Question E7B12:
What type of amplifier circuit is shown in Figure E7-1?  

Choices:  
A: Common base  
B: Common collector  
C: Common emitter  
D: Emitter follower    

Correct Answer is  C: Common emitter  

Correct answer: Common emitter

1) Why “common emitter” is correct
- In a common-emitter (CE) amplifier, the input is applied to the base, the output is taken from the collector, and the emitter is the node common to both (usually at AC ground via a bypass capacitor).
- Typical exam-figure clues: input coupling into the base, collector resistor to the supply with output taken from the collector via a coupling capacitor, and an emitter resistor that is bypassed to ground for AC.
- CE stages provide voltage gain and invert phase (output is 180° out of phase with the input).

2) Why the other choices are incorrect
- Common base (A): The base would be at AC ground (bypassed), input applied to the emitter, and output at the collector. The figure for CE shows input at the base, not the emitter.
- Common collector (B): Also called an emitter follower; the output would be taken from the emitter and the collector is effectively at AC ground. The CE figure takes output from the collector, not the emitter.
- Emitter follower (D): This is the same as common collector, so for the same reasons as (B), it does not match a CE schematic.

3) Useful formulas and rules of thumb
- Naming rule: the “common” terminal is the one at AC ground shared by input and output.
  - Common emitter: emitter is AC common; input at base; output at collector; inverts phase.
  - Common collector (emitter follower): collector is AC common; input at base; output at emitter; no inversion; unity voltage gain.
  - Common base: base is AC common; input at emitter; output at collector; no inversion.
- CE small-signal voltage gain (with emitter bypassed):
  $$A_v \approx -g_m\,(R_C \parallel R_L) \approx -\frac{R_C \parallel R_L}{r_e}$$
  where:
  - $g_m = \dfrac{I_C}{V_T}$, with $V_T \approx 25\,\text{mV}$ at room temp
  - $r_e \approx \dfrac{25\,\text{mV}}{I_E}$ (in amps)
- CE input resistance (approx.): $R_{in} \approx \beta\,r_e$ (plus bias network effects).
- Emitter follower voltage gain: $A_v \approx 1$; very high input impedance, low output impedance.
- Common base current gain: $\approx \alpha \lesssim 1$; very low input impedance at the emitter.