JPH0822021B2 - Video amplifier circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video amplifier circuit capable of blanking.

[0002]

2. Description of the Related Art Conventionally, there is known a flat cathode ray tube in which a fluorescent screen is formed so as to be inclined at a relatively small angle with respect to a central axis of an electron gun. FIG. 5 shows a drive system of this flat cathode ray tube.

In the figure, 1 is a flat cathode ray tube,
K is a cathode electrode, G1 is a first grid electrode, G2 is a second grid electrode forming an accelerating electrode, G3 is a third grid electrode forming a focus electrode, and 2 is a fluorescent screen.
The phosphor screen 2 has a flyback transformer (hereinafter, “FB”).
A high voltage HV obtained by rectifying the pulse voltage output from the T.) is applied.

As the cathode electrode K, unlike the indirectly heated electrode used in a general cathode ray tube, a directly heated electrode which operates at low power and has a quick rising is adopted. A pulse voltage is supplied as a heater voltage 3 from the FBT to the cathode electrode K, and so-called pulse ignition is performed.

In this case, since the FBT winding is connected to the cathode electrode K, the stray capacitance of the cathode electrode K increases, and when the cathode drive system for applying a video signal to the cathode electrode K is adopted, Loss increases. In other words, in order to improve the frequency characteristics, it is necessary to reduce the impedance of the output stage of the video circuit so that the influence of the stray capacitance can be ignored, and the emitter follower configuration or the like will be adopted. Will lead to an increase.

Therefore, a G1 drive system is adopted in which a video signal is applied to the first grid electrode G1 having a relatively small stray capacitance. That is, 4 is an NPN type transistor which constitutes a video amplifier.
The video signal SV is supplied from the terminal 5 to the base of the.
The emitter of this transistor 4 is grounded via a parallel circuit of a resistor 6 and a capacitor 7, and its collector is connected to a power supply terminal + B (for example, 50V) via a resistor 8. Then, the video signal obtained at the connection point of the collector of the transistor 4 and the resistor 8 is applied to the first grid electrode G1 of the cathode ray tube 1 via the capacitor 9.

Voltage source + B1 (for example, 900V)
Is grounded via a series circuit of a variable resistor 10 for focus adjustment, a variable resistor 11 for cutoff adjustment, and a resistor 12. The voltage obtained at the movable terminal of the variable resistor 10 is applied to the third grid electrode G3 of the cathode ray tube 1 via the resistor 13. The mover of the variable resistor 11 is grounded via the capacitor 14, and the voltage obtained at the connection point between the mover and the capacitor 14 is applied to the second grid electrode G2 of the cathode ray tube 1.

Voltage source + B2 (for example, 140V)
Is grounded via a semi-fixed resistor 15 for adjusting the sub-brightness, a resistor 16, a variable resistor 17 for adjusting the brightness and a resistor 18 forming a constant current circuit. The voltage obtained at the mover of the variable resistor 17 is applied to the cathode electrode K via the resistor 19.

The cutoff adjustment of the cathode ray tube 1 is performed by changing the voltage applied to the second grid electrode G2 and the cathode electrode K. That is, the voltage applied to the second grid electrode G2 is varied by the variable resistor 11 to determine the cutoff of the cathode ray tube 1 and the semi-fixed resistor 15
Therefore, the sub-brightness is determined by changing the voltage applied to the cathode electrode K.

As described above, since the voltage applied to the cathode electrode K is varied by the sub-brightness adjustment by the semi-fixed resistor 15 and the brightness adjustment by the variable resistor 17, the resistance value of the resistor 19 is relatively high, for example, 10
It is set to 0 KΩ. However, as is well known, since a beam current proportional to a video signal flows into the cathode electrode K, it is necessary to lower the circuit impedance. Therefore, the cathode electrode K of the cathode ray tube 1 is grounded via the capacitor 20 to lower the impedance in terms of AC.

Reference numeral 21 is a horizontal blanking pulse HBL.
K is supplied to the terminal, 22 is a vertical blanking pulse VB
This is the terminal to which LK is supplied. The terminal 21 is connected to the base of the NPN transistor 25 via the resistor 23 and the diode 24. The terminal 22 is connected to the base of the transistor 25 via the resistor 26. The base of this transistor is grounded via a resistor 27, its emitter is grounded, and its collector is connected via resistor 28 to the junction of transistor 4 and resistor 8.

During the horizontal and vertical blanking periods, a blanking pulse (positive pulse) HBL is applied to the terminals 21 and 22.
Since K and VBLK are supplied, the transistor 25 is turned on. As a result, the voltage at the connection point between the transistor 4 and the resistor 8 becomes approximately 0V, and the blanking operation is performed.

[0013]

In the example of FIG. 5, the voltage of the power supply terminal + B is high, and the transistor 25 constituting the blanking circuit must have a high breakdown voltage. Also,
Since a relatively large current flows in the ON period of the transistor 25, the power loss becomes large. Further, since the blanking circuit is added as a stray capacitance during the video period which is the off period of the transistor 25, the frequency characteristic is deteriorated. Further, it has a time constant due to the capacitor 9 for coupling with the first grid electrode G1 and operates at a very high speed when the transistor 25 is turned on, but operates because it is charged by the time constant of the resistor 8 and the capacitor 9 when turned off. Is slow and affects the response characteristics of the video signal.

Therefore, in the present invention, the blanking circuit does not require a high breakdown voltage transistor, the power loss of the blanking circuit can be reduced, and the blanking is performed without affecting the frequency characteristic and the response characteristic. It is an object of the present invention to provide a video amplifier circuit capable of

[0015]

The present invention has an emitter-coupled differential amplifier which supplies a video signal only to the first grid electrode G1 of a cathode ray tube, and a constant current source constituting this differential amplifier is blanked. Blanking is applied by switching control with pulses.

[0016]

According to the present invention, the constant current source constituting the differential amplifier is switching-controlled by the blanking pulse, so that a transistor having a high withstand voltage as in the conventional case is not required and a large current flows during the ON period. None,
Power loss is also reduced. Further, since the video amplifier circuit is composed of the differential amplifier, the operation of the video amplifier circuit is stable, and the blanking can be executed without affecting the frequency characteristic and the response characteristic.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. In FIG. 1, parts corresponding to those in FIG. 5 are designated by the same reference numerals. The cathode electrode K, the second grid electrode G2, the third grid electrode G3
The connection of the part is the same as that of the example of FIG. 5, and the illustration thereof is omitted.

Reference numeral 61 is a terminal to which the video signal SV is supplied. The terminal 61 is connected to the base of the NPN transistor 63 via the capacitor 62. In addition, power supply terminal +
A series circuit of resistors 64 and 65 is connected between Vcc and ground, and the connection point of the resistors 64 and 65 is connected to the base of the transistor 63, whereby the base bias is supplied to the transistor 63.

The collector of the transistor 63 is connected to the power supply terminal + Vcc, and the emitter of the transistor 63 is grounded via the resistor 66 to form an emitter follower structure. The connection point between the emitter of the transistor 63 and the resistor 66 is grounded through a series circuit of a variable resistor 67 for contrast adjustment, a resistor 68 and a capacitor 69, and the connection point of the resistor 68 and the capacitor 69 is an emitter-coupled differential. It is connected to the base of one NPN transistor 70 which constitutes an amplifier. The mover of the variable resistor 67 is connected to the base of the other NPN transistor 71 which constitutes the emitter-coupled differential amplifier.

The collector of the transistor 70 is a resistor 72.
Is connected to the power supply terminal + B (for example, 50V) via.

The collector of the transistor 71 is the power supply terminal +
Connected to Vcc. A series circuit of resistors 74 and 75 is connected between the emitters of the transistors 70 and 71, and the connection point of the resistors 74 and 75 is the collector / emitter and resistor of the NPN transistor 76 that constitutes a constant current source. It is grounded through a series circuit of 77.

A series circuit of a resistor 78, a resistor 79 and an NPN transistor 80 is connected between the power supply terminal + Vcc and ground, and the connection point of the resistors 78 and 79 is connected to the base of the transistor 76. The collector and base of the transistor 80 are connected to form a diode connection, and the transistors 76 and 80 constitute a current mirror circuit as a constant current source.

Reference numeral 81 is a horizontal blanking pulse HBL.
A terminal to which K (for example, a positive pulse having a pulse width of 10 to 12 μsec) is supplied, and 82 is a vertical blanking pulse VBLK
(For example, a positive pulse having a pulse width of 1 msec) is supplied to the terminal. The terminal 81 is grounded via a series circuit of resistors 83 and 84, and the connection point of these resistors 83 and 84 is connected to the base of the transistor 76 via a diode 85. The terminal 82 is also connected to the base of the transistor 76 via the resistor 86.

The connection point between the collector of the transistor 70 and the resistor 72 is connected to the first grid electrode G1 of the cathode ray tube 1 via the capacitor 87.

The present example is constructed as described above, and the other parts are constructed similarly to the example of FIG.

In the above structure, the same DC voltage is supplied as a bias from the emitter output of the transistor 63 to the bases of the transistors 70 and 71. A video signal is supplied to the base of the transistor 71 from the mover of the variable resistor 67. for that reason,
An amplified video signal is obtained at the connection point between the collector of the transistor 70 and the resistor 72, and this video signal is transmitted via the capacitor 87 to the first grid electrode G1 of the cathode ray tube 1.
Is applied to Here, by moving the position of the mover of the variable resistor 67, the magnitude of the video signal supplied to the base of the transistor 71 changes, and the contrast can be adjusted accordingly.

When the blanking pulses HBLK and VBLK are supplied to the terminals 81 and 82, the transistor 76 is turned on and the collector current of the transistor 70 increases, so that the collector of the transistor 70 and the resistor 7 are connected.
A voltage of blanking level (retrace line erasing voltage) is obtained at the connection point of 2, and a blanking operation is performed.

FIG. 2 shows an equivalent circuit of the emitter-coupled differential amplifier. Si is the input video signal supplied to the base of the transistor 71, and So is the transistor 7
It is the output video signal obtained at the junction of the collector of 0 and the resistor 72. In the figure, the base bias voltage of the transistors 70 and 71 is VB, the base-emitter voltage of the transistor 76 is VBE1, the base-emitter voltage of the transistor 80 is VBE2, and the resistors 72, 74, 75,
The resistance values of 77, 78, 79 are RL, Re2, Re respectively.
1, Re3, R1 and R2.

Here, the collector current Ic flowing through the transistor 76 is as shown in equation 1.

[0030]

[Equation 1]

Further, the gain A of the amplifier is represented by the equation 2, and the output video signal So is determined by the equation 3.

[0032]

[Equation 2]

[0033]

(Equation 3)

In this example, the transistors 70, 71
In the emitter-coupled differential amplifier constituted by, the blanking pulses HBLK and VBLK are supplied to the base of the transistor 76 constituting the constant current source to turn it on to obtain a blanking level voltage. Therefore, the blanking circuit does not need a high breakdown voltage transistor. Further, a large current does not flow during the ON period of the transistor 76, and power loss is reduced. Also,
The operation of the differential amplifier is stable, and blanking can be executed without affecting the frequency characteristic and the response characteristic.

FIG. 3 is a block diagram showing another embodiment of the present invention. 3, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the figure, the connection point between the resistors 74 and 75 is grounded via a resistor 88 forming a constant current source.
In parallel with this resistor 88, a diode 89 and an NPN
The collector-emitter series circuit of the transistor 90 is connected. The base of the transistor 90 is grounded via the resistor 91. In addition, horizontal blankin pulse HBLK
Is supplied to the terminal 81 through the resistor 83 and the diode 8
A terminal 8 connected to the base of the transistor 90 via 5 and supplied with a vertical blanking pulse VBLK.
2 is connected to the base of the transistor 90 via the resistor 86.

The present example is constructed as described above, and the other parts are constructed similarly to the example of FIG.

In this example, when the blanking pulses HBLK and VBLK are supplied to the terminals 81 and 82, the transistor 90 is turned on and the collector current of the transistor 70 increases, so that the collector and the resistor of the transistor 70 are increased. A blanking level voltage is obtained at the connection point of 72, and a blanking operation is performed.

FIG. 4 shows an equivalent circuit of a portion of the emitter-coupled differential amplifier, and portions corresponding to those of FIG. 2 are designated by the same reference numerals. In the example of FIG. 3, the diode 89 may be removed.

In this example, the transistors 70, 71
In the emitter-coupled differential amplifier configured by, the blanking pulse HBLK, VBLK is supplied to the base of the transistor 90 connected in parallel with the resistor 88 forming the constant current source to turn on the blanking level. Is obtained, and the same effect as that of the example of FIG. 1 can be obtained.

In the above-described embodiment, an example in which a video signal is supplied to the first grid electrode G1 of the flat cathode ray tube 1 has been shown, but the present invention is applied to a predetermined electrode of another cathode ray tube. The same applies to the case of supplying a signal, which is suitable.

[0042]

According to the present invention, a constant current source forming a differential amplifier is switching-controlled by a blanking pulse, which eliminates the need for a high withstand voltage transistor in a blanking circuit as in the prior art, and it is turned on. No large current flows during the period, and power loss can be reduced. In addition, since the video amplifier circuit is configured by the differential amplifier, the operation of the video amplifier circuit is stable, and blanking can be performed without affecting the frequency characteristic and the response characteristic.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an embodiment of a video amplifier circuit according to the present invention.

FIG. 2 is a diagram showing a part of an equivalent circuit of the example of FIG.

FIG. 3 is a connection diagram showing another embodiment of the video amplifier circuit according to the present invention.

FIG. 4 is a diagram showing a part of an equivalent circuit of the example of FIG.

FIG. 5 is a connection diagram showing a drive system of a flat cathode ray tube.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Flat cathode ray tube 2 Phosphor screen 67 Variable resistor 70,71 for contrast adjustment NPN which comprises an emitter coupling differential amplifier
Type transistor 76, 80 NPN type transistor constituting a constant current source 88 Resistor constituting a constant current source 90 NPN type transistor constituting a switching element G1 First grid electrode

Claims (4)

[Claims] 1. An emitter-coupled differential amplifier for supplying a video signal only to a first grid electrode G1 of a cathode ray tube, wherein a constant current source constituting the differential amplifier is switching-controlled by a blanking pulse. Video amplifier circuit characterized by ranking. 2. The video amplifier circuit according to claim 1, wherein a current mirror circuit is used as the constant current source, and a transistor forming the current mirror circuit is switching-controlled by the blanking signal. 3. A switch circuit provided with an emitter-coupled differential amplifier for supplying a video signal only to the first grid electrode G1 of the cathode ray tube, and a switch circuit provided in parallel with a constant current source constituting the differential amplifier. A video amplifier circuit characterized by applying blanking by controlling switching with a blanking pulse. 4. The video amplifier circuit according to claim 3, wherein the switch circuit is composed of a series circuit of a diode and a transistor.