JP3070714B2 - 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 improvement in a filter circuit, and more particularly to an improvement in a filter circuit suitable for use in a television receiver or the like for signal processing for image quality correction. It is.

[0002]

2. Description of the Related Art A technique for forming a filter by using an IC is described in, for example, Japanese Patent Publication No. 1-54884. The filter is constituted by a combination of a variable gain amplifier and a capacitor. The cutoff frequency as a filter characteristic can be changed by controlling the gain of the variable gain amplifier. When the filter is configured as an IC, since the resistance value and the capacitance value inside the IC have manufacturing variations, the cutoff frequency, which is the filter characteristic, also varies. By controlling the gain of a variable gain amplifier, variation compensation can be performed.

FIG. 10 is a circuit diagram showing an example of such a conventional filter circuit. A signal from the input signal source INPUT is output from the output terminal OUT. C is a capacitor constituting the filter. By adjusting the variable voltage source VV, the gain of the variable gain amplifier constituting the filter can be controlled to compensate for the variation in the cutoff frequency due to the production variation in the resistance and capacitance values inside the IC constituting the filter. It has become.

[0004]

The filter according to the prior art described above is used for frequency switching for switching the cutoff frequency from a certain frequency to another frequency, for fine adjustment for compensating for variation in the cutoff frequency, and Therefore, if the gain control means is configured so that the cutoff frequency can be arbitrarily switched, such gain control means can compensate for variations in the cutoff frequency. If the gain control means is configured so as not to be able to compensate for the variation of the cutoff frequency, the cutoff frequency cannot be arbitrarily switched by such a gain control means.

By the way, it is possible to switch a filter characteristic such as a cut-off frequency characteristic from a certain characteristic to another characteristic, and when there is a variation in the switched characteristic, this characteristic variation can be compensated. Filters may be required. For example, in a television receiver that receives both a high-definition broadcast television signal and a double-scanned standard television signal, these two signals have different original signal bands. Correction)
It is necessary to switch the center frequency to be subjected to image quality correction according to which television signal is received.

When the image quality correction circuit is constituted by an IC, a phase shift circuit, which is one of filters, is often used in place of a required delay line. Since the frequency is determined by the delay time of the phase shift circuit, the delay time (that is, the filter characteristic) must be switched. Further, if there is a variation in characteristics at the switching destination, it is required to compensate for this.

SUMMARY OF THE INVENTION An object of the present invention is to provide a filter circuit capable of switching filter characteristics, which may be required under the circumstances described above, and capable of compensating for any characteristic variations at the point of switching. Is to do.

[0008]

According to the present invention, there is provided a filter circuit comprising a combination of a variable conductance amplifier as a voltage controlled current source and a capacitor.

As the differential amplifier constituting the variable conductance amplifier, at least first and second two differential amplifiers having a common load are provided, and depending on which one is selected and operated. First characteristic control means for switching a mutual conductance of the variable conductance amplifier to switch a filter characteristic such as a cutoff frequency;

[0010] Of the two differential amplifiers, the output current of the differential amplifier that is selected and operating is variably controlled, so that the mutual conductance of the variable conductance amplifier is variably controlled to cut off the cutoff. And second characteristic control means for changing a filter characteristic such as a frequency.

In a filter circuit constituted by a combination of a variable conductance amplifier as a voltage control current source and a capacitor, at least first and second two capacitors are provided as the capacitors. First characteristic control means for switching a filter characteristic such as a cutoff frequency depending on which one is selected to function;

A second characteristic for variably controlling the mutual conductance of the variable conductance amplifier by variably controlling the output current of the differential amplifier constituting the variable conductance amplifier to vary a filter characteristic such as a cutoff frequency; Control means.

[0013]

The filter characteristics can be switched by using the first characteristic control means, and the variation of the filter characteristics at the switching destination can be compensated by using the second characteristic control means. In this way, it is possible to arbitrarily switch the filter characteristics such as the cutoff frequency characteristic, and to realize a filter in which the characteristic variation is compensated at each switching destination.

[0014]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
explain. FIG. 1 is a circuit diagram showing one embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes an independent voltage source for supplying a power supply voltage to the circuit, and 2 denotes a variable independent voltage source, which is a differential pair including transistors 9 and 10 and a transistor 1.
It is a variable independent voltage source that inputs a differential pair of 5 and 16, and can change the voltage in both positive and negative directions. An independent voltage source 3 biases the differential pair.

An input signal voltage source 4 is input to the bases of the transistors 17 and 21.
The emitter of the transistor 17 is connected to the emitter of the transistor 18 via the resistor 25. The emitter of the transistor 17 is connected to the collector of the current source transistor 19, and the emitter of the transistor 18 is connected to the collector of the current source transistor 20, respectively. And constitutes a first differential amplifier.

The emitter of the transistor 21 is connected to the emitter of the transistor 22 via the resistor 26. The emitter of the transistor 21 is connected to the collector of the current source transistor 23. The emitter of the transistor 22 is connected to the current source transistor 24. The second differential amplifier is connected to the collector.

The loads of the first differential amplifier and the second differential amplifier are shared, that is, the transistor 1
The collectors of 7 and 21 are common, and the collectors of transistors 18 and 22 are common. Transistor 1
The collectors of the transistors 7 and 21 are connected to the independent voltage source 1, and the collectors of the transistors 18 and 22 are connected to the emitters of the transistors 15 and 16 forming a differential pair.

The collector of the transistor 15 is connected to the independent voltage source 1, and the collector of the transistor 16 is connected to the PN
The output current i _{o of} the first differential amplifier or the second differential amplifier is output from the contact point of the collector of the P transistor 14 and the collector of the PNP transistor 14 and the collector of the transistor 16.

The output current i _o is determined by the load capacitor 2
Flows to 7, and the capacitance value of the capacitor 27 is C, the output voltage v _o = i _o / sC (where s is the Laplace operator, As far steady frequency response, be replaced by s → j [omega] Is supplied to the base of the transistor 28, and the diodes 29, 30, and 31 for level shift are provided.
Through the transistor 1 constituting the first differential amplifier
Negative feedback is input to the base of transistor 8 and the base of transistor 22 forming the second differential amplifier.

Reference numeral 32 denotes a transistor 28 and a diode 2
A bias current source for driving 9, 30, 31. The current I ₁ flowing from the current source 5 is supplied to the collector of the transistor 7 or 8 via the switch 6.
The transistor 7 has a collector and a base connected, and this contact is connected to the base of the transistor 11 and the bases of the current source transistors 19 and 20 of the first differential amplifier to form a current mirror circuit. doing.

Similarly, the collector and the base of the transistor 8 are connected, and their contacts are connected to the base of the transistor 12 and the bases of the current source transistors 23 and 24 of the second differential amplifier. Construct a mirror circuit. The collectors of the transistors 11 and 12 are connected to the emitters of transistors 9 and 10 forming a differential pair.
Is connected to independent voltage source 1 and transistor 1
The collector of 0 is connected to the collector of PNP transistor 13.

The transistor 13 has a collector and a base connected to each other, and its contact is connected to the base of the PNP transistor 14. The transistors 13 and 14 form a current mirror circuit. The base of transistors 9 and 10 forming a differential pair is the transistor 1 forming a differential pair.
The bases of the transistors 9 and 15 are connected to one terminal of the variable voltage source 2, and the bases of the transistors 10 and 16 are connected to the other end of the variable independent voltage source 2. Terminal. 33 is a terminal for inputting a switching signal for controlling the switch 6.

Next, the operation of this embodiment will be described. The first characteristic control means used for switching the filter characteristic is as follows.
First, it will be clarified that the second characteristic control means, which is the switching means of the switch 6 and is used for compensating the variation in the filter characteristics, is variable control in the variable independent voltage source 2.

[0025] In FIG. 1, for simplicity of explanation, the value of the resistor 25 and R _1, the value of the resistor 26 as R _2, which are the emitter resistance of the transistor r _e = kT / qI _E
And the value of the variable voltage source 2 is set to 0 V (where k is Boltzmann's constant, T is absolute temperature, q is elementary charge, and _IE is the emitter current of the transistor). .

When the switch 6 is tilted to the left, the current source 5
As a result, the current I ₁ flows through the transistor 7, and the current I ₁ also flows through the collectors of the transistors 11, 19 and 20, which form a current mirror with the transistor 7. On the other hand, since no current flows through the transistor 8, no current flows through the transistors 12, 23 and 24 which form a current mirror with the transistor 8. Therefore, the first differential amplifier including the transistors 17 and 18 operates, but the second differential amplifier including the transistors 21 and 22 does not operate.

[0027] AC current i _o1, which is output from the collector of this time, the transistor _{18, i o1 = (v i -v} o) / R
_Is one. This i _o1 is determined by transistors 15 and 16
Since the variable voltage source 2 is at 0 V, the base voltages of the transistors 15 and 16 are equal, the shunt ratio is 1: 1, and the alternating current flowing through the transistors 15 and 16 is i _o1 / 2.

Therefore, the collector of transistor 16 and P
AC current i output from the contact of NP transistor 14
_{Since the} AC current supplied from the collector of the transistor 14 is zero, the AC current output from the collector of the transistor 16 is all, and _o = _io1 / 2 = (v
_i the -v _o) / 2R _1. Since the input voltage of the differential amplifier is (v _i -v _o), the differential amplifier, to the input (v _i -v _o), the current _{i o = (v i -v o} ) /
A voltage controlled current source for outputting a 2R _1, the transconductance g _m1 at this time is 1 / 2R _1.

[0029] If the switch 6 is lying on the right side, the current I ₁ from the current source 5 flows through the transistor 8, which a current mirror - transistor constituting 12,23,24
Also flows. On the other hand, since no current flows through the transistor 7, the transistor 1 forming a current mirror with the transistor 7 does not flow.
No current flows through 1, 19, and 20, and the first differential amplifier does not operate. Thus, the same concept as when the switch 6 is fallen to the left, the output current i _o is _{i o} = (v i -v
_o) becomes / 2R _2, the transconductance g _{m @ 2} at this time is 1 / 2R _2.

The mutual conductance g _{m1 of} this circuit
Alternatively, g _m2 can also be changed by changing the value of the variable voltage source 2 in both the positive and negative directions around 0V. When the variable voltage source 2 is sufficiently low in the negative direction, the transistor 16 is cut off, and no current flows. At this time, the transconductance g _m is therefore zero.

On the other hand, if the variable voltage source 2 is sufficiently high in the positive direction, i _o1 or i _o2 all flow through the transistor 16, and
_{i o = i o1 = (v} i -v o) / R 1, or, i _o = i _o2
= Become _{(v i -v o) / R} 2, the value of g _m in this case is 1 / R ₁ or 1 / R _2. Therefore, g _m1 is ２
In the range of 0 to 1 / R ₁ centered on R _1, g _m2 is 1 / 2R ₂
Can be changed in the range of 0 to 1 / R ₂ around.

[0032] The present embodiment as described above, that the switching signal inputted from the terminal 33, it is possible to switch the value of the transconductance g _m, changing the value of g _m by changing the variable voltage source 2 Can be.

It should be noted, by changing the variable voltage source 2, focusing on direct current even I _1/2 sucked from the collector of the transistor 16, the DC varies to 0 to I _1, which is sucked from the collector of the transistor 10 The current changes in a similar manner, and thus the DC current flowing through the PNP transistor 13 also changes. Since the PNP transistors 13 and 14 constitute a current mirror, the DC current flowing from the collector of the transistor 14 also changes.

[0034] That is, until 0 to I ₁ around the I _1/2, exactly changed in the same way as direct current drawn from the collector of the transistor 16. Therefore, the current output from the contact point between the collector of the transistor 14 and the collector of the transistor 16 is only the AC current i _o , and no DC current is output. Therefore, the base DC voltage of the transistor 17 or 21 is always equal to the base DC voltage of the transistor 18 or 22, and the output voltage does not change with respect to the input voltage even when the variable voltage source 2 is changed.

FIG. 2 is a circuit diagram showing an equivalent circuit of the embodiment shown in FIG. 2, a voltage control current source (variable conductance amplifier) 101 includes a current source 5, a switch 6, transistors 7 to 24, a resistor 25,
26 are collectively represented as an equivalent circuit. For simplicity, the emitter follower and the level shift circuit including the transistor 28 and the like are omitted.

When the transfer function is calculated by the equivalent circuit of FIG. 2, H (s) = ( _gm1 / C) / (s + _gm1 / C) or H (s) = ( _gm2 / C) / (s + g) _m2 / C). That is, this is a first-order LPF (low-pass filter), and a transfer function can be selected from either of them according to a switching signal input from a terminal 33. Therefore, it is possible to switch the cutoff frequency to one of f _c = gm ₁ / 2πC or f _c = gm ₂ / 2πC. Further, since the mutual conductance gm ₁ or gm ₂ can be arbitrarily changed by the variable voltage source 2, the variation in the resistance value and the capacitance value can be compensated by controlling the variable power supply 2.

In this embodiment, the plus input terminal of the voltage controlled current source (variable conductance amplifier) 101 is grounded, and the input signal source 4 is connected to the output terminal of the voltage controlled current source 101 via the capacitor 27. By the HPF
(High-pass filter), and the cutoff frequency can be switched similarly to the LPF.

FIG. 3 is a circuit diagram in which the present invention is applied to a primary phase shift circuit. That is, in the embodiment shown in FIG. 2, a phase inversion circuit 102 is provided, and one end of the capacitor 27 is connected to the output terminal of the phase inversion circuit 102, and the other end is connected to the output of the voltage control current source 101 in the same manner as in FIG. The transfer function is H (s) = (− s + gm
_{1 / C) / (s +} gm 2 / C) or, H (s) = (-s
+ Gm ₂ / C) / (s + gm ₂ / C) and functions as a phase shift circuit.

In this case, a frequency sufficiently lower than the cutoff frequency can be used as a delay circuit. If the delay time is τ _d , τ _d is τ _d = 2 (C / g
m ₁ ) or τ _d = 2 (C / gm ₂ )
Any of the delay times can be selected according to the switching signal input from No. 3.

FIG. 4 is a circuit diagram in which the present invention is applied to a secondary phase shift circuit. That is, the secondary phase shift circuit shown in FIG.
It can be used as a delay circuit up to a much higher frequency than the primary phase shift circuit shown in FIG. 3, and its delay time is the same as that of the primary phase shift circuit shown in FIG. 3, and the delay time can be switched. .

The circuit shown in FIG. 4 can also realize a BPF (bandpass filter) or a trap circuit by changing the connection position of the input signal 4 and the connection position of the capacitor 27 (of course, the secondary circuit). LPF and HPF are also feasible). It is also possible to configure a third-order or higher-order filter.

FIG. 5 is a circuit diagram showing another embodiment of the present invention. 5, parts common to those in FIG. 1 are denoted by the same symbols as those in FIG. 1 is an independent voltage source for supplying a power supply voltage, 2 is a variable independent voltage source input to the bases of transistors 9, 10 and 15, and 16 forming a differential pair, and 3 is an independent voltage source for biasing the transistors Reference numeral 4 denotes an independent voltage source for generating an input signal.

The input signal source 4 is connected to the base of the transistor 17.
The transistor 17 is connected to the resistor 25 and the collector of the current source transistor 19. Further, the transistor 17 is connected via a resistor 25 to the emitter of the transistor 18 and the collector of the current source transistor 20 to constitute a differential amplifier. The collector of the transistor 17 is connected to the independent voltage source 1, and the collector of the transistor 18 is connected to the collector. It is connected to the emitters of transistors 15 and 16 forming a differential pair.

The base of the transistor 15 is the variable voltage source 2
The base of the transistor 16 is connected to the other end of the variable voltage source 2, the collector of the transistor 15 is connected to the independent voltage source 1, and the collector of the transistor 16 is connected to the collector of the PNP transistor 14. , the output current i _o of the differential amplifier from the connection point of the collectors of the transistors 14 of the transistor 16 is output.

The connection point between the collector of transistor 16 and the collector of transistor 14 is connected to capacitors 40 and 41
Is connected to the base of a transistor 28, the collector of the transistor 28 is connected to the independent voltage source 1, and the emitter is connected to three diodes 29, 3 connected in series.
The base of the transistor 18 constituting the differential amplifier and the current source 32 are connected via 0 and 31.

The current source 32 is a bias current source for driving the transistor 28 and the diodes 29, 30, and 31. The current source 5 is connected to the collector of the transistor 7, and the transistor 7 has a collector and a base connected to each other, and this connection point is connected to the bases of the transistors 11, 19, and 20, which are current mirrors. -Constitutes a circuit; The collector of the transistor 11 is connected to the emitters of the transistors 9 and 10 forming a differential pair. The collector of the transistor 9 is connected to the independent voltage source 1, and the collector of the transistor 10 is connected to the collector of the PNP transistor 13.

The bases of the transistors 9 and 10 are common to the bases of the transistors 15 and 16 which also constitute a differential amplifier, that is, the base of the transistor 9 is connected to one end of the variable voltage source 2. The base of the transistor 10 is connected to the other end of the variable voltage source 2. The collector of the PNP transistor 13 is also connected to the base of the transistor 13, and this connection point is connected to the base of the PNP transistor 14, so that the transistors 13 and 14 constitute a current mirror circuit.

The current source 43 is connected to the collector of a transistor 38 or 39 via a switch 44. The collectors of the transistors 38 and 39 are also connected to their respective bases.
The base of the transistor 39 is connected to the base of the transistor 37, respectively, and these constitute a current mirror circuit.

The DC voltage source 42 is connected to the bases of the transistors 34 and 35, and their collectors are connected to the independent voltage source 1. The emitter of the transistor 34 is connected to the collector of the current source transistor 36 and the transistor 3
5 is connected to the collector of the current source transistor 37,
These are connected to each other to form emitter followers as viewed from the voltage source 42.

Further, the emitter of the transistor 34 is connected to the collector of the
Connected to the connection point of the collector of the NP transistor 14,
The emitter of the transistor 35 is connected via a capacitor 41 to a connection point between the collector of the transistor 16 and the collector of the PNP transistor 14. 33 is an input terminal for a signal for controlling the switch 44.

The emitters of the transistors constituting the current mirror are all grounded in the case of NPN transistors, and are all connected to the voltage source 1 in the case of PNP transistors.

Next, the operation of this embodiment will be described. The first characteristic control means used for switching the filter characteristics is as follows.
First, it should be noted that the second characteristic control means, which is the switching means of the switch 44 and is used for compensating the variation in the filter characteristics, is a variable control in the variable independent voltage source 2.

Referring to FIG. 5 to 25 constitute a voltage-controlled current source, which is different from the voltage-controlled current source in the embodiment of FIG. 1 in that a switch 6 and a second differential amplifier including 21 to 24 and 26 in FIG. That is not to do. Therefore, the transconductance g _m of this voltage-controlled current source shown in FIG. 5 is g _m = 1 when the value of the resistor 25 is R.
/ 2R mainly, by changing the variable voltage source 2, it is possible to vary in a range of 0 to 1 / R, as in the embodiment of FIG. 1, switching the g _m impossible It is.

FIG. 6 is a circuit diagram showing an equivalent circuit of the embodiment shown in FIG. The voltage-controlled current source (variable conductance amplifier) 103 in FIG. 6 is a circuit in which 5 to 25 in FIG. 5 are collectively represented as an equivalent circuit. Hereinafter, description will be made with reference to FIG. If the switch 44 is lying on the left, flowing current I ₂ from the current source 43 to the transistor 38, which a current mirror - also same current flows to the transistor 36 constituting the.

On the other hand, since no current flows through the transistor 39, the transistor 37 which forms a current mirror with the transistor 39 does not flow.
No current flows. Therefore, the transistor 34 is turned on, but the transistor 35 is not turned on and is in a cutoff state. Therefore, the output current i _o = g _{m of the} voltage control current source 103
(V _i -v _o), all flowing to the capacitor 40. The transistor 34 functions as an emitter follower, and its base voltage is a DC voltage from the DC voltage source 42.
The emitter voltage is also DC, and is therefore equivalent to ground in terms of AC.

Assuming that the capacitance value of the capacitor 40 is C ₁ ,
The output voltage at this _{time, v 0 = i o / sC} 1 , and the transfer function, H (s) = (g m / C 1) / (s + g m /
C ₁ ). Conversely, when the switch 44 is tilted to the right, the operation is the reverse of the above, and a current flows through the transistor 39, and the transistor 37 forms a current mirror with the current.
The same current flows through

On the other hand, since no current flows through the transistor 38, no current flows through the transistor 36 which forms a current mirror with the current. Therefore, the transistor 35 is turned on, but the transistor 34 is in a cutoff state. Therefore, at this time, the output current i _o of the voltage control current source 103 is all
The flow, the output voltage, when the capacitance value of the capacitor 41 and C _2, a _{v o = i o / sC 2} . The transfer function in this case, H (s) = (g m / C 2) / (s + g m / C 2)
Becomes

As described above, this embodiment shown in FIG. 6 is a first-order LPF, and by controlling the switch 44, the cutoff frequency can be changed to f _c = g _m / 2πC ₁ or
f _c = g _m / 2πC ₂ . or,
Variations in the resistance value and the capacitance value can be compensated by controlling the variable voltage source 2.

FIG. 7 is a circuit diagram in which this embodiment is applied to an HPF. In this case, the transfer function is H (s) = s / (s
+ G _m / C ₁ ) or H (s) = s / (s + g _m / C
₂ ), and the cutoff frequency can be switched as in the example of FIG.

FIG. 8 is a circuit diagram in which the present embodiment is applied to a phase shift circuit. Reference numeral 102 in FIG. 8 is the same as the phase inversion circuit in FIG. The circuit shown in FIG. 8 can be used as a delay circuit for a frequency sufficiently lower than the cutoff frequency. The transfer function in this case is H (s) =
_{(-S + g m / C 1} ) / (s + g m / C 1) or, H (
s) = (− s + g _m / C ₂ ) / (s + g _m / C ₂ ), and at a frequency sufficiently lower than the cutoff frequency, the delay time τ _d is τ _d = 2 (C ₁ / g _m ) Or τ _d = 2 (C ₂ / g _m ), and switching can be performed by controlling the switch 44.

FIG. 9 is a circuit diagram in which the present embodiment is applied to a secondary phase shift circuit, and can be used as a delay circuit up to a much higher frequency than the case of FIG. The delay time in this case is the same as that in FIG. 8 and can be switched.

Further, in the embodiment shown in FIG. 6, a BPF or a trap circuit can be realized by using a secondary filter (naturally, a secondary LPF or HPF can also be realized). It is also possible to configure a third-order or higher-order filter.

[0063]

According to the present invention, the filter characteristics such as the cutoff frequency can be switched, and at the point where the filter characteristics are switched, the filter characteristics are caused by the variation in the resistance value and the capacitance value in the IC constituting the filter circuit. If there is a variation in characteristics, there is an advantage that a filter circuit capable of compensating for the variation can be provided.

In particular, of the television receivers capable of receiving both a high-definition broadcast television signal and a double-scanned standard television signal, when any of the television signals is received. Also,
It is useful to use the filter circuit according to the present invention when it is desired to perform image quality correction (for example, contour correction) properly.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment (primary LPF) of the present invention.

FIG. 2 is a circuit diagram showing an equivalent circuit of the embodiment shown in FIG.

FIG. 3 is a circuit diagram showing a primary phase shift circuit configured as an application example of the embodiment shown in FIG. 1 using the equivalent circuit of FIG. 2;

4 is a circuit diagram showing a secondary phase shift circuit configured as an application example of the embodiment shown in FIG. 1 using the equivalent circuit of FIG. 2;

FIG. 5 is a circuit diagram showing another embodiment (primary LPF) of the present invention.

FIG. 6 is a circuit diagram showing an equivalent circuit of the embodiment shown in FIG.

7 is a circuit diagram showing a primary HPF configured as an application of the embodiment shown in FIG. 5 using an equivalent circuit of FIG. 6;

8 is a circuit diagram showing a primary phase shift circuit configured as an application example of the embodiment shown in FIG. 5, using an equivalent circuit of FIG. 6;

9 is a circuit diagram showing a secondary phase shift circuit configured as an application example of the embodiment shown in FIG. 5, using an equivalent circuit of FIG. 6;

FIG. 10 is a circuit diagram showing an example of a conventional filter circuit.

[Explanation of symbols]

1-3, 42 ... independent voltage source, 4 ... input signal source, 5, 3
2,43 ... independent current source, 6,44 ... switch, 7-1
2, 15 to 24: NPN transistors forming a voltage controlled current source, 13, 14: PN forming a voltage controlled current source
P transistors, resistors for determining the g _m of 25 and 26 ... voltage control current source, 27, 27 ', 40,41,4
0 ', 41', capacitor, 28, NPN transistor for impedance conversion, 29-31, diode for level shift, 33, terminal for inputting a signal for controlling a switch, 34-39, 34 '~ 37': NPN transistor constituting a circuit for switching a capacitor
101, 101 '... voltage controlled current source can be switched g _m, 102 ... phase inverter, 103, 103'
... voltage controlled current source.

Continuation of the front page (72) Inventor Chiharu Ishino 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Hitachi, Ltd. AV Equipment Division (56) References JP-A-4-217113 (JP, A) JP-A-4- 103288 (JP, A) JP-A-4-299610 (JP, A) JP-A-5-114836 (JP, A) JP-A-63-245007 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 11/04

Claims (2)

(57) [Claims] 1. A filter circuit comprising a combination of a variable conductance amplifier as a voltage controlled current source and a capacitor, wherein at least a first and a second load are provided as differential amplifiers constituting the variable conductance amplifier. 2 which shared
A first characteristic control means for switching the mutual conductance of the variable conductance amplifier and switching a filter characteristic such as a cutoff frequency, depending on which of the differential amplifiers is selected and operated. By variably controlling the output current of the differential amplifier that is selected and operating from the two differential amplifiers,
A second characteristic control means for variably controlling a mutual conductance of the variable conductance amplifier to vary a filter characteristic such as a cutoff frequency. 2. A filter circuit comprising a combination of a variable conductance amplifier as a voltage control current source and a capacitor, wherein at least a first and a second 2
A first characteristic control means for switching filter characteristics such as a cut-off frequency depending on which one of the capacitors is selected to function, and an output current of a differential amplifier constituting the variable conductance amplifier. And a second characteristic control means for variably controlling the mutual conductance of the variable conductance amplifier to vary a filter characteristic such as a cutoff frequency.