For years, bipolar operational amplifiers (op amps) have been the standard for technology given its better matching, higher transconductance efficiency, and a tighter noise corner than CMOS. However, advances in technology enable CMOS to use digital techniques to correct precision characteristics such as offset and offset drift. These digital techniques are not feasible to implement in bipolar op amps.
The result is that manufacturers can now make CMOS op amps that offer higher precision than bipolar op amps. In addition, the use of digital techniques and other innovations allow op amp manufacturers to create new architectures that can provide greater precision, better power efficiency, and higher bandwidth all in a smaller package.
This revolution in op amp technology has another, wider impact on circuit design: it completely changes how engineers can select components. Often, op amps provide basic all-around performance. No particular parameter has exceptional performance, but at the same time, every parameter generally performs well enough to get the job done.
With CMOS, designers now have access to a wider range of precision op amps that go beyond “general purpose”. These devices provide exceptional performance in a key parameter. For example, if power efficiency is a priority, op amps are available that trade off less important parameters. These other parameters still perform well enough to get the job done, but the op amp provides better overall performance where it matters most.
Designers will still have “general-purpose” op amps they tend to rely upon. The difference is that now they can have a favorite for low power, for higher bandwidth, for better noise and so on.
The truth is, not every general-purpose op amp is suited for every application. There is no such thing as a “perfect” op amp. And so, many applications will benefit by migrating from bipolar components to modern CMOS op amps.
Beyond general purpose
The primary question to ask when selecting an op amp is to identify 1) the parameters that are most important for an application and 2) the parameters which have a minimal or relatively insignificant effect on the application. This second set needs to be identified because improving one parameter often means relaxing another.
Low Power: Low power is becoming increasingly important all across application types. The tradeoff for lower power in an op amp is typically an increase in noise and a decrease in bandwidth. CMOS can improve power efficiency substantial. For example, the quiescent current of a bipolar LM358 is 350μA with noise of 40nV/√Hz and bandwidth of 1 MHz. In comparison, the CMOS NCS20062 has a quiescent current of 125μA, noise of 20nV/√Hz and bandwidth of 3 MHz. Thus, the NCS20062 gives twice the noise performance and three times better bandwidth for half the quiescent current.
It’s important to note that low power CMOS op amps from ON Semiconductor cover a broad range of capabilities and corresponding tradeoffs. Consider the NCS20062 versus the NCS20092, with quiescent currents (Iq) of 125μA and 20μA, noise of 40nV/√Hz and 20nV/√Hz, and bandwidth (BW) of 3 MHz and 0.35MHz, respectively. The advantage for engineers is not that one modern CMOS op amp is simply more efficient than another but rather that each op amp offers different tradeoffs, providing almost a full magnitude of bandwidth performance range.
Rail-to-Rail: CMOS amplifiers with a rail-to-rail input accept a wider dynamic signal range. This has the benefit of allowing designers to disregard common-mode voltage limitations (see Figure 1). Examples of rail-to-rail amplifiers are the NCS2006/8/9 and NCS325/NCS333 . For comparison, a bipolar LM358 or LM324 general purpose op amp has a common mode input range limitation of Ground to VDD - 1.7 V. These means that when using the LM358/324 in a common 5V system, the input must be restricted between 0V and 3.3V (5V-1.7V).
Precision: Precision CMOS op amps provide low VOS and VOS drift over temperature, making them ideal for sensors and low-side current sensing that require greater precision. For low-side current sensing, the input offset voltage adds error to measurements, especially as the differential voltage across the sense resistor approaches the offset voltage. Figure 2a shows how using a general-purpose bipolar op amp (LM358) results in a high offset error due to the input offset voltage of the op amp. A 450 mV shunt drop is required to achieve a 2% offset error. Figure 2b shows how this offset error can be minimized using a precision CMOS op amp (NCS333) to achieve a 0.02% (100 X better) offset error using a 50 mV shunt drop. Note that if a 50 mV shunt drop were used with the bipolar op amp, the offset error would be 18%.
Advantages CMOS brings to precision amplifiers and current sense amplifiers include low input bias current (Ib). CMOS input transistors need very little bias current to operate compared to standard bipolar transistors. In applications with high source impedance, input bias current can result in offset voltage errors that are greater than the input offset voltage.
Another advantage of CMOS op amps for precision current sensing is that precision resistors can be cost effectively integrated into the circuit. This enables higher accuracy when measuring small differential voltages, as well as a more compact design. For example, the NCS210R from ON Semiconductor eliminates the need for the two to four resistors required in a typical op amp current sensing circuit. Such op amps are ideal for both high-side and low-side current sensing and can be used as difference amplifiers.
High Speed: High-speed applications require op amps that support higher bandwidth and/or faster slew rate to prevent unwanted filtering of signals. A fast slew rate also allows the op amp to respond more quickly to feedback control circuits. For example, the bipolar LM358 has a bandwidth of 1 MHz and 0.5 V/μs slew rate compared to the NCS2005 with a bandwidth of 8 MHz and 2.8 V/μs slew rate.
Noise and Distortion: Noise and distortion negatively impact measurement accuracy. All analog signal applications, including sensor interfaces, instrumentation amplifiers, and audio preamplifiers/power amplifiers, can benefit from lower noise and distortion. The CMOS NCS20072, for example, offers noise of 30 nV/√Hz and distortion of 0.002% THD+N. In comparison, the LM358 has noise of 40 nV/√Hz and distortion of 0.015% THD+N.
The new general purpose
As a leader in op amp technology, ON Semiconductor has built on its extensive op amp experience with investment in numerous technologies – including precision thin film, zero-drift architectures, and state-of-art semiconductor processes – to provide industry-leading CMOS op amp components. By providing a wide portfolio of op amp choices, ON Semiconductor enables engineers to choose the right general-purpose op amp for their application and design better products to compete in the market (see Figure 3).
The superior performance of CMOS op amps also gives OEMs the ability to improve existing designs with minimal reworking. Figure 4, for example, shows how a developer could migrate from a bipolar LM321 to a CMOS-based op amp and achieve differences levels of improvement in reducing the sense resistor voltage, lowering power dissipation, and improving system efficiency.
Indeed, the advantages of CMOS op amps go far beyond those often referred to as “general purpose”. In fact, many modern applications stand to benefit from using CMOS components. Learn more about how CMOS op amps can help improve your next design.
Images
Figure 1: CMOS amplifiers with a rail-to-rail input accept a wider dynamic signal range, allowing designers to disregard common-mode voltage limitations.
Figure 2: Offset error in low-side current sensing can be minimized by using a precision CMOS op amp.
Figure 3: By providing a wide portfolio of operational amplifier choices, ON Semiconductor enables engineers to choose the right general-purpose op amp for their application and design better products to compete in the market.
Figure 4: Developers can improve existing designs by migrating to CMOS op amps to achieve different levels of improvement in reducing the sense resistor voltage, lowering power dissipation, and improving system efficiency.