What is the effect of load impedance on the output of a voltage follower (buffer) using an op-amp?

• A. The ideal buffer has zero output impedance and high input impedance; real op-amps have finite output impedance, so heavy loads cause a voltage drop and reduced buffering accuracy.
• B. The output impedance increases with light loads but remains unchanged under heavy loads.
• C. The buffer boosts the load's impedance, increasing drive capability.
• D. There is no effect; the buffer always maintains perfect buffering.

Answer: (A)

A voltage follower uses negative feedback to reproduce the input at the output with unity gain, so the source sees high input impedance and the load should see a low, stable output. In a real op-amp, the output can’t be a perfect zero-impedance driver and has finite output resistance and a limit on how much current it can deliver. The feedback greatly reduces the effective output impedance, but it isn’t zero. When a load is connected, especially a low-impedance one, the op-amp must source more current. If that current demand approaches or exceeds what the device can supply without the output voltage sagging, the output drops below the input, reducing buffering accuracy. With light loads, the current is small and the drop is negligible, so buffering is nearly ideal; with heavy loads, the droop becomes noticeable. This is why a buffer is described as having near-zero output impedance in theory, but real op-amps show a finite output impedance that causes voltage drop under heavy loading.