TC 9-62
7-66. The output signal (which is opposite in phase to the input signal) causes a feedback
current (Ifdbk) to flow through R2. The left-hand side of R2 is at 0 volts (point A) and the
right-hand side is at EOUT. Therefore, the feedback current is computed by using the
following formula:
- E OUT
I fdbk =
R2
NOTE: The minus sign indicates that EOUT is 180 degrees out of phase with
EIN and should not be confused with output polarity.
7-67. Since no current flows into or out of the inverting input of the operational
amplifier, any current reaching point A from R1 must flow out of point A through R2.
Therefore, the input current (IIN) and the feedback current (Ifdbk) must be equal. Now we
can develop a mathematical relationship between the input and output signals and R1 and
R2 by computing the following:
IIN = Ifdbk
By substitution:
E IN - E OUT
=
R2
R1
If you multiply both sides of the equation by R1:
-(EOUT)(R1)
EIN =
R2
If you divide both sides of the equation by EOUT:
E IN
R1
=-
R2
E OUT
By inverting both sides of the equation:
EOUT
R2
=-
E IN
R1
You should recall that the voltage gain of a stage is defined as the output voltage divided
by the input voltage:
E OUT
E IN
7-20
TC 9-62
23 June 2005
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