Op amps in small-signal audio design - Part 1: Op amp history, properties

Relatively few discussions of op-amp behavior deal with non-linear distortion, perhaps because it is a complex business. Op-amp 'accuracy' is closely related, but the term is often applied only to DC operation. Accuracy here is often specified in terms of bits, so '20-bit accuracy' means errors not exceeding one part in 2²⁰, which is -120 dB or 0.0001%. Audio signal distortion is of course a dynamic phenomenon, very sensitive to frequency, and DC specs are of no use at all in estimating it.

Distortion is always expressed as a ratio, and can be quoted as a percentage, as number of decibels, or in parts per million (ppm). With the rise of digital processing, treating distortion as the quantization error arising from the use of a given number of bits has become more popular. Figure 4.2 hopefully provides a way of keeping perspective when dealing with these different metrics.

There are several different causes of distortion in op-amps. We will now examine them.

Op-Amp Internal Distortion
This is what might be called the basic distortion produced by the op-amp you have selected. Even if you scrupulously avoid clipping, slew-limiting, and common-mode issues, op-amps are not distortion free, though some types such as the 5532 and the LM4562 have very low levels. If distortion appears when the op-amp is run with shunt feedback, to prevent common-mode voltages on the inputs, and with very light output loading, then it is probably wholly internal and there is nothing to be done about it except pick a better op-amp.

If the distortion is higher than expected, the cause may be internal instability provoked by putting a capacitive load directly on the output, or neglecting the supply decoupling. The classic example of the latter effect is the 5532, which shows high distortion if there is not a capacitor across the supply rails close to the package; 100 nF is usually adequate. No actual HF oscillation is visible on the output with a general-purpose oscilloscope, so the problem may be instability in one of the intermediate gain stages.

Slew-Rate-Limiting Distortion
While this is essentially an overload condition, it is wholly the designer's responsibility. If users whack up the gain until the signal is within a hair of clipping, they should still be able to assume that slew limiting will never occur, even with aggressive material full of high frequencies.

Arranging this is not too much of a problem. If the rails are set at the usual maximum voltage, i.e. ±18 V, then the maximum possible signal amplitude is 12.7 Vrms, ignoring the saturation voltages of the output stage. To reproduce this level cleanly at 20 kHz requires a minimum slew rate of only 2.3 V/µs. Most op-amps can do much better than this, though with the OP27 (2.8 V/µs) you are sailing rather close to the wind. The old LM741 looks as though it would be quite unusable, as its very limited 0.5 V/µs slew rate allows a full output swing only up to 4.4 kHz.

Horrific as it may now appear, audio paths full of LM741s were quite common in the early 1970s. Entire mixers were built with no other active devices, and what complaints there were tended to be about noise rather than distortion. The reason for this is that full-level signals at 20 kHz simply do not occur in reality; the energy at the HF end of the audio spectrum is well known to be much lower than that at the bass end.

This assumes that slew limiting has an abrupt onset as level increases, rather like clipping. This is in general the case. As the input frequency rises and an op-amp gets closer to slew limiting, the input stage is working harder to supply the demands of the compensation capacitance. There is an absolute limit to the amount of current this stage can supply, and when you hit it the distortion shoots up, much as it does when you hit the supply rails and induce voltage clipping. Before you reach this point, the linearity may be degraded, but usually only slightly until you get close to the limit.

It is not normally necessary to keep big margins of safety when dealing with slew limiting. If you are employing the usual suspects of the audio op-amp world – the 5532 and TL072, with maximal slew rates of 9 and 13 V/µs respectively – you are most unlikely to suffer any slew-rate non-linearity.

Distortion Due to Loading
Output stage distortion is always worse with heavy output loading because the increased currents flowing exacerbate the gain changes in the Class-B output stage. These output stages are not individually trimmed for optimal quiescent conditions (as are audio power amplifiers) and so the crossover distortion produced by op-amps tends to be both higher and more variable between different specimens of the same chip. Distortion increases with loading in different ways for different op-amps. It may rise only at the high-frequency end (e.g. the OP2277) or there may be a general rise at all frequencies. Often both effects occur, as in the TL072.

The lowest load that a given op-amp can be allowed to drive is an important design decision. It will typically be a compromise between the distortion performance required and opposing factors such as number of op-amps in the circuit, cost of load-capable op-amps, and so on. It even affects noise performance, for the lower the load resistance an amplifier can drive, the lower the resistance values in the negative feedback can be, and hence the lower the Johnson noise they generate. There are limits to what can be done in noise reduction by this method, because Johnson noise is proportional to the square root of circuit resistance, and so improves only slowly as op-amp loading is increased.

Thermal Distortion
Thermal distortion is that caused by cyclic variation of the properties of the amplifier components due to the periodic release of heat in the output stage. The result is a rapid rise in distortion at low frequencies, which gets worse as the loading becomes heavier.

Those who have read my work on audio power amplifiers will be aware that I am highly sceptical – in fact totally sceptical – about the existence of thermal distortion in amplifiers built from discrete components [3]. The power devices are too massive to experience per-cycle parameter variations, and there is no direct thermal path from the output stage to the input devices. There is no rise, rapid or otherwise, in distortion at low frequencies in a properly designed discrete power amplifier.

The situation is quite different in op-amps, where the output transistors have much less thermal inertia and are also on the same substrate as the input devices. Nonetheless, op-amps do not normally suffer from thermal distortion; there is generally no rise in low-frequency distortion, even with heavy output loading. Integrated-circuit power amplifiers are another matter, and the much greater amounts of heat liberated on the substrate do appear to cause serious thermal distortion, rising at 12 dB/octave below 50 Hz. I have never seen anything resembling this in any normal op-amp.

Common-Mode Distortion
This is the general term for extra distortion that appears when there is a large signal voltage on both the op-amp inputs. The voltage difference between these two inputs will be very small, assuming the op-amp is in its linear region, but the common-mode (CM) voltage can be a large proportion of the available swing between the rails.

It appears to be by far the least understood mechanism, and gets little or no attention in op-amp textbooks, but it is actually one of the most important influences on op-amp distortion. It is simple to separate this effect from the basic forward-path distortion by comparing THD performance in series and shunt-feedback modes; this should be done at the same noise gain. The distortion is usually a good deal lower for the shunt-feedback case where there is no common-mode voltage. Bipolar and JFET input op-amps show different behavior and they are treated separately below.

Coming up in Part 2: Distortion in bipolar and JFET input op-amps.

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.

References
[1] A. Blumlein, UK patent 482,470, 1936.
[2] W. Jung (Ed.), Op-Amp Applications Handbook, Newnes, 2006 (Chapter 8).
[3] D. Self, Audio Power Amplifier Design Handbook, fifth ed, Focal Press, 2009, pp. 186–189.
[4] D. Self, Audio Power Amplifier Design Handbook, fifth ed, Focal Press, 2009, p. 96.
[5] W. Jung (Ed.), Op-Amp Applications Handbook, Newnes, 2006, p. 399 (Chapter 5).
[6] D. Self, Audio Power Amplifier Design Handbook, fifth ed, Focal Press, 2009, p. 380.

Related links:
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