I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 442

limitations of such models and give guidance for when to use nonlinear behavioral
models.

11.2 PUSH-PULL TRANSMITTERS

11.2.1 Operation

The simplest type of output circuit is a push-pull transmitter, which can be
implemented using a simple CMOS inverter (paired with a receiver that is also an
inverter in order to preserve the logic state), as shown in Figure 11-1. Push-pull
transmitters were popular in the early days of CMOS ICs due to their ease of
implementation and low power consumption. They can be used in interconnect
systems without termination, with series termination, and/or with parallel ter-
mination. A comprehensive description of the operation of CMOS transistors
and inverters is provided in a book by Rabaey et al. [2003]. We offer a brief
overview here to make sure that the reader understands the fundamentals of
push-pull transmitter operation.
We start by providing the expression relating the current conducted by a MOS
transistor, i D , as a function of the voltage potential applied across the terminal
nodes:

0 v GS − v T < 0 ( subthreshold )

(v GS − v T )v DS −

( 1

k W
L

v 2 DS
2

+ λv DS ) ≤ v DS <v GS − v T

0 ( triode )

I D =

W
L

(v GS − v T ) 2 ( 1

k
2

+ λv DS ) 0 <v GS − v T ≤ v DS ( saturation )

(11-1)

V DD

MP 1

v in v out

i out

MN 2

V SS

Figure 11-1 CMOS inverter transmitter circuit. (From Dabral and Maloney [1998].)