JPH0349235B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a filter circuit used in a VTR (video tape recorder) capable of recording, for example, a SECAM system color television signal.

Background technology and its problems In the SECAM system, the carrier frequency is
The R-Y signal (red difference signal) FM-modulated at 4.40625 MHz and the B-Y signal (blue difference signal) FM-modulated at a carrier frequency of 4.25 MHz are transmitted alternately every horizontal period. . In this case, the FM-modulated R-Y signal and B-Y signal are transmitted after their amplitudes are suppressed in the vicinity of the carrier frequency by a bandpass filter having frequency characteristics as shown in FIG. This is to suppress the carrier wave amplitude near achromatic colors to reduce dot interference.

In a television receiver, the signal is processed after passing through a filter circuit having a frequency characteristic as shown in FIG. 2, which is opposite to the frequency characteristic shown in FIG.

Now, when recording this SECAM system color television signal on a VTR, the FM modulated R
The -Y signal and the BY signal are converted into flat signals through a filter circuit (also called a bell filter) having frequency characteristics as shown in FIG. 2, and then recorded.

This is because in a VTR, the luminance signal is usually FM modulated, and the color signal (RY signal, BY signal) is low frequency converted and recorded.
In the case of the SECAM method, the color signal is FM-modulated and distributed over a wide band, and if the frequency characteristics shown in Figure 1 remain, there will be interference with the luminance signal at the tail of the color signal, which is inconvenient. Therefore, this is to avoid this. Also, if the frequency characteristics shown in Figure 1 remain as they are, the SN near the carrier frequency will be particularly bad when noise is mixed in during tape recording, so it is necessary to make the SN equal over the entire band. It is.

As a filter circuit used in this VTR and having frequency characteristics as shown in Fig. 2,
For example, the one shown in FIG. 3 has been proposed.

In the same figure, 1 is an FM modulated R-Y signal having frequency characteristics as shown in FIG.
This is an input terminal to which a signal is supplied, and a color signal is supplied from this terminal 1 to the base of an npn transistor 2. The emitter of this transistor 2 is grounded via an impedance circuit 3. This impedance circuit 3 is composed of, for example, resistors 3a and 3b, a capacitor 3c, and a variable coil 3d. Further, the collector of the transistor 2 is connected via a resistor 4 to a power supply terminal 5 to which DC voltage +B is supplied. An output terminal 6 from which an output signal is obtained is led out from the connection point between the collector of the transistor 2 and the resistor 4.

In this example in Fig. 3, G = R / Z ... (1) R: resistance value of resistor 4 Z: impedance In order to realize the frequency characteristics of the impedance of circuit 3, that is, the frequency characteristics as shown in Fig. 2. However, in reality, the frequency characteristic G' is related to G'=R/ _re +Z (2) and the emitter resistance r _e of the transistor 2. In this case, in addition to the gain changing depending on the emitter resistance r _e , the Q also changes, resulting in the inconvenience that stable frequency characteristics cannot be obtained.

Therefore, conventionally, the emitter current of transistor 2 is made considerably large (for example, about 5 mA),
The influence of the emitter resistance r _e on the impedance Z is made negligibly small, the above-mentioned disadvantages are eliminated, and stable frequency characteristics approximately expressed by equation (1) can be obtained.

Therefore, in this conventional filter circuit,
Since the emitter current of the transistor 2 is considerably large, there is a disadvantage that power consumption is large due to this.

Purpose of the Invention The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to propose a filter circuit that can obtain stable frequency characteristics without increasing power consumption.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention connects an impedance circuit having a predetermined frequency characteristic to the emitter of a transistor whose output side is connected to the base of the differential amplifier circuit, and An input signal is applied to the input terminal, the other input terminal is connected to the emitter of the transistor to form a return path, and an output signal is obtained from the collector of the transistor.

By configuring the present invention in this manner, the influence of the emitter resistance r _e of the transistor on its frequency characteristics is reduced, the emitter current of the transistor can be reduced, and stable frequency characteristics can be obtained without increasing power consumption. be able to.

Embodiment Hereinafter, an embodiment of a filter circuit according to the present invention will be described with reference to FIG. In FIG. 4, parts corresponding to those in FIG. 3 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In FIG. 4, input terminal 1 is connected to the positive terminal of differential amplifier circuit 7, and the output side of differential amplifier circuit 7 is connected to the base of transistor 2. In FIG.
Further, the emitter of the transistor 2 is connected to the negative terminal of the differential amplifier circuit 7, thereby forming a feedback path.

The rest of the structure is the same as the example shown in FIG. 3 described above.

In the case of the example in FIG. 4, if the gain of the differential amplifier circuit 7 is A, its frequency characteristic G'' is: G''=(A/1+A)R/ _re (1/1+A)+Z)
...(3), and the influence of the emitter resistance r _e becomes extremely small.

FIG. 5 shows a specific example of the example shown in FIG. In this example, when the emitter current of transistor 2 was set to 1.1 mA, substantially ideal characteristics (actually measured values) as shown by the solid line in FIG. 6 were obtained. 6th
What is shown by a broken line in the figure is a theoretical value.

Also, the impedance circuit 3 is configured as shown in Figure 7, and the emitter current of the transistor 2 is set to 0.2 mA.
Even when this was done, substantially the same characteristics as in the example shown in FIG. 5 were obtained. However, it goes without saying that the dynamic range decreases as the emitter current decreases.

In this way, according to the example in Fig. 4, the frequency characteristic
The effect of the emitter resistance r _e of the transistor 2 on G'' is reduced, and stable frequency characteristics can be obtained without increasing the emitter current flowing through the transistor 2. Therefore, the emitter current may be small. Power consumption can be reduced compared to conventional filter circuits.

Next, FIG. 8 shows another embodiment of the present invention, which is an example of an integrated circuit. In FIG. 8, parts corresponding to those in FIG. 4 are designated by the same reference numerals.

In the same figure, the impedance circuit 3 is npn
It consists of a type transistor 8, resistors 9, 10, 11, capacitors 12, 13, and a coil 14. A bias voltage V _v is applied to the base of the transistor 8 from a terminal 15 . The emitter current of the diode 2 is controlled by the transistor 8 and the resistor 9.

In this case, the transistor 8 and resistor 9 constituting the impedance circuit 3 are integrated into an integrated circuit like the differential amplifier circuit 7 and the transistor 2. However, resistors 10, 11, capacitors 12, 13,
The coil 14 is connected to the outside using a pin 16 led out from the emitter of the transistor 2.

The structure shown in FIG. 8 is also similar to that of the example shown in FIG. 4, and the same effects can be obtained.

Effects of the Invention As is clear from the embodiments described above, according to the filter circuit according to the present invention, the emitter resistance of the transistor constituting the filter circuit has almost no effect on its frequency characteristics, and the emitter current of the transistor is small. Stable frequency characteristics can be obtained with both. Therefore, since the emitter current only needs to be small, compared to conventional filter circuits,
The power consumption can be reduced.

[Brief explanation of drawings]

1 and 2 are diagrams showing frequency characteristics, FIG. 3 is a block diagram showing an example of a conventional filter circuit, and FIG. 4 is a block diagram showing an example of a filter circuit according to the present invention. 5 and 7 are connection diagrams showing their specific circuits, FIG. 6 is a diagram showing the frequency characteristics of the example shown in FIG. 5, and FIG. 8 is a connection diagram showing another embodiment of the present invention. . 1 is an input terminal, 2 is a transistor, 3 is an impedance circuit, 4 is a resistor, 5 is a power supply terminal, 6 is an output terminal, and 7 is a differential amplifier circuit.

Claims (1)

[Claims] 1 An impedance circuit having predetermined frequency characteristics is connected to the emitter of a transistor whose output side is connected to the base of the differential amplifier circuit, and an input signal is applied to one input terminal of the differential amplifier circuit, and an input signal is applied to the other input terminal. A filter circuit whose terminal is connected to the emitter of the transistor to form a feedback path, and an output signal is obtained from the collector of the transistor.