A CMOS Implementation of the uA741 Op Amp

The uA741 Operational Amplifier was a widely popular op amp in the 1960s and 1970s. It is used as an example in many tutorials, including the Wikipedia entry on Operational Amplifiers (at least at the time of this writing, May 2012). Bipolar circuit design may be less familiar to many 21st century readers, so here (above) is a CMOS version of the design.  

The 741 architecture leaves little headroom for voltage swing. That was not a problem when analog circuits had +/-15V supplies. Modern analog CMOS or mixed signal designs usually have supplies in the 1V to 3.3V range. Other architectures are much better for low voltage. So it is not CMOS that killed the 741 architecture, it is low voltage.

If you are nerdy enough to want the details, read the history chapter of  Op Amp Applications, Edited by Walt Jung, Analog Devices, Inc.

For an easy intro to CMOS amplifiers, Gray & Meyer's 1982 tutorial on MOS Op Amps is reported to be one of the most popular papers on IEEE Xplore.

Typical CMOS Processes: Critical Dimensions Lmin & tox

For CMOS processes, the min length for a MOSFET (Lmin), the oxide thickness (tox), and the typical supply voltage (VDD) all track each other. They all go down together with smaller dimension processes. Below 130nm, things get wierd. For these processes, tox may even go up, and VDD levels out at about 1V. Although in some cases, the process is designed to work with a VDD > 1.0V to achieve higher speeds.

For an excellent discussion of how and why this parameters track each other, see this paper by Berkeley Prof. Chenming Hu, "Gate Oxide Scaling Limits and Projection".

For quick reference, here is a table Lmin, VDD typ, & tox for some typical CMOS processes:

Lmin (nm)  VDD typ.  tox (nm)
500        5.0      11.0
350        3.3       6.6 ,7.0, 8.5
250        2.5       4.8, 5.0
180        1.8       3.0, 3.2
130        1.2       2.0
 90        1.0       2.3
 65        1.0       2.6

The Elevator Pitch

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• You don't have the Analog expertise.
• You have an excellent team but you need someone with the expertise who can work with the team complete the project.
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• You've found me. 30 years of Analog and Mixed Signal experience without a lengthy candidate search.
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