Inverting Op Amp
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Feedback is the signal from the Op Amp output, fed back to the input, through Rf. The inverting amplifier uses negative feedback to control the gain.
For the Falstad Circuit Simulation, CTRL+Click Inverting Amplifier Gain = -4.7
In options, check European Resistors and uncheck Conventional Current.
Alternatively view Inverting_Amplifier.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.
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Explanation: Operational amplifiers have a very large open loop gain. So if the output is a few volts, the inverting input voltage must be very small (a few microvolts). This voltage is so small, it is called a virtual earth.
The inverting input, labelled "P", is a Virtual Earth because the voltage here is approximately zero as long as the amplifier is not saturating or limiting.
Ohm's Law can be used to predict the gain of the Inverting Op Amp.
No current flows into or out of ideal Op Amp inputs. Therefore the current in Rf is equal to the current in R1. Using Ohm's law (V = I R) and since the voltage at P is zero ...
Vin = -I R1 so I = - Vin / R1
Vout = I Rf so I = Vout / Rf
- Vin / R1 = Vout / Rf
Gain = Vout / Vin = - Rf / R1
Build this inverting amplifier and measure the gain. The volt meters should be set to the 20 Volt range.
Plot a graph showing Vin on the X axis and Vout on the Y axis. The graph gradient gives you the gain.
Gain = -Rf / R1 = - 100/10 = -10.
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