Page 125 - Op Amps Design, Applications, and Troubleshooting

108 AMPLIFIERS

will have a major effect on the final input impedance, as RI will be close to the
same value as R/, and it is Rj that determines the input impedance of the amplifier.
Thus, in order to meet the requirements for an input impedance of at least 3000
ohms, we must choose a value for Rj that is larger than 3000 ohms,

Determine the Value of R F. R F can be computed from the voltage gain equa-
tion shown in Equation (2.28):

.tYf ~ AVJ \J t.y x/

For the present design example, we compute Rp as follows:

We select the nearest standard value of 75 kilohms to use as R F.

Determine the Required Unity Gain Frequency. We can compute the mini-
mum unity gain frequency for our op amp with Equation (2.22):

where/ UG is the minimum required unity gain frequency for the op amp and bw is
the highest operating frequency. Thus, we must select an op amp that has a mini-
mum unity gain frequency of at least 180 kilohertz. Because the 741 has a 1.0-
megahertz unity gain frequency, it should be fine for our purposes. Now let us
determine the slew-rate requirement.

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

slew mte(mm) - ff SRLv 0(max)

In our case, let us assume that we want to deliver a full output swing (±13 V) at the
highest frequency (15 kHz). The minimum slew rate is computed as follows:

slew rate(min) = 3.14 x 15 kHz x 26 V = 1.2 V/jjs
The 741 has a slew rate of 0.5 volts per microsecond, so it will not be adequate for
this application. There are many alternatives, but let us choose the MC1741SC op
amp (Appendix 4). It has a unity gain frequency similar to that of the 741, but it
offers a slew rate of 10 volts per microsecond.

Select RI. R! is chosen to be the same value as the parallel combination of the
feedback resistor (R F) and the input resistor (R 7). This following value is computed: