Page 40 - Op Amps Design, Applications, and Troubleshooting

Introduction to Practical Op Amps 23

1.4.6 Output Impedance
The output impedance of an ideal op amp is 0. This means that regardless of the
amount of current drawn by an external load, the output voltage of the op amp
remains unaffected. That is, no loading occurs.
In the case of a practical op amp, there is some amount of output impedance.
The ideal output voltage is divided between this internal resistance and any exter-
nal load resistance. Generally this is an undesired effect so we prefer the op amp to
have a very low output impedance.
The manufacturer's specification sheet in Appendix 1 lists the typical value
of output resistance for a 741 as 75 ohms. What is not clear from the data sheet is
that this value refers to open-loop output resistance. In most practical applica-
tions, the op amp is provided with feedback (i.e., closed-loop). Under these condi-
tions, the effective output impedance can be dramatically reduced with values as
low as Mooth of the open-loop output impedance being reasonable.

1.4.7 Temperature Effects
Although we want an ideal op amp to be unaffected by temperature, some effects
are inevitable since the op amp is constructed from semiconductor material that
has temperature-dependent characteristics. In a practical op amp, nearly every
parameter is affected to some degree by temperature variations. Whether the
changes in a particular characteristic are important to us depends on the applica-
tion being considered and the nature of the operating environment. We will exam-
ine methods for minimizing the effects of temperature problems as we progress
through the remainder of the text.

1.4.8 Noise Generation
Under ideal conditions, an amplifying or signal-processing circuit should have no
signal voltages at the output that do not have corresponding signal voltages at the
input. When the circuit has additional fluctuations in the output we call these
changes noise.
There are many sources of electrical noise generation inside of the op amp. A
detailed analysis of the contribution of each source to the total circuit noise is a
complex subject and well beyond the goal of this text. We will, however, examine
techniques that can be used to minimize problems with noise. It is fortunate that
noise problems are most prevalent in circuits operating under low-signal condi-
tions. Most other circuits do not require a detailed analysis of the circuit noise and
can be adequately controlled by applying some basic guidelines and precautions
for minimizing noise.
We can get an appreciation for the noise generated in a 741 by examining
the data supplied by the manufacturer and shown in Appendix 1. Several
graphs describe the noise performance of the 741. The op amp noise is effectively
added to the desired signal at the input of the op amp. If the input signal is small
or even comparable in amplitude to the total op amp noise, then the noise volt-
ages will likely cause erroneous operation. On the other hand, if the desired sig-
nal is much larger than the noise signal, then the noise can be ignored for many
applications.