In these cases, your top priority for operational amplifier selection is likely one or more signal-chain performance parameters. These might include AC performance terms, such as distortion and broadband noise performance, or DC terms, such as input offset and offset drift.
But, in energy-constrained applications for which you must wring out every last bit of unnecessary power dissipation, the temptation is to start by looking for the operational amplifier with the lowest quiescent current. Unfortunately, this intuitively reasonable approach all too often identifies numerous candidates that meet the application’s power requirement but not necessarily its GBWP (gain-bandwidth product) needs.
For a given circuit topology, GBWP and Iq (quiescent current) go hand in hand—essentially in direct proportion. The reasons for this behavior are several and tied to the detailed topology of specific amplifiers.
At the top level, however, consider that the operational amplifier you choose must charge and discharge internal capacitances at signal speed. The resulting displacement currents flow from the internal bias current of the amplifier, which determines the net Iq. Therefore, for a given topology, as bandwidth increases so must the amplifier’s Iq.
A helpful figure of merit
The challenge for low-power design, then, is not simply to find low-power operational amplifiers, but to find operational amplifiers that most efficiently provide bandwidth. A simple figure of merit to assess operational-amplifier bandwidth efficiency is the ratio of GBWP to Iq.
In an example in the article, a performance comparison and figure-of-merit calculation for four devices of similar architecture—in this case, the Microchip MCP644X, MCP640X, MCP628X and MCP629X—shows a figure of merit that varies barely more than an octave, while the GBWP and Iq vary by a little more than three orders of magnitude. Indeed, at the lower bandwidths, the figure of merit for these devices is nearly constant.
In practice, your selection process will focus, of course, on competing devices of similar GBWP, not a group covering several orders of magnitude. First, though, you must determine the answer to one key question: "GBWP: How much is enough?"
This article explores the tradeoff and balance of gain-bandwidth versus quiescent current and in detail, with analysis, equations, and examples. To read it as a pdf document, click here; to read it as a Microsoft Word document, click here.
About the author:
Namrata Pandya is a Product Marketing Engineer for the Analog and Interface Products Division of Microchip Technology Inc. in Chandler, Ariz. She is responsible for the strategic marketing of operational amplifiers, as well as tactical marketing support for Microchip’s Analog and Interface products in the South Pacific and ASEAN countries. Prior to joining Microchip in 2007, Namrata spent two years with Cypress Semiconductor in San Jose, CA in Product Marketing. She earned a BSEE degree from Mumbai University in August 2001 and a Masters of Electrical Engineering degree from San Jose State University in December 2006.