Op amps in small-signal audio design - Part 4: Selecting the right op amp (JFET-input types reviewed)

August 30, 2011 //By Douglas Self
Op amps in small-signal audio design - Part 4: Selecting the right op amp (JFET-input types reviewed)
Douglas Self continues to examine the selection criteria for op-amp technology and surveys the most popular options.

[Part 1 reviews a brief history of op amps and then looks at various op amp properties from a perspective of audio design. Part 2 looks at distortion in BJT and JFET-input op amps, and using rail bootstrapping to reduce common-mode distortion. Part 3 examines various op amps and their key performance specs from a perspective of audio design.]

Op-Amps Surveyed: JFET Input Types
Op-amps with JFET inputs tend to have higher voltage noise and lower current noise than BJTinput types, and therefore give a better noise performance with high source resistances. Their very low bias currents often allow circuitry to be simplified.

The TL072 Op-Amp
The TL072 is one of the most popular op-amps, having very-high-impedance inputs, with effectively zero bias and offset currents. The JFET input devices give their best noise performance at medium impedances, in the range 1–10 kΩ.

The TL072 has a modest power consumption at typically 1.4 mA per op-amp section, which is significantly less than with the 5532. The slew rate is higher than for the 5532, at 13 V/µs against 9 V/µs. The TL072 is a dual op-amp. There is a single version called the TL071, which has offset null pins.

However, the TL072 is not THD free in the way the 5532 is. In audio usage, distortion depends primarily upon how heavily the output is loaded. The maximum loading is a trade-off between quality and circuit economy, and I would put 2 kΩ as the lower limit. This op-amp is not the first choice for audio use unless the near-zero bias currents (which allow circuit economies by making blocking capacitors unnecessary), the low price, or the modest power consumption are dominant factors.

It is a quirk of this device that the input common-mode range does not extend all the way between the rails. If the common-mode voltage gets to within a couple of volts of the V- rail, the op-amp suffers

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