JPH07123295A - Video output circuit - Google Patents

Abstract

PURPOSE:To obtain a broad band frequency characteristic by connecting a new peaking element in parallel with a series circuit comprising a switching element and a resistor to compensate deficient peaking when the circuit gain is switched. CONSTITUTION:The circuit consists of a polarity reversion section 1, a differential amplifier section 2, a gain switching section 3 and a cathode ray tube 4. The frequency characteristic of the cathode ray tube 4 due to a capacitive load is compensated by using a peaking element comprising inductors L1, L2, L3 and capacitors C1, C2. A primary color signal from the polarity reversion section 1 is fed to a base of a transistor(TR) Q2 and when an emitter voltage of the TRQ2 is lower than that of the TRQ3, the TRQ3 is conductive. A resistor R5 is connected in parallel with a resistor R6 to reduce a common emitter resistor of TRS Q2, Q4. As a collector current of the TRQ4 increases to increase the gain, the capacitor C2 is connected in parallel with the resistor R5 to compensate deficient peaking.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cathode ray tube driving circuit in a color television receiver, and more particularly to a video output circuit for outputting a video signal to the cathode ray tube.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional example. FIG. 5 shows a video output circuit used in a conventional color television receiver.

FIG. 5 is a circuit diagram of a video output circuit capable of changing the gain. The gains of the circuits of three systems of red, green and blue are color difference signals (RY), (GY), respectively. (B
-Y) is inputted to the emitter terminals of the transistors Q1, Q2 and Q3, and the luminance signal -Y is inputted to the transistor Q.
The combined resistance between the four emitter terminals and the transistor Q1,
It is determined by the ratio of the load resistances R1, R3 and R6 connected between the collector terminals of Q2 and Q3 and the voltage source Vcc.
VR1 and VR2 are variable resistors, and by adjusting these, the gain of the circuit is changed and the white balance is adjusted. A series circuit composed of a diode D1 and a resistor R5, and a diode D2 and a resistor R8 is connected in parallel to the resistors R4 and R7 connected to the emitter terminals of the transistors Q2 and Q3, respectively.

When the amplitude of the luminance signal -Y increases, that is, when the voltage of the emitter terminal of the transistor Q4 decreases, the color difference signals are input to the transistors Q1, Q2, Q.
The current flowing through the resistor connected between the emitter terminal of the transistor 3 and the emitter terminal of the transistor Q4 increases, and the resistor R2
The voltage drop of R4 and R7 becomes large. Then the resistance R
4, diode D1, which is connected in parallel to the voltage of R7
D2 becomes conductive, resistors R5 and R8 are connected in parallel to resistors R4 and R7, respectively, to reduce the combined resistance between the emitter terminals of the transistors Q2 and Q3 and the emitter terminal of the transistor Q4, increase the gain, and balance the white balance. It was a switch.

As an example of a video output circuit having such a circuit configuration, there is the invention disclosed in Japanese Patent Laid-Open No. 53-99721.

[0006]

There are the following two problems in the video output circuit showing the above-mentioned prior art. First
Is to secure the necessary frequency band. Although the peaking means for obtaining the necessary frequency band is not explicitly described in the conventional example, as a configuration of a general cascode amplifier, Q
Series-parallel peaking is used on the collector side of 1, Q2, Q3, and emitter peaking (adding a capacitor in parallel, etc.) is used on the emitter side of Q1, Q2, Q3. However, when the diodes D1 and D2 are turned on, the combined resistance between the emitter terminals of the transistors Q2 and Q3 and the emitter terminal of the transistor Q4 decreases, and the peaking amount becomes insufficient. As a result, there arises a problem that the amplitude characteristic changes depending on the frequency band.

The second is to secure the correction accuracy. When applying the conventional example to the white balance correction of the projection type television that projects the images of the three projection tubes of red, green and blue on the screen,
In particular, the brightness characteristics of the blue projection tube are greatly different from those of the red and green projection tubes, and it is necessary to accurately correct the gain of the video output circuit. However, since the conventional video output circuit uses a diode as a switching element,
Since the threshold voltage for switching the gain is constant and the series circuit of the diode and the resistor for switching the gain is one system, it is extremely difficult to correct it accurately.

A first object of the present invention is to provide a video output circuit capable of obtaining a wide frequency band.

A second object of the present invention is to provide a video output circuit capable of highly accurately correcting the gain of the circuit according to the brightness characteristics of the projection tube.

[0010]

In order to achieve the first object, a new peaking element is connected in parallel to the resistors R5 and R8 which are connected to switch the gain of the circuit.

In order to achieve the above-mentioned second object, the diode D which determines the threshold value for switching the gain of the circuit.
To use a switching element for which a threshold value can be arbitrarily set for 1 and D2, and to switch a gain composed of a resistor and a capacitor and a switching element for which the threshold value can be arbitrarily set. A plurality of circuits are provided and are connected in parallel to the resistors R4 and R7.

[0012]

With the above-mentioned means 1, the diode D
1 and D2 become conductive, and resistors R5 and R8 are connected in parallel to resistors R4 and R7, respectively, and at the same time, resistor R5 and
A new peaking element connected in parallel with R8 is connected in parallel with the resistors R4 and R7 to compensate for the shortage of the peaking amount.

By adopting the above-mentioned means 2, the threshold value for switching the gain of the circuit and the number of stages are arbitrarily set,
Highly accurate correction can be performed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a diagram showing an example in which the present invention is applied to a television device. In FIG. 1, 1 is a polarity reversal unit, 2 is a differential amplification unit, 3 is a gain switching unit, 4 is a cathode ray tube, 5 is an input terminal, and 6 is an output terminal. Further, R1 to R10 are resistors, C1 and C2 are capacitors, L1 to L3 are inductors, E1 to E3 are power supply voltage supply terminals, V1 to V3 are voltage sources, D1 and D2 are diodes, and Q1 to Q7 are transistors.

The input terminal 5 is connected to the base of the transistor Q1, the emitter of the transistor Q1 is connected to the ground potential via the resistor R2, and the collector of the transistor Q1 is connected to the power supply voltage supply terminal E1 via the resistor R1. Has been done. The collector of the transistor Q1 and the contact point of the resistor R1 are connected to the base of the transistor Q2, the emitter of the transistor Q2 is connected to the power supply voltage supply terminal E2 via the resistor R3, and the collector of the transistor Q2 is connected to the ground potential. And the transistor Q2
The contact point between the emitter of R1 and the resistor R3 is connected to the emitter of the transistor Q4 via the resistor R5. A series circuit of a resistor R4 and a capacitor C1 and a gain switching unit 3 are connected in parallel to the resistor R5. In the gain switching unit 3, the resistor R6 is connected to the emitter of the transistor Q3, and the other of the resistors R6 is connected to the collector of the transistor Q2 and the resistor R5.
Connected to the contact. The base of the transistor Q3 is connected to the voltage source V1, and the collector of the transistor Q3 is connected to the contact point between the resistor R5 and the emitter of the transistor Q4. Further, the capacitor C2 is connected to the resistor R6.
And are connected in parallel. The emitter of the transistor Q4 is connected to the ground potential via the resistor R7, the collector of the transistor Q4 is connected to the emitter of the transistor Q5, and the base of the transistor Q4 is the voltage source V.
Connected to 2. The base of the transistor Q5 is connected to the voltage source V3, and the collector of the transistor Q5 is a parallel circuit of the resistor R8 and the inductor L2 and the diode D.
2, the diode D1, the inductor L1, and the resistor R9, and is connected to the power supply voltage supply terminal E3. The collector of the transistor Q6 is connected to the power supply voltage supply terminal E3, and the base of the transistor Q6 is connected to the contact point of the diode D1 and the inductor L1.
The emitter of 6 is connected to the emitter of transistor Q7. The base of the transistor Q7 is connected to the contact point of the parallel circuit of the diode D2, the resistor R8 and the inductor L2, and the collector of the transistor Q7 is connected to the ground potential. The contact point between the emitter of the transistor Q6 and the emitter of the transistor Q7 is a resistor R10 and an inductor L3.
Is connected to the output terminal 6 through the parallel circuit of, and the cathode ray tube 4 is connected to the output terminal 6.

The polarity reversing unit 1 inverts the polarity of the primary color signal input to the input terminal 5 and outputs it. The differential amplifier 2 is a differential input cascode amplifier, and the gain of this circuit is determined by the ratio of the load resistance R9 of the transistor Q4 and the common emitter resistance of the transistor Q2 and the transistor Q4. The primary color signal output from the polarity reversing unit 1 is the transistor Q2.
Is inputted to the base of the transistor Q2, the emitter potential of the transistor Q2 is changed by the primary color signal. on the other hand,
Since the base of the transistor Q4 is fixed by the voltage source V2, the emitter potential of the transistor Q4 is kept at a potential lower than the voltage source V2 by V _BE , and has a substantially constant value. Therefore, a change in the emitter potential of the transistor Q2 results in a change in the collector current of the transistor Q4. The change in the collector current of the transistor Q4 depends on the transistor Q
5, the emitter current of the transistor Q5 is almost equal to the collector current of the transistor Q5 because the transistor Q5 is used with its base grounded. A change in the collector current of the transistor Q5 is converted into a voltage by the load resistor R9. The transistors Q6 and Q7 are cathode ray tubes 4 connected to the output terminal 6 according to the voltage waveform appearing at the collector of the transistor Q5.
To drive. Since the cathode ray tube 4 is a capacitive load, the inductors L1, L2, L3, the capacitors C1, C
The frequency characteristic is compensated by the peaking element of No.2. Further, the resistors R4, R8, and R10 prevent excessive peaking.

In this circuit, when a signal such that the emitter voltage of the transistor Q2 becomes lower than the emitter voltage of the transistor Q3 is input, the transistor Q3 becomes conductive, the resistor R6 is connected in parallel with the resistor R5, and the transistor Q2 is connected. The common emitter resistance of transistor Q4 is reduced. Then, the collector current of the transistor Q4 increases and the gain increases. The resistor R6 is connected in parallel with the resistor R5, and at the same time, the capacitor C2 is connected in parallel with the resistor R5 to compensate for the peaking amount shortage due to the reduction of the emitter resistance.

A second embodiment of the present invention will be described with reference to FIG. C21, C22, C2n are capacitors, R61, R
62 and R6n are resistors, and V11, V12, and V1n are voltage sources. This is different from the first embodiment in that the gain switching section 3 is provided with an n-system circuit including a gain switching resistor, a transistor, and a peaking element. According to the present embodiment, the gain of the circuit can be switched to n steps by setting the base voltages of the transistors Q31, Q32, ... Q3n to different values. Furthermore, resistors R61 and R6
By changing the values of 2, ... R6n, it is possible to freely set the inclination of the operating characteristics between the transistors Q31, Q32 ,. For example, in a projection television having three projection tubes for red, green, and blue, if it is attempted to match the brightness characteristics of the green projection tube with the brightness characteristics of the green projection tube, the brightness characteristics of the blue projection tube Can be corrected with high accuracy according to the brightness characteristics of the green projection tube. Therefore, good white balance performance can be realized.

FIG. 3 shows a third embodiment of the present invention. Second
The embodiment is different from the embodiment in that the primary color signal is directly input to the base of the transistor Q4 forming the cascode amplifier. Therefore, the input terminal 5 is connected to the base of the transistor Q4, and the emitter of the transistor Q4 has a resistance R
It is connected via 5 to the ground potential. The other circuit configuration is similar to that of the second embodiment.

In the present embodiment, the gain of the circuit is determined by the ratio of the load resistance R9 and the emitter resistance R5 of the transistor Q4. Also in this embodiment, the same effect as that of the second embodiment can be obtained by connecting the gain switching unit 3 in parallel with the emitter resistance R5 of the transistor Q4 that determines the gain of the circuit.

FIG. 4 shows a fourth embodiment of the present invention. Unlike the first to third embodiments, FIG. 4 shows a circuit in which a diode is used as a switch element to improve the accuracy of correction. The configuration of the embodiment is such that the resistors R8, R9, R21,
R22, R23, R24, capacitors C21, C22,
C23, transistors Q4, Q5, Q9, diode D
21, D22, inductors L1 and L2, and a voltage source V3. The circuit configuration of FIG. 4 is a differential input type cascode type video output circuit. The description of the circuit operation in the cascode format is the same as in the first embodiment,
A color difference (BY) signal is input to the base terminal of Q4, and a luminance (-Y) signal is input to the base terminal of Q9. The operation of the element connected between the transistors Q4 and Q9, which is a feature of this embodiment, will be described. When the input signal amplitude from the luminance signal input terminal 22 is small, the diodes D21 and D22 are in the cutoff state, and the resistors R23 and R24 and the capacitors C22 and C23 are in the disconnected state. The capacitor C21 is an element for performing emitter peaking. Therefore, the gain of the circuit is almost determined by R9 / (R21 + R22). When the luminance signal amplitude increases (the emitter potential of Q9 decreases), the emitter current of Q4 increases, and the voltage across the resistor R21 exceeds the threshold voltage of the diode D21, the resistor R23 is connected in parallel with the resistor R21. , Q4 and Q9 have a small combined resistance, and the gain of the circuit increases. The capacitor C22 compensates for a decrease in emitter peaking that accompanies an increase in gain. Furthermore, when the brightness signal amplitude increases, the voltage across R21 and R22 becomes
When the threshold voltage of the diode D22 is exceeded, the resistor R24 is further connected in parallel, the combined resistance between Q4 and Q9 becomes smaller, and the gain of the circuit becomes larger than the above. That is, the gain of the circuit can be switched in three steps, and optimal settings can be made at low brightness, medium brightness, and high brightness according to the brightness characteristics of the projection tube used, for more accurate correction. Can be realized. Further, the capacitor C22, like the capacitor C21, compensates for a decrease in emitter peaking and can secure a necessary frequency band. Further, in this embodiment, two series elements of a diode and a resistor are used, but it goes without saying that the accuracy can be improved by further increasing the series elements to three and four.

[0022]

As described above, according to the present invention, the peaking amount can be always kept optimum, and a wide band frequency characteristic can be obtained even when the gain of the circuit is switched. When the present invention is applied to a projection television, the white balance can be corrected with high accuracy by setting the gain of the video output circuit according to the brightness characteristics of the projection tube.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a video output showing a first embodiment of the present invention.

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

FIG. 3 is a video output circuit diagram showing a third embodiment of the present invention.

FIG. 4 is a circuit diagram of a video output showing a fourth embodiment of the present invention.

FIG. 5 is a circuit diagram of a conventional video output.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Polarity inversion part, 2 ... Differential amplification part, 21 ... Color difference signal input terminal, 22 ... Luminance signal input terminal, 3 ... Gain switching part,
4 ... Cathode ray tube, 5 ... Input terminal, 6 ... Output terminal, R1 to R
10, R61, R62, R6n ... Resistors, C1, C2, C
21, C22, C2n ... Capacitors, V1, V2, V
3, V11, V12, V1n ... Voltage source, Q1 to Q7, Q
31, Q32, Q3n ... Transistors, L1, L2, L
3 ... Inductor, D1, D2 ... Diode, E1, E
2, E3 ... Power supply voltage supply terminal.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenichi Matsumoto Kenji Matsumoto, 216 Totsuka-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi, Ltd. Information & Video Media Division (72) Akira Ueno 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi Image Information Systems Co., Ltd. (72) Inventor Toshimitsu Watanabe 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa Prefecture

Claims (3)

[Claims] 1. An NPN transistor in which an image signal is input to a base terminal, an emitter terminal is connected to a ground potential via a resistor, and a collector terminal is connected to a power supply voltage supply terminal via a load resistor, and the transistor. In a video output circuit in which a peaking element and a series circuit including at least one switching element and a resistor are connected in parallel to a resistor connected between the emitter terminal of the A video output circuit in which a new peaking element is connected in parallel to the resistance of the series circuit configured by. 2. The video output circuit according to claim 1, wherein
Video output circuit characterized in that the new peaking element is capacitance. 3. The video output circuit according to claim 1, wherein
A video output circuit, wherein ON / OFF of the switching element is controlled by an amplitude of an input image signal.