Ideal versus Real
| Ideal property | Real limitation |
|---|---|
| Infinite open-loop gain | Large but finite, and it falls with frequency. |
| Infinite input impedance | Bias and leakage currents exist. |
| Zero output impedance | Output resistance and current limits exist. |
| Infinite bandwidth | Gain-bandwidth and internal poles limit response. |
| Infinite slew rate | Large signals have finite dV/dt. |
| Zero offset and drift | Offset voltage changes with device and temperature. |
| Infinite CMRR and PSRR | Common-mode and supply rejection are finite and frequency dependent. |
| No noise | Voltage noise, current noise, and 1/f noise are present. |
Virtual short
In a stable negative-feedback circuit with ample loop gain, V+ and V- are nearly equal. This approximation fails during saturation, slew limiting, invalid input common-mode voltage, or instability.
Virtual ground
If the non-inverting input is at ground, feedback may hold the inverting summing node near ground potential. It is called a virtual ground.
Finite gain and bandwidth
As frequency rises, open-loop gain falls. Closed-loop gain then becomes less exact, distortion can rise, and phase shift increases.
Output current
An amplifier that swings close to the rails with a 100 kΩ load may swing far less into 600 Ω. Output-current capability and thermal limits are part of the circuit design.
Input protection
Inputs frequently contain protection structures. Excess voltage or excessive differential voltage can force current through them, so source resistance and power sequencing sometimes matter.