JPS634391B2 - - 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, but other grid The electrodes are arranged in common for each signal. As a result, each cathode potential and signal amplification of the electron gun can be adjusted to compensate for variations in the characteristics of the triode consisting of the cathode of the electron gun, the first grid, and the second grid, that is, white balance adjustment. This is generally done by doing the following: A conventional video output circuit that has been applied for this purpose is shown in FIG.

In FIG. 1, CRT is a cathode ray tube of the type described above, and K _R , K _G and K _B are primary color signals R, G,
B is a cathode electrode to which voltage is applied.
G ₁ and G ₂ are first and second grids having a common structure for the cathode electrode. The color signals R, G, and B added to this cathode ray tube CRT are
It is taken out from the collector of each video output transistor Q ₁₁ , Q ₂₁ and Q ₃₁ . The collectors of transistors Q ₁₁ , Q ₂₁ , Q ₃₁ are connected to DC power source E ₁ through load resistors R ₁₁ , R ₂₁ and R ₃₁ , respectively. 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 . In addition, the emitter is connected to a variable resistor VR ₁₂ for adjusting the drive of this transistor and a resistor, as described for transistor Q ₁₁ , for example.
R is grounded through a series circuit of ₁₂ and resistor
It is connected via _R13 to the sliding terminal of variable resistor _VR11 for cut-off adjustment of the cathode ray tube. Variable resistance VR ₁₁
The voltage of the bias DC power supply _E2 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 cathode ray tubes for each color are adjusted using variable resistors VR ₁₁ , VR ₂₁ , and VR ₃₁ .
Video output transistors Q ₁₁ , Q ₂₁ , after adjustment
The emitter current of Q ₃₁ changed according to the variation in the cut-off voltage of the electron gun for each color. Therefore, each emitter current in the dark signal region near the cutoff is the transistor Q ₁₁ ,
Q ₂₁ and Q ₃₁ are not uniform, and this causes variations in the high frequency parameters of the transistor, mainly f _T (gain bandwidth product).
This means that the high frequency characteristics of each color in the video output stage become uneven, resulting in a color shift phenomenon appearing on the screen of the cathode ray tube. Although this color shift phenomenon is tolerable to some extent for use in ordinary color television receivers, it is unsuitable in cases where precision is required.

The present invention aims to solve the conventional problems that cause the color shift phenomenon as described above.
Further, the present invention provides a new circuit arrangement of a video output circuit for a cathode ray tube having an electron gun in which the cathode electrodes are independent for three input primary color signals, but the first and second grid electrodes are common. Therefore, this type of cathode ray tube can be effectively applied not only to general color television receivers but also to color display devices that require broadband video frequency characteristics.
Therefore, according to the present invention, a bias setting circuit is provided in series with the collector of each video output transistor for taking out the primary color signal applied to the cathode electrode of the cathode ray tube and the load resistor. This bias setting circuit works as a collector power supply (constant voltage source) for each video output transistor, and when adjusting the cut-off voltage of the cathode ray tube, the collector voltage, that is, the cathode ray tube The cathode potential of can be adjusted. As a result, after the cutoff voltage is adjusted, the emitter current of each video output transistor is not affected by variations in the cutoff voltage of the cathode ray tube.

Embodiments of the present invention are shown in FIGS. 2 and 3 below.
This will be explained with a diagram.

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, and different configurations will be mainly explained. In addition, each video output stage for the red, green, and blue signals R, G, and B has the same configuration, so to simplify the explanation, the red signal R
I will mainly discuss the transmission path.

The video output transistor _Q11 eliminates the circuit connection to the emitters of the elements _R13 and _VR11 shown in Figure 1, and instead connects the load resistor _R11 and the DC power supply on the collector side.
It has the following bias setting circuit between it and _E1 .
That is, the bias setting circuit includes a transistor _Q12 having a collector-emitter path between the DC power source _E1 and the load resistor _R11 . A divided voltage consisting of a variable resistor VR ₁₃ and a resistor R ₁₃ connected in series between the DC power source E ₁ and the ground is applied to the base of the transistor Q ₁₂ . Further, the emitter of transistor _Q12 is grounded through capacitor _C11 for high frequency bypass.

Here, a bipolar type transistor is used as the transistor _Q12 . As is well known, a bipolar transistor has constant voltage characteristics between its base and emitter, and constant current characteristics between its collector and emitter. Therefore, if the bipolar transistor _Q12 is configured as an emitter follower as shown in Fig. 2, the potential of the cathode _KR at cut-off of the cathode ray tube CRT will be changed by the variable resistor _VR13 to the transistor _Q12.
This means that the base DC potential can be set. At this time, the emitter current of the video output transistor _Q11 is determined by the constant current characteristic between the collector and emitter of the transistor _Q12 , and the potential of the cathode _KR at the time of cut-off of the cathode ray tube CRT, which is set by the variable resistor _VR13 .
That is, it is not affected by the collector voltage of the video output transistor _Q11 . The emitter current of the video output transistor Q ₁₁ is determined by the constant voltage characteristics between the base and emitter of the transistor Q ₁₂ , and the DC potential of the input terminal _TR plus the signal input amplitude and the variable resistor VR ₁₂ for drive adjustment and the resistor R _12. Determined by These considerations apply to the other signals G and B as well.

It is generally known that the f _T :gain bandwidth product of a bipolar transistor is given by the following equation in the hybrid π-type equivalent circuit shown in FIG.

g _n = qI _E /kT, f _T = g _n /2π (C _b ′ _c + C _b ′ _e ), where g _n : mutual conductance, f _T : gain bandwidth product, q : electron charge, k : Boltzmann Constant, T: Absolute temperature, I _E : Transistor emitter current (DC).

Here, f _T :The gain-bandwidth product is proportional to the emitter current I _E of the transistor.

In the circuit arrangement of the video output circuit shown in FIGS. 1 and 2, the emitter current of the video output transistor _Q11 on the black level side of the cathode ray tube is smaller than the emitter current on the white level side. As a result, it is important to make the frequency characteristics of the video output transistors Q ₁₁ , Q ₂₁ and Q ₃₁ uniform, especially on the black level side where the emitter current is small. That is, the emitter currents of the video output transistors Q ₁₁ , Q ₂₁ and Q ₃₁ can be made equal regardless of the cut-off adjustment of the cathode ray tube CRT, and the high frequency characteristics of each video output stage are made uniform.

FIG. 3 shows a circuit layout relating to another embodiment of the present invention, representatively showing a transmission path for a red signal R.
In the embodiment shown in FIG. 2 described above, the video output stage is a grounded emitter amplifier circuit with one transistor, but in FIG. 3, a cascode amplifier circuit with two transistors is used. That is, the red signal from the color demodulation circuit is input to the terminal _TR ' and applied to the base of the video drive transistor _Q13 . The emitter of transistor Q ₁₃ is grounded by a series circuit of variable resistor VR ₁₄ and resistor R ₁₄ for drive adjustment.
The collector is connected to the emitter of video output transistor _Q14 . A DC power supply E ₃ is connected to the base of the transistor Q ₁₄ , and a high frequency bypass filter capacitor C ₁₂ is connected between the transistor Q 14 and the ground.
The collector of the output transistor Q ₁₄ is the load resistor R ₁₁
is connected to a bias setting circuit similar to that described in FIG. This circuit arrangement is particularly suitable for color display devices requiring wideband video frequency characteristics.

Since the present invention has the configuration described above, it is possible to prevent unevenness in the high frequency parameters of the video output transistors and provide a video output circuit suitable for cases where wideband video frequency characteristics are required. be.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional video output circuit, FIG. 2 is a circuit diagram of a video output circuit according to an embodiment of the present invention, and FIG. 3 is a circuit diagram according to another embodiment of the present invention.
FIG. 4 is a hybrid π-type equivalent circuit diagram of a bipolar transistor. CRT...Color cathode ray tube, Q ₁₁ , Q ₂₁ and Q ₃₁
...Video output transistor, Q ₁₂ , Q ₂₂ and Q ₃₂
...A transistor that makes up the bias setting circuit.

Claims (1)

[Scope of Claims] 1. A color cathode ray tube having an integrated electron gun electrode structure in which cathode electrodes are independent according to three primary color signals applied, and a first grid electrode and a second grid electrode are commonly provided. a video output circuit for driving the cathode ray tube, the video output circuit having a bias setting circuit provided in series with the collector of each video output transistor and each load resistor for taking out a primary color signal applied to the cathode electrode of the cathode ray tube; A video output circuit characterized in that a bias setting circuit sets a cutoff voltage of the cathode ray tube and operates as a constant voltage source for the video output transistor. 2. A bias setting circuit comprising an emitter follower-connected bipolar transistor, the emitter of which is connected in series with a load resistor and the collector of a video output transistor, and whose base potential determines the cut-off voltage of the cathode ray tube. The video output circuit according to item 1. 3. The video output circuit according to claim 1 or 2, wherein each video output transistor is a common emitter transistor amplifier circuit. 4. Claim 1 or 2, characterized in that each video output transistor constitutes a cascode amplifier circuit together with its drive transistor.
Video output circuit described in section.