JPH0998066A - Filter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease the capacitance the number of elements by using a single filter for many purposes. SOLUTION: A filter circuit is provided with a 1st differential amplifier 1, a 2nd differential amplifier 5, a 1st reactance element 8 which is connected to the collector of a 2nd transistor TR 3, the 5th and 6th TR 10 and 11 whose emitters are connected in common to each other, a 3rd differential amplifier 9 which applies voltage across the element 8 to the base of the TR 10 and feeds its output voltage back to the amplifier 9 itself and also to the amplifier 1, and a 2nd reactance element 12 which has one of its both ends connected to an input terminal and the other end connected to the collectors of the 4th and 6th TR 7 and 11 respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter circuit using a differential amplifier, and more particularly to a filter circuit which can reduce the capacitance and the number of elements by enabling one filter to be used for multiple purposes. .

[0002]

2. Description of the Related Art In a luminance signal processing IC for a TV, when a luminance signal containing a chroma signal is applied, a trap (band elimination) circuit is required to attenuate the chroma signal. When a luminance signal that does not include a chroma signal is applied, the trap (band elimination) circuit is short-circuited to pass the signal.

However, if this is done, the delay times of the two signal paths will differ. Therefore, instead of simply short-circuiting, an all-pass filter (passing from a low frequency band to a high frequency band) is used to pass a signal through the filter to balance the delay time. By doing so, a desired delay time can be obtained even if the input signal does not include the chroma signal.

[0004]

However, when a filter is used for each signal path, two filters are required, so that the number of elements such as capacitors and transistors required for the filter is increased, and the chip area of the IC is increased. Increase,
There was a problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has first and second transistors whose emitters are commonly connected, and the base of the first transistor is connected to an input terminal from the input terminal. A first differential amplifier to which an input signal is applied, a second differential amplifier having third and fourth transistors whose emitters are commonly connected and connected to the collector of the first transistor, and the collector of the second transistor It has a first reactance element connected to it, and fifth and sixth transistors whose emitters are commonly connected. The voltage across the first reactance element is applied to the base of the fifth transistor to output the output voltage to itself and A third differential amplifier that returns to the first differential amplifier, and a second reactor having one end connected to the input terminal and the other end connected to the collectors of the fourth and sixth transistors. A Nsu element, and wherein the changing the frequency characteristic to trap characteristics from the all-pass characteristic by varying the ratio of currents flowing through the third and fourth transistors constituting said second differential amplifier.

[0006]

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows the present invention.
(1) has first and second transistors (2) and (3) whose emitters are commonly connected, and the first transistor (2)
A first differential amplifier in which an input signal from an input terminal (4) is applied to the base of the third transistor, and a third and fourth transistor (5) whose emitters are commonly connected and which are connected to the collector of the first transistor (2) (6) A second differential amplifier having (7), (8) is a first reactance element connected to the collector of the second transistor (3), and (9) is a fifth and fifth emitter whose emitters are commonly connected. 6 transistors (10) (1
1), the voltage across the first reactance element (8) is applied to the base of the fifth transistor (10),
A third differential amplifier for returning the output voltage to the base of the sixth transistor (11) and the first differential amplifier (1), (1
2) is a second reactance element whose one end is connected to the input terminal (4) and the other end is connected to the collectors of the fourth and sixth transistors (7) and (11), and (13) and (14) are current I0 and First and second constant current sources for supplying 2I0.

The circuit of FIG. 1 changes the values of the currents flowing through the third and fourth transistors (6) and (7) of the second differential amplifier (5) to change the input terminal (4) and the output terminal (15). During), the frequency characteristic from the all-pass characteristic to the trap characteristic can be changed. In FIG. 1, the input signal of the input terminal (4) is V1, the output signal of the output terminal (15) is V0, the transconductance of the first differential amplifier (1) is gm1, and the capacitance value of the first reactance element (8). To C2,
S = jω, the collector voltage of the second transistor (3) is V
If the value is 2, the following relationship holds in the first differential amplifier (1).

[0008]

[Equation 1]

Further, the transconductance of the third differential amplifier (9) is gm2, the capacitance value of the second reactance element (12) is C1, and the third and fourth transistors (6) of the second differential amplifier (5). The ratio of the currents flowing in (7) is α (α = 0,
If the third transistor (6) is turned on, the fourth transistor (7) is turned off, α = 1, the third transistor (6) is turned off, and the fourth transistor (7) is turned on), the third differential amplifier ( In 9), the following relation holds.

[0010]

[Equation 2]

Here, the expression (1) is represented by the voltage V2 and is substituted into the expression (2). Then, rearranging equation (2), the transfer function (V0 / V1) of FIG.

[0012]

(Equation 3)

[0013] In FIG. 1, α = 0 and the third
The transistor (6) is turned on and the fourth transistor (7) is turned off. Then, the collector signal of the first transistor (2) is not transmitted to the other end of the second reactance element (12). Then, the circuit of FIG. 1 becomes a trap circuit as shown by the following equation.

That is, when α = 0 is substituted into the equation (3), the equation (3) becomes

[0015]

[Equation 4]

[0016] The transfer function of Expression (4) indicates a trap circuit. Therefore, the circuit of FIG. 1 can operate as a trap circuit. Next, in FIG. 1, setting α = 1,
The third transistor (6) is turned off and the fourth transistor (7) is turned on. Then, the first transistor (2)
The collector signal of is transmitted to the other end of the second reactance element (12). Then, the circuit of FIG. 1 becomes an all-pass filter circuit as shown below.

First, in order to configure the all-pass filter circuit, it is necessary to set the mutual conductance gm1 of the first differential amplifier (1) to gm2 = 2gm1. Substituting the condition into the equation (3), the equation (3) becomes

[0018]

(Equation 5)

## EQU1 ## Then, by substituting α = 1 into the equation (5), the equation (5) becomes

[0020]

(Equation 6)

## EQU1 ## The transfer function of Expression (6) indicates an all-pass filter circuit. Therefore, the circuit of FIG. 1 can operate as an all-pass filter. The above explanation is α
= 1 and α = 0, but an intermediate value may be used. By doing so, desired characteristics can be obtained as shown in FIG. The circuit of FIG. 1 has an attenuation point at the frequency where the numerator of equation (4) becomes zero.

Next, a mode in which the circuit of FIG. 1 is also used as an LPF will be described. FIG. 2 shows such an embodiment, the emitter follower transistor (16) having a base connected to the input terminal (4) and an emitter connected to one end of the second reactance element (12).
And a grounding circuit (20) which is composed of a constant current source (17), a switch (18) and a transistor (19) and grounds one end of the second reactance element (12) in an AC manner.
2 is different from FIG. 1 in the two circuits, and the other points are the same as in FIG.

First, it will be described that the circuit of FIG. 2 can operate in the same manner as the circuit of FIG. In this case, switch (18) is opened. In FIG. 2, if the switch (18) is opened as shown, the transistor (19) is turned off. The emitter follower transistor (16) operates as an emitter follower transistor and receives an input signal from the input terminal (4) as a second reactance element (12).
Applied to one end of. This state is AC-wise the same as the connection of FIG.

Therefore, the circuit of FIG. 2 operates similarly to the circuit of FIG. Next, in FIG. 2, if the switch (18) is closed, the transistor (19) is turned on. Then, the power supply voltage is applied to one end of the second reactance element (12). Emitter follower transistor (16)
Stops applying the input signal from the input terminal (4) to one end of the second reactance element (12). One end of the second reactance element (12) will be grounded AC-wise. Then, the circuit of FIG. 2 operates as an LPF.

The transfer function (V0 / V1) of FIG. 2 is

[0026]

(Equation 7)

[0027] In the equation (7), if α = 0, the equation (7) becomes

[0028]

(Equation 8)

It becomes Expression (8) indicates that it is an LPF. Therefore, the circuit of FIG. 2 operates as a low pass filter.

[0030]

As described above, according to the present invention, it is possible to provide a filter circuit which also serves as a reactance element, a differential amplifier and the like and which can obtain trap, low pass and all pass characteristics. Further, according to the present invention, it is possible to provide a filter circuit that can obtain an intermediate characteristic between the trap characteristic and the all-pass characteristic.

[Brief description of drawings]

FIG. 1 is a block diagram showing a filter circuit of the present invention.

FIG. 2 is a block diagram showing a filter circuit of the present invention.

FIG. 3 is a characteristic diagram for explaining characteristics of the present invention.

[Explanation of symbols]

 (1) First differential amplifier (5) Second differential amplifier (8) First reactance element (9) Third differential amplifier (12) Second reactance element

Claims (2)

[Claims] A first and a second emitters are connected in common.
A first differential amplifier having a transistor, a base of the first transistor to which an input signal from an input terminal is applied, and third and fourth transistors having emitters commonly connected to a collector of the first transistor A second differential amplifier having: a first reactance element connected to the collector of the second transistor; and a fifth and a sixth transistor having emitters commonly connected, and the fifth transistor has a base connected to the first reactance element. A third differential amplifier to which the output voltage of the first differential amplifier is applied and which feeds back the output voltage to the sixth transistor and the first differential amplifier; and one end of the third differential amplifier and the other ends of the fourth and sixth differential amplifiers. A second reactance element connected to the collector of the transistor,
And a filter circuit characterized by changing frequency characteristics. 2. A first and a second emitters whose emitters are commonly connected.
A first differential amplifier having a transistor, a base of the first transistor to which an input signal from an input terminal is applied, and third and fourth transistors having emitters commonly connected to a collector of the first transistor A second differential amplifier having: a first reactance element connected to the collector of the second transistor; and fifth and sixth transistors having their emitters commonly connected, and the fifth transistor has a base connected to the first reactance element. A third differential amplifier to which a voltage across both ends of one reactance element is applied and which feeds back an output voltage to itself and the first differential amplifier, one end of which is the input terminal, and the other end of which is the collector of the fourth and sixth transistors And a grounding means for grounding the one end of the second reactance element in an alternating current manner. By changing the ratio of the currents flowing through the third and fourth transistors constituting the second differential amplifier, the frequency characteristic is changed from the all-pass characteristic to the trap characteristic, and the grounding means is operated to improve the low-pass characteristic. A filter circuit characterized by being obtained.