Pass transistor logic

In electronics, pass transistor logic (PTL) describes several logic families used in the design of integrated circuits. It reduces the count of transistors used to make different logic gates, by eliminating redundant transistors. Transistors are used as switches to pass logic levels between nodes of a circuit, instead of as switches connected directly to supply voltages.^[1] This reduces the number of active devices, but has the disadvantage that output levels can be no higher than the input level. Each transistor in series has a lower voltage at its output than at its input.^[2] 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 always switches transistors to the power supply rails, so logic voltage levels in a sequential chain do not decrease. 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.^[3] Simulation of circuits may be required to ensure adequate performance.

Complementary pass-transistor logic

Complementary pass-transistor logic or "Differential pass transistor logic" refers to a logic family which is designed for certain advantages. It is common to use this logic family for multiplexers and latches.

CPL uses series transistors to select between possible inverted output values of the logic, the output of which drives an inverter to generate the non-inverted output signal. Inverted and non-inverted inputs are needed to drive the gates of the pass-transistors.

Double pass transistor logic

Double-pass transistor logic eliminates some of the inverter stages required for complementary pass transistor logic by using both N and P channel transistors, with dual logic paths for every function. While it has high speed due to low input capacitance, it has only limited capacity to drive a load.

Other forms

Static and dynamic types of pass transistor logic exist, with differing properties with respect to speed, power and low-voltage operation.^[4] As integrated circuit supply voltages decrease, the disadvantages of pass transistor logic become more significant; the threshold voltage of transistors becomes large compared to the supply voltage, severely limiting the number of sequential stages. Because complementary inputs are often required to control pass transistors, additional logic stages are required.

References

1. ^ Jaume Segura, Charles F. Hawkins CMOS electronics: how it works, how it fails, Wiley-IEEE, 2004 ISBN 0471476692, page 132
2. ^ Clive Maxfield Bebop to the boolean boogie: an unconventional guide to electronicsNewnes, 2008 ISBN 1856175073, pp. 423-426
3. ^ Albert Raj/latha Vlsi Design PHI Learning Pvt. Ltd. ISBN 8120334310 pp. 150-153
4. ^ Cornelius T. Leondes Digital signal processing systems: implementation techniques Elsevier, 1995 ISBN 0120127687 page 2

• Weste and Harris, CMOS VLSI Design, Third Edition (ISBN 0-321-14901-7; ISBN 0-321-26977-2 (international edition))
• Douglas A. Pucknell and Kamran Eshraghian, Basic VLSI Design, Third Edition (ISBN 978-81-203-0986-9 (Indian Edition))