SLOA011B January 2018 – July 2021 LF347 , LF353 , LM348 , MC1458 , TL022 , TL061 , TL062 , TL071 , TL072 , UA741
The Thevenin amplifier model shown in Figure 1-1 is redrawn in Figure 1-2 showing standard op amp notation. An op amp is a differential to single-ended amplifier. It amplifies the voltage difference, V_{d} = V_{p} - V_{n}, on the input port and produces a voltage, V_{o}, on the output port that is referenced to ground.
We still have the loading effects at the input and output ports as noted above. The ideal op amp model was derived to simplify circuit calculations and is commonly used by engineers in first-order approximation calculations. The ideal model makes three simplifying assumptions:
Applying these assumptions to Figure 1-2 results in the ideal op amp model shown in Figure 1-3.
Other simplifications can be derived using the ideal op amp model:
Because R_{i} = ∞, we assume I_{n} = I_{p} = 0. There is no loading effect at the input.
Because R_{o} = 0 there is no loading effect at the output.
If the op amp is in linear operation, V_{0} must be a finite voltage. By definition V_{o} = V_{d} × a. Rearranging, V_{d} = V_{o} / a . Since a = ∞, V_{d} = V_{o} / ∞ = 0. This is the basis of the virtual short concept.
The ideal voltage source driving the output port depends only on the voltage difference across its input port. It rejects any voltage common to V_{n} and V_{p}.
No frequency dependencies are assumed.
There are no changes in performance over time, temperature, humidity, power supply variations, etc.