Common Emitter Amplifier

With an emitter resistor $R_E$ (unbypassed):

$A_v \approx -\frac{R_C \parallel R_L}{r_e + R_E} \qquad r_e = \frac{V_T}{I_E} \approx \frac{25\ \text{mV}}{I_E}$

With $R_E$ fully bypassed, $A_v \approx -g_m R_C$, where $g_m = I_C/V_T$ — much higher but less predictable and more distorting.

Characteristics: moderate input impedance, output impedance ≈ $R_C$, output inverted (180° phase shift). The common-source MOSFET amplifier is its FET equivalent.