Definition of signal voltage gain In amplifier terminology, the small-signal voltage gain Av is defined as which ratio?

Introduction / Context:
Voltage gain is one of the most fundamental amplifier metrics. It relates the output signal voltage to the input signal voltage and determines how much an amplifier increases the amplitude of an applied signal. Precise definitions prevent confusion with transresistance (vo/io) or transconductance (io/vi) parameters.

Given Data / Assumptions:

• Linear, small-signal operation around a fixed Q-point.
• vo and vi represent incremental (AC) voltages, not DC levels.
• Load and source impedances are part of the overall gain behavior.

Concept / Approach:
The voltage gain Av is the ratio of the output voltage to the input voltage. In a common-emitter BJT stage, for example, Av ≈ −gm * RC (magnitude considered), but the definition itself is simply vo/vi. Distinguishing between gain definitions avoids misapplication of formulas and incorrect design conclusions.

Step-by-Step Solution:
Identify the quantity asked: signal voltage gain, not current or mixed units.Write the definition: Av = vo / vi.Recognize that specific stage formulas (e.g., −gmRC) are implementations that yield Av but are not the definition.Conclude the correct ratio is output voltage over input voltage.

Verification / Alternative check:
Dimensional analysis: vo and vi both in volts; Av is dimensionless, consistent with a pure voltage ratio.

Why Other Options Are Wrong:
io/vi: that is transconductance, gm.IC × RC: has units of volts and relates to a stage's internal swing, not a definition of gain.vo/io: that is transresistance, Rout or a transimpedance measure.

Common Pitfalls:
Confusing specific gain formulas with the general definition; always start from Av = vo/vi and then substitute the stage model to compute its value.

Final Answer:
Av = vo / vi