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.