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Current flow in an inverting amplifier

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In this standard inverting amplifier configuration, the arrows show the direction of current flow. The voltage across the input resistor R1 is equal to Vin because the right end of R1 (the - input of the op amp) is virtually at ground potential). This generates a current Vin/R1 through R1. This current flows almost entirely through the feedback resistor R2, 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 (Vin/R1) flowing through R2 generates a voltage equal to VinR2/R1. Thus the gain of the circuit is -R2/R1.

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 R1 and R2, 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.

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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
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