Page 387 - Op Amps Design, Applications, and Troubleshooting

Adder 363

The results of our first simplification are shown in Figure 9.3(b). The Thevenin
equivalents computed above are shown inside the dotted box. We can reapply
Thevenin's Theorem to the remaining circuit to obtain our final simplified cir-
cuit.
First we see that we have series-aiding voltage sources for an effective volt-
age source equal to

The portion of this effective voltage that drops across R 3 can be found with the
voltage divider formula.

The resulting Thevenin voltage can now be found with Kirchhoff's Voltage Law.

V TH = -8 V + 10 V = +2 V

We don't really need the value of Thevenin resistance for the remainder of the
problem, but, in the name of completeness, we will compute it as

The fully simplified circuit is shown in Figure 9.3(c). Our equivalent circuit
reconnected to the amplifier portion of the circuit is shown in Figure 9.3(d). The
output voltage can be easily computed by applying our basic gain equation.

This value confirms the correct operation of our adder, which should provide the
algebraic sum of its inputs (i.e., +2 V + 12 V - 8 V = +6 V},

9.1,3 Practical Design Techniques
Let us now design a noninverting adder that will satisfy the following design
goals:

1. Accept four inputs -10 volts < sum <+10 volts
2. Minimum input impedance >6kilohms
3. Frequency range 0 to 1 kilohertz

Select the Value for RI and for R 3 to 1^. The design of the noninverting
adder circuit is very straightforward, since all resistor values are the same with the