In this standard inverting amplifier configuration, the arrows show the direction
of current flow. The voltage across the input resistor R_{1} is equal to
V_{in}
because the right end of R_{1} (the - input of the op amp)
is virtually at ground potential).
This generates a current V_{in}/R_{1}
through R_{1}.
This current flows almost entirely through the feedback resistor
R_{2}, rather
that into the operational amplifier inputs, because the voltage
between the
- and + inputs of the op amp is very small (typically microvolts) and the
resistance between the inputs (the differential input resistance) is large
(typically megohms). The current (V_{in}/R_{1}) flowing
through R_{2} generates a voltage equal to
V_{in}R_{2}/R_{1}.
Thus the gain of the circuit is -R_{2}/R_{1}.

Note that the current flowing into the op amp (the differential input
current) is vanishingly
small compared to the current flowing through the resistors.
Also, note that the
differential input voltage is very small compared to
the other voltages. That's why we say that the top (-) input of the op and is
at "virtual ground".

You can change the values of R_{1} and R_{2},
the differential
input resistance (typically 1 to 100 Megohm), and the
open-loop gain
typically 50000 to 200000). Just click on the value with the mouse pointer and
edit like any text field.

This page is maintained by Prof. T. C. O'Haver , Department of Chemistry and
Biochemistry, The University of Maryland at College Park.
Comments, suggestions and questions should be directed to
Prof. O'Haver at toh@umd.edu.
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