Q. Common-Emitter Configuration?

The emitter part of a circuit being common to both the input and the output portions, Figure (a) illustrates a common-emitter (CE) BJT amplifier. The resistors R₁, R₂, R_C, and R_E are primarily set by biasing. The input ac source is represented by its Thévenin equivalent. The amplified output ac voltage v_L appears across the load resistor R_L, which could represent the input resistance of the next stage in a cascade. Capacitors CB,CC, and CE are so chosen that they represent short circuits at the lowest frequency of interest. CE would be made large enough so that 1/ω_CE is small relative to RE in parallel with the impedance looking into the emitter at the smallest ω of interest.

Similarly, the reactances of CC and CB would be chosen small relative to the resistances in their parts of the circuit. Capacitors CB and CC appear as short circuits to the ac signals, but block the dc voltages and currents out of one part of the circuit from coupling with  another part. Capacitor CE, known as the bypass capacitor, bypasses the ac current around RE so that no significant ac voltage is generated  across RE, and helps to increase the gain.

The small-signal ac equivalent circuit is shown in Figure (b), in which the small- signal model of Figure for the transistor is used. While omitting the details of analysis and summarizing the results, we have