Operational Auto Zero Amplifiers
(May 13 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 recommendations:
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
Auto zero amplifiers used for analog digital conversion and very
detailed design of filters:
Gregorian, Temjes
'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