JPH06314951A - Band pass filter - Google Patents

Abstract

PURPOSE:To make the performance of the band pass filter constant by devising the circuit such that a voltage to current conversion means converts an output voltage of a resonance circuit into a current and feeds back the current to the resonance circuit to vary the Q while a resonance frequency f0 is kept constant thereby realizing a phase characteristic of a constant Q regardless of the frequency f0. CONSTITUTION:A gm converter 1 converts an input voltage V1 into a current i1 and the current is injected to an LCR parallel resonance circuit 3 comprising a parallel circuit of a coil Lo, a capacitor Co and a resistor Ro. A gm converter 2 converts an output voltage Vo of the resonance circuit 3 into a current i2 and it is fed back to the resonance circuit 3. Thus, the current fed to the resonance circuit 3 is the sum of the currents i1, i2. Thus, an admittance gm2 of the gm converter 2 is changed to vary the Q while keeping the resonance frequency fo constant without revising components of the resonance circuit 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency Q of several mega-hundreds of mega-Hz used in a tuner, VIF (video intermediate frequency) and SIF (audio intermediate frequency detection) circuit of a TV receiver. Of a large (10 to 30) bandpass filter (BPF).

[0002]

2. Description of the Related Art Conventionally, in this type of BPF, a gm converter 11 converts an input voltage into a current and is composed of a parallel circuit of a coil Lo, a capacitor Co and a resistor Ro, as shown in FIG. It is injected into the LCR parallel resonance circuit 13 and the frequency at the resonance point of the impedance of the circuit 13 is extracted. The resonance (center) frequencies fo and Q at this time are represented by the following (Equation 1) and (Equation 2).

[0003]

[Equation 1]

[0004]

[Equation 2]

Further, conventionally, in the audio signal detection circuit of a TV receiver, in order to realize a multi-system corresponding to a plurality of broadcasting systems having different modulated carrier frequencies fc, B
Since the resonance frequency fo of the PF is required for the number of different modulated carrier frequencies fc, as shown in FIG. 6, for example, three coils L ₁ , L ₂ , L ₃ are arranged in parallel with the resistor Ro and the capacitor Co. Are connected and switched by the switch SW.

[0006]

The conventional BPF shown in FIG.
When changing the resonance frequency fo, if the value of the coil Lo or the value of the capacitor Co is changed, Q also fluctuates. Therefore, in order to keep Q constant, it is necessary to change the value of the resistor Ro. In the case of changing the value, if the value of the coil Lo or the value of the capacitor Co is changed, fo changes. Therefore, it is limited to the change of the value of the resistor Ro. Therefore, the degree of freedom in setting the constants of the constituent elements of the resonance circuit 13 is small. Has become.

In the conventional BPF of FIG. 6, Q changes when the resonance frequency fo is changed. Due to this fluctuation, the detection circuit changes the characteristics of the detection output level, distortion, D range, and the like, and the performance cannot be kept uniform. In the AFC circuit, the frequency control accuracy cannot be kept constant, and the BPF itself cannot be maintained. However, there is an inconvenience that the frequency characteristic (attenuation amount), the C / N characteristic, and the like vary.

The present invention has been made in view of such a situation, and Q in the state where the resonance frequency fo is kept constant.
It is an object of the present invention to provide a bandpass filter capable of changing.

[0009]

A bandpass filter according to the present invention comprises a resonance circuit (for example, the resonance circuit 3 in FIG. 1) and a voltage / current for converting an output voltage of the resonance circuit into a current and returning the current to the resonance circuit. Converting means (for example, gm converter 2 in FIG. 1)
It is characterized by including.

[0010]

In the bandpass filter of the present invention, the voltage-current conversion means converts the output voltage of the resonance circuit into a current and feeds it back to the resonance circuit.

[0011]

1 is a block diagram of a bandpass filter (BP) of the present invention.
F) shows the configuration of an embodiment. The gm converter 1 converts the input voltage V _I into a current i ₁ and outputs the coil Lo and the capacitor C.
It is injected into the LCR parallel resonance circuit 3 formed of a parallel circuit of o and a resistor Ro. The gm converter 2 converts the output voltage Vo of the resonance circuit 3 into a current i ₂ and feeds it back to the resonance circuit 3. Therefore, the current supplied to the resonance circuit 3 is the sum of i ₁ and i ₂ . When the admittances of the gm converters 1 and 2 are gm1 and gm2, respectively, the currents i ₁ and i ₂ are represented by the following (formula 3) and (formula 4).

[0012]

[Equation 3]

[0013]

[Equation 4]

The impedance Z of the resonance circuit 3 is expressed by the following (formula 5).

[0015]

[Equation 5]

The output voltage Vo is expressed by the following (formula 6).

[0017]

[Equation 6]

From the equations (5) and (6), the transfer function T
When (s) is obtained, it becomes as shown in (Equation 7).

[0019]

[Equation 7]

The resonance frequencies fo and Q at this time are expressed by the following (Equation 8) and (Equation 9).

[0021]

[Equation 8]

[0022]

[Equation 9]

Therefore, according to the embodiment of FIG. 1, by changing the admittance gm2 of the gm converter 2,
It is possible to change Q while maintaining the resonance frequency fo constant, without changing the values of the elements forming the resonance circuit 3 (however, the gain changes with Q).

FIG. 2 shows a configuration example of a gm converter that can be used in the embodiment of FIG. This gm converter has an input voltage source V
NPN transistor Q ₁ whose one terminal is connected to the base, NPN transistor Q ₂ whose other terminal of the input voltage source Vi is connected to the base, and DC voltage source V ₁ which is connected to the input voltage source Vi And transistors Q ₁ and Q ₂
Of the emitter resistance R ₁ connected between the bases of the transistor Q ₁ , the constant current source of I ₁ connected between the emitter of the transistor Q ₁ and the ground point, and the constant current source of I ₁ connected between the emitter of the transistor Q ₂ and the ground point. a constant current source, the NPN transistor Q ₃ having an emitter connected to the collector of the transistor Q _1, the NPN transistor Q ₄ having an emitter connected to the collector of the transistor Q _1, emitter connected to the collector of the transistor Q ₂ transistor NPN transistor Q ₅ whose collector is connected to the collector of Q ₄ and transistor Q ₂
NPN transistor Q ₆ whose collector is connected to the emitter and whose collector is connected to the collector of transistor Q _3.
And an NPN transistor Q ₇ having collectors connected to the bases of the transistors Q ₄ and Q _{6 and} a base connected to one terminal of the control voltage source V _k , and transistors Q ₃ and Q
An NPN transistor Q ₈ whose collector is connected to the base of _{5 and} whose base is connected to the other terminal of the control voltage source V _k ;
DC voltage source V ₂ connected to the other terminal of the control voltage source V _k
, An emitter resistor R ₂ connected between the bases of the transistors Q ₇ and Q ₈ , a constant current source of I ₂ connected between the emitter of the transistor Q ₇ and the ground, and an emitter of the transistor Q ₈ and the ground. a constant current source I ₂ connected between an NPN transistor Q ₉ in which the base is connected to a DC voltage source V ₃ together with the emitter to the collector of the transistor Q ₇ is connected, the emitter to the collector of the transistor Q ₈ And an NPN transistor Q ₁₀ which is connected and whose base is connected to the DC voltage source V ₃ .

In the gm converter of FIG. 2 configured as described above, the input voltage Vi is reduced to the current i ₁ by the resistance R ₁ between the differential pair formed by the transistors Q ₁ and Q ₂ and the emitter R _1.
And the current i ₁ is multiplied by a coefficient K by the full balance circuit including the transistors Q _{3 to} Q ₆ . Current i
₁ is represented by the following (Equation 10).

[0026]

[Equation 10]

The coefficient K is the control voltage V given as the voltage between the bases of the differential pair composed of the transistors Q ₇ and Q _8.
It is determined by k as follows. That is, the control voltage Vk is converted into the current I ₄ by the differential pair formed by the transistors Q ₇ and Q ₈ and the resistance R ₂ between the bases. Current I
₄ is represented by the following (Equation 11).

[0028]

[Equation 11]

The currents flowing through the transistors Q ₉ and Q ₁₀ , respectively, are (I ₂ + I ₄ ) and (I ₂ -I ₄ ). The relationship between the base-emitter voltage of each of the transistors Q _{3 to} Q ₆ and Q ₉ and Q ₁₀ is expressed by the following (Equation 12) and (Equation 13).

[0030]

[Equation 12]

[0031]

[Equation 13]

Therefore, the output current Ki ₁ is calculated by
4).

[0033]

[Equation 14]

From (Equation 11) and (Equation 14), K is
It can be expressed as the following (Equation 15).

[0035]

[Equation 15]

Therefore, it can be seen that K changes linearly with the change of the control voltage Vk. Further, by making I ₂ the inverse temperature coefficient of R, the temperature change of K disappears.

FIG. 3 shows an example of a concrete IC circuit of the embodiment shown in FIG. gm converter 1, an NPN transistor Q ₁ to one terminal of the input voltage source V _I is connected to the base, the NPN transistor Q ₂ to which the other terminal of the input voltage source V _I is connected to the base, the transistor Q A resistor R ₁ connected between the emitters of ₁ and Q ₂ , a constant current source of I connected between the emitter of the transistor Q ₁ and ground, and an I connected between the emitter of the transistor Q ₂ and ground. a constant current source, the NPN transistor Q ₃ to one terminal of the input voltage source V _I is connected to the base, the NPN transistor Q ₄ and the other terminal of the input voltage source V _I is connected to the base, the transistor Q A resistor R connected between the emitters of ₃ and Q _4.
3 , the constant current source of I connected between the emitter of the transistor Q ₃ and the ground, the constant current source of I connected between the emitter of the transistor Q ₄ and the ground, and the emitter of the collector of the transistor Q _3. NPN transistor Q whose collector is connected to the collector of transistor Q _2
5, an NPN transistor Q ₆ which collector to the collector of the collector emitter connected to the transistor to Q ₁ transistor Q3 is connected, the collector to the collector of the transistor Q emitter to the collector of the ₄ connected transistors Q ₁ and Q ₆ are NPN transistor Q connected
₇ and an NPN transistor Q ₈ whose emitter is connected to the collector of the transistor Q _{4 and} whose collectors are connected to the collectors of the transistors Q ₂ and Q ₅ .

The resonance circuit 3 includes a capacitor Co / 2 connected between the collectors of the transistors Q ₁ , Q ₆ and Q _{7 and} the collectors of the transistors Q ₂ , Q ₅ and Q ₈ , and in parallel with this capacitor. The connected coil 2L, the resistor Ro / 2 connected between one end of the capacitor Co / 2 and the coil 2L and the bias voltage line, and the resistor Ro / 2 connected between the other end of the capacitor Co / 2 and the coil 2L and the bias voltage line. And a resistor Ro / 2.

The gm converter 2 includes an NPN transistor Q ₉ whose bases are connected to the collectors of the transistors Q ₁ , Q ₆ and Q ₇ and a constant current source I which is connected between the emitter of the transistor Q ₉ and the ground point. An NPN transistor Q ₁₀ whose base is connected to the collectors of the transistors Q ₂ , Q ₅ and Q ₈ , a constant current source of I connected between the emitter of the transistor Q ₁₀ and the ground point, and an emitter of the transistor Q ₁₀ . Is connected to the base of an NPN transistor Q ₁₁ and the emitter of the transistor Q ₉ is connected to the base of an NPN transistor Q _11.
A transistor Q ₁₂ , an emitter resistor R ₂ connected between the bases of the transistors Q ₁₁ and Q ₁₂ , a constant current source of I connected between the emitter of the transistor Q ₁₁ and the ground, and an emitter of the transistor Q ₁₂ . a constant current source I connected between the ground point, the NPN transistor Q ₁₃ having an emitter connected to the collector of the transistor Q _11, the NPN transistor Q ₁₄ having an emitter connected to the collector of the transistor Q _11, the transistor Q An NPN transistor Q ₁₅ having an emitter connected to the collector of _{12 and} a collector connected to the collector of a transistor Q ₁₄ ;
NPN transistor Q whose emitter is connected to the collector of _{12 and} whose collector is connected to the collector of transistor Q _13.
16 and.

The coefficient control circuit 4 includes a transistor Q ₆ ,
NPN whose collectors are connected to the bases of Q ₈ , Q ₁₄ and Q _{16 and} whose bases are connected to one terminal of the control voltage source V _k
An NPN transistor Q having a collector connected to the bases of the transistor Q ₁₇ and the transistors Q ₅ , Q ₇ , Q ₁₃ and Q _{15 and} a base connected to the other terminal of the control voltage source V _k.
18 , a DC voltage source V ₂ connected to the other terminal of the control voltage source V _k , an emitter resistor R ₄ connected between the bases of the transistors Q ₁₇ and Q ₁₈ , and an emitter of the transistor Q ₁₇ and a ground point. The constant current source of I connected between them, the constant current source of I connected between the emitter of the transistor Q ₁₈ and the ground point, the emitter of the collector of the transistor Q _{17 and} the direct current voltage source V _B are connected. N to which the base is connected
A PN transistor Q ₁₉ and an NPN transistor Q ₂₀ whose emitter is connected to the collector of the transistor Q _{18 and} whose base is connected to the DC voltage source V _B are provided.

The collectors of the transistors Q ₂ , Q ₅ and Q ₈ , the base of the transistor Q ₁₀ , and the collectors of the transistors Q ₁₄ and Q ₁₅ are connected to one terminal of the output voltage Vo, and the transistors Q ₁ , Q ₆ and The collector of Q _7, the base of transistor Q ₉ , and the collectors of transistors Q ₁₃ and Q ₁₆ are connected to the other terminal of output voltage Vo.

In the circuit of FIG. 3 configured as described above, the coefficient K set by the coefficient control circuit 4 is expressed by the following (Equation 16).

[0043]

[Equation 16]

The gm converter 1 uses the transistors Q ₁ and Q ₂ and the resistor R ₁ between the emitters to input the input voltage V _I.
Is converted into a current i ₁ , and the input voltage V is increased by the transistors Q ₃ and Q ₄ and the resistance R ₃ between the emitters.
Convert _I to current i ₃ . The currents i ₁ and i ₃ can be expressed by the following (Equation 17) and (Equation 18), respectively.

[0045]

[Equation 17]

[0046]

[Equation 18]

The current i ₃ is multiplied by a coefficient K by a full balance circuit composed of transistors Q _{5 to} Q ₉ ,
The current i ₁ is added with the opposite polarity, and the current (i ₁ −Ki ₃ ) is output. The admittance gm1 of the gm converter 1 can be expressed by the following (Formula 19).

[0048]

[Formula 19]

On the other hand, the gm converter 2 includes a transistor Q ₁₁
The output voltage Vo is converted into the current i ₂ by the differential pair formed by Q 1 and Q ₁₂ and the resistor R ₂ . The current i ₂ can be expressed by the following (Equation 20).

[0050]

[Equation 20]

The current i ₂ is multiplied by a coefficient K by a full balance circuit composed of transistors Q _{13 to} Q ₁₆ ,
i ₂ is supplied to the resonance circuit 3. Therefore, the current supplied to the resonance circuit 3 by both the gm converters 1 and 2 is (i ₁ −Ki ₃ + Ki ₂ ).

The admittance gm2 of the gm converter 2 is
It can be shown by the following (Equation 21).

[0053]

[Equation 21]

Therefore, by substituting (Equation 19) and (Equation 21) into (Equation 7) to obtain the transfer function T (s), the following (Equation 22) is obtained.

[0055]

[Equation 22]

Here, if the resistor R ₃ is selected as in the following (Equation 23), the first-order terms of the numerator and denominator of (Equation 22) match, and the resonance frequencies fo, Q and gain G are
The following (Equation 24), (Equation 25) and (Equation 26) are obtained.

[0057]

[Equation 23]

[0058]

[Equation 24]

[0059]

[Equation 25]

[0060]

[Equation 26]

Therefore, according to the circuit example of FIG. 3, the value of Q is set to K without changing the resonance frequency fo and the gain G.
Can be changed freely.

Therefore, even if the constant value (L or C) of the resonance circuit 3 is changed to change the resonance frequency fo,
By switching the coefficient setting voltage Vk, it is possible to always keep the Q and the gain G constant.

Therefore, the circuit of FIG.
When used in the FM FM and AM detection circuit and the tank circuit of the AFC circuit (using the BPF characteristic), Q can be corrected by changing the constant in setting the resonance frequency fo.

FIG. 4 shows the phase characteristic of the embodiment of FIG. 1 in comparison with the phase characteristic of the conventional BPF. The broken line shows a conventional example, and the solid line shows an embodiment of the present invention. In the conventional example, the characteristic of the resonance circuit alone becomes the phase characteristic of the BPF as it is, and when the resonance frequency fo is set high, Q also becomes large and the slope of the phase characteristic becomes steep. Therefore, when this is used for the detection circuit, the characteristics such as the level, distortion, and D range of the detection output change with the switching of the system, and the performance becomes uneven. On the other hand, in the embodiment of the present invention, the BPF characteristic that makes Q constant is obtained regardless of the value of the resonance frequency fo, and even when the detection circuit is configured, the characteristic variation due to the system switching is eliminated and the performance is improved. Can be made uniform. Also, in the AFC circuit, in the conventional circuit,
While the accuracy of frequency control changes with the change of Q,
In the embodiment of the present invention, there is no change in Q and the accuracy of frequency control is also kept constant.

In the embodiment of the present invention shown in FIG. 1, even when used as the BPF itself, the frequency characteristics (attenuation amount) due to the fluctuation of Q and the performance such as C / N can be made uniform.

[0066]

As apparent from the above description, according to the bandpass filter of the present invention, the voltage-current conversion means converts the output voltage of the resonance circuit into a current and feeds it back to the resonance circuit. , Q can be changed while keeping the resonance frequency fo constant. Therefore, since a constant Q characteristic can be realized regardless of fo, the detection output can be made uniform in performance when the detection circuit is constructed, and the frequency control accuracy can be made constant when the AFC circuit is constructed.
Further, since a constant Q frequency characteristic can be realized regardless of fo, the performance of the bandpass filter can be made constant.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a bandpass filter (BPF) of the present invention.

FIG. 2 is a circuit diagram showing a configuration example of a gm converter that can be used in the embodiment of FIG.

FIG. 3 is a circuit diagram showing a specific circuit example of the embodiment shown in FIG.

FIG. 4 is a diagram showing the phase characteristic of the embodiment of FIG. 1 in comparison with the phase characteristic of a conventional BPF.

FIG. 5 is a block diagram showing an example of a conventional BPF.

FIG. 6 is a circuit diagram showing an example of a conventional multi-system compatible SIF (audio intermediate frequency detection) BPF.

[Explanation of symbols]

 1, 2 gm converter 3 resonance circuit 4 coefficient control circuit

Claims (3)

[Claims] 1. A bandpass filter comprising: a resonance circuit; and a voltage-current conversion means for converting an output voltage of the resonance circuit into a current and feeding back the current to the resonance circuit. 2. The bandpass filter according to claim 1, wherein the admittance of the voltage-current conversion means is set by voltage control. 3. A further voltage-current conversion means for converting an input voltage into a current and injecting it into the resonance circuit, the admittance setting voltage of the voltage-current conversion means and the admittance setting voltage of the feedback voltage-current conversion means. The band-pass filter according to claim 1, wherein and are commonly used.