Pass transistor logic: Difference between revisions

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{{Short description|Group of logic families in electronics}}

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 [[transistor|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>{{cite book |first1=Jaume |last1=Segura |first2=Charles F. |last2=Hawkins |title=CMOS electronics: how it works, how it fails |publisher=Wiley-IEEE |date=2004 |isbn=0-471-47669-2 |pages=132 |url=}}</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 (since pass transistors have some resistance and do not provide level restoration). Each transistor in series is less saturated at its output than at its input.<ref>{{cite book |first=Clive |last=Maxfield |title=Bebop to the boolean boogie: an unconventional guide to electronics |publisher=Newnes |date=2008 |isbn=978-1-85617-507-4 |pages=423–6 |url=}}</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 (resembling an [[open collector]] scheme), so logic voltage levels in a sequential chain do not decrease.

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