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

[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 phase reversal and the inputs swap their polarities. There may be really horrible clipping, where the output hits the bottom rail and then shoots up to hit the top one, or the stage may simply latch up until the power is turned off.

TL072s are relatively relaxed about supply-rail decoupling, though they will sometimes show very visible oscillation if they are at the end of long thin supply tracks. One or two rail-to-rail decoupling capacitors (e.g. 100 nF) per few centimeters is usually sufficient to deal with this, but normal practice is to not take chances, and allow one capacitor per package as with other op-amps.

Because of common-mode distortion, a TL072 in shunt configuration is always more linear. In particular compare the results for 3k3 load in Figures 4.32 and 4.33. At heavier loadings the difference is barely visible because most of the distortion is coming from the output stage.

Figure 4.32: Distortion versus loading for the TL072, with various loads. Shunt-feedback configuration eliminates CM input distortion. Output level 3 Vrms, gain 3.23×, rails ±15 V. No output load except for the feedback resistor. The no-load plot is indistinguishable from that of the testgear alone. Distortion always gets worse as the loading increases. This factor, together with the closed-loop NFB factor, determines the THD

Figure 4.33: Distortion versus loading for the TL072, with various loads. Series-feedback configuration, output level 3 Vrms, gain 3.23×, rails ±15 V. Distortion at 10 kHz with no load is 0.0015% compared with 0.0010% for the shunt configuration. This is due to the 1 Vrms CM signal on the inputs

TL072/71 op-amps are prone to HF oscillation if faced with significant capacitance to ground on the output pin; this is particularly likely when they are used as unity-gain buffers with 100% feedback. A few inches of track can sometimes be enough. This can be cured by an isolating resistor, in the 47–75 Ω range, in series with the output, placed at the op-amp end of the track.

The TL052 Op-Amp
The TL052 from Texas Instruments was designed to be an enhancement of the TL072, and so is naturally compared with it. Most of the improvements are in the DC specifications. The offset voltage is 0.65 mV typical, 1.5 mV max, compared with the TL072's 3 mV typical, 10 mV max. It has half the bias current of the TL072. This is very praiseworthy, but rarely of much relevance to audio.

The distortion, however, is important, and this is worse rather than better. THD performance is rather disappointing. The unloaded THD is low, as shown in Figure 4.34, in series-feedback mode. As usual, practical distortion depends very much on how heavily the output is loaded. Figure 4.35 shows that it deteriorates badly for loads of less than 4k7.

Figure 4.35: Distortion of the TL052 at 5 Vrms output with various loads. At 1 kΩ and 2k2 loading the residual is all crossover distortion at 1 kHz. Gain 3.23, non-inverting (series feedback)

The slew rate is higher than for the TL072 (18 against 13 V/µs) but the lower figure is more than adequate for a full-range output at 20 kHz, so this enhancement is of limited interest. The power consumption is higher, typically 2.3 mA per op-amp section, which is almost twice that of the TL072. Like the TL072, the TL052 is relatively relaxed about supply-rail decoupling. At the time of writing (2009) the TL052 costs at least twice as much as the TL072.