
Ampliers
359
For this scheme, the gain, termed open-circuit voltage gain A
v
(unitless) is
0
0
=
=
A
v
v
o
i
i
This is a voltage amplier, so to understand how it behaves we add a voltage
source (modeled by a Thévenin equivalent) v
s
with a series internal resistance R
s
,
and a load R
L
to which the output voltage is supplied (Figure 10.15).
The resistance R
s
of the voltage source
and the input resistance R
i
of the ampli-
er form a voltage divider, as well as the
output resistance of the amplier R
o
and
the resistance of the load R
L
. Accordingly
we can write
=
+
v
R
RR
o
L
and
=
+
v
R
RR
v
i
i
s
Therefore, the overall voltage gain
o
can be found as
=
v
v
R
RR
A
R
o