2/18/2011
The NonInverting Configuration lecture
1/4
The Non-Inverting Configuration
R1
R2
i1
i2
i- =0
vin
i+ =0
v-
-
ideal
v+
vout
+
Good heavens! The
inverting input (v+) of this
configuration is not at
virtual ground (i.e., v + ≠ 0 )!
Recall that v- =v+ (the virtual short) ALWAYS for feedback amplifiers.
Jim Stiles
The Univ. of Kansas
Dept. of EECS
2/18/2011
The NonInverting Configuration lecture
2/4
No virtual ground here!
Notice also that for the circuit above, the
voltage at the non-inverting terminal is the input
voltage vin :
∴ v − = vin ≠ 0
We use this fact to analyze this non-inverting
configuration.
First, we use KCL to determine that:
i1 = i− + i2
R1
R2
i1
i2
i- =0
and since i− = 0 , we again find that:
v-
i1 = i2
and from Ohm’s Law:
i1 =
Jim Stiles
0 −v −
R1
=
−v −
R1
vin
i2 =
i+ =0
-
ideal
v+
vout
+
v − −vout
R1
The Univ. of Kansas
Dept. of EECS
2/18/2011
The NonInverting Configuration lecture
3/4
i- = 0 is the key
These results are of course very similar to the expressions we derived when
analyzing the inverting configuration.
The main difference is of course that v- is not equal to zero.
Instead, we know that v − = vin . Thus:
i1 =
−vin
and since i1 = i2 , we determine
a relationship involving vin and
vout only:
−vin
R1
=
vin −vout
R2
R1
i2 =
vin −vout
R1
R1
R2
i1
i2
i- =0
vin
Jim Stiles
The Univ. of Kansas
i+ =0
v-
-
ideal
v+
vout
+
Dept. of EECS
2/18/2011
The NonInverting Configuration lecture
4/4
Note the gain is a positive number
Performing some simple algebra, we rearrange this expression and find the
open-circuit voltage gain of the non-inverting configuration:
oc
vout
R
=1+ 2
Avo =
vin
R1
Note that the open-circuit voltage gain for
this configuration is a positive number.
We conclude then that the input and output
voltage will have the same sign (i.e., ± ).
This is why we call the configuration noninverting.
Jim Stiles
The Univ. of Kansas
Dept. of EECS