Op-Amp Properties: Bias Current & Cost
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 Rcompen, 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 Rcompen 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.