Explain the application of BJT as an amplifier.

BJT as an amplifier (common-emitter, NPN, voltage-divider bias):

                 +Vcc
            R1    |   Rc
            |     +--/\/\/--+------||-----o  Vout
  Vin o--||-+---- B   C     |     Cc2      (to RL)
       Cc1  |        \|/    |
            R2        E
            |         |
           GND    RE || CE (bypass)
                      |
                     GND

Principle: the transistor is biased in the active region (base-emitter junction forward biased, collector-base junction reverse biased) with a suitable Q-point near the middle of the load line.

• R₁, R₂ provide base bias; R_E gives bias stability; C_E bypasses R_E for AC; C_C1, C_C2 are coupling capacitors that pass AC and block DC.
• A small AC input voltage v_in applied between base and emitter changes the base current by a small amount i_b.
• Since \( i_c = \beta i_b \), the collector current changes by a much larger amount.
• This large change of collector current flowing through R_C produces a large voltage change: \( v_o = -i_cR_C \) (or −i_c(R_C∥R_L)).
• Voltage gain \( A_v = \dfrac{v_o}{v_{in}} \approx -\dfrac{\beta R_C}{r_{in}} \) is large (tens to hundreds). The negative sign shows 180° phase shift between input and output in CE configuration.
• As V_in increases, I_B and I_C increase, the drop I_CR_C increases and V_CE = V_CC − I_CR_C decreases (phase reversal).

The energy for the larger output signal comes from the DC supply V_CC; the transistor only controls it. CE configuration gives both current and voltage gain (highest power gain). Applications: audio amplifiers, pre-amplifiers, RF amplifiers.