Digital Electronics: Principles, Devices and Applications

140 Digital Electronicsfeatures of ALS-TTL and AS-TTL have been discussed at length in the preceding paragraphs. As isobvious from the internal circuit schematic of the AS-TTL NAND gate, it has some additional circuitsnot found in ALS-TTL devices. These are added to enhance the throughput of AS-TTL family devices.Transistor Q 10 provides a discharge path for the base-collector capacitance of Q 5 . In the absence ofQ 10 , a rising voltage across the output forces current into the base of Q 5 through its base-collectorcapacitance, thus causing it to turn on. Transistor Q 10 turns on through D 9 , thus keeping transistor Q 5in the cut-off state.5.3.8.1 Characteristic FeaturesCharacteristic features of this family are summarized as follows: V IH = 2V; V IL = 0.8 V; I IH = 20 A;I IL = 0.5 mA; V OH = (V CC −2) V; V OL = 0.5 V; I OH = 2 mA; I OL = 20 mA; V CC = 4.5–5.5 V; propagationdelay (for a load resistance of 50 , a load capacitance of 50 pF, V CC = 4.5–5.5 V and an ambienttemperature of minimum to maximum) = 4.5 ns/5 ns (max.) for LOW-to-HIGH and 4 ns/5 ns (max.) forHIGH-to-LOW output transitions (74AS/54AS); worst-case noise margin = 0.3 V; fan-out = 40; I CCH(for all four gates) = 3.2 mA; I CCL (for all four gates) = 17.4 mA; operating temperature range = 0–70 °C(74-series) and −55 to +125 °C (54-series); speed–power product = 13.6 pJ; maximum flip-flop togglefrequency = 200 MHz.5.3.9 Fairchild Advanced Schottky TTL (74F/54F)The Fairchild Advanced Schottky TTL family, commonly known as FAST logic, is similar to theAS-TTL family. Figure 5.23 shows the internal schematic of a Fairchild Advanced Schottky TTLVCCR 110KR 210KR 34.1KR 445Input AD1Q 1Q 4D 11D3D5Q 3R55KInput BD2D6D7Output YQ 2Q 5D12D 4R 615KR 72KR 83KD8 D 9D10Q 7Q 6GNDFigure 5.23NAND gate in the FAST TTL.