Page 354 - Op Amps Design, Applications, and Troubleshooting

332 SIGNAL PROCESSING CIRCUITS

impedance is established by JR.!. In practice, the actual minimum impedance may
never go as low as that but it is a good approximation for worst-case analysis,
The output impedance increases with frequency, but can still be estimated as
described in Chapter 2 for inverting amplifiers.

7.7*3 Practical Design Techniques
We will now design a differentiator circuit that will satisfy the following design
goals:

1. Input waveform Triangle (dual ramp)
2. Input voltage ±2 volts
3. Input frequency 2kilohertz
4. Output voltage ±10 volts for the given input signal
5. Op amp 741

Compute i?2. Resistor K 2 is selected to establish the basic range of operation. A
good rule of thumb for the initial selection of R 2 is given by Equation (7.18),

where p+(max) is the highest expected output voltage and I sc is the short-circuit
current rating of the op amp. For our present design, we compute R 2 as

We will select the nearest standard value of 12 kilohms.
Compute Ci. The time constant for R 2Q is determined by the expected rate of
change of input voltage as compared to the resulting output voltage. In equation
form, we can compute the value of Q as

Utilization of this equation requires us to know the rate of change of input voltage.
The design specifications tell us that we will have a ramp voltage that goes from
-2 volts to +2 volts and back at a frequency of 2 Mohertz. Thus, the At?j is 4 volts
(i.e., -2V to +2V), and the At is one half of the period (t) of one input cycle. That is,