AN-29| Application Note

IC Op Amp Beats FETs on Input Current

IC Op Amp Beats FETs on Input Current
Note: National Semiconductor recommends replacing 2N2920 and 2N3728 matched pairs with LM394 in all application circuits.

National Semiconductor Application Note 29 Robert J. Widlar September 2002

mance is often limited by leakages in capacitors, diodes, analog switches or printed circuit boards, rather than by the op amp itself.

Abstract
A monolithic operational amplifier having input error currents in the order of 100 pA over a -55 C to 125 C temperature range is described. Instead of FETs, the circuit used bipolar transistors with current gains of 5000 so that offset voltage and drift are not degraded. A power consumption of 1 mW at low voltage is also featured. A number of novel circuits that make use of the low current characteristics of the amplifier are given. Further, special design techniques required to take advantage of these low currents are explored. Component selection and the treatment of printed circuit boards is also covered.

Introduction
A year ago, one of the loudest complaints heard about IC op amps was that their input currents were too high. This is no longer the case. Today ICs can provide the ultimate in performance for many applications -- even surpassing FET amplifiers. FET input stages have long been considered the best way to get low input currents in an op amp. Low-picoamp input currents can in fact be obtained at room temperature. However, this current, which is the leakage current of the gate junction, doubles every 10 C, so performance is severely degraded at high temperatures. Another disadvantage is that it is difficult to match FETs closely.1 Unless expensive selection and trimming techniques are used, typical offset voltages of 50 mV and drifts of 50 uV/ C must be tolerated. Super gain transistors2 are now challenging FETs. These devices are standard bipolar transistors which have been diffused for extremely high current gains. Typically, current gains of 5000 can be obtained at 1 uA collector currents. This makes it possib
le to get input currents which are competitive with FETs. It is also possible to operate these transistors at zero collector base voltage, eliminating the leakage currents that plague the FET. Hence they can provide lower error currents at elevated temperatures. As a bonus, super gain transistors match much better than FETs with typical offset voltages of 1 mV and drifts of 3 uV/ C. Figure 1 compares the typical input offset currents of IC op amps and FET amplifiers. Although FETs give superior performance at room temperature, their advantage is rapidly lost as temperature increases. Still, they are clearly better than early IC amplifiers like the LM709.3 Improved devices, like the LM101A,4 equal FET performance over a -55 C to 125 C temperature range. Yet they use standard transistors in the input stage. Super gain transistors can provide more than an order of magnitude improvement over the LM101A. The LM108 uses these to equal FET performance over a 0 C to 70 C temperature range. In applications involving
125 C operation, the LM108 is about two orders of magnitude better than FETs. In fact, unless special precautions are taken, overall circuit perfor00687501

FIGURE 1. Comparing IC op Amps with FET-Input Amplifier

Effects of Error Current
In an operational amplifier, the input current produces a voltage drop across the source resistance, causing a dc error. This effect can be minimized by operating the amplifier with equal resistances on the two inputs.5 The error is then proportional to the difference in the two input currents, or the offset current. Since the current gains of monolithic transistors tend to match well, the offset current is typically a factor of ten less than the input currents.

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FIGURE 2. Illustrating the Effect of Source Resistance on Typical Input Error Voltage Naturally, error current has the greatest effect in high impedance circuitry. Figure 2 illustrates this point. The offset voltage of the LM709 is degraded significantly with source resistances greater than 10 k. With the LM101A this is

AN-29

2002 National Semiconductor Corporation

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