How does a buffer amp not remove all loading effect?

[itzernie]

Homework Statement

I recently completed a lab where I set up a buffer amp. Its input was a function generator set at 5Vp amplitude and we placed various load resistors on the output to test Vout on an oscilloscope ( 75kΩ, 3300Ω, 510Ω and 68Ω.) From my data I noticed that the Vout was the same for the first 3 (which makes sense since the buffer amp overcomes the loading effect), but for the 68Ω the Vout value decreased.

Since Vout on a buffer = Vin, how is this possible ? Or does the buffer amp only decrease the loading effect of a practical source? Not remove it.

Homework Equations

Vout = Vin

The Attempt at a Solution

I know that a smaller load resistor would increase the current coming out of the Op Amp, but I am not sure how that could effec the output voltage.

 

[gneill]

I know that a smaller load resistor would increase the current coming out of the Op Amp, but I am not sure how that could effec the output voltage.

This is the difference between ideal components and real-world ones. We know that the ideal op amp has an output impedance of zero Ohms. But how about real components that you can actually buy?

It can be informative to consult the datasheet for the op amp that you're using and look for the output impedance spec. If it's listed in a table it's usually called Zo, but sometimes they'll only supply an output impedance vs frequency plot (generally towards the end of the datasheet).

Also check for the maximum output current. See if your expected output voltage swings are going to want to drive the load with anything approaching the maximum current.