Design Article
Op amps in small-signal audio design - Part 4: Selecting the right op amp (JFET-input types reviewed)
Douglas Self
8/24/2011 11:47 AM EDT
The OPA627 from Burr-Brown is a laser-trimmed JFET-input op-amp with excellent DC precision, the input offset voltage being typically ±100 mV. The distortion is very low, even into a 600Ω load, though it is increased by the usual common-mode distortion when series feedback is used.
The OPA627 is a single op-amp and no dual version is available. The OPA637 is a decompensated version only stable for closed-loop gains of 5 or more. This op-amp makes a brilliant DC servo for power amplifiers, if you can afford it; it costs about 50 times as much as a 5532, which is 100 times more per op-amp section, and about 20 times more per op-amp than the OPA2134, which is my usual choice for DC servo work.
The current noise in is very low, the lowest of any op-amp examined in this book, apparently due to the use of Difet (dielectrically isolated JFET) input devices, and so it will give a good noise performance with high source resistances. Voltage noise is also very respectable at 5.2 nV/√Hz, only fractionally more than that of the 5532.
The series-feedback case barely has more distortion than the shunt one, and only at the extreme HF end. It appears that the Difet input technology also works well to prevent input non-linearity and CM distortion. See Figures 4.40 and 4.41.
Figure 4.41: OPA627 driving the usual loads at 5 Vrms. Shunt feedback, gain = 2.23× but noise gain = 3.23×. 'Gen-Mon' trace shows the distortion produced by the AP System 2 generator alone
References:
[6] D. Self, Audio Power Amplifier Design Handbook, fifth ed, Focal Press, 2009, p. 380.
Printed with permission from Focal Press, a division of Elsevier. Copyright 2010. "Small Signal Audio Design" by Douglas Self. For more information about this title and other similar books, please visit www.elsevierdirect.com.
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Related links:
Op amps in small-signal audio design - Part 1: Op amp history, properties | Part 2: Distortion in bipolar and JFET input op-amps | Part 3: Selecting the right op amp
PRODUCT HOW-TO: Differential line driver with excellent load drive
Using Op Amps with Data Converters - Part 1 | Part 3 | Part 4 | Part 5
Yet More On Decoupling, Part 4: Op amp macromodels: A cautionary tale
Discrete audio amplifier basics - Part 1: Bipolar junction transistor circuits | Part 2: JFETs, MOSFETs and other circuit configurations
Op amps: to dual or not to dual? Part 1 | Part 2
Are you violating your op amp’s input common-mode range?
Distortion in power amplifiers, Part I: the sources of distortion | Part II: The input stage | Part III: The voltage amplifier stage | Part VII: frequency compensation and real designs
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Dr. HD
8/25/2011 7:30 AM EDT
There are some JFET-input op-amps that have been released since Doug wrote his book that are worth considering:
The LME49880 is a dual op-amp with the usual JFET-input traits of negligible bias currents and low current noise. Voltage noise isn't too shabby at 7 nv/sqrt(Hz). The only drawback is a relatively high input offset voltage of ±5 mV (typ), ±10 mV (max). As one would expect for an LME49xxx part, it has exceptionally low distortion (0.00003 % into 600 ohms; yes, that's four zeros!) and isn't too expensive.
The ADA4627-1 (unity-gain stable) and ADA4637-1 (decompensated), as the part numbers suggest, are decent alternatives to the OPA627/637 with very similar performance but at half the price (still very expensive!).
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