Page 70 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Using Transistors to Build Primitive Logic Functions BJ 51

Pas s-t ran s i s tor Log ic

In the BUF, NOT, AND, NAND, OR, and NOR gates described earlier,
the input signals and internal data signals are only used to drive control
terminals on the transistors. By comparison, transistors Tr, and Tr4 in the
XQR and XNOR gates shown above are connected so that input and internal
data signals pass between their data terminals. This technique is known as
pawtransistor logic.. It can be attractive in that it minimizes the number of
transistors required to implement a function, but it’s not necessarily the best
approach. Strange and unexpected effects can ensue if you’re not careful and
you don’t know what you’re doing.
An alternative solution for an XOR is to invert the output of the XNQR
shown above with a NOT. Similarly, an XNOR can be constructed by invert-
ing the output of the XOR shown above with a NOT. Although these new
implementations each now require six transistors rather than four, they are
more robust because the NOT gates buffer the outputs and provide a higher
drive capability. In many cases, XORs and XNORs are constructed from
combinations of the other primitive gates. This increases the transistor count
still further, but once again results in more robust solutions.
Having said all. this, pass-transistor logic can be applicable in certain
situations for designers who do know what they’re doing. In the discussions
above, it was noted that it is possible to create an AND using a single transistor
and a resistor. Similarly, it’s possible to create an OR using a single transistor
and a resistor, and to create an XOR or an XNOR using only two transistors
and a resistor. If you’re feeling brave, try to work out how to achieve these
minimal implementations for yourself (solutions are given in Appendix G).