By Douglas Self 0

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]. Partnered Content: NGK contributing to the speard of IoT devices through new ceramic Li-ion rechargeable batteries

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.

Bipolar-input op-amps not only have larger noise currents than their JFET equivalents; they also have much larger bias currents. These are the base currents taken by the input transistors. This current is much larger than the input offset current, which is the difference between the bias current for the two inputs. For example, the 5532A has a typical bias current of 200 nA, compared with a much smaller input offset current of 10 nA. The LM4562 has a lower bias current of 10 nA typical, 72 nA maximum. In the case of the 5532/4 the bias current flows into the input pins as the input transistors are NPN. Bias currents are a considerable nuisance; when they flow through variable resistors they make them noisy when moved. They will also cause significant DC offsets when they flow through high-value resistors. It is often recommended that the effect of bias currents can be canceled out by making the resistance seen by each op-amp input equal. Figure 4.1(a) shows a shunt-feedback stage with a 22 kΩ feedback resistor. When 200 nA flows through this it will generate a DC offset of 4.4 mV, which is rather more than we would expect from the input offset voltage error. If an extra resistance R compen , of the same value as the feedback resistor, is inserted into the non-inverting input circuit then the offset will be canceled. This strategy works well and is done almost automatically by many designers. However, there is a snag. The resistance R compen generates extra Johnson noise, and to prevent this it is necessary to shunt the resistance with a capacitor, as in Figure 4.1(b). Figure 4.1: Compensating for bias-current errors in a shunt-feedback stage. The compensating resistor must be bypassed by a capacitor C2 to prevent it adding Johnson noise to the stage This extra component costs money and takes up PCB space, so it is questionable if this technique is actually very useful for audio work. It is usually more economical to allow offsets to accumulate in a chain of op-amps, and then remove the DC voltage with a single output blocking capacitor. This assumes that there are no stages with a large DC gain, and that the offsets are not large enough to significantly reduce the available voltage swing. Care must also be taken if controls are involved, because even a small DC voltage across a potentiometer will cause it to become crackly, especially as it wears. FET input op-amps have very low bias current at room temperature; however, it doubles for every 10°C rise. This is pretty unlikely to cause trouble in most audio applications, but a combination of high internal temperatures and high-value pots could lead to some unexpected crackling noises. Op-Amp Properties: Cost

While it may not appear on the data sheet, the price of an op-amp is obviously a major factor in deciding whether or not to use it. Table 4.3 was derived from the averaged prices for 1+ and 25+ quantities across a number of UK distributors. At the time of writing (September 2009) the cheapest popular op-amps are the TL072 and the 5532, and these happened to come out at exactly the same price, so their price is taken as unity and used as the basis for the price ratios given. TABLE 4.3 Op-amps ranked by price (2009) relative to 5532 and TL072 Table 4.3 was compiled using prices for DIL packaging and the cheapest variant of each type. Price is per package and not per op-amp section. It is obviously only a rough guide. Purchasing in large quantities or in different countries may change the rankings somewhat (even going from 1+ to 25+ causes some changes) but the basic look of things will not alter too much. One thing is obvious – the 5532 is one of the great op-amp bargains of all time.