Find all voltages (and currents) in an electronic circuit that contains operational amplifiers.
Verify that the following conditions are met:
Negative feedback circuit
Ideal operational amplifier
Add the following “rules” to Node Voltage Analysis:
Write a KCL equation at both input nodes (V+, V-) of the opamp (except if the voltage is known, e.g. set by a source)
Do not write an equation for the output node of the opamp (Vout).
Add V+ = V- to the equations to solve the circuit.
Example: Differencing Amplifier
Let’s use NVA to determine Vout as a function of Vin+ and Vin- (ignore the part number, the circuit works with any opamp).
Write KCL equations for nodes Vin+ and Vin- (steps 5 to 7 have been combined for brefity). No equations are needed for Vout (NVA opamp analysis rule) or the inputs Vin+ and Vin-.
Solve the equations to get
The circuit amplifies the difference of the input voltage, a very useful function for example with bridge circuits.
Reference: Schaum’s Outline of Electric Circuits, Seventh Edition, Section 5.4.