Define an ideal operational amplifier and state the golden rules for negative feedback.

Define an ideal operational amplifier and state the golden rules for negative feedback.

• A. An ideal op-amp has finite open-loop gain, infinite input impedance, nonzero input current, and zero output impedance; with negative feedback V- ≈ V+, and the input current is approximately zero.
• B. An ideal op-amp has infinite open-loop gain, infinite input impedance, zero input current, and zero output impedance; with negative feedback V- ≈ V+, and the input current is approximately zero.
• C. An ideal op-amp has infinite open-loop gain, infinite input impedance, nonzero input current, and nonzero output impedance; with negative feedback V- ≈ V+, and the input current is approximately zero.
• D. An ideal op-amp has infinite open-loop gain, zero input impedance, zero input current, and zero output impedance; with negative feedback V- ≈ V+, and the input current is approximately zero.

Answer: (B)

The idea being tested is how an ideal op-amp behaves when negative feedback is present. In an ideal op-amp, the open-loop gain is infinite and the input impedance is infinite, so no current flows into the input terminals and the output can drive any required load without dropping voltage (zero output impedance). Because the gain is infinite, even a tiny difference between the noninverting and inverting inputs would produce an unbounded output. In a closed-loop configuration with negative feedback, the circuit adjusts its output to reduce that difference as much as possible. In the linear operating region, that means the voltages at the two input terminals become equal, a situation described as a virtual short: V- is forced to be very close to V+. At the same time, the input currents remain essentially zero, since the input impedance is infinite. The result is that the op-amp behaves like a perfect voltage source at the output (zero output impedance) while drawing no current at its inputs, and enforcing V- ≈ V+ through the feedback network. So the best description is the one that states infinite open-loop gain, infinite input impedance, zero input current, and zero output impedance, with negative feedback enforcing V- ≈ V+ and maintaining zero input current into the device. The other descriptions fail because they assign finite gain, nonzero input or output impedance, or nonzero input current, which contradict the ideal model.

The idea being tested is how an ideal op-amp behaves when negative feedback is present. In an ideal op-amp, the open-loop gain is infinite and the input impedance is infinite, so no current flows into the input terminals and the output can drive any required load without dropping voltage (zero output impedance).

Because the gain is infinite, even a tiny difference between the noninverting and inverting inputs would produce an unbounded output. In a closed-loop configuration with negative feedback, the circuit adjusts its output to reduce that difference as much as possible. In the linear operating region, that means the voltages at the two input terminals become equal, a situation described as a virtual short: V- is forced to be very close to V+. At the same time, the input currents remain essentially zero, since the input impedance is infinite. The result is that the op-amp behaves like a perfect voltage source at the output (zero output impedance) while drawing no current at its inputs, and enforcing V- ≈ V+ through the feedback network.

So the best description is the one that states infinite open-loop gain, infinite input impedance, zero input current, and zero output impedance, with negative feedback enforcing V- ≈ V+ and maintaining zero input current into the device. The other descriptions fail because they assign finite gain, nonzero input or output impedance, or nonzero input current, which contradict the ideal model.