Digital Signal Processing Reference
In-Depth Information
V
V
s
V
ss
t
Figure 11-5
Example transient response for a linear transmitter model.
is typically a pulse or a piecewise linear source, characterized by minimum and
maximum voltage levels (
V
SS
and
v
S
in the figure) and rise and fall times, as
illustrated by Figure 11-5. The resistor represents the effective output impedance
of the transmitter. The ability to change the behavior of linear models by mod-
ifying the output resistance and/or rise and fall times makes them particularly
useful for exploring the potential working solution space for a design. In addition,
since they require only a couple of passive elements, simulations that make use
of them execute with maximum speed. As a final note, the model may include a
capacitor at the output to account for the capacitance of the transmitter, as shown
in Figure 11-4b.
Limitations of the linear model
The nonlinear relationship between output cur-
rent and node voltages for real MOS transistors creates the potential for significant
behavioral differences between the linear model and a real transmitter, which we
illustrate by example.
Example 11-1
Load-Line Analysis Using a Linear Transmitter Model We com-
pare the falling-edge behavior of interconnect circuit shown in Figure 11-6a with
that of the linear model in Figure 11-6b. The current versus voltage character-
istic for the pull-down transistor is described in Figure 11-2a for
v
GS
=
2
.
5V.
To calculate the actual impedance of the transmitter, we use a technique known
as a
load-line plot
[Rabaey et al., 2003] We start with a plot of output current
versus output voltage for the NMOS transistor. Recognizing that the falling-edge
transition for this circuit begins from a state in which no current flows (
i
=
0)
and the voltage is 2.5 V, we construct a load line for the transmission line that
expresses the output current as a function of the output voltage using Ohm's
law:
−
v
out
2
.
5V
=
50
·
i
out
The current versus voltage plots for both the transistor and transmission line are
shown in Figure 11-7. The intersection of the two lines gives the values for the
Search WWH ::
Custom Search