Page 66 - Op Amps Design, Applications, and Troubleshooting

Inverting Amplifier 49

rent is increased (load resistance decreased), the actual output voltage begins to
drop as shown in the previous section. Finally, if the load resistance is reduced all
the way to 0 ohms, the output current will be limited to a safe value. This value
can be found in the data sheet (Appendix 1), and is 20 milliamps for the 741
device.
As the load resistance varies from infinity (open) to zero (short), the output
current from the op amp varies from i F to 20 milliamps. The limiting factor is the
amount of reduction that can be tolerated on the output voltage.
The amount of current (i F) flowing through the feedback resistor is easily
computed with Ohm's Law as

On an unprotected op amp, the value of load current plus the value of feedback
current must be kept below the stated output current rating. If this value is not
supplied in the data sheet, then it can be estimated by using the maximum power
dissipation data; recall that power = voltage x current.

Minimum Value of Load Resistance. The minimum value of load resis-
tance is determined by the maximum value of output current (determined in the
previous section). The actual computation is essentially Ohm's Law:

where i L is the maximum allowable output current of the op amp minus the cur-
rent (if) flowing through the feedback circuit, and V L is the minimum acceptable
output voltage.
Note that in many, if not most, applications, the value of output current
needed for the load is substantially below the limiting value, so no significant
loading occurs.
Let us assume that the application shown in Figure 2.3 requires us to have at
least 1.19 volts across the load when 100 millivolts is applied to the input terminal.
Let us further assume that the frequency of interest is 5 kilohertz. From previous
calculations we know that the voltage gain (A v) is 12.2 (ignoring the effects of
bandwidth described in the next section) and that the output resistance at 5 kilo-
hertz is 4.96 ohms.
Figure 2.5 shows the equivalent circuit at this point. The value of i 0 can be
computed with Ohm's Law.