Page 73 - Op Amps Design, Applications, and Troubleshooting

56 AMPLIFIERS

For our particular case, the minimum power supply voltages will be

Anything greater than ±11.48 volts for the DC supply will be adequate; therefore,
let us choose the standard values of ±15 volts for our application.

Determine the Required Slew Rate. The required slew rate of the op amp
is affected by the highest operating frequency and the maximum output voltage
swing, In our present case, the highest input frequency has been specified as 2.5
kilohertz. The maximum peak-to-peak output voltage swing (z; 0(max)) was previ-
ously computed as 16.96 volts. The minimum required slew rate for the op amp is
determined by rearranging Equation (2.11) to yield

Since the slew rate of the 741 exceeds this minimum value, we can continue
with our initial op amp selection. If the above calculation indicates a higher
requirement than our preliminary op amp selection can deliver, then another op
amp must be selected mat has a higher slew rate.

Calculate the Value of Compensation Resistor (R B)« The compensation
resistor (R B) reduces the error in the output voltage caused by the voltage drops
that result from the op amp's input bias currents. To achieve maximum error
reduction, we try to place equal resistances between both op amp input terminals
and ground. If we were to apply Thevenin's Theorem to the inverting input circuit,
we would see that resistors R F and Rj are effectively in parallel. This means that the
optimum value for JR B is simply the combined value of R F and Rj in parallel.

For the present example, we compute R B as follows: