Page 86 - Op Amps Design, Applications, and Troubleshooting

Noninverting Amplifier 69

Thus, we must select an op amp that has minimum unity gain frequency of at least
40.1 kilohertz. Since the 741 has a 1.0-megahertz unity gain frequency, it should be
adequate for this application with respect to bandwidth.

Determine the Minimum Supply Voltages. The minimum supply voltages
are computed simply by ensuring that the maximum expected output voltage
swing is no greater than the ±V$AT values. The maximum output swing can be
found by using the basic equation for voltage gain, Equation (2.1).

In our particular example, the maximum output voltage will be

Notice the multiplying factor 1.414 to convert our input voltage (given in
RMS) to a peak or worst-case value. The manufacturer's data sheet in Appendix 1
indicates that the 741 op amp will produce at least a ±12-volt output swing with a
±15-volt supply voltage and a load resistance of at least 10 kilohms. Thus, we can
infer that we have a worst-case internal voltage drop of 15 - 12, or 3 volts. The
minimum power supply voltage can be determined with Equation (2.25):

Anything greater than ±12.05 volts for the DC supply will be adequate, so we choose
the standard values of ±15 volts for our application. Realize that this is a worst-case
calculation; a more typical internal drop would be 2 volts rather than 3 volts.

Determine the Required Stew Rate. The minimum slew rate for the op
amp is computed by transposing Equation (2.11).