In electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

, pass transistor logic (PTL) describes several logic families

Logic family

In computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and power supply...

 used in the design of integrated circuit

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s. It reduces the count of transistors used to make different logic gate

Logic gate

A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...

s, by eliminating redundant transistors. Transistors are used as switches to pass logic level

Logic level

In digital circuits, a logic level is one of a finite number of states that a signal can have. Logic levels are usually represented by the voltage difference between the signal and ground , although other standards exist...

s between nodes of a circuit, instead of as switches connected directly to supply voltages. 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. 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. 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

Inverter (logic gate)

In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. The truth table is shown on the right.This represents perfect switching behavior, which is the defining assumption in Digital electronics. In practice, actual devices have electrical characteristics that...

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. 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.