Electronics Handbook/Components/Amplifier

Transformer

 * $$\frac{V_o}{V_i} = \frac{N_2}{N_1}$$

When $$N_2 > N_1 V_o > V_i$$ Transformer acts as Voltage Amplifier

Transistor

 * $$\frac{V_o}{V_i} = \frac{R_2}{R_1}$$

When $$R_2 > R_1 V_o > V_i$$ Transistor acts as Voltage Amplifier

Op Amp

 * $$\frac{V_o}{V_i} = 1 + \frac{R_2}{R_1}$$

When $$R_2 > R_1 V_o > V_i$$ Transistor acts as Voltage Amplifier