Which factor is NOT typically a method to increase amplifier gain?

In the context of amplifiers, the goal is usually to enhance the output power or voltage relative to the input signal. The correct choice, which is employing a less efficient amplifier, is not a typical method for increasing amplifier gain because a less efficient amplifier would generally result in poorer overall performance and lower gain.

Increasing gain through adjusting feedback resistor values is a common practice in feedback amplifier designs, as it directly influences the overall gain of the circuit by modifying the feedback loop. Using a transformer can also be an effective method for increasing gain, especially in audio applications or RF designs where impedance matching is crucial. Increasing the input signal strength is a straightforward approach as well; by providing a stronger input, the amplifier can produce a greater output signal, thus evoking an apparent increase in gain.

In contrast, employing a less efficient amplifier would inherently suggest a decrease in output relative to the input under normal conditions, therefore not contributing to an increase in gain.