Page 27 - Op Amps Design, Applications, and Troubleshooting

10 BASIC CONCEPTS OF THE INTEGRATED OPERATIONAL AMPLIFIER

You can obtain the Thevenin equivalent circuit by applying the following
sequential steps:

1. Short all voltage sources and open all current sources. (Replace all sources
with their internal impedance if it is known.) Also open the circuit at the
point of simplification.
2. Calculate the value of Thevenin's resistance as seen from the point of
simplification.
3. Replace the voltage and current sources with their original values and open
the circuit at the point of simplification.
4. Calculate Thevenin's voltage at the point of simplification.
5. Replace the original circuit components with the Thevenin equivalent for
subsequent analysis of the circuit beyond the point of simplification.

Consider, for example, the circuit in Figure 1.8. Here four different values of
R x are connected to the output of a voltage divider circuit. The value of loaded
voltage is to be calculated. Without a simplification theorem such as Thevenin's
Theorem, each resistor value would require several computations. Now let us
apply Thevenin's Theorem to the circuit.
First we will define the point of simplification to be the place where R x is
connected. This is shown in Figure 1.9(a). The voltage source is shorted and the

FIGURE 1.9 Thevenizing the circuit of Figure 1.8.