BJT Applications: CE configuration as amplifier

BJT Applications: CE configuration as amplifier

1. What CE Configuration Is

CE (Common-Emitter) = emitter common to input and output: input at base-emitter, output at collector-emitter (emitter grounded for AC via capacitor). It is the most used BJT amplifier configuration because it gives high voltage, current, and power gain, and phase inversion ($180°$).

Alternatives: CB (common-base, voltage gain but $α≈1$, low input $Z$, high output) and CC (emitter follower, $β$ current gain, voltage gain ≈1, buffer) — but CE is standard.

2. Circuit (bias + AC)

Vcc (12V) → Rc (collector resistor, e.g., 2kΩ) → Collector → Emitter → Re (emitter resistor with bypass Ce) → Ground
Base → voltage divider R1,R2 (e.g., 40k/10k) setting Vb≈2V ⇒ Ve=1.3V ⇒ Ie≈1.3mA, Q-point → coupling capacitors Cin, Cout block DC, pass AC.

Bias (Q-point): sets $I_C$ and $V_{CE}$ in active region (e.g., $V_{CE}≈6V$) so signal swing not clipped.

3. Working as Amplifier

• Small AC input $v_{be}$ (e.g., 10mV) modulates $I_B$:

where

(e.g., 1mA ⇒ 25Ω? Actually $r_π=β/gm$, $gm=I_C/V_T$).

• Collector current

with transconductance

mS (e.g., 1mA ⇒40mS).

• Voltage gain:

where

, negative sign = 180° phase inversion (when base goes up, collector goes down due to $V_{Rc}$ drop).

• For $R_C=2kΩ$, $I_E=1mA$ ⇒ $A_v≈ -80$ (40mS·2k).

4. Characteristics

Feature	CE Amplifier
Voltage gain $A_v$	High (-50 to -500)
Current gain	High $β≈100$
Power gain	High
Input $Z$	Moderate $≈R1||R2||r_π$ (~kΩ)
Output $Z$	Moderate-high $≈R_C$
Phase	180° inverted (input up ⇒ output down)

5. Worked Design

Need Av=-100 with Ic=2mA: gm=80mS ⇒ Rc=Av/gm=1.25kΩ ⇒ choose 1.2kΩ.
Input rπ=β/gm≈100/80mS=1.25kΩ, with bias divider 10k ⇒ Zin≈1.1kΩ.

6. Exam Points

• Draw CE amplifier with voltage divider bias, $R_C$, $C_{in/out}$, $R_E$ — 7 marks, very frequent (almost certain to ask CE circuit).
• Explain $I_C=β I_B$, $V_{CE}$ bias, and $A_v=-g_m R_C$ with 180° — 7-10 marks, very frequent (the most common amplifier question).
• Mention high voltage/current/power gain — 3-5 marks, frequent.
• Distinguish CE vs CC vs CB (gain, $Z$) — 5 marks, frequent.