Thomson Consumer Electronics Inc.

Charles M White

Thomas D Gurley

William A Lagoni

Jeffery B Lendaro

A differential transconductance amplifier having a fixed value of differential transconductance provides a video output signal current directly proportional to the transconductance of the amplifier and to a difference between first and second differential video input signals supplies thereto. A pair of buffer amplifiers couples the video input signals to respective inputs of the differential transconductance amplifier via respective diode switches which provides Vber protection for the buffer transistors and provides bus isolation under power down conditions. An output circuit regulates the output voltage of the differential transconductance amplifier at a substantially fixed voltage and converts the output signal current to an output signal voltage. Advantageously, the receiver provides very accurate signal gain control unconditional stability, a wide common mode input voltage range, avoids the need for stand-by power and will not load the input bus under power-down conditions.

Video differential bus receiver for audio/video interconnection

A complementary push-pull emitter follower amplifier is coupled between the output of a high voltage driver amplifier and the cathode of a kinescope for reducing the effective capacitance presented to the driver amplifier that is attributable to the kinescope cathode, socket, spark gaps and related stray capacitances. A secondary undesired capacitance loading of the amplifier attributable to the collector to base capacitances of the follower amplifier is effectively reduced by regulating the collector emitter voltages of the push-pull follower output transistors at respective substantially constant values thereby improving parameters such as the slew rate and bandwidth of the overall video display system.

Kinescope driver apparatus

In a projection television system, a blue phosphor roll-off compensation network is included in the emitter circuit of a common emitter configured transistor comprising the driver amplifier for the blue cathode ray tube. The roll-off compensation network includes a voltage divider connected between the emitter of the common emitter configured transistor and a source of a bias voltage, and a second transistor having its base connected to a point within the voltage divider and its collector-emitter path connected in series with a gain changing resistor between the emitter of the common emitter configured transistor and the source of the variable bias voltage. The voltage divider determines the level of drive current at which the second transistor is rendered conductive and thereby the point at which roll-off compensation is applied. The gain changing resistor determines the amount of compensation.

Circuit for compensating the blue phosphor roll off at high drive levels

Audio bus receiver includes a pair of bipolar transistors each having a base coupled via a respective resistor to a respective bus receiver input terminal, each having an emitter that is diode coupled to a respective current source and each having a collector coupled via a respective load resistor to a source of reference potential. The emitters are coupled together via a gain control resistor and the collectors are coupled to an output terminal via a differential amplifier. Advantageously, the receiver avoids loading the bus under power-down conditions without requiring the use of stand-by power supplies. Additionally, high common mode rejection is achieved without need for precision matching of components and only a single supply voltage is required for operation.

Audio differential bus receiver for audio/video interconnection

A driver circuit especially for driving scan velocity modulation (SVM) coils or similar loads is characterized by low quiescent current loading and high peak output. The driver is coupled to an input signal varying between a quiescent signal level and a peak signal level. A transistor is coupled to a power supply and to the input signal, so as to conduct according to the input signal. A nonlinear element such as a diode is coupled in series with the emitter-collector junction of the transistor, and is biased to a voltage slightly less than a forward biased conducting diode voltage drop. Thus the diode has a higher resistance when the transistor is conducting at the quiescent signal level, and a lower resistance when the transistor is conducting at the peak signal level. The quiescent bias conditions are maintained by resistors in series and parallel with the diode. The driver may be configured as a complementary push-pull stage.

Power stage bias circuit with improved efficiency and stability

A driver for a cathode ray tube includes a black level compensation circuit which generates an offset voltage which tracks the current flowing to the output of the driver and therefore the average picture level (APL). The offset voltage is added as a component of the output voltage so as to increasingly shift the output voltage toward black as the APL increases. This compensates for CRT gamma non-linearities and for light scattering effects which might otherwise cause the loss of perceived detail and sharpness in dark areas of a high APL image. In the disclosed embodiment, the driver comprises a cascode amplifier and the offset voltage generating element includes a capacitively bypassed resistor connected in a reference voltage network connected to the emitter circuit of the lower (common emitter configured) transistor of the cascode amplilfier.

Black compensation circuit for a video display system

In a video display system including a cathode ray tube (CRT) as a display device and video output amplifiers for amplifying relatively low level red, green an blue video input voltages to produce respective relatively high level video output voltages suitable for application to the CRT, a 'black tracking' circuit is coupled to the inputs of video output amplifiers. Voltage to current converters are utilized as preamplifiers for the video output amplifiers. The 'black tracking' circuit includes terminating resistors connected at one end to respective outputs of the voltage to current converters for converting output currents of the converters to respective ones of the low level video input voltages, and a common resistor connected between the other ends of the terminating resistors and a signal ground point. A filter capacitor is connected in parallel with the common resistor. A common 'black tracking' voltage representative of the average picture level of a reproduced image is developed across the common resistor and, as a result of the connection to individual ones of the terminating resistors, is added to each one of the low level video input voltages at the inputs of respective ones of the video output amplifiers.

Cathode ray tube driver with input black tracking provisions

A differential current source (50) provides first and second complementary output currents (I1, I2) responsive to a video input signal (S1) which are coupled via respective load networks (52,54) to a common supply terminal (22) and develop first and second complementary video output signals (V1,V2). A first output circuit (56) applies the first output signal to a first output terminal (16) via a voltage follower transistor (Q6) and applies the second output signal to the first output terminal via a current source transistor (Q8). A second output terminal (18) is similarly driven by a second voltage follower transistor (Q5) and a second current source transistor (Q7), all transistors being of the same conductivity type. A tri-state control circuit (Q1,Q2) coupled to the differential current source enables the differential current source in a first operating mode to produce the first and second complementary currents whereby push pull output signals are produced at the output terminals. In a second operating mode, the tri-state control circuit forces the complementary currents to zero whereby the common supply terminal (22) applies turn-off bias via the two load networks to the control electrodes of all of the output transistors thereby 'tri-stating' (isolating) the output terminals. Advantageously, in the tri-state mode, there is zero quiescent power dissipation in the output circuits, the load networks, the differential current source and the tri-state control circuit.

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Noblesville, IN, United States of America

Charles M White