Page 92 - Op Amps Design, Applications, and Troubleshooting

Voltage Follower 75

Output Impedance. The output impedance of the voltage follower can be
computed as follows:

where A OL is the open-loop gain of the op amp at the specified frequency. You can
determine the value of A OL at the desired operating frequency as in Equation (2.16):

where/ JN is the specific input frequency being considered.
For the circuit in Figure 2.16, the open-loop gain at 5 kilohertz, for example, is

The output impedance then becomes Equation (2.31).

As with most op amp circuits, the output impedance is so low relative to any prac-
tical load resistance that its effects may be ignored.

Output Current Capability. The total current flowing in or out of the output
terminal of the op amp in Figure 2.16 may be delivered directly to the load. That is,
the feedback current is extremely small and can be disregarded in nearly all cases.
As the load resistance varies from infinity (open) to 0 (short), the output current
from the op amp varies from 0 to the short-circuit value of 20 milliamps (given in
the data sheet). The limiting factor is the amount of reduction that can be tolerated
on the output voltage swing.
On an unprotected op amp, the value of load current must be kept below
the stated output current rating. If this value is not supplied in the data sheet, it
can be estimated by using the maximum power dissipation data; recall that power =
voltage x current.