Page 402 - Op Amps Design, Applications, and Troubleshooting

378 ARITHMETIC FUNCTION CIRCUITS

FIGURE 9.14 A sign changing circuit
allows selectable inversion.

9.5.2 Numerical Analysis
The numerical analysis for this circuit must be done in two stages (one for each
position of the switch). Each of these calculations is literally identical to the calcu-
lations presented in Chapter 2 for inverting and noninverting amplifiers, so they
are not repeated here.

9.5.3 Practical Design Techniques

Let us now design a simple sign changer circuit that satisfies the following design
requirements:

1. Input impedance >47 kilohms
2. Frequency range 0-8.5 kilohertz

Compute JRi and R*. The value of Rj is established by the input impedance
requirement. Its value is computed with Equation (2.7).

Let us choose a standard value of 100 kilohms. Resistor R 2 will be the same value
as RI, since we want the circuit to have a voltage gain of -1.

Compute R 3. The value of R 3 should be equal to the value of Rj so that we can
maintain a constant input impedance for both modes. Therefore,

It should be noted that this selection does not minimize the effects of input bias
current. The ideal value for JR 3 is different for the two modes if we want to mini-
mize those effects.

Select the Op Amp. We will select the op amp on the basis of bandwidth and
slew rate. The minimum unity gain frequency is found as

This value should be doubled (ie., 17 kHz) for critical applications. The minimum
required slew rate (based on a full output swing) is calculated with Equation (2.11).