Pass transistor logic: Difference between revisions

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

Pass transistors are a design technique in digital circuits that utilize transistors themself as switches. They offer a way to create logic gates with a potentially smaller transistor count compared to traditional CMOS logic. Here's a deeper dive:

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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Simulation of circuits may be required to ensure adequate performance.

== Applications ==

Traditional CMOS vs. Pass Transistors:

[[Image:SRAM Cell (6 Transistors).svg|thumb|250px|A six-transistor CMOS [[Static random-access memory|SRAM]] cell. M5 and M6 are bidirectional pass transistors.]]

[[Image:Multiplexer-based latch using transmission gates.svg|thumb|250px|a 10-transistor CMOS [[flip-flop (electronics)#Gated D latch|gated D latch]], similar to the ones in the CD4042 or the CD74HC75 integrated circuits. ]]

CMOS Logic Gates: Typically built using a combination of N-channel (NMOS) and P-channel (PMOS) transistors to create logic functions like AND, OR, NOT, etc. Each logic function has a dedicated circuit design with specific transistor configurations.

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.<ref>

Pass Transistors: Here, transistors act as electronically controlled switches. By applying a voltage to the gate terminal of a transistor, you can control whether it allows current to flow between its drain and source terminals (like a switch turning on or off). This allows for creating logic functions using these "pass transistors" in specific arrangements.

Norimitsu Sako.

[https://patents.google.com/patent/US7171636 "Patent US7171636: Pass-transistor logic circuit and a method of designing thereof"].

'It is known in the art to employ a "pass-transistor logic circuit" to reduce a number of elements and power consumption, and to improve operating speed.'

</ref>

XOR has the worst-case [[Karnaugh map]]—if implemented from simple gates, it requires more transistors than any other function. Back when transistors were more expensive, designers of the [[Z80]] and many other chips were motivated to save a few transistors by implementing the XOR using pass-transistor logic rather than simple gates.<ref>

Benefits of Pass Transistors:

{{cite web |first=Ken |last=Shirriff

|url=http://www.righto.com/2013/09/understanding-z-80-processor-one-gate.html |title=Reverse-engineering the Z-80: the silicon for two interesting gates explained |date=2013}}

</ref>

==Basic principles of pass transistor circuits==

Reduced Transistor Count: For some logic gates, pass transistors can potentially require fewer transistors compared to a traditional CMOS implementation. This can be advantageous in situations where minimizing chip area is crucial.

MOSFET pass transistors are [[Electronic switch|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 [[Static random-access memory|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]] 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.

Conceptual Simplicity: The concept of using transistors as switches can sometimes offer a more intuitive understanding of how certain logic functions work.

Simulation of circuits may be required to ensure adequate performance.

Drawbacks of Pass Transistors:

=={{anchor|CPL}}Complementary pass transistor logic==

Degraded Signal Levels: Unlike a true CMOS gate with dedicated pull-up and pull-down networks, pass transistors introduce some resistance in the current path. This can lead to a decrease in the voltage swing (difference between high and low logic levels) at the output.

Some authors use the term ''complementary pass transistor logic'' to indicate a style of implementing logic gates that uses [[transmission gate]]s composed of both NMOS and PMOS pass transistors.<ref>

Charge Sharing Concerns: When multiple transistors are connected together in a pass-transistor logic circuit, there's a risk of unintended charge sharing between them during switching. This can lead to glitches or errors in the output signal.

{{cite book |first=Gary K. |last=Yeap |title=Practical Low Power Digital VLSI Design |publisher=Springer |orig-year=1998 |date=2012 |isbn=978-1-4615-6065-4 |pages=197 |url=https://books.google.com/books?id=sXTdBwAAQBAJ}}

</ref>

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.<ref>

Limited Functionality: Not all logic functions can be efficiently implemented using just pass transistors. Complex gates might still require traditional CMOS design for better performance.

{{cite book |first=Vojin G. |last=Oklobdzija |title=Digital Design and Fabrication |publisher= CRC Press|date= 19 December 2017|isbn= 9780849386046|pages=2–39 |url=https://books.google.com/books?id=VOnyWUUUj04C}}

</ref><ref name="IEEE_1990"/><ref name="ULVD_2015"/>

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.<ref>

Applications of Pass Transistors:

{{cite book |editor-first=Wai-Kai |editor-last=Chen |title=Logic Design |publisher=CRC Press |___location= |date=2003 |isbn=978-0-203-01015-0 |pages=15–7 |url=https://books.google.com/books?id=X0a3BgAAQBAJ |oclc=1029500642}}

</ref><ref>

{{cite book |editor-first=Vojin G. |editor-last=Oklobdzija |title=The Computer Engineering Handbook |publisher=Taylor & Francis |___location= |date=2001 |isbn=978-0-8493-0885-7 |pages=2-23–2-24 |url=https://books.google.com/books?id=38Aj3CjHgc8C}}

</ref><ref>

{{cite book |first=Ajit |last=Pal |title=Low-Power VLSI Circuits and Systems |publisher=Springer |date=2014 |isbn=978-81-322-1937-8 |pages=109–110 |url=https://books.google.com/books?id=0I1xBQAAQBAJ |chapter=5.2.3 Pass-Transistor Logic Families |chapter-url={{GBurl|0I1xBQAAQBAJ|p=109}}}}

</ref>

''Complementary pass transistor logic'' or ''differential pass transistor logic refers'' to a [[logic families|logic family]] which is designed for certain advantage. It is common to use this logic family for [[Multiplexer#Digital multiplexers|multiplexers]] and [[Latch (electronics)|latches]].{{citation needed|date=April 2015}}

Transmission Gates: A common application is in transmission gates, which are essentially electronically controlled switches used for routing digital signals within a circuit.

CPL uses series transistors to select between possible inverted output values of the logic, the output of which drives an [[Inverter (logic gate)|inverter]]. The CMOS [[transmission gate]]s consist of nMOS and pMOS transistor connected in parallel.

Simple Logic Gates: For some basic logic gates like inverters or buffers, pass transistors can be a viable option.

==Other forms==

Custom Logic Design: In specific design scenarios where minimizing transistor count is a priority, pass transistors might be considered, but careful analysis of potential drawbacks is essential.

Static and dynamic types of pass transistor logic exist, with differing properties with respect to speed, power and low-voltage operation.<ref>{{cite book |first=Cornelius T. |last=Leondes |title=Digital signal processing systems: implementation techniques |publisher=Elsevier |date=1995 |isbn=0-12-012768-7 |pages=2 |url=}}</ref> 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==

In conclusion, pass transistors offer a technique for potentially reducing transistor count in logic circuits. While they can be conceptually simpler, their use comes with trade-offs in terms of signal degradation, charge sharing concerns, and limited functionality for complex gates. Understanding these limitations is crucial when deciding if pass transistors are a suitable design choice for a particular application.

{{Reflist|refs=

<ref name="IEEE_1990">{{cite journal |title=A 3.8-ns CMOS 16x16-b multiplier using complementary pass-transistor logic |author-last1=Yano |author-first1=Kuniaki |author-last2=Yamanaka |author-first2=Toshiaki Yamanaka |author-last3=Nishida |author-first3=Takeshi |author-last4=Saito |author-first4=Mitsuo |author-last5=Shimohigashi |author-first5=Katsuhiro |author-last6=Shimizu |author-first6=Atsushi |date=1990 |journal=[[IEEE Journal of Solid-State Circuits]] |volume=25 |issue=2 |pages=388–395 |doi=10.1109/4.52161|bibcode=1990IJSSC..25..388Y }}</ref>

<ref name="ULVD_2015">{{cite book |title=Ultra-Low-Voltage Design of Energy-Efficient Digital Circuits |first1=Nele |last1=Reynders |first2=Wim |last2=Dehaene |series=Analog Circuits And Signal Processing (ACSP) |date=2015 |publisher=Springer Switzerland |isbn=978-3-319-16135-8 |issn=1872-082X |doi=10.1007/978-3-319-16136-5 |lccn=2015935431}}</ref>

}}

==Further reading==

*{{cite book |last1=Weste |last2=Harris |title=CMOS VLSI Design |year=2005 |publisher= Pearson/Addison-Wesley|edition=3rd |isbn=0-321-14901-7 |pages= |url=}}

*{{cite book |first1=Douglas A. |last1=Pucknell |first2=Kamran |last2=Eshraghian |title=Basic VLSI Design |year=1994 |publisher= Prentice-Hall Of India Pvt. Limited|edition=3rd |isbn=978-81-203-0986-9 |pages= |url=}}

{{Logic Families}}

[[Category:Logic families]]