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

Technology News | May 29, 2011

By eeNews Europe

Audio design has for many years relied on a very small number of op-amp types; the TL072 and the 5532 dominated the audio small-signal scene for many years. The TL072, with its JFET inputs, was used wherever its negligible input bias currents and low cost were important.

For a long time the 5534/5532 was much more expensive than the TL072, so the latter was used wherever feasible in an audio system, despite its inferior noise, distortion, and load-driving capabilities. The 5534 was reserved for critical parts of the circuitry. Although it took many years, the price of the 5534 is now down to the point where you need a very good reason to choose any other type of op-amp for audio work.

The TL072 and the 5532 are dual op-amps; the single equivalents are TL071 and 5534. Dual op-amps are used almost universally, as the package containing two is usually cheaper than the package containing one, simply because it is more popular.

There are, however, other op-amps, some of which can be useful, and a selected range is covered here.

A Very Brief History of Op-Amps
The op-amp is today thought of as quintessentially a differential amplifier, responding to the difference of the input voltages while (hopefully) ignoring any common-mode component. The history of differential amplifiers goes back to that great man Alan Blumlein, and his 1936 patent [1] for a pair of valves with their cathodes connected to ground through a common resistor.

However, the first valve-based operational amplifiers, i.e. those intended to be capable of performing a mathematical operation, were in fact not differential at all, having only one input. That had to be an inverting input, of course, so you could apply negative feedback.

The first op-amp to get real exposure in the UK was the Fairchild uA709, designed by the renowned Bob Widlar and introduced in 1965. It was a rather awkward item that required quite complicated external compensation and was devoid of output short-circuit protection. One slip of the probe and an expensive IC was gone. It was prone to latch-up with high common-mode voltages and did not like capacitive loads. I for one found all this most discouraging, and gave up on the 709 pretty quickly. If you’re going to quit, do it early, I say.

The arrival of the LM741 was a considerable relief. To my mind, it was the first really practical op-amp, and it was suddenly possible to build quite complex circuitry with a good chance of it being stable, doing what it should do, and not blowing up at the first shadow of an excuse. I have given some details of it in this chapter for purely historical reasons. There is also an interesting example of how to apply the LM741 appropriately in Chapter 17.

The first IC op-amps opened up a huge new area of electronic applications, but after the initial enthusiasm for anything new, the audio market greeted these devices with less than enthusiasm. There were good reasons for this. The LM741 worked reliably; the snag with using it for audio was the leisurely slew rate of 0.5 V/µs, which made full output at 20 kHz impossible.

For a period of at least 5 years, roughly from 1972 to 1977, the only way to obtain good performance in a preamp was to stick with discrete transistor Class-A circuitry, and this became recognized as a mark of high quality. The advent of the TL072 and the 5532 changed this situation completely, but there is still marketing cachet to be gained from a discrete design. An excellent and detailed history of operational amplifiers can be found in Ref. [2].

Op-Amp Properties: Noise, Slew Rate, CM Range & Input Offset Voltage

There is no point in regurgitating manufacturers’ data sheets, especially since they are readily available on the internet. Here I have simply ranked the op-amps most commonly used for audio in order of voltage noise (Table 4.1).

TABLE 4.1 Op-amps ranked by voltage noise density (typical)

The great divide is between JFET input op-amps and BJT input op-amps. The JFET op-amps have more voltage noise but less current noise than bipolar input op-amps, the TL072 being particularly noisy. If you want the lowest voltage noise, it has to be a bipolar input. The difference, however, between a modern JFET-input op-amp such as the OPA2134 and the old faithful 5532 is only 4 dB, but the JFET part is a good deal more costly.

The bipolar AD797 seems to be out on its own here, but it is a specialized and expensive part. The LT1028 is not suitable for audio use for reasons described later. The LM741, which is included in this chapter for purely historical reasons, is omitted from Table 4.1 because there are no noise specs on its data sheets.

Op-amps with bias-cancellation circuitry are normally unsuitable for audio use due to the extra noise this creates. The amount depends on circuit impedances, and is not taken into account in Table 4.1. The general noise behavior of op-amps in circuits is dealt with in Chapter 1.

Op-Amp Properties: Slew Rate
Slew rates vary more than most parameters; a range of 100:1 is shown in Table 4.2. The slowest is the 741, which is the only type not fast enough to give full output over the audio band. There are faster ways to handle a signal, such as current-feedback architectures, but they usually fall down on linearity. In any case, a maximum slew rate greatly in excess of what is required appears to confer no benefits whatever.

TABLE 4.2 Op-amps ranked by slew rate (typical)

The 5532 slew rate is typically ±9 V/µs. This version is internally compensated for unity-gain stability, not least because there are no spare pins for compensation when you put two op-amps in an eight-pin dual package. The single-amp version, the 5534, can afford a couple of compensation pins, and so is made to be stable only for gains of 3× or more. The basic slew rate is therefore higher at ±13 V/µs.

Compared with power-amplifier specs, which often quote 100 V/µs or more, these speeds may appear rather sluggish. In fact they are not; even ±9 V/µs is more than fast enough.

Assume you are running your op-amp from±18V rails, and that it can give a±17V swing on its output. For most op-amps this is distinctly optimistic, but never mind. To produce a full-amplitude 20 kHz sine wave you only need 2.1 V/µs, so even in the worst case there is a safety margin of at least four times. Such signals do not of course occur in actual use, as opposed to testing. More information on slew limiting is given in the section on op-amp distortion.

Op-Amp Properties: Common-Mode Range
This is simply the range over which the inputs can be expected to work as proper differential inputs. It usually covers most of the range between the rail voltages, with one notable exception. The data sheet for the TL072 shows a common-mode (CM) range that looks a bit curtailed at -12 V. This bland figure hides the deadly trap this IC contains for the unwary.

Most op-amps, when they hit their CM limits, simply show some sort of clipping. The TL072, however, when it hits its negative limit, promptly inverts its phase, so your circuit either latches up, or shows nightmare clipping behavior with the output bouncing between the two supply rails. The positive CM limit is, in contrast, trouble-free. This behavior can be especially troublesome when TL072s are used in high-pass Sallen-and-Key filters.

Op-Amp Properties: Input Offset Voltage
A perfect op-amp would have its output at 0 V when the two inputs were exactly at the same voltage. Real op-amps are not perfect and a small voltage difference – usually a few millivolts – is required to zero the output. These voltages are large enough to cause switches to click and pots to rustle, and DC blocking is often required to keep them in their place. This issue is examined in depth in Chapter 11.

The typical offset voltage for the 5532A is ±0.5 mV typical, ±4 mV maximum at 25°C; the 5534A has the same typical spec but a lower maximum at ±2 mV. The input offset voltage of the new LM4562 is only ±0.1 mV typical, ±4 mV maximum at 25°C.

Op-Amp Properties: Distortion

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.

Figure 4.2: The relation between different ways of quoting THD: decibels, percentages, bit accuracy, and parts per million

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.