Operational Auto Zero Amplifiers

(June 05 2005, Update May 12 2017)

Auto zeroing operational amplifiers and chopper amplifiers are used when the low offset voltage requirements can not be fullfilled by good layout alone. Very often auto zero technologies are presented as a way to make amy amplifier reach good performance. This is not quite true. A poor amplifier combined with auto zeroing will still give a mediocre result. Only a good operational amplifier combined with autozeroing will lead to an excellent design.

The Basic Circuits

There are three main concepts.
For high precision analog amplifiers with feedack an 'in the loop adjustment' is used.
For open loop applications such as comparators or for medium performance applications the so called 'ping-pong-topology' is used.
In system ICs with predictable timing the output signal only needs to be stable at a known clock edge. Here a 'stripped ping-pong-topology' can be used. In this topology only one amplifier that is operating as an amplifier and is adjusted intermittently is needed. Removing the second amplifier reduces die area and cost.

Literature

There is a lot of literature for more interrested reades available. Some of it dates back to the 1960s! Here are some examples:

Basic concepts of comparators can be found in chapter 6 of:
Geiger, Allen, Strader
'VLSI design techniques for analog and digital circuits'
Mc. Graw Hill, 1990

A book specialized on filters. An auto 0 amplifier can be regarded as a special filter canceling a DC voltage. See chapter 6:
Gregorian, Temes
'Analog MOS inegrated circuits for signal processing'
John Wiley & Sons, 1986

The gain stage is not necessarily a transistor. A varactor serves well too. See page H33 of:
Walt G. Jung
'Op Amp applications'
Analog Devices, 2002

A discrete transistor chopper amplifier is described in chapter 10 of:
Gelder, Hirschmann
'Schaltungen mit Halbleiterbauelementen Band 1'
Siemens & Halske AG. Berlin. 1961