Browse Prior Art Database

Method of Extending Op Amp Input Signal Range

IP.com Disclosure Number: IPCOM000110532D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 2 page(s) / 72K

Publishing Venue

IBM

Related People

Schulz, RA: AUTHOR

Abstract

Operational amplifiers implemented in the complementary metal oxide semiconductor (CMOS) technology have been available for some time now. One problem introduced by low voltage CMOS is that the input common mode signal range becomes severely restricted when using conventional input stage designs.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method of Extending Op Amp Input Signal Range

       Operational amplifiers implemented in the complementary
metal oxide semiconductor (CMOS) technology have been available for
some time now.  One problem introduced by low voltage CMOS is that
the input common mode signal range becomes severely restricted when
using conventional input stage designs.

      An n-channel differential input pair is able to handle input
voltages which approach the power supply, but it cannot accept
signals near ground because the tail current sink quits working.
Conversely, p-channel input pairs can accept input signals which
approach ground, but cannot handle large positive voltages because
the tail current source quits working.  An amplifier which can accept
both signal extremes can be constructed by paralleling an n-channel
input stage with a p-channel input stage.  One or the other of the
segments will always have sufficient voltage to work properly.
However, a new problem is created because uncontrolled currents from
the unused half of the input stage will disturb the second stage
biasing.

      A circuit which is based on this approach is described in [*].
The purpose of this article is to describe a simpler method of coping
with the problem.

      A schematic of a new input stage is shown in the figure.
Transistors T1 and T2 along with current sink I1 form a conventional
n-channel differential input stage.  Similarly, transistors T3 and T4
along with current source I2 form a conventional p-channe...