Q1: If an op-amp has one input grounded and the other input has a signal feed to it, then it is operating as what?

ANS:B - Single-ended

Single-ended Operation:

When a differential amplifier is operated in this mode, one input is ground and the signal voltage is applied only to the other input. Differential Operation.

In this mode, two opposite polarity (out of phase) signals are applied to the inputs.

This type of operation is also referred to as a double-ended operation.

The input signal at each input terminal causes the signal to appear at both output terminals. The resultant output signals have a peak value twice the value for the single-ended operation.

Common Mode Operation:

If two in-phase and equal signals are applied at the two input terminals, the resultant output signal at each output terminal would be zero. In other words, the output between collectors would be zero.