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Browse Prior Art Database

Compound Fet Device With High Drain-Source Resistance

IP.com Disclosure Number: IPCOM000102258D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 55K

Publishing Venue

IBM

Related People

Jenkins, M: AUTHOR

Abstract

Disclosed is a two-transistor circuit which operates as a single FET device with dramatically increased drain-source resistance. This implementation is much simpler and more general than known circuits.

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This is the abbreviated version, containing approximately 63% of the total text.

Compound Fet Device With High Drain-Source Resistance

       Disclosed is a two-transistor circuit which operates as a
single FET device with dramatically increased drain-source
resistance.  This implementation is much simpler and more general
than known circuits.

      Field-effect transistors exhibit finite drain-to-source
resistance caused by the change in junction width of the source and
drain diffusions, modulating the effective channel length.  This
effect is a limiting factor in numerous areas of circuit design,
including current sources and CMOS gain stages.

      The figure shows two cascode devices with both gates tied
together.  Since the objective in biasing the top device (Q1) is to
guarantee that the bottom device (Q2) remains saturated, it is, at
first intuition, apparent that the bottom device would be in triode
mode, resulting in low output resistance.  However, for a reasonable
set of design constraints, the bottom device can be guaranteed to be
operating above pinch-off, that is, in saturation.  These constraints
and their underlying reasons are:
1.   The dimensions of the bottom device (Q2) are chosen as follows:
      a.   Width-to-length ratio is chosen for desired bias current
and gate-to-source voltage.
      b.   Absolute dimensions are chosen for tracking and bias
tolerance considerations, if any.
      c.   Consistent with the first two constraints, Q2 is made as
long and narrow as possible to enhance the...