The op amp as an amplifier
For the op amp to work as a amplifier the
difference between the two input voltages V
2 and V
1 must be less than 150
mV. (More generally it must be less than V
S/A
o.)
This is achieved in different ways by the non-inverting and the inverting voltage
amplifiers.
The non-inverting voltage amplifier
In this circuit, shown in Figure 1, the input voltage is
applied to the non-inverting input (Q), This gives an output voltage that is in phase with the
input voltage.
A certain fraction (
b) of this output is then
fed back to the inverting input. This gives negative feedback and the feedback fraction is
given by the equation:
Feedback fraction (
b) =
R
2 /[R
1 +
R
2]
However for negative feedback we have
V
o = A
o(v
i -
bV
o) but since the closed
loop gain (A) is V
o/v
i we can write A = A
o(1 +
bA
o).
However since usually
bA
o » 1 this gives A is approximately 1/
b.
Therefore for the non-
inverting amplifier:
This is dependent only on the external
resistors R
1 and R
2.
