Figure 7.7: Ideal I-V curve with a fixed impedance.
Recall that the impedance in the saturation region is very high; thus, operation in this region
should be avoided when matched impedance is required. If the buffer is designed for
operation primarily within the triode region, the impedance will vary much less, as shown in
Figure 7.8
. Generally, highly linear behavior of the buffer is a desired feature. In early stages
of design, when actual buffer curves have not yet been defined, a linear model of the buffer
can be used. The information gained from this model can be simulated with a larger system
and help in defining the characteristics of the buffer-to-be. In the
next section
we detail the
linear approximation of a buffer.
Figure 7.8: Different regions of the I-V curve exhibit different impedance values.
Let us consider the effect on total impedance of the series resistor (either integrated or
discrete) as shown in
Figure 7.1
. The variation (with process, temperature, etc.) of the I-V
curve of the transistor will vary. This will, of course, affect the impedance.
Figure 7.9
shows
three curves for a transistor at a given V
gs
. This variation should be accounted for. Good
estimates may be obtained from silicon designers as to the total variation of a given buffer
type. Otherwise, assumptions can be made at this point and system simulations can be
conducted to determine the allowable constraints. Constraints on the buffer curves for a
given design may ultimately be derived from these simulations and communicated back to
buffer designers as design rules.
Summary :
In early stages of design, when actual buffer curves have not yet been defined, a linear model of the buffer can be used. Constraints on the buffer curves for a given design may ultimately be derived from these simulations and communicated back to buffer designers as design rules.
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