Page 108 - Op Amps Design, Applications, and Troubleshooting

Inverting Summing Amplifier 91

The worst-case output will be

i? o(max) = i; Ol(max) + u O2(max) + v 03(max)
= (-1.75 V) + (-2 V) + (-8.48 V]
= -12.2 V

Unless the internal drop on the output of the selected op amp is unusually
high, we should be able to use standard ±15-volt supplies. Suppose, for example,
we decide to use a 741 op amp. The manufacturer's data sheet in Appendix 1 indi-
cates that the op amp can deliver at least ±12 volts to a load >10 kilohms when
±15-volt supplies are used. We will plan to use a 741 unless we encounter prob-
lems with bandwidth or slew rate (verified in subsequent sections).

Determine the Required Unify Gain Frequency. The minimum unity gain
frequency for each input is computed with Equation (2.22):

In all cases, the required minimum unity gain bandwidth is substantially below
the 1.0-megahertz limit of a 741. Therefore, we will initially plan to use a 741 in our
design. If the minimum bandwidth requirement were greater than 1.0 megahertz,
we would have to select a different op amp.

Determine the Required Slew Rote. The minimum acceptable slew rate for
the op amp is given by the following equation, Equation (2.11):

slew rate(min) = jf SRLv 0(max)

Let us determine the minimum slew rate for each input:

slew mte(mm) 1 = 3.14 x 2.7 kHz x 3.5 V = 0.03 VIpS

slew rate(min) 2 = DC input
slew nzte(min) 3 = 3.14 x 500 Hz x 16.97 V = 0.027 V/j/S

In all cases, the required slew rate is substantially below the 0.5-volts-per-
mkrosecond rating of the 741. Therefore, we will select this device as our final
choice.
The schematic of our design is shown in Figure 2.21. The actual performance
of the circuit is evident from the oscilloscope displays in Figure 2.22. The mea-
sured performance is contrasted with the original design goals in Table 2.4.