![]() ![]() Building larger chips out of TTL was not feasible, as TTL logic tends to dissipate fairly large amounts of heat, which is what led to its eventual superceding by MOS. The earliest TTL chips are classed as SSI, but eventually TTL chips ranged up to the largest MSI. This produced a gate which has symmetric high->low and low->high switching.Īnother sub-set of TTL are open collector gates. The two transistors are then driven by the desired logic signal and its negation. One sub-set of TTL uses a pair of transistors in a so-called 'totem pole' output arrangement, with the collector of one attached to the supply voltage, the emitter of the other attached to ground, and the remaining emitter and collector are both connected to the output. These approaches are used in the 74H and 74L TTL families, respectively. ![]() Use of a smaller pull-up resistor results in a gate which can switch faster, but draws more power similarly a larger one produces a transistor that is slower, but uses less power. When the gate is off, the output is pulled up via a resistor connected to the supply voltage. In TTL, gates produce low outputs by sinking current to ground (drawing it into the gate through the output, seemingly paradoxically): the collector of the gate's output transistor is connected to the gate's output, and the emitter to ground, and when the gate is on, for a low output, current flows through the transistor to ground. ![]()
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