Page 28 - Op Amps Design, Applications, and Troubleshooting

Review of Important Basic Concepts 11

circuit is opened at the point of simplification in Figure 1.9(b). We can now calcu-
late the Thevenin resistance (RTH)- By inspection, we can see that the 5-kilohm and
the 20-kilohm resistors are now in parallel. Thus the Thevenin resistance is found
in this case by the parallel resistor equation:

In this particular case,

Next we determine the Thevenin voltage by replacing the sources (step 3).
This is shown in Figure 1.9(c). The voltage divider equation, Equation (1.7), is
used in this case to give us the value of Thevenin's voltage.

Figure 1.9(d) shows the Thevenin equivalent circuit. Calculations for each of
the values of R x can now be quickly computed by simply applying the voltage
divider equation. The value of Thevenin's Theorem would be even more obvious
if the original circuit were more complex.
The preceding discussion was centered on resistive DC circuits. The techniques
described, however, apply equally well to AC circuits with inductive and/or capaci-
tive components. The voltages and impedances are simply expressed in their com-
plex form (e.g., 5 + j7 would represent a 5-ohm resistance and a 7-ohm reactance).

1.2.5 Norton's Theorem
Norton's Theorem is similar to Thevenin's Theorem in that it produces an equiva-
lent, simplified circuit. The major difference is that the equivalent circuit is com-