MXPA99007826A - A video signal driver including a cascode transistor - Google Patents

Abstract

A kinescope video driver includes a series connection of a main cascode transistor (T4) coupled to a cathode of a cathode ray tube and a video signal amplifying transistor (T2). A second cascode transistor (T3) is coupled between the main cascode transistor and the video signal amplifying transistor. The second transistor is coupled to the video signal amplifying transistor through a short wire conductor (SHORT WIRE CONNECTION) and to the main cascode transistor through a long wire conductor (LONG WIRE CONNECTION).

Description

A VIDEO SIGNAL EXCITER INCLUDING A CASCODO TRANSISTOR The present invention relates to kinescope drive circuits. Specifically, the present invention operates on a projection television receiver to reduce the radio frequency interference generated in high-speed digital circuits, used to generate an auxiliary video signal, by coupling them to the input circuits of the antenna due to tips. of cable relatively long between the auxiliary video signal generator and the kinescope drive circuits. An auxiliary video signal generator in the current television receivers may include an on-screen display (OSD) circuit to display information useful to an observer, such as channel number and / or time, on the screen. In addition, an auxiliary video signal generator may also be used to display useful patterns for adjusting and calibrating the data stored in a digital convergence integrated circuit (DCIC) such as vertical and / or horizontal lines, dots or color bars. For example, an on-screen display generator may be provided in a receiver to generate a video signal that represents the image of the screen display. The signal representative of the image of the screen display is usually coupled directly to the final exciter of the kinescope, ignoring the television signal processing circuits related to the channel. The representative signal of the on-screen display image can be applied alone or can be multiplexed in time with the signal of the video component derived from the received television signal and the resulting video signal can be coupled to the cathode electrode of the kinescope . As is well known, the representative signal of the screen display image is generated by digital circuits in a screen display generator. An on-screen display generator responds to a high-frequency clock signal, and includes digital circuits at that clock frequency. The signals generated by such circuits may have a substantial harmonic content in the frequency range at which the tuner circuits are sensitive. In a standard television receiver, such harmonics are attenuated by placing the tuner and its associated circuits in a metal shield. The input and output terminals of the tuner are isolated by low pass filters where they pass through the shield. However, in a projection television receiver the housing containing the tuner and the auxiliary video signal generator used for the digital convergence integrated circuit is physically separated from the housing containing the three kinescopes that generate the projected images on the screen of passive display in a known manner. The conducting wire carrying, for example, the auxiliary video signal representative of the image to the kinescopes is therefore relatively long, ie several meters in length. This wire acts as a transmitting antenna, and the relatively high frequency harmonics of the signal carried by this wire are retransmitted back to the input terminal of the tuner antenna. The frequency of the RF radio frequency energy radiated by the harmonics in the signal representative of the image is in the range to interfere with the primary television signal. It is possible to minimize the radio frequency interference using LC filters. However, filters that have the characteristics to pass the portion of the signal representative of the image sufficient to produce a display of reasonable quality, and simultaneously attenuate the radio frequency interference sufficiently, is a relatively complex filter and requires substantial numbers of components and is expensive to manufacture and assemble. It is desirable to have an apparatus for attenuating radio frequency interference without degrading the image and which is relatively inexpensive. A typical kinescope exciter includes a pair of transistors coupled in a helmet configuration. One of the transistors, called the lower transistor, acts as an amplifier to convert the voltage of the video signal to a current. The other transistor, termed the upper transistor, is coupled in a common base configuration to the cathode of the kinescope to isolate the video signal component from the cathode voltage of the kinescope of the collector of the lower transistor. When the auxiliary video signal is applied to the cathode of the tube, it may be desirable to avoid having a long wire conductor between the collector of the corresponding lower transistor and the emitter of the upper transistor in the signal path of the auxiliary video signal. This is because the signal developed in a long wire can undesirably act with Miller capacitance between the base and the collector of the lower transistor to amplify the aforementioned interference producing harmonics related to the clock signal. In accordance with one aspect of the invention, a third transistor is interposed between the lower and upper transistors. The third transistor is coupled in a horn configuration with respect to the lower transistor. The conductor wire that couples the collector of the lower transistor to the emitter of the third transistor is conveniently short. On the other hand, the wire that couples the collector of the third transistor to the emitter of the upper transistor can be long. That is, the third transistor to island the lower transistor amplifier of the long wire. Therefore, conveniently, the signal developed in such a long wire can no longer act with the aforementioned Miller capacitor of the lower transistor. Thus, the radio frequency interference is conveniently reduced. A video excitation step for a cathode ray tube electrode including an aspect of the invention includes a source of a first video signal, a transistor amplifier that responds to the first video signal to amplify the first video signal and a first horn transistor coupled to the first transistor. A second horn transistor is coupled to the first horn transistor so that the first horn transistor is coupled in a signal path between the amplifier transistor and the second horn transistor. The second horn transistor is coupled to the cathode ray tube to apply the first video signal to the electrode of the cathode ray tube. In the drawing: The only Figure is a diagram, partially in block form, partly in schematic form illustrating a kinescope driver embodying the present invention. In the Figure, a front end of a television receiver (not shown) produces a video component signal in response to a received television signal, in a known manner. Such a front end of the receiver includes an antenna and / or cable input terminal, radio frequency amplifiers, intermediate frequency amplifiers, a tuner, as well as video signal processing circuits of known design. The video processing circuitry produces a video signal representing the image included in the television signal or a video signal representing each color component that makes up that image. The illustrated embodiment of the present invention is in a projection television system in which each of the color components (red, green and blue) is coupled to a separate kinescope. In the Figure, the circuits that provide a color component image signal to only one of the three kinescopes are illustrated. Those skilled in the art will understand that each of the kinescopes has similar circuits coupled thereto, and will understand which portion of the illustrated circuits is shared in common among all the kinescopes and which portion of the circuits is separately provided for each kinescope. Those skilled in the art will also understand that the illustrated embodiment can also be used in a standard television receiver in which a single kinescope includes three electron guns, one for each of the color components, or a single electron gun shared by the three colors. In the Figure, a video signal from the front end of the receiver (not shown) is coupled to a base electrode of a video amplifier transistor T1. An emitting electrode of the video amplifier transistor T1 is coupled to a reference potential source (ground) through a emitter resistor R1. A polarization circuit (not shown) may be coupled to the base electrode of the video amplifier transistor T1 in a known manner.

An auxiliary video signal generator 10 such as, for example, a screen display generator (OSD) produces a signal representative of the image at an output terminal 10a. The generator 10 can be an auxiliary video signal generator such as, for example, a pattern generator used to calibrate a digital convergence integrated circuit (DCIC). The generator 10 receives a clock signal of relatively high frequency (CLK). In the illustrated embodiment, the clock signal has a frequency of 8.56 MHz. The generator 10 includes digital circuits, not shown in detail, which is timed at the frequency of the clock signal. The circuits possibly include a processor that can respond to the observer input, and generate a signal representing the auxiliary image, as described above. The output terminal 10a of the generator 10 is coupled to a base electrode of a transistor amplifier T2 through a first bias resistor R2. A second polarization resistor R3 is coupled between the output terminal of the generator 10 and ground. An emitting electrode of the transistor amplifier T2 is coupled to ground through a transmitting resistor R4. The transistor T2 converts the voltage of the video signal developed in the output terminal 10a of the generator 10 to a collector current of the transistor T2. In carrying out an aspect of the invention, a collector electrode of the transistor amplifier T2 is coupled to an emitting electrode of a helmet transistor T3. A source of a polarization potential is coupled to the base electrode of the cascode transistor T3. In the illustrated embodiment, the polarization potential is 3 volts. In addition, a radio frequency bypass capacitor C1 is coupled between the base electrode of the helmet transistor T3 and ground. The collector electrode of the cascode transistor T3 is coupled to a collector electrode of the video amplifier transistor T1 through a collector resistor R5. The collector of the video amplifier transistor T1 is coupled to a transmitting electrode of a main horn transistor T4 through a series connection of respective resistors R6 and R7. An extinguishing circuit 20 of known design monitors the vertical and horizontal scanning of the deflection coils (not shown) associated with the kinescopes on the projection television in a known manner. An output terminal of the extinguishing circuit 20 is coupled to the junction of the resistor R7 and the resistor R6. A source of a main-head bias voltage is coupled to a base electrode of the main-head transistor T4. In the illustrated embodiment, the main helmet bias voltage is 10 volts. An AC filter capacitor C2 is coupled between the base electrode of the main helmet transistor T4 and ground. A collector electrode of the main horn transistor T4 is coupled to a source of an operation potential through a series connection of a compensation coil L1, and a load resistor R8. In the illustrated embodiment, the operating potential is 225 volts. A compensating amplifier 30 has an input terminal coupled to the resistor R8. In the illustrated embodiment, the compensating amplifier 30 may be a tilting amplifier (class B) having complementary transistors connected in series, not shown. The output terminal of the compensating amplifier 30 is coupled to a control electrode of a kinescope 40. In the illustrated embodiment, the output terminal of the compensating amplifier 30 is coupled to a cathode electrode of an electron gun in the kinescope 40. In operation, the video signal from the front end of the receiver (not shown) is coupled to the kinescope 40 by a horn amplifier formed of the video amplifier transistor T1 and the main horn transistor T4, as in the prior art configurations. It may be undesirable to couple the auxiliary video signal of the generator 10 to the kinescope 40 by coupling the collector electrode of the amplifying transistor T2 directly to the emitting electrode of the main horn transistor T4 through a long wire conductor. This is so because the high-frequency harmonics in the auxiliary video signal, which are produced from the digital circuits in the generator 10, as described above, could have been significantly amplified by the operation of the transistor T2. The amplification could occur due to the interaction of the impedance of such a long wire with a capacitance Miller CT2, between the collector and the base of transistor T2. If the connection between the collector of the transistor amplifier T2 and the main horn transistor T4 had been made directly through a relatively long wire that could be several meters long, and without interposing the transistor T3, the high frequency harmonics developed in this long wire they could have been amplified and transmitted to the tuner input terminal of the front end of the receiver, and substantially degrade the operation of the tuner. In carrying out an aspect of the invention, the cascode transistor T3 is coupled between the amplifier transistor T2 and the main helmet transistor T4 through a short wire conductor connection, called SHORT WIRE, of less than 2.54 cm . Transistor T3 is configured to isolate the collector of transistor T2 from the signal developed in the long wire connection between transistors T3 and T4 which could be several meters long. Harmonics of relatively high frequency can not develop due to the low impedance formed in the collector of transistor T2. The low impedance is obtained due to the short wire connection, the base emitter low impedance and the use of the radio frequency bypass capacitor C 1. These harmonics are placed in parallel to ground through the low impedance before passing to the long wire that is coupled to the main helmet transistor T4. Therefore, the signal developed in the impedance of that long wire can not interact with the capacitance Miller CT2 of the transistor T2. Accordingly, the operation of the tuner is not adversely affected by any raising of these harmonics. Additionally, the cascode transistor T3 and the radio frequency deviation capacitor C 1 do not significantly degrade the image. Conveniently, this solution requires only one additional transistor T3 and one radio frequency offset capacitor C1 which are relatively inexpensive components.

Claims (8)

1. CLAIMS I.A video exciter stage for an electrode of a cathode ray tube comprising: a source of a first video signal (10); a transistor amplifier (T2) that responds to said first video signal to amplify said first video signal; a first horn transistor (T3) coupled to said first transistor (T2); a second horn transistor (T4) coupled to said first horn transistor (T3) so that said first horn transistor (T3) is coupled in a signal path between said transistor amplifier (T2) and such second transistor cascode (T4), said second cascode transistor (T4) is coupled to the electrode of the cathode ray tube to apply said first video signal of such a cathode ray tube electrode.
2. 2. An exciting stage according to claim 1, wherein said first video signal comprises an auxiliary video signal.
3. 3. An exciting stage according to claim 1, wherein said video signal source comprises one of a display video signal generator and a pattern video generator.
4. 4. An exciting stage according to claim 1, further comprising a source of a second video signal and a fourth transistor (T1) responsive to said second video signal and coupled to said second horn transistor (T4) of so that said second horn transistor (T4) is coupled to said fourth transistor (T1) in a horn configuration.
5. An exciting stage according to claim 1, wherein said second horn transistor (T4) is coupled to said first horn transistor (T3) via a substantially longer conductor than a conductor coupling said amplifier transistor (T2) ) to said first cascode transistor (T3).
6. An exciting stage according to claim 1, further comprising a radio frequency bypass capacitor C1 coupled to a control electrode of said first horn transistor (T3) so that a low impedance develops in a conductive terminal of main current of such amplifier transistor (T2) at said high frequency.
7. An exciting stage according to claim 1, wherein said first video signal is applied to a cathode electrode of said cathode ray tube (40).
8. 8. A video driver stage for a cathode ray cathode electrode (40), comprising: a first transistor (T4) coupled to a cathode electrode of said cathode ray tube (40); a second transistor (T2) that responds to a first video signal and coupled to such front transistor face (T4) to form a first horn video amplifier; and a second cascode video amplifier (T1) that responds to a second video signal and coupled to said first transistor (T4) via a signal path that ignores such a second transistor.