The Unity Gain
Buffer 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:
-
A bridge circuit or a voltage
divider circuit - maybe where there is a resistive sensor in the bridge or
voltage divider - and drawing current when you measure the voltage would be
enough to change the voltage.
-
A rectifier/filter where the
output is across a capacitor. In that situation drawing current from the
capacitor will discharge the capacitor, reducing the voltage.
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.
|
