electronics Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...

, pass transistor logic (PTL) describes several

logic families In computer engineering, a logic family is 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 compati ...

used in the design of

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

s. It reduces the count of

transistors A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

used to make different

logic gate A logic gate is a device that performs a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has, for ...

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

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

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 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. 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 Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type ...

switches transistors so the output connects to one of the power supply rails (resembling an

open collector Open collector, open drain, open emitter, and open source refer to integrated circuit (IC) output pin configurations that process the IC's internal function through a transistor with an exposed terminal that is internally unconnected (i.e. "ope ...

scheme), so logic voltage levels in a sequential chain do not decrease. Simulation of circuits may be required to ensure adequate performance.

Applications

Pass transistor logic often uses fewer transistors, runs faster, and requires less power than the same function implemented with the same transistors in fully complementary CMOS logic. XOR has the worst-case

Karnaugh map A Karnaugh map (KM or K-map) is a diagram that can be used to simplify a Boolean algebra expression. Maurice Karnaugh introduced the technique in 1953 as a refinement of Edward W. Veitch's 1952 Veitch chart, which itself was a rediscovery of ...

—if implemented from simple gates, it requires more transistors than any other function. Back when transistors were more expensive, designers of the

Z80 The Zilog Z80 is an 8-bit microprocessor designed by Zilog that played an important role in the evolution of early personal computing. Launched in 1976, it was designed to be software-compatible with the Intel 8080, offering a compelling altern ...

and many other chips were motivated to save a few transistors by implementing the XOR using pass-transistor logic rather than simple gates.

Basic principles of pass transistor circuits

MOSFET pass transistors are electronic switches that turn on or off the path between their drain and source depending on their gate's voltage signal (for instance the clock signal in the SRAM cell or gated D latch). Because pass transistors do not provide level restoration and because their conducting path has a small non-zero resistance, there is increased

RC delay The RC time constant, denoted ' (lowercase tau), the time constant of a resistor–capacitor circuit (RC circuit), is equal to the product of the circuit Electrical resistance and conductance, resistance and the circuit capacitance: : \tau = RC \ ...

for charging the next logic stage's input capacitance (which includes parasitic capacitance in addition to the next stage's gate capacitance) towards valid logic-high or logic-low voltage levels. Simulation of circuits may be required to ensure adequate performance.

Complementary pass transistor logic

Some authors use the term ''complementary pass transistor logic'' to indicate a style of implementing logic gates that uses

transmission gate A transmission gate (TG) is an analog gate similar to a relay that can conduct in both directions or block by a control signal with almost any voltage potential. It is a CMOS-based switch, in which PMOS passes a strong 1 but poor 0, and NMOS passes ...

s composed of both NMOS and PMOS pass transistors. Other authors use the term ''complementary pass transistor logic'' (CPL) to indicate a style of implementing logic gates where each gate consists of a NMOS-only pass transistor network, followed by a CMOS output inverter. Yet other authors use the term ''complementary pass transistor logic'' (CPL) to indicate a style of implementing logic gates using dual-rail encoding. Every CPL gate has two output wires, both the positive signal and the complementary signal, eliminating the need for inverters. ''Complementary pass transistor logic'' or ''differential pass transistor logic refers'' to a

logic family In computer engineering, a logic family is 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 compati ...

which is designed for certain advantage. 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 A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the op ...

. The CMOS

s consist of nMOS and pMOS transistor connected in parallel.

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.

Applications

Basic principles of pass transistor circuits

Complementary pass transistor logic

Other forms

References

Further reading