The Unity Gain Buffer Amplifier - (a.k.a. The Isolation Amplifier)


The circuit:


What does the circuit do?

        The circuit essentially just makes a copy - at the output - of the the input voltage, Vin.  It does that without drawing any current from wherever the input voltage terminal is attached.  However, at the output terminal you can draw whatever amount of current the operational amplifier can supply (and that depends upon the kind of operational amplifier you use).


Why is this circuit useful?

        There are many situations where you do not want to draw current from a circuit (i.e. "load" the circuit).  Some of those situations are:

In all of these situations - and others - the goals is to ensure that doing the measurement of a voltage does not disturb the circuit producing the voltage to be measured.

Analysis of the circuit

      If the difference between V+ and V- is negligibly small so that V+ = V- we must have:


The Circuit on a Circuit Board

        Here is a drawing of what the circuit should look like on a circuit board.  In this drawing we have shown connections that are actually made under the board - connections that are not visible from the top of the board.  Those are shown with lighter lines that go through the connecting "holes" in the drawing.  They are there to show you how the various wires are actually connected to the operational amplifier through metallic connections under the board.  And, don't forget, the integrated circuit is a 741 operational amplifier chip.