation,
The slope of the attenuation in the attenuation region can be kept constant.

(Third Embodiment) In the third embodiment,
An example is shown in which the filter circuit of the second embodiment is integrated together with a differential amplifier circuit.

As shown in FIG. 5, this circuit has the same output terminal 41 and NPN transistor as in the second embodiment (FIG. 3).
31, current source 32, resistors 11, 16 and capacitors 13, 14, 15
NPN transistors 33 and 34 having a common emitter, a resistor 17 connected between a connection portion of the resistor 11 and the capacitor 13 and a collector of the transistor 33,
A resistor 18 connected between the power supply voltage Vcc and the collector of the transistor 33, a resistor 19 connected between the power supply voltage Vcc and the collector of the transistor 34, and an input terminal 42 connected to the base of the transistor 34; , An input terminal 43 connected to the base of the transistor 33, and a current source 35 connected between the emitter of the transistor 33 and ground.

This circuit is different from the second embodiment in that a differential amplifier circuit including a resistor 18, a resistor 19, a transistor 33, a transistor 34, a power supply 35, an input terminal 42, and an input terminal 43 is added. The difference is that the resistor 10 is changed to the resistor 17. At this time, the resistor 17 is obtained by subtracting the resistor 18 from the resistor 10.

This circuit sets the gain of the differential amplifier circuit to A
When [dB] is set, V is applied between the input terminal 42 and the input terminal 43.
When a signal having one-phase amplitude of in-A [dBm] is input, the same operation as that of the first embodiment is performed.

As described above, the filter circuit of the present invention is also suitable for integration with other circuits to constitute a bipolar semiconductor integrated circuit or the like.

Further, by incorporating this semiconductor integrated circuit into a wireless device such as a portable telephone, the size and weight of the wireless device can be reduced.

[0058]

As is apparent from the above description, the low-pass filter circuit of the present invention can keep the slope of attenuation in the attenuation region constant and can increase the accuracy of the cutoff frequency to be set. .

The filter circuit can be integrated with other circuits to form an integrated circuit. By incorporating this integrated circuit into a wireless device, the size and weight of the wireless device can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a low-pass filter circuit according to a first embodiment of the present invention;

FIG. 2 is a frequency characteristic diagram of the low-pass filter circuit according to the first embodiment;

FIG. 3 is a circuit diagram of a low-pass filter circuit according to a second embodiment of the present invention;

FIG. 4 is a frequency characteristic diagram of a low-pass filter circuit according to a second embodiment;

FIG. 5 is a circuit diagram of a low-pass filter circuit according to a third embodiment of the present invention;

FIG. 6 is a circuit diagram of a conventional secondary low-pass filter circuit,

FIG. 7 is a frequency characteristic diagram of a conventional secondary low-pass filter circuit,

FIG. 8 is a circuit diagram of another conventional second-order low-pass filter circuit,

FIG. 9 is a frequency characteristic diagram of another conventional second-order low-pass filter circuit,

FIG. 10 is a circuit diagram in which another conventional second-order low-pass filter circuit is modified,

FIG. 11 is a frequency characteristic diagram of a modified second-order low-pass filter circuit.

[Explanation of symbols]

 10, 11, 12, 16, 17, 18, 19, 20, 21, 24 Resistor 13, 14, 15, 22, 23 Capacitor 30 Amplifier 31, 33, 34 NPN transistor 32, 35 Current source

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 11/04 H03F 1/22 H03H 11/12

Claims (6)

(57) [Claims] In a low-pass filter circuit having an amplifier having a gain of 1 and having an output impedance as a component, one end of a first resistor is connected to an output side of the amplifier.
The first resistor is connected between the other end of the first resistor and the input side of the amplifier.
Are connected to the first resistor and the first capacitor.
Between the connection point of the capacitor and the output terminal
And a second resistor connected to the input side of the amplifier.
One end is connected, and the other end of the second resistor is connected to an input terminal.
A resistor constituting a primary filter is connected in series between the
A primary fill is provided between a resistor and the other end of the second resistor.
Grounded through a capacitor constituting the
The resistor and the capacitor constituting the filter are connected to the first
First-order filter derived from resistor and first capacitor
A low-pass filter circuit characterized by canceling the function of (1 ). 2. The amplifier according to claim 1 , wherein said first resistor is connected to an output resistor of said amplifier.
2. The low-pass filter circuit according to claim 1, wherein the low-pass filter circuit is resistant . 3. An emitter follower circuit is included in a component.
In a low-pass filter circuit, the emitter follower circuit connects a transistor and a current source.
And a third transistor between the emitter of the transistor and a current source.
The first resistor is connected to the other end of the first resistor and the transformer.
The first capacitor is connected to the base of the transistor.
And a connection point between the first resistor and the first capacitor and an output.
Between the power terminal and the power terminal via a second capacitor.
One end of a second resistor is connected to the base of the transistor.
Between the other end of the second resistor and the input terminal.
A resistor constituting a filter is connected in series, and the resistor and the second resistor are connected in series.
The other end of the second resistor constitutes the first-order filter.
Grounded via a capacitor to form the primary filter
The formed resistor and the capacitor are the first resistor and the transistor.
The resistance component of the transistor and the first capacitor.
It is characterized by canceling the action of the primary filter generated
Low-pass filter circuit that. 4. The low-pass filter according to claim 1,
A semiconductor integrated circuit with a built-in filter circuit. 5. The low-pass filter circuit according to claim 3,
Semiconductor integrated circuit. 6. The semiconductor integrated circuit according to claim 4, wherein
Radio including road .