A little over a decade ago, semiconductor design and applications engineers high fived each other when they had working silicon on CMOS because it achieved sub 100uV of input offset voltage in amplifiers with a yield of 80%. Back then, the titans of the industrial space like Allen Bradley, John Deere, Rockwell automation, Siemens and a few others considered CMOS amplifiers for lower cost platforms, but seldom did they choose them for performance.
While bipolar technology is alive and well, new CMOS amplifiers are pushing the boundaries of process limitations with clever design techniques, sophisticated trim methodologies and yield improvements.
Historically, bipolar devices have been on the “preferred” list of engineers for applications requiring high precision. These devices can achieve sub 1uV/ºC of offset drift. CMOS input stages, on the other hand, would yield as much as 5uV/ºC.
The challenge of achieving a very low offset in CMOS input op amps stems from the difference between the threshold voltages (input differential pair) as well as the difference between the gate-source voltage and the threshold voltage (VGS-VTH). Unlike bipolar devices, offset and offset drift are not correlated in CMOS devices whether in weak or strong inversion.
Other challenges in CMOS amplifier design include higher voltage noise flicker and white noise, and generally a much smaller open loop gain which is attributed to a lower transonconductance value compared to that of bipolar input.
One way around the above challenges is to use techniques such as auto zeroing, chopping, or a combination of both, which reduce the offset and the drift greatly (in CMOS) at the expense of more complex circuitry. Chopper stabilized amplifiers provide the lowest drift over extended temperature ranges, but their internal structure puts some limitations on their use.
Another way is to select a very well-trimmed device. To better appreciate the results of such a well-trimmed op amp, check out the newly released OPA192. A true milestone achievement in the design of CMOS amplifiers, this device is capable of rivaling the best available bipolar and JFET technologies.
So the next time you’re in the market for a true precision op amp, take a look the OPA376 if your system requires a low voltage operation, or the OPA192 for higher voltages.
Or, read “Time for a trim?” a blog I wrote on various trim methodologies.
