JP2505821Y2 - Video output circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a video output circuit provided in a television receiver or a display device using a CRT, and particularly to a video output circuit suitable for color display. It is a thing.

(B) Conventional technology There are various types of video output circuits for displaying an image in a display device using a CRT (for example, the actual output circuit 1-
(See Japanese Patent No. 179671).

FIG. 2 shows an example of a conventional DC voltage superimposing type video output circuit. Note that FIG. 2 shows only the R circuit out of the three circuits of R, G, and B.

The video signal is input to the base of the transistor Q ₁ , is further input to the base of the transistor Q ₂ whose emitter is grounded, and is amplified by the transistor Q ₂ and the transistor Q ₃ whose ground is grounded. Then, the amplified video signal is input to the cathode of the CRT via the resistor. The amplification degree at this time is determined by the variable resistor VR ₁ . The peaking coils L ₁ and L ₂ and the resistor R ₆ are for improving high frequency characteristics during amplification, and the capacitor C ₁ is for stabilizing frequency characteristics when the amplification degree is changed. is there.

The transistor Q ₄ , variable resistor VR ₂ , resistors R ₉ , R ₁₀ , diode D ₁ , and capacitor C ₃ constitute a constant current source circuit, and the constant current I ₁ determined from the collector of the transistor Q ₄ by the variable resistor VR ₂ . Is output.

Further, in the variable current output circuit having the transistor Q ₅ as a main element, the current I ₂ flows from the collector of the transistor Q ₅ to the emitter through the resistor R ₁₁ . Therefore, the resistance R
_14, the current flows I ₁ -I _2, the both ends R ₁₄ ×
A DC voltage of (I ₁ −I ₂ ) is generated.

This DC voltage is superimposed as a CRT bias voltage on the amplified video signal to drive the CRT appropriately.

At this time, the variable resistor VR ₂ is used for cutoff adjustment, and the variable resistor VR ₃ changes the current I ₂ by changing its resistance value, whereby the current value of I ₁ −I ₂ changes and the bias is changed. Since the voltage is controlled, it is used to control the brightness of the displayed screen.

Vc ₁ and Vc ₂ are power supply voltages, and it is necessary to lower Vc ₂ in order to reduce the power consumption of the transistor Q ₃ , and Vc ₂ <Vc ₁ is set.

Figure 3 shows the light output characteristics of a color CRT. The horizontal axis is the cathode-grid voltage of the CRT, and the vertical axis is the optical output.
E _KR , E _KG , and E _KB represent cutoff voltages of R, G, and B, that is, voltages when the optical output is 0.

When adjusting the white balance in the circuit as shown in Fig. 2, first change the resistance value of the variable resistor VR ₂ so that the R, G, B light outputs are the same when the screen is dark. I do. Next, change the resistance value of the variable resistor VR ₁ to increase or decrease the amplitude of the video signal applied to the cathode of the CRT, and adjust so that the R, G, B light outputs are the same when the screen is bright. . Thereby, the white balance is adjusted.

(C) Problems to be solved by the invention As described above, the white balance is adjusted by adjusting the bias voltage with the variable resistor VR ₂ and then adjusting the amplitude of the video signal with the variable resistor VR _1. At this time, the resistance value of the variable resistor VR ₁ changes, so that the collector current of the transistor Q ₂ increases or decreases. Transistor Q ₃
Since the collector current of the transistor Q ₂ is almost equal to the collector current of the transistor Q _2, the collector current of the transistor Q ₃ also changes due to the variable resistor VR ₁ .

Since the coil L ₁ causes almost no voltage drop in terms of direct current, the potential E _{0 at} the connection point of the resistor R ₆ and the parallel circuit of the coil L ₁ and the resistor R ₁₄ becomes substantially equal to the collector voltage of the transistor Q _3. , It changes according to the change of the resistance value of the variable resistor VR ₁ .

Then, the cut-off voltage E _KR of the R cathode is E _KR = E ₀ + R ₁₄ × (I ₁ c−I ₂ c) (where I ₁ c and I ₂ c are the transistors Q ₄ and It is the collector voltage of Q _5. ).

As can be seen from this equation, when E ₀ changes by ΔE ₀ , E _KR also changes by ΔE ₀ , and the adjustment of the cutoff voltage deviates.

Then, it becomes necessary to repeat the white balance adjustment again.

The present invention has been made in view of the above circumstances, and provides a video output circuit in which the cutoff voltage does not change due to amplitude adjustment. (D) Means for Solving the Problems Includes an amplification means for amplifying a video signal, a variable constant current source for variably generating a constant current, and a variable current output means for variably outputting a current, and the current generated from the variable constant current source and the The current difference from the current output from the variable current output means is superimposed on the video signal amplified by the amplification means.
A video output circuit for supplying to the cathode of a CRT, a transistor having an emitter connected to the cathode of the CRT and having a collector supplied with a video signal amplified by an amplification means, and a power supply voltage supplied to the base of the transistor. It comprises a resistance means to be connected and a capacitor for supplying the video signal amplified by the amplification means to the cathode of the CRT.

(E) Action The base potential of the transistor is kept constant by the resistance means, and the AC component of the video signal is supplied to the emitter of the transistor and the cathode of the CRT by the capacitor. The transistor performs a switching operation according to the alternating current signal supplied to the emitter, and the video signal whose black level is kept constant is supplied to the cathode of the CRT.

(F) Embodiment FIG. 1 is a circuit diagram of an embodiment of the circuit of the present invention. The same parts as those in FIG. 2 are designated by the same reference numerals and the description thereof is omitted.

Similar to FIG. 2, FIG. 1 shows an R circuit, which includes an amplifier circuit (1) for amplifying a video signal whose main elements are a transistor Q ₁ , a transistor Q ₂ , and a transistor Q ₃ . Transistor Q ₄ , variable resistor VR ₂ , resistors R ₉ , R
_A constant current source circuit (2) composed of a diode ₁₀ , a diode D ₁ and a capacitor C ₃ and a variable current output circuit (3) having a transistor Q ₅ as a main element are provided.

In one embodiment of the present invention shown in FIG. 1, a collector is connected to a connection point where a coil L ₁ , a resistor R ₆ and a coil L ₂ of an amplifier circuit (1) are connected, and a resistor R is connected to a cathode of a CRT.
A transistor Q ₆ having an emitter connected through ₁₂ is provided. The collector of the transistor Q ₄ is connected to the base of the transistor Q ₆ , and the power supply voltage Vc ₂
A resistor R ₈ as a resistance means connected to is connected.

Further, a capacitor C ₂ connected to the emitter and collector of the transistor Q ₆ (where the connection point on the collector side is M and the connection point on the emitter side is N), the anode is connected to the base of the transistor Q ₆ , and the cathode is the emitter. A diode D ₂ connected to is provided.

Now, in such a circuit, when the video signal is input to the base of the transistor Q ₁ , it is amplified by the amplifier circuit (1), and the amplified video signal appears at the point M. As shown in FIG. 4, for example, video signals appearing at the point M include a low average signal level (a) and a high average signal level (b), and a high average signal level is low. DC level is low compared to.

When such a video signal appears at the point M, an alternating current component of the signal (having an amplitude of v) appears at the point N via the capacitor C ₂ .

On the other hand, the current I ₁ -I ₂ obtained by subtracting the collector current I ₂ of the transistor Q ₅ of the variable current output circuit (3) from the collector current I ₁ of the transistor Q ₄ of the constant current source circuit (2) passes through the resistor R ₈ . since the flow through the power supply voltage Vc ₂ Te base potential E _B of the transistor Q ₆ has a _{E B = Vc 2 + R 8} × (I 1 -I 2).

Therefore, the AC component of the signal (a) and the signal (b) to the point N is applied, the high periods of the voltage _{(t 1 ~t 2, t 5} ~t 6),
The potential at the point N transistor Q ₆ is turned ON certain E _B + V _BE (V _BE is the base of the transistor Q ₆ - emitter voltage) it becomes. Also, the periods of low voltage (t ₃ ~t _4), the transistor Q ₆ is turned OFF, the diode D
_The capacitor C ₂ is charged via ₂ . As a result, the black level (the period from t _{1 to} t ₂ and t _{5 to} t ₆ ) is always constant (E _B
+ V _BE ) with amplitude v (c) or (d) (signal (c)
Corresponds to the signal (a), and the signal (d) corresponds to the signal (b)) is applied to the cathode of the CRT via the resistor R ₁₂ .

As described above, the DC component of the video signal applied to the cathode of the CRT is irrelevant to the change in the resistance value of the variable resistor VR ₁ ,
It is determined by the collector currents of the transistors Q ₄ and Q ₅ and the resistance value of the resistor R ₈ .

Therefore, by the amplitude adjustment in the white balance adjustment,
Even if the resistance value of the variable resistor VR ₁ changes and the collector current of the transistor Q ₃ also changes, the variable resistor of the constant current source circuit (2) changes.
The cutoff voltage determined by VR ₂ does not change.

(G) Effect of the Invention As is clear from the above description, the present invention is a constant that determines the cutoff voltage even if the DC component of the video signal changes due to the amplitude adjustment of the white balance adjustment. Since it is separated from the current source circuit by the transistor Q ₆ , the cutoff voltage does not change.

Therefore, after adjusting the white balance when the screen is dark,
When the white balance adjustment is performed when the screen is bright, the white balance adjustment when the screen is bright does not deteriorate the white balance adjustment when the screen is dark, and the good white balance state is maintained.

In addition, even if the DC operating point changes due to the temperature change of the transistors Q ₂ and Q ₃ for amplifying the amplitude of the video signal,
It is possible to display a stable image without a change in luminance without being affected by the influence.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the circuit of the present invention, FIG. 2 is a circuit diagram showing an example of a conventional video output circuit, and FIG. 3 is a color CR.
FIG. 4 is a diagram showing the optical output characteristic of T, and FIG. 4 is a signal waveform diagram for explaining the operation according to one embodiment of the present invention. (1) ... Amplifying circuit (amplifying means), (2) ... Constant current source circuit (variable constant current source), (3) ... Variable current output circuit (variable current output means), Vc ₂ ... Power supply voltage , Q ₆ ... Transistor, C ₂ ... Capacitor, R ₈ ... Resistor, D ₂ ...
diode.

Claims (1)

(57) [Scope of utility model registration request] 1. An amplification means for amplifying a video signal, a variable constant current source for variably generating a constant current, and a variable constant current output means for variably outputting a current, which are generated from the variable constant current source. In the video output circuit that superimposes the current difference between the current output from the variable current output means on the video signal amplified by the amplification means and supplies it to the cathode of the CRT, the emitter is connected to the cathode of the CRT. A transistor to which the video signal amplified by the amplification means is supplied to the collector,
A video output circuit comprising: a resistance means for connecting a power supply voltage supplied to the base of the transistor, and a capacitor for supplying the video signal amplified by the amplification means to the cathode of the CRT.