Pass Transistor Logic [All lecture notes are adapted from Mary Jane Irwin, Penn State, which were adapted from Rabaey’s Digital Integrated Circuits , ©2002, J. Rabaey et al.]

• XOR/XNOR gates usually require a large number of transistors in static CMOS logic • Remember: pass transistors –NMOS switch closes if gate input is high • But, NMOS pass strong 0 but weak 1 515 A B X Y = X if A and B X A B Y = X if A or B note that this is true in both directions

I am trying to plot the voltage transfer characteristic of this pass-transistor AND gate in Ltspice. in order to obtain the following graph: However, when I tried to do it, I did not obtain the correct output: I tried to choose a value for C, but I am not sure if it is correct.

When A=VDD, and B makes a transition from 0 to 1, T2 is turned on until VDD/2 and Output =0. Once T2 is turned off, output follows the input B minus a threshold drop. Hence, pass transistor gates cannot be cascaded by connecting the output of a pass gate to the gate input of another pass transistor. They can only be cascaded in series....

When A=V DD, and B makes a transition from 0 to 1, T2 is turned on until VDD/2 and Output =0. Once T2 is turned off, output follows the input B minus a threshold drop. Out 0.5 m/0.25 m 0 .5 m/025 m . Hence, pass transistor gates cannot be cascaded by connecting the output of a pass gate to the gate input of another pass transistor .

Pass transistor gates should never be cascaded as on the left Logic on the right suffers from static power dissipation and reduced noise margins

We can view the complementary CMOS gate as switching the output pin to one of power or ground. A slightly more general gate is obtained if we switch the output to one of power; ground; or any of the input signals. In such designs the MOSFET is considered to be a pass transistor.

" NMOS and pMOS transistor at minimum gate length (L) " Capacitance directly proportional to gate width (W) # C = W*C " Conductance directly proportional to gate width (W) # G = W*G

The Pass transistor logic is required to reduce the transistors for implementing logic by using the primary inputs to drive gate terminals, source and drain terminals. In complementary CMOS logic primary inputs are allowed to drive only gate terminals.

In this lecture, we will talk about another way to implement logic functions using transistors: pass-transistor logic (NMOS only) and transmission-gate logic (NMOS and CMOS transistors). For some types of functions, this can lead to much more …