Editor’s note: As you can see, Michael Steffes puts a great deal of ‘blood, sweat, and tears”, with a generous helping of his long and extensive Op Amp knowledge, into these blogs. This series of highly technical and informative blogs are awesome tutorials for all levels of designers. It’s the kind of articles I would cut and paste from print magazines at EDN and EETimes for my future designs when I was a circuit designer for 40 years. I hope you enjoy these bits of wisdom and experience that will surely help you in your work and pique your creative imagination as well as stimulate your brain’s knowledge centers. Enjoy.
Some years back, an excellent blog appeared on EDN stepping through common issues and questions surrounding lower speed Voltage Feedback Amplifiers (VFA) (ref. 1). Those first appeared, with questions and comments, on the TI E2E forum where it seemed a fair number of those queries were on similar issues with higher speed op amps. At that time, those types of questions were referred to the ‘TI high speed E2E forum’. Those may well have been answered there, but a wider discussion of the unique issues that come with higher speed op amps will be the intent here.
While my colleague Bruce Trump did a fabulous job in succinctly stepping through quite a few op amp issues for the slower VFA type of device (ref. 2), in high speed we have the VFA plus Current Feedback Amplifiers (CFA) and Fully Differential Amplifier (FDA) types that bring their own set of issues. Very roughly speaking, most vendors segment off devices with >50MHz “bandwidth” as being in their high-speed product mix. This discussion will lean heavily on Bruce’s original blog for VFA types, but will add any unique or new considerations for CFA’s and FDA’s in the most pertinent topic areas. This first blog will pick up the I/O range issues (ref. 3) and include industrywide parametric tables where useful. To get some overlap, VFA’s down to 20MHz bandwidth will be included in those tables.
It might be fair to ask, “Who the heck is Michael Steffes?”. After 8 years of analog IC design at IBM and StorageTek, I concluded my high-speed analog IC design career contributing (slightly) to the design of one of the first integrated CFA op amps, the CLC400 – which is basically an IC version of the hybrid CLC231. From there, I traversed a 30-year career in applications, marketing, and business management across 5 suppliers, always focused on high speed amplifier development, introduction, and design-in tools. Along the way, I participated in the introduction of over 70 high speed amplifiers while publishing >100 application notes plus contributed articles. My product development contributions were always some combination of device definition, specification line items and limits, characterization plots, applications text, macro-modeling, and design-in support. Similar to Bruce, I also have had the good fortune to work closely with some of the top analog IC designers where basically no amplifier question was off limits. All of the best questions come from customers - where having those IC designers the next cube over for help is almost unfair.
Bruce Trump’s “”The Signal” came out weekly covering only VFA devices. This monthly blog will necessarily be a little longer covering the 3 types of high speed devices appearing in modern amplifier portfolios. Questions are welcome.
Input/Output Range issues for higher speed Voltage Feedback Amplifiers (VFAs)
In IC design, the output swing range is a separate issue from the input swing design requirements. The working assumption here is that overdrive into the swing limits is best avoided. For all speed ranges and topologies, the outputs usually have a symmetric swing range relative to the supplies. If it is Rail-to-Rail Output (RRO) it is Rail-to-Rail towards both supplies. If it is not RRO, it is not RRO towards both supplies. Also, if the outputs are not RRO, then the input range is usually not Rail-to-Rail Input (non-RRI) with a few exceptions. Again, op amps have no ground pin so any combination of voltages on the two supply pins are allowed as long is the total supply voltage (Vs) across the device is in the specified operating range. The available output swing is always a “headroom” to the supplies; how much overhead voltage is needed to allow for “linear” operation. Rail-to-Rail output stages reduce that headroom as far as possible. But there is still a 10mV to 200mV overhead requirement for proper op amp operation. Non-RRO outputs require much higher headroom, but often offer other benefits in terms of output linearity vs quiescent power.
The early op amp “slang” for output swing was an available +/-Vout on some specified supply voltages. That is a bit cumbersome where a more useful “headroom to the supplies” sometimes appears in more recent product releases like the OPA838 specification (ref. 4) shown in Figure 1. This headroom specification remains the same across any combination of supply voltages.
At the same time, a relatively new, and very useful, output swing range vs output current plot has come to be a standard RRO characterization curve. This “claw” curve has emerged as a typical way to give a little more information on the increased headroom requirement that comes with increasing source/sink output current and is even included in the most recent TINA (ref. 5) macromodels (ref. 6). Figure 2 shows this curve from the RRIO, 38MHz, OPA350 data sheet (ref. 6). The left scale is the available output swing referenced off the +/- supply voltages (V+ & V-). The delta from the top and bottom maximum is the headroom vs. source or sink current.
Example “claw” curve for the RRIO OPA350.
This claw curve is in fact one way to estimate another very important op amp output specification – linear output current. This is easily one of the more difficult specifications to either deliver, or extract out consistently, across decades of data sheets from different vendors. Lacking any more definitive specification, the parametric tables shown below used a 0.5V headroom limit from this type of claw curve (if available) to estimate a “linear” output current using the minimum of source or sink. This 0.5V headroom with load current is approximately at the limit of where you would call a part RRO (Rail-to-Rail Output) and crossing over into the regime of non-RRO devices. The other approach used, lacking this curve or a clear specification line, is ½ of the short circuit current. A more recent effort at showing an available “linear” output current appears in the unity gain stable OPA837 (ref. 8) data sheet as Figure 3. The “A” at the far right indicates 100% ATE screened where the conditions show those test setups and limits on a +/-2.5V supply.
While very useful, the “claw” curve of Figure 2 is implicitly showing a 2-quadrant output VI capability. Some older high-speed op amps like the OPA690 (ref. 9) attempted to show a more thorough 4-quadrant VI capability as illustrated in Figure 4. While there is a lot of information crammed into this plot (including the claw curve portion), it didn’t generate much interest and has largely been dropped at this point from the TI datasheets – while it does show up in the newer Microchip data sheets (Figures 4-6, ref. 10). Note the higher output currents available from this 5.5mA quiescent current device. That is the main advantage of a non-RR output design in that they can have much higher “power gain” from the required quiescent supply current to the available output current.
Example 4-Quadrant output VI operating range for the non-RRO OPA690.