Page 71 - Op Amps Design, Applications, and Troubleshooting
P. 71
54 AMPLIFIERS
3. Frequency range
4. Load resistance
5. Maximum input voltage
As an example of the design process, let us design an inverting amplifier with the
following characteristics:
1. Voltage gain 12
2. Maximum input current 250 microamperes RMS
3. Frequency range 20 hertz to 2.5 kilohertz
4. Load resistance lOOkilohms
5. Maximum input voltage 500 millivolts RMS, 0-volt reference
Determine an Initial Value for R,. The minimum value for R, is determined
by the maximum input voltage and the maximum input current and is computed
with Ohm's Law as follows:
In this case, the calculations are
As a general rule, you should avoid designing amplifiers with input resis-
tances of less than 1000 ohms unless you have a specific need for them. In our
present case, the computed iriinimum (2.0 kilohms) is greater than 1000 ohrns, so
we will use the computed value. It should also be noted that the minimum input
impedance is often determined by the needs of the application.
Determine the Value of R& R f can be computed from the voltage gain equa-
tion, Equation (2.6):
Note that the inversion sign is omitted from the equation when computing a resis-
tance value. For the present example, we compute R F as follows:
