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Properties of ideal op-amp

Short answer

An ideal op‑amp has infinite open‑loop gain, infinite input resistance, zero output resistance, infinite bandwidth, infinite CMRR, and zero offset. In negative feedback, input currents are zero and the two input terminals are virtually shorted (equal voltage).

State any (four/six) important properties of an ideal operational amplifier.

An operational amplifier (op-amp) is a high-gain, direct-coupled differential amplifier with a high input impedance and a low output impedance. An ideal op-amp has the following properties:

          +Vcc
      |\   |
 V- o-| \  |
      |  >--o Vo
 V+ o-| /
      |/   |
          -Vcc
  1. Infinite open-loop voltage gain (AOL = ∞): a tiny input difference gives a large output, so the differential input voltage is practically zero in linear operation (virtual short).
  2. Infinite input resistance (Ri = ∞): no current flows into either input terminal.
  3. Zero output resistance (Ro = 0): the output behaves as an ideal voltage source and can drive any load without a voltage drop.
  4. Infinite bandwidth: the gain is constant at all frequencies from dc to infinity.
  5. Infinite common-mode rejection ratio (CMRR = ∞): a common-mode signal produces no output.
  6. Zero offset voltage: the output is exactly zero when both inputs are zero.
  7. Infinite slew rate: the output can change instantly.
  8. Zero drift: the characteristics do not change with temperature.
PropertyIdealTypical 741
Gain∞2 × 10⁵
Ri∞2 MΩ
Ro075 Ω
Bandwidth∞1 MHz (unity gain)
CMRR∞90 dB
Slew rate∞0.5 V/µs

Two golden rules follow for circuits with negative feedback: (i) no current enters the input terminals, and (ii) V+ = V− (virtual short).

✓ Answer checked by Claude Opus 5.5 (high effort)

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