Page 399 - Op Amps Design, Applications, and Troubleshooting

Absolute Value Circuit 375

For negative input voltages, the output is equal in amplitude but opposite in
polarity. This, coupled with our previous analysis for positive input voltages,
means that the output voltage is positive for either polarity of input voltage and
equal in amplitude to the input voltage. Of course, this is the proper behavior for
an absolute value circuit where V^ = I Vj I.
The input impedance, output impedance, frequency response, and so on, are
computed in the same manner as are similar circuits previously discussed in
detail. These calculations are not repeated here,

9.4.3 Practical Design Techniques
Since all resistor values are the same in the absolute value circuit, the calculations
for design are fairly straightforward. Let us design an absolute value circuit that
will perform according to the following design goals:

1. Input voltage -10 volts < V } < 10 volts
2. Input impedance >18kilohms
3. Frequency range 0 to 100 kilohertz
Select the Value for R. The minimum value for all of the resistors is deter-
mined by the required input impedance. The maximum value is limited by the
nonideal characteristics of the circuit (refer to Chapter 10), but is generally below
100 kilohms. The minimum value for R can be found by applying Equation (2.7).

R = input impedance = 18 kQ.

Let us choose to use 22-kilohm resistors for our design.

Select the Op Amps. As usual, slew rate and small signal bandwidth are used
as the basis for op amp selection. The required unity gain frequency for A 2 can be
computed with Equation (2.22).

The bandwidth for AI will be somewhat lower because it has a voltage gain of ~1.
But in either case, the standard 741 will be more than adequate with regard to
bandwidth.
The required slew rate for either amplifier can be computed with Equation
(2.11). The voltage and frequency limits are stated in the design goals.

slew rate = jijV o(max) = 3.14 x 100 kHz x 20 V = 6.28 V/^Jts