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CMOS Operational Amplifier Output Stage

IP.com Disclosure Number: IPCOM000110491D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 3 page(s) / 92K

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 CMOS is that the analog signal dynamic range (largest possible signal/smallest recognizable signal) has been severely reduced, compared to traditional bipolar implementations, because of the low power supply voltage. Bipolar processes used for operational amplifier circuits typically use 30 volts across the chip, while current CMOS processes use 5 or 10 volts. In order to get the highest dynamic range possible, it is important to have an output stage which can develop full rail to rail voltages.

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CMOS Operational Amplifier Output Stage

       Operational amplifiers implemented in the complementary
metal oxide semiconductor (CMOS) technology have been available for
some time now.  One problem introduced by CMOS is that the analog
signal dynamic range (largest possible signal/smallest recognizable
signal) has been severely reduced, compared to traditional bipolar
implementations, because of the low power supply voltage.  Bipolar
processes used for operational amplifier circuits typically use 30
volts across the chip, while current CMOS processes use 5 or 10
volts.  In order to get the highest dynamic range possible, it is
important to have an output stage which can develop full rail to rail
voltages.

      A CMOS output stage should also have a low output resistance
and it should be capable of a stage gain somewhat greater than unity.
A gain greater than unity means that the output stage input signal is
not required to be rail to rail, simplifying the design of the
previous stage.

      The purpose of this article is to describe a CMOS circuit which
successfully addresses these needs.

      A schematic of the new output stage is shown in Fig. 1.  The
circuit is actually a feedback amplifier with transistors T1 and T2
providing the forward gain.  Transistors T3 and T4 provide a voltage
to current conversion (analogous to an input resistor, but with a
very high input resistance), and transistors T5 and T6 act as a
feedback resistor.  The amplifier feedback action controls the output
voltage so...