Pass transistor logic: Difference between revisions

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In [[electronics]], '''pass transistor logic''' (PTL) describes several [[logic family|logic families]] used in the design of [[integrated circuit]]s. It reduces the count of transistors used to make different [[logic gate]]s, by eliminating redundant transistors. Transistors are used as switches to pass [[logic level]]s between nodes of a circuit, instead of as switches connected directly to supply voltages.<ref>Jaume Segura, Charles F. Hawkins ''CMOS electronics: how it works, how it fails'', Wiley-IEEE, 2004 ISBN 0-471-47669-2, page 132</ref> This reduces the number of active devices, but has the disadvantage that the difference of the voltage between high and low logic levels decreases at each stage. Each transistor in series is less saturated at its output than at its input.<ref>Clive Maxfield ''Bebop to the boolean boogie: an unconventional guide to electronics''Newnes, 2008 ISBN 1-85617-507-3, pp. 423-426</ref> If several devices are chained in series in a logic path, a conventionally constructed gate may be required to restore the signal voltage to the full value. By contrast, conventional [[CMOS logic]] switches transistors so the output connects to one of the power supply rails, so logic voltage levels in a sequential chain do not decrease.

<!-- needs a diagram -->Simulation of circuits may be required to ensure adequate performance.

Since there is less isolation between input signals and outputs, designers must take care to assess the effects of unintentional paths within the circuit. For proper operation, design rules restrict the arrangement of circuits, so that sneak paths, charge sharing, and slow switching can be avoided.<ref>Albert Raj/latha ''Vlsi Design'' PHI Learning Pvt. Ltd. ISBN 81-203-3431-0 pp. 150-153</ref> Simulation of circuits may be required to ensure adequate performance.

== Applications ==