An op amp is ideal if it has the following characteristics:
- Infinite open-loop gain, \(A \simeq \infty\).
- Infinite input resistance, \(R_{i} \simeq \infty\,\Omega\).
- Zero output resistance, \(R_{o} \simeq 0\,\Omega\).
An ideal op amp is an amplifier with infinite open-loop gain, infinite input resistance, and zero output resistance.
Two important characteristics of the ideal op amp are:
-
The currents into both input terminals are zero.
\(\boxed{i_{1} = i_{2} = 0}\)
This is due to infinite input resistance. An infinite resistance between the input terminals that an open circuit exists there and current cannot enter the op amp.
-
The voltage across the input terminals is equal to zero.
\(\displaystyle v_{d} = v_{2} - v_{1} = 0\)
\(\boxed{v_{1} = v_{2}}\)
When negative feedback is added to an op amp, the input terminals become identical. Meaning, whatever is the voltage present in the non-inverting input is also present in the inverting input.
The two characteristics can be exploited by noting that for voltage calculations the input port behaves as a short circuit, while for current calculations the input port behaves as an open circuit.