It's been about 40 years since the first monolithic op amps were introduced. Those first devices, some of which are still in production, were designed exclusively in bipolar technology. Today most single chip op amps are still designed using bipolar transistors or their close cousin, the JFET, even though most 40 year-old IC innovations have long since become CMOS turncoats. Perhaps even more astonishing is that no company has yet invented a one-size-fits-all device. In fact, there aren't five, or ten, or even one hundred fits-all devices. Try over 1000! Texas Instruments alone lists nearly 700 op amp products to select from through their web site search engine. What is it that keeps 21st century op amp designers living in the '60s? Where is the Pentium IV op amp? The answers lie deep inside the thousands of manufacturer datasheets: The Specifications.
Specifications: General versus Application
Although not found in every datasheet, cumulatively there are at least 30 different specifications that relate the performance of an op amp. Each specification falls into one of two possible categories: General and Application Specific. The General category, shown in Table I, is comprised of items like input offset voltage and current, common mode rejection ratio (CMRR), and others. The Application category, also shown in Table I, is comprised of items such as noise density, gain-bandwidth product, output load drive, supply voltage, and current. What sets these two categories apart is the definition of what constitutes the best performance.
Table 1: There are dozens of specifications op amp designers are trying to juggle.
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All of the specifications in the General category have the same definition of best. The best offset voltage for all applications is that one closest to zero volts. The best CMRR for all applications is that one closest to zero volts/volt. General specifications primarily relate the degree of mismatch between circuit elements on a properly designed IC. If the input differential pair of a op amp is perfectly matched, the input offset voltage, offset current, and drift are nearly zero " a universal best. Many op amp products are available whereby a post manufacturing trim is executed to remove the random mismatches that result from normal processing. The result is a dramatic improvement in General specification performance for some parameters.
Conversely, the Application category has no single definition of best. There are only tradeoffs based mostly on electrical theory. For example, the best broadband noise performance is generally available on parts that exhibit relatively high operating currents. If your application demands very low operating current with no concern for broadband noise, your definition of best may have you choosing a part with 150 nV/√Hz of voltage noise density at 5μA of supply current. On the other hand, your colleague down the hall may need 1 nV/√Hz, but has a substantially higher operating current budget. She has a substantially different definition of best. Given this very fluid definition of best, the Application category has become the Achilles heel of the op amp industry and the reason there are over one thousand different amplifiers to choose from when specifying a device.
A good methodology for selecting an op amp from a myriad of search engine parameters is to start with defining the minimum acceptable General specifications for the circuit application. During this step of the selection process it is extremely important to not over specify the performance limits. There are two reasons to be cautious.
First, the highest performance General specifications are typically available on devices that have circuit topologies optimized primarily for matching. An example of such a topology is the OP177 manufactured by Analog Devices and shown in Figure 1. While the OP177 has impressive matching performance in the form of low offset voltage and high CMRR, such an input topology is not preferred for high speed or large common mode input range.
Figure 1: Analog Devices' OP177 input stage optimized for low offset voltage and high common mode rejection.
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On the other hand, the LT6220 manufactured by Linear Technology and shown in Figure 2 has an input stage that is capable of handling signals over the entire power supply range and at frequencies 100 times higher than the OP177. Alas, that same LT6220 exhibits a CMRR that is a factor of 100 less than the OP177. The point here is that by over specifying the General requirements, one will be left with selecting from a group of parts that severely restrict the potential performance of other aspects of the circuit.
Figure 2: Linear Technology's LT6220 input stage optimized for rail to rail operation and high bandwidth.
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Secondly, and with only a few exceptions, those devices with the highest General specification performance have the lowest manufacturing yields and/or require post manufacturing trimming. Both of these translate to a higher manufacturing cost and hence a higher sale price. One exception, although not a classical op amp topology, would be a chopper-stabilized or auto-zeroed amplifier where sampled-data techniques are used to eliminate mismatch yielding issues.
Once a search list of amplifiers with acceptable General performance limits is established, the list can be culled down to a best fit part based upon the Application performance requirements and ultimately cost.
CMOS is King. NOT!
For the past 20 years analog circuit designers have been slowly moving numerous analog IC product lines to CMOS technology and in some cases leaving bipolar as a footnote in history. One can see this transition in areas such as analog to digital and digital to analog converters, low frequency filters, the complete receive and transmit channels of single chip radios, portable power management, sample/track and hold amplifiers, and others. Conspicuously absent from this list though are classical high performance op amps; arguably the most analog of all analog circuits.
Sure, sprinkled among the endless lists of op amps you can find a multiplicity of modern, non-classical, auto-zero style CMOS amplifiers with magnificently high common mode rejection, unheard of low offset drift, and virtually no 1/f noise, but outside of very low frequency applications one will be sorely disappointed by the switching noise, aliasing, distortion, overload recovery time, and other unexpected and annoying artifacts that accompany those specialized architectures.
Since the recent turn of the century, CMOS amplifier manufacturers such as Texas Instruments, Analog Devices, and others have made significant inroads into supplanting bipolar as the leading technology for high performance op amps. Despite their impressive results to date, they must ultimately overcome three major hurdles before they will be successful.
Perhaps the most oft sighted hurdle is the superior elemental matching obtainable with a bipolar process. Poor elemental matching in a process restricts the General specification performance of an analog IC. While it is beyond the scope of this article to offer a detailed technical presentation to explain why this is true, consider that PSPICE, a popular circuit simulation software program, requires over 7 pages of mathematical variables to describe the operation of a MOS transistor to a computer. Compare this to the bipolar model used by the same program where about one page of variables is adequate. Needless to say, if there are a substantially greater number of variables that define the behavior of an MOS transistor above that necessary to define a bipolar transistor, the relative overall performance variation is bound to be greater.
Figure 3: Bipolar transistor current flows vertically and below the IC surface.
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Equally as limiting as poor matching is the lower transconductance efficiency of MOS transistors. Low transconductance manifests itself in an op amp as high transistor broadband noise, low output load driving capability, and low current gain. Here the difference lies in how the transconductance of a transistor scales with bias current. In a bipolar transistor the scaling is linear while an MOS device scales with a much weaker square root dependence on bias current.
Finally, MOS transistors are notorious for being a source of high 1/f noise. It is not unusual to find CMOS op amps with 1/f noise corner frequencies approaching 100 kHz while a similarly biased bipolar op amp operates with a noise corner near 1 Hz. Here the problem lies with how MOS transistors are manufactured. Whereas a bipolar transistor exhibits gain through a conduction path buried well below the surface of the IC [see Figure 3], an MOS transistor has its gain occur at the surface and, most importantly, in a region with numerous noise-generating defects [see Figure 4]. Perhaps less understood than the simple 1/f noise effects, these defects contribute additional performance limiting artifacts such as input offset voltage recovery time following a large voltage step transient.
Figure 4: MOS current flow occurs laterally near the surface and directly under defects in the gate oxide.
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The times, they are a-changing
To borrow a line from a 60's Bob Dylan song, the times are a-changing. In the past few years, op amp manufacturers have begun introducing several new high performance CMOS op amps. Chief among these are amplifiers designed for high speed with gain-bandwidth products that are knocking on the door of 100 MHz. Despite these admirable new developments though, the scene for highest performance op amps has not changed much in regards to the three hurdles described earlier. Bipolar still leads the pack and is holding on strong although perhaps not for long.
Late last year Texas Instruments introduced the OPA727 MOS op amp with a bipolar class input offset voltage and perhaps more important, an impressive low offset voltage drift of 1.5 μV /degree-C max over the temperature range of 0 C to 85 C. To obtain this result TI claims they trim the offset voltage after packaging and over the temperature range of operation. In true CMOS style though, the low frequency noise for the OPA727 is 10μVp-p.
And not to break the hearts of true audiophiles, but the advent of several new portable audio products is leading an amplifier mini-revolution towards class D amplification; a perfect fit for MOS technology.
But bipolar is not going away soon
While CMOS technology may be the cheapest IC manufacturing technology, it takes the will of a company and market pressure to move product research and development in that direction. The experts in analog op amp design reside in companies whose product lines stem from a bipolar family tree dating back decades. These veteran op amp designers take huge pride in the power of a single bipolar transistor that admittedly can do a better job of amplification than a 10 transistor CMOS circuit. The truth be told though, this is probably more a statement about designing CMOS op amp architectures that try to mimic those that work best for bipolar rather than inventing new architectures that trade transistor count for equivalently sized area.
To ultimately supplant bipolar op amp technology from its perch at the top of the op amp market will undoubtedly require new architectural approaches rather than simply repackaging past bipolar success stories. Texas Instruments' e-Trim and Analog Devices' DigiTrim technologies are certainly steps in that direction.
In the end, the cold hard facts are that if you can design an adequate analog product in CMOS, that's where it will ultimately be. The same cannot be said about bipolar. Said another way, bipolar is a technology of last resort. Today in op amp design, all of the leading analog IC manufacturers still avail themselves of that last resort quite frequently and in more than 1000 different ways.
J. Scott Elder of Analogue Integrated Technology is a design consultant based in Orlando, FL. He can be reached at