Inverting Amplifier and TV Trigger Circuit

Current from one transistor of the conducting pair of transistors chosen by SLOPE switch S464 is applied to U480C pin 10. Current from the other side of the Comparator is applied to pin 14 at the output side of U480C through R468. Pin 11 of U480C is at a LO logic level except when TV FIELD triggering is enabled. This LO does not affect circuit operation in either AUTO or NORM triggering.

NOR-gate U480C is an emitter-coupled logic (ECL) device that is operated in the linear region. In the linear region, U480C acts as a high-speed inverting amplifier. Common-mode signals such as noise or thermal drift in the Comparator output signal are cancelled by U480C and associated circuitry. These types of offsets equally affect the outputs from both sides of the Comparator. Changing current to pin 10 of U480C causes a corresponding voltage change at U480C pin 14. The voltage change at one end of R468 is equal in amount and opposite in direction to the voltage change at the other end since the same common-mode signal from the other half of the Comparator is applied to the other end of R468.

When the TRIGGER MODE switch is set to TV FIELD, +8.6 V is applied to the TV Trig Enable signal line. Transistors Q474 and Q476 are biased on via R474, and U480C pin 11 is set HI, causing the output of U480C at pin 14 to be LO.

Current flowing through R466 from either U460C or U460D causes a voltage drop that establishes the bias voltage on the base of 0474. Current flowing through R473 and R472 produces a voltage drop across R473 that establishes the bias voltage on the base of Q476. The circuit components are selected such that when the Comparator output voltages from both halves are equal, the base voltages to both Q474 and Q476 will be the same. With equal base voltages, each transistor will conduct an equal amount of current.

When the Comparator output becomes unbalanced, due to an input trigger signal, unequal biasing of Q474 and Q476 occurs. In response to a changing bias condition, the collector currents vary proportionally.

The collector current changes from Q474 are filtered by a network composed of C476, C477, R477, and R478. The filter network rejects TV video information and averages the TV horizontal-sync pulses. Setting the triggerlevel threshold at near the center of the horizontal-sync-pulse swing establishes the untriggered level. When the TV vertical-sync block occurs, the output of the filter rises to a level that will cause the Schmitt Trigger circuit to switch. Precise TV field synchronization is obtained as a result of the filtering action.

The output signal from the filter is applied to U480B pin 6. The Schmitt Trigger circuit responds only to the TV sync signal because pin 7 is held LO by the output of U480C.

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