I presume you are taking about CMOS driver?
In case of CMOS driver the small-signal-input impedance depends upon the
circuit topology, process parameters, device size,
frequency of operation, input power fed to the driver and
as well as the load etc.. CMOS device at high frequencies
cannot be approximated as a unilateral device and thereby the
feed-back from output to input leads to a significant change
in the input impedance of a driver. All of these effects lead to
a nonlinear behavior of the input impedance vs all the circuit and
process parameters mentioned above.
The best way to find the small-signal-input impedance (even for a
moderate frequencies this may be necessary) is by
doing SPICE simulation. For most of the present day digital applications
a good SPICE model for CMOS devices leads to acceptable results.
Additionally, SPICE models for CMOS devices
generally do not include second-order effects. In such a
case a (expensive!) high frequency measurement set-up will be generally
required. Most of RF-companies working with CMOS technology
have in general such a in-house measurement setup. Moreover,
the concept of small-signal-impedance (as well as scattering parameters)
should be approached with caution for high power devices (driving large
current) due to the inherent nonlinearities present in V_in-I_in curve,
especially for digital as well as RF applications.
Sincerely,
Alok Tripathi
alok@ece.orst.edu
On Tue, 13 Oct 1998 Shimon_Turgeman@amat.com wrote:
> what is the input impedance of CMOS devices (for calculating the reflection
> coff.)as a function of input capacitance ? how I calculate this impedance ?
> what is the formula ?
>
> Best Regards
> Shimon Turgeman
> Applied Materials
>
>
>
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