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

Technology News | August 29, 2011

By eeNews Europe

[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 4.34: Distortion versus frequency at two output levels for the TL052CP, with no load. Series feedback

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.

The OPA2134 and OPA604 Op-Amps

The OPA2134 Op-Amp

The OPA2134 is a Burr-Brown product, the dual version of the OPA134. The manufacturer claims it has superior sound quality, due to its JFET input stage. Regrettably, but not surprisingly, no evidence is given to back up this assertion.

The input noise voltage is 8 nV/√Hz, almost twice that of the 5532. The slew rate is typically ±20 V/µs, which is ample. The OPA2134 does not appear to be optimized for DC precision, the typical offset voltage being ±1 mV, but this is usually good enough for audio work. I have used it many times as a DC servo in power amplifiers, the low bias currents allowing high resistor values and correspondingly small capacitors.

The OPA2134 does not show phase reversal anywhere in the common-mode range, which immediately marks it as superior to the TL072.

The two THD plots in Figures 4.36 and 4.37 show the device working at a gain of 3× in both shunt and series-feedback modes. It is obvious that a problem emerges in the series plot, where the THD is higher by about three times at 5 Vrms and 10 kHz. This distortion increases with level, which immediately suggests common-mode distortion in the input stage. Distortion increases with even moderate loading, see Figure 4.38.

Figure 4.36: The OPA2134 working in shunt-feedback mode. The THD is below the noise until frequency reaches 10 kHz; it appears to be lower at 5 Vrms simply because the noise floor is relatively lower.

Figure 4.37: The OPA2134 in series-feedback mode. Note much higher distortion at HF

Figure 4.38: The OPA2134 in shunt-feedback mode (to remove input CM distortion) and with varying loads on the output. As usual, more loading makes linearity worse. Output 5 Vrms, gain = 3.33×

This is a relatively modern and sophisticated op-amp. When you need JFET inputs (usually because significant input bias currents would be a problem) this definitely beats the TL072; it is, however, four to five times more expensive.

The OPA604 Op-Amp
The OPA604 from Burr-Brown is a single JFET-input op-amp, which claims to be specially designed to give low distortion. The simplified internal circuit diagram in the data sheet includes an enigmatic box intriguingly labeled ‘Distortion Rejection Circuitry’. This apparently ‘linearizes the open-loop response and increases voltage gain’, but no details as to how are given; whatever is in there appears to have been patented so it ought to be possible to track it down.

However, despite this, the distortion is not very low even with no load (see Figure 4.39), and is markedly inferior to the 5532’s. The OPA604 is not optimized for DC precision, the typical offset voltage being ±1mV. The OPA2604 is the dual version, which omits the offset null pins.

Figure 4.39: An OPA2604 driving various loads at 7.75 Vrms. Series feedback, gain = 3.23×

The data sheet includes a discussion that attempts to show that JFET inputs produce a more pleasant type of distortion than BJT inputs. This unaccountably omits the fact that the much higher transconductance of BJTs means that they can be linearized by emitter degeneration so that they produce far less distortion of whatever type than a JFET input [6]. Given that the OPA604 costs five times as much as a 5532, it is not very clear underwhat circumstances this op-amp would be a good choice.

The OPA627 Op-Amp

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 4.40: OPA627 driving the usual loads at 5 Vrms. Series feedback, gain = 3.23×. ‘Gen-Mon’ is the test-gear output

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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