Page 82 - Op Amps Design, Applications, and Troubleshooting
P. 82
Noninverting Amplifier 65
stated in the manufacturer's data sheets. The value of effective output impedance
can be approximated by applying Equation (2.15).
where A OL is the open-loop gain of the op amp at a particular frequency. For the
circuit in Figure 2.12, the open-loop gain at 2500 hertz is computed with Equation
(2.16) as
The output impedance at 2500 hertz can then be estimated as
In most cases, the output impedance is so low relative to the value of load
resistance that the output voltage is essentially unaffected, but you can always be
sure by performing the voltage divider calculation outlined in Section 2.2.2.
Output Current Capability. If the output of the op amp is short-circuit pro-
tected (as in the 741), then the output current capability is limited by the maxi-
mum allowable drop in output voltage for the given application. This can be
estimated with Ohm's Law as discussed in the preceding section. Recall from our
discussion of inverting amplifiers that the output must supply both load resistor
current and the current through the feedback resistor. The feedback current is
computed using Ohm's Law. For this particular circuit, the calculations are
With no output current being supplied to the load, the output voltage stays
at the expected v 0 level and the total output current is equal to i f. As the load cur-
rent is increased (load resistance is decreased), the actual output voltage begins to
