Page 407 - Op Amps Design, Applications, and Troubleshooting

CHAPTER TEN

Nonideal Op Amp Characteristics

For purposes of analysis and design in the preceding chapters, we considered many
of the op amp parameters to be ideal. For example, we generally assumed the input
bias current to be 0, we frequently ignored output resistance, and we disregarded
any effects caused by drift or offset voltage. This approach not only greatly simpli-
fies the analysis and design techniques, but is a practical method for many situa-
tions. Nevertheless, more demanding applications require that we acknowledge the
existence of certain nonideal op amp characteristics. This chapter will describe
many of these additional considerations.

10.1 NONIDEAL DC CHARACTERISTICS

We will classify the nonideal characteristics of op amps into two general cate-
gories: DC and AC. Let us first consider the effects of nonideal DC characteristics.

10.1.1 Input Bias Current
As briefly noted in Chapter 1, the first stage of an op amp is a differential amplifier.
Figure 10.1 shows a representative circuit that could serve as an op amp input stage.
Clearly, the currents that flow into or out of the inverting (-) and noninverting (+) op
amp terminals are actually base current for the internal transistors. So, for proper
operation, we must always insure that both inputs have a DC path to ground. They
cannot be left floating, and they cannot have series capacitors. These currents are
very small (ideally 0), but may cause undesired effects in some applications.
Figure 10.2 can be used to show the effect of nonideal bias currents. It illus-
trates a basic op amp configured as either an inverting or a noninverting amplifier
with the input signal removed (i.e., input shorted to ground). The direction of cur-
rent flow for the bias currents and the resulting output voltage polarities are
essentially arbitrary, since different op amps have different directions of current
flow. However, for a given op amp, both currents will flow in the same direction

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