Page 24 - Op Amps Design, Applications, and Troubleshooting

Review of important Basic Concepts 7

eral examples. In each case, the current entering and leaving a given point is the
same. This law is generally stated mathematically in the form of

where J T is the total current leaving a point (for instance) and I l7 I 2, and so on, are
the various currents entering the point. In the case of Figure 1.4(c), we can apply
Equation (1.4) as

Here again, though, it is important for you to strive to develop an intuitive, non-
mathematic appreciation for what the law is telling you.
Consider the examples in Figure 1.5. Without using your calculator, can you
estimate the effect on the voltage drop across Rj when resistor R 3 opens? Try it
before reading the next paragraph.
In the first case, Figure 1.5(a), your reasoning might be like this. Since the
open resistor (JR 3) was initially very small compared to parallel resistor R 2, it will
have a dramatic effect on total current when it opens. That is, Kkchhofif's Current
Law would tell us that the total current (Zj) is composed of Z 2 and I 3. Since K 3 was
initially much smaller than R 2, its current will be much greater (Ohm's Law).
Therefore, when R 3 opens, the major component of current Z a will drop to zero. This
reduced value of current through R l will greatly reduce the voltage drop across Rj.
In the second case, Figure 1.5(b), R 3 is much larger than the parallel resistor
R 2 and therefore contributes very little to the total current 1^ When R 3 opens, there
will be no significant change in the voltage across Rj.
Again be reminded of the value of a solid intuitive view of electronic circuits.

FIGURE 1.4 Examples of Kirchhoff's Current Law illustrate that the
current entering a point must equal the current leaving that same point.