Page 42 - Op Amps Design, Applications, and Troubleshooting

Circuit Construction Requirements 25

1.4.10 Troubleshooting Tips
At this point in our study of op amps, there is little difference between ideal and
nonideal devices. You will recall from Section 1.3.9 that the following items are
necessary for proper operation of the op amp:

+
1. There should be positive 15 volts DC on the V connection with respect to
ground.
2. There should be negative 15 volts DC on the V~ connection with respect to
ground.
3. As long as the differential input voltage is greater than zero, the output
should be at either of two extreme voltages (approximately ±15 volts).

Items 1 and 2 are essential checks regardless of the circuit being evaluated
and whether it is viewed as ideal or nonideal. In the case of Item 3, we can now
refine our expectations of normal operation. We saw from Figure 1.2 that the out-
put of an op amp has two transistor/resistor pairs between the output and the
±V CC connections. As you know, when current flows through these components a
portion of the supply voltage is dropped. For most bipolar op amps, the internal
voltage drop is approximately 2 volts regardless of the polarity of the output
voltage. Thus, if the output of an op amp was forced to its positive extreme and
was being operated from a ±15 volt supply, we would expect the output to mea-
sure approximately +13 volts. This is called the positive saturation voltage (+V SAT).
Similarly, the negative extreme of the output is called the negative saturation volt-
age (-V SAT) and is about 2 volts above (i.e., less negative than) the negative power
supply voltage.

1.5 CIRCUIT CONSTRUCTION REQUIREMENTS

This is one of the most important sections in the text and yet the most likely to be
skipped or skimmed. Every technician and engineer believes he or she knows
how to build circuits. Perhaps you do know how to build circuits, but you are
urged to study the following sections anyway. It is difficult to convince people of
the value of many of the techniques discussed. The reason for the lack of accep-
tance is that many of the techniques can be skipped or slighted without any
observable deterioration in circuit performance in many cases. But it is equally
true that some of the most elusive problems experienced when building and test-
ing circuits are a direct result of poor, or at least inappropriate, circuit construc-
tion techniques. So, to repeat, you are urged to apply the following techniques on
a consistent basis whether or not there appears to be an observable change in per-
formance,

1.5.1 Prototyping Methods
There are numerous ways to construct a circuit for purposes of testing prior to
committing the design to a printed circuit board. The techniques and precautions