Page 103 - Op Amps Design, Applications, and Troubleshooting
P. 103
86 AMPLIFIERS
Finally we compute the estimated value of output resistance, from Equation (2,35):
Since this value was computed at the highest input frequency (worst case), and
since it is very low compared to the value of the load resistor, its effects on output
voltage can be safely ignored.
Output Current Capability. The maximum value of load current occurs
when the output reaches its highest instantaneous value. The maximum voltage
was previously computed as 12.2 volts. The worst-case load current can be com-
puted with Ohm's Law:
The output of the op amp must also supply the feedback current. In most
applications, this current can be ignored because it is generally much smaEer than
the load current. Our present circuit is no exception. That is, we can see by inspec-
tion that the feedback path has over 10 times as much resistance as the load.
The data sheet in Appendix 1 indicates that even under worst-case condi-
tions, the output can maintain at least 10 volts across a 2000-ohm load. By Ohm's
Law, we can conclude that this corresponds to an output current of
Of course, the typical value of current is even higher. In any case, the current capa-
bility of the output clearly exceeds our requirements and therefore poses no prob-
lem. If our load resistor were smaller, we could anticipate a reduced output voltage.
Bandwidth. For a meaningful discussion on bandwidth, we must consider the
response of each input individually. When the responses are considered separately,
we can estimate the bandwidth of any given input by applying the bandwidth
equation, Equation (2.22), used in previous analyses:
