JPH05183928A - Crt cathode current detecting circuit - Google Patents

Abstract

PURPOSE:To eliminate smear by the CRT cathode current detecting circuit in a CRT drive circuit. CONSTITUTION:The base of a PNP transistor 12 connected to a CRT cathode 11 in emitter follower is connected to the cathode of a diode 14 added between the two emitters of a complementary emitter follower in the preceding step. A diode 13 is added between the anode of the diode 14 and the CRT cathode 11 while setting the side of the CRT cathode 11 as an anode. Further, in addition to the two diodes between the bases of the two transistors of the complementary emitter follower, a diode 15 is added in the same direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video display device such as a television and a monitor, and a CRT such as an auto white balance circuit.
The present invention relates to a circuit for solving smear that occurs when a cathode current detection circuit is configured.

[0002]

2. Description of the Related Art In recent years, an automatic white balance circuit has become an IC and has become relatively easy to use. However, this auto white balance circuit is designed to detect the cathode current of the CRT, which is why the PNP is used.
Is connected to the cathode of the CRT with an emitter follower. Therefore, when the transistor is turned off, that is, when the signal changes from white to black, the cathode voltage is hard to rise, and smear appears on a screen where the contrast is particularly small and the brightness is floating.

A conventional circuit will be described below. Figure 3
Shows a conventional circuit. In FIG. 3, 31 is a CRT cathode. Reference numeral 32 is a PNP detection transistor. 33 is a diode. A video signal is input from 34, and a current flows through the 31 CRT cathode. This cathode current becomes the collector current of 32 transistors and is detected.

In the CRT drive circuit configured as described above, the PNP transistor is added to detect the CRT cathode current, so that smear occurs. The operation will be described below. First, the inverted signal comes into the base of the 32nd transistor, so when the signal is large, 3
The base potential of the 2nd transistor drops, and the 32nd transistor turns on, so that a cathode current flows. However, when the signal suddenly decreases from this state and the base potential of the transistor 32 rises, the potential of the emitter hardly rises and smear occurs on the screen.

[0005]

However, in the above-mentioned conventional structure, the diode 33 is turned on so as to increase the cathode voltage of 31 to act to increase the voltage of the emitter, but the emitter is still higher than the base. The voltage cannot be increased and the smear cannot be eliminated. The present invention solves the above-mentioned conventional problems, and an object thereof is to provide a circuit for eliminating smear in a CRT drive circuit.

[0006]

In order to achieve this object, the CRT drive circuit of the present invention has a base of a PNP transistor connected to the CRT cathode by an emitter follower for detecting a CRT cathode current, and a complementary emitter of the preceding stage thereof. It is connected to the cathode of a diode that is added between the two emitters of the follower, and the anode and CRT cathode of the diode that is added are connected to the CRT.
It has a circuit configuration characterized in that a diode is added with the cathode side as an anode, and a diode is added in the same direction in addition to the two diodes between the bases of the two transistors of the complementary emitter follower.

[0007]

With this configuration, the emitter of the transistor connected to the cathode of the CRT is always biased appropriately and turned on as compared with the base thereof, so that even when the signal suddenly rises from white to black. The signal is correctly transmitted to the cathode.

[0008]

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, 11 is a CRT cathode and 12
Is a detection transistor, and 13, 14 and 15 are diodes.

Twelve Ps connected by an emitter follower to eleven CRT cathodes for detecting the CRT cathode current.
The base of the NP transistor is added between the two emitters of the complementary emitter follower in the preceding stage.
It is connected to the cathode of the diode of No. 4, and the diode of 13 is added between the anode of the diode of No. 14 and the CRT cathode of No. 11 with the CRT cathode side of No. 11 as the anode.
In addition to the two diodes between the bases of one transistor, 15 diodes are added in the same direction.

The operation of the CRT drive circuit configured as described above will be described. When the input signal changes from white to black, the voltage on the cathode tends to rise. Here, 14 creates a potential difference between the base of the transistor of 12 and the anode of 13, and by setting the forward voltage of the diode of 15 appropriately, 11
Since the CRT cathode voltage of, that is, the emitter voltage of 12 can always be set higher than the base voltage,
The 12 transistors are always turned on. Therefore, smear does not occur because signals are transmitted correctly.

As described above, according to this embodiment, the CRT
Smear can be eliminated by additionally connecting an appropriate diode between the base and emitter of the detection transistor of the drive circuit and the base of the emitter follower of the preceding stage.

(Second Embodiment) A second embodiment of the present invention will be described below with reference to the drawings.

In FIG. 2, reference numeral 21 is a cathode, 22 is a detection transistor, and 23 and 24 are diodes. The difference from the configuration of FIG. 1 is that 15 diodes are not added and 16 resistors are not added.

The operation of the second embodiment constructed as described above is the same as that of the first embodiment, and smear can be eliminated. In particular, the second embodiment is advantageous in that it is simpler than the first embodiment, without the 15 diodes and the 16 resistors.

[0016]

As described above, the smear is eliminated by adding the diode to the base and the emitter of the transistor connected to the cathode for detecting the CRT cathode current, and adding the diode to the complementary emitter follower circuit of the preceding stage. You can

[Brief description of drawings]

FIG. 1 C for eliminating smear in the first embodiment
RT drive circuit diagram

FIG. 2 C for eliminating smear in the second embodiment
RT drive circuit diagram

FIG. 3 is a CRT drive circuit diagram for explaining the operation of a conventional circuit in which smear appears.

[Explanation of symbols]

 11 CRT Cathode 12 Detection Transistor 13 Diode 14 Diode 15 Diode 16 Resistor 21 CRT Cathode 22 Detection Transistor 23 Diode 24 Diode 31 CRT Cathode 32 Detection Transistor 33 Diode

Claims (2)

[Claims] 1. A base of a PNP transistor connected to a CRT cathode by an emitter follower for detecting a CRT cathode current in a CRT drive circuit is connected to a cathode of a diode added between two emitters of a complementary emitter follower in the preceding stage. , The diode and the CRT cathode of the added diode are added with the CRT cathode side as the anode, and the diode is added in the same direction in addition to the two diodes between the bases of the two transistors of the complementary emitter follower. CR characterized by
T cathode current detection circuit. 2. A base of a PNP transistor connected to a CRT cathode by an emitter follower for detecting a CRT cathode current in a CRT drive circuit is connected to a cathode of a diode added between two emitters of a complementary emitter follower in the preceding stage. Also, a CRT cathode current detection circuit characterized in that a diode is added with the anode and CRT cathode of the added diode as the anode of the CRT cathode side.