In a feedback amplifier, what condition is used to approximate linear operation in the ideal model?

• A. Output equals input
• B. V- equals V+
• C. V- equals ground
• D. Input currents are infinite

Answer: (B)

In this ideal feedback amplifier, linear operation is achieved because the feedback forces the voltages at the two input terminals to be essentially equal. With the amplifier’s very high gain, any tiny difference between the noninverting and inverting inputs would drive the output to extreme values, so the feedback network acts to cancel that difference. The result is a virtual short between the inputs: V- equals V+. This equality makes the overall behavior linear and predictable, determined by the feedback network rather than by a large, uncontrolled open-loop gain.

Other statements don’t describe the general condition for linear operation. Output being equal to the input would imply a specific, not universal, gain of one. Having the inverting input at ground happens only in particular configurations (often called virtual ground), not a universal requirement. And assuming infinite input currents contradicts the ideal model, which actually uses zero input currents.