JPS6142473B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a video output circuit for driving a color cathode ray tube in which convergence is automated.

In recent color television receivers, so-called self-convergence type color cathode ray tubes that do not require a convergence correction circuit have come to be used. The electron gun structure of this type of cathode ray tube is the same as the conventional one in that the cathode electrodes are independent for each color, corresponding to the three primary color signals R, G, and B applied. Grid electrodes are commonly arranged for each signal. As a result, correction adjustment for variations in the characteristics of the triode consisting of the cathode of the electron gun and the first and second grids, that is, white balance adjustment, can be done by varying the cathode potential and signal amplification of each electron gun. It is generally done by
A conventional video output circuit that has been applied to this purpose is shown in FIG.

In FIG. 1, CRT is a cathode ray tube of the type described above, K _R , K _G and K _B are primary color signals R,
G and B are cathode electrodes to which voltage is applied, respectively. G ₁ and G ₂ are first and second grids having a common structure for the cathode electrode.
The primary color signal R applied to this cathode ray tube CRT,
G and B are respective video output transistors
It is taken from the collectors of Q ₁₁ , Q ₂₁ , and Q ₃₁ . The collectors of the video output transistors Q ₁₁ , Q ₂₁ , Q ₃₁ are connected to load resistors R ₁₁ , R ₂₁ and
Connected to DC power supply E ₁ through R ₃₁ . Primary color signals R, _G , and B are applied to each base from the color demodulation circuit at the preceding stage via input terminals T _R , T G , and T _B . Furthermore, as described for transistor Q ₁₁ , for example, the emitter is grounded through a series circuit of variable resistor VR ₅ and resistor R ₅ for adjusting the drive of transistor Q ₁₁ , and also connected to a series circuit of variable resistor VR ₅ and capacitor C ₁₁ . grounded through. This variable resistor VR5 and capacitor C11 constitute a circuit that compensates for the frequency characteristics of the video output transistor Q11. The emitter of the video output transistor _Q11 is further connected via a resistor _R6 to a sliding terminal of a variable resistor _VR11 for cut-off adjustment of the cathode ray tube. Variable resistor VR ₁₁ is DC power supply E ₂ for bias
The voltage is divided and taken out from the sliding terminal.

In the conventional circuit layout as described above, the cutoff level of the cathode ray tube CRT, that is, the black level of the cathode potential, is connected to the video output transistor.
The voltage drops are given by the load resistances R ₁₁ , R ₂₁ , and R 31 of Q ₁₁ , _{Q 21 ,} and Q ₃₁ , respectively. Therefore,
Variations in the cutoff of the cathode ray tube for each color are adjusted by variable resistors VR ₁₁ , VR ₂₁ , VR ₃₁ , or the emitter currents of the video output transistors Q ₁₁ , Q ₂₁ , Q ₃₁ after adjustment are adjusted by the cutoff of the electron gun. This means that it has changed in accordance with each variation in voltage. Therefore, each emitter current in the dark signal region near the cutoff is not uniform among transistors Q ₁₁ , Q ₂₁ , and Q ₃₁ , and this causes variations in the high-frequency parameters of the transistors, mainly f _T (gain-bandwidth product). cause it to occur. This means that the high frequency characteristics of each color in the video output stage become uneven, which causes a problem that appears as a color shift phenomenon on the screen of the cathode ray tube.

SUMMARY OF THE INVENTION An object of the present invention is to equalize the frequency characteristics of video output transistors for each color in a video output circuit in order to prevent the above-mentioned color shift phenomenon from occurring.

Hereinafter, the configuration of the present invention will be explained according to one embodiment.

FIG. 2 is a circuit diagram of a video output circuit showing one embodiment of the present invention.

Note that elements corresponding to the circuit arrangement in FIG. 1 are given the same reference numerals. In addition, the video output stages for red, green, and blue signals all have the same configuration, so
In order to simplify the explanation, the transmission path of the red signal R will be mainly described.

A CRT is a cathode ray tube of the type mentioned above, with K _R ,
K _G and K _B are cathode electrodes to which primary color signals RGB are respectively applied. G ₁ and G ₂ are first and second grids having a common structure for the cathode electrode. The primary color signals R, G, and B applied to this cathode ray tube CRT are taken out from the respective collectors of the respective video output transistors Q ₁₂ , Q ₂₂ and Q ₃₂ . video output transistor
The collectors of Q ₁₂ , Q ₂₂ and Q ₃₂ are connected to the DC power source E ₁ through load resistors R ₁₂ , R ₂₂ and R ₃₂ respectively.
connected to. Each _base has _a primary color signal _R ,
G, and B are given. Also, Emitsuta is
For example, regarding the video output transistor _Q12 , it is grounded through a series circuit of a variable resistor V _R5 for adjusting the drive of the video output transistor _Q12 and a resistor _R5 . Also, variable resistor V _R5 and capacitor C ₁₂
The emitter of the frequency characteristic compensation transistor _TR1 is connected to the collector of the frequency characteristic compensation transistor TR1 through a series circuit of the frequency characteristic compensation transistor _TR1 . A series circuit of the variable resistor _VR5 and the frequency characteristic compensation capacitor _C12 is connected to the collector of the transistor _TR1 , and the feedback resistor _Rb1 is connected between the base and collector of the transistor _TR1 . The base of the transistor TR ₁ and the sliding terminal of the cut-off adjustment variable resistor VR ₁₂ are connected via a cut-off voltage detection resistor R ₃₁ , and one end of the cut-off adjustment variable resistor VR ₁₂ is connected to the bias DC power supply E ₂ . and the other end is grounded. The emitter of video output transistor _Q12 is
Variable resistor for cut-off adjustment via resistor R ₆
Connected to the sliding terminal of VR ₁₂ .

Next, the operation of this circuit will be explained. The gain-bandwidth product of the video output transistor is proportional to the emitter current. If the emitter current increases, the frequency characteristics, particularly the high frequency characteristics, of the video output transistor will improve, and conversely, if the emitter current decreases, the frequency characteristics will deteriorate. It is difficult to change the capacitance of capacitor _C12 to compensate for the frequency characteristics.
Therefore, the cut-off voltage is detected by the action of the cut-off voltage detection resistor R _a1 and the cut-off adjustment variable resistor VR ₁₂ . When the sliding terminal voltage of the cutoff adjustment variable resistor VR ₁₂ decreases, the emitter current of the video output transistor Q ₁₂ increases and the collector potential decreases. As a result, the equivalent resistance value of the circuit consisting of the frequency characteristic compensation transistor TR ₁ and the feedback resistor R _b1 increases, and even though the capacitance of the capacitor C ₁₂ itself remains unchanged, the capacitance of the capacitor C ₁₂ increases. This is equivalent to a decrease. Conversely, when the sliding terminal voltage of the cut-off adjustment variable resistor VR ₁₂ increases, the emitter current of the video output transistor Q ₁₂ decreases, the collector voltage increases, and the frequency characteristic compensation transistor TR ₁ and the feedback resistor R _b1 The equivalent resistance value of the circuit consisting of C12 decreases, and this is equivalent to an increase in the capacitance of the capacitor _C12 , although its capacitance remains unchanged. As described above, in this embodiment, a voltage variable resistance element is used in series with the frequency characteristic compensation capacitor _C12 , and the transistor _Tr1 is used, but it is also possible to use other variable resistance elements. By having the video output circuit according to the present invention configured as described above, after adjusting the cutoff voltage, the emitter current of each video output transistor is not affected by variations in the cutoff voltage of the cathode ray tube, and the frequency Characteristics are made uniform. It is possible to provide a video output circuit suitable for cases where wideband video frequency characteristics are required, and to prevent color shift on the screen of a cathode ray tube. Furthermore, it provides a new circuit layout for the video output circuit for cathode ray tubes with electron guns in which the cathode electrodes are independent, or the first and second grid electrodes are common, depending on the input three primary color signals. Therefore, this type of cathode ray tube can be applied not only to general color television receivers but also to color display devices that require band-specific video frequency characteristics.

[Brief explanation of the drawing]

FIG. 1 is a conventional video output circuit diagram, and FIG. 2 is a circuit diagram of a video output circuit according to an embodiment of the present invention. CRT...Color cathode ray tube, Q ₁₂ , Q ₂₂ and Q ₃₂
...Video output transistor, C ₁₂ , C ₂₂ and C ₃₂
... Capacitor for frequency characteristic compensation, TR ₁ , TR ₂ and TR ₃ ... Transistor for frequency characteristic compensation, R
_b1 , R _b2 and R _b3 ... Collector base feedback resistors, R _a1 , R _a2 and R _a3 ... Resistors for cut-off voltage detection, VR ₁₂ , VR ₂₂ and VR ₃₂ ... Variable resistors for cut-off voltage detection.

Claims (1)

[Claims] 1. R, G,
B Applied to a color cathode ray tube having each cathode electrode, the cut-off voltage of the color cathode ray tube is obtained by changing the bias voltage of each of the video output transistors and applying different DC voltages to each of the cathode electrodes. In the video output circuit, the video output circuit includes a frequency characteristic compensation circuit for compensating the frequency characteristics of each of the video output transistors, wherein each cut-off voltage of the color cathode ray tube is detected and the frequency characteristic between each of the video output transistors is determined. A video output circuit comprising means for controlling each of the frequency characteristic compensation circuits so as to equalize the frequency characteristics.