Question E7B09:
What is characteristic of an emitter follower (or common collector) amplifier?  

Choices:  
A: Low input impedance and phase inversion from input to output  
B: Differential inputs and single output  
C: Acts as an OR circuit if one input is grounded  
D: Input and output signals in-phase    

Correct Answer is  D: Input and output signals in-phase  

Correct answer: D — Input and output signals in-phase

Why D is correct:
- An emitter follower (common-collector) takes the output from the emitter, which “follows” the base voltage.
- For AC signals, the emitter voltage tracks the base with no 180° inversion; it is in-phase and slightly smaller in amplitude.
- Approximate small-signal voltage gain:
  $$A_v \approx \frac{R_E}{R_E + r_e} \lesssim 1 \quad \text{(positive, in-phase)}$$
  where $r_e \approx \frac{25\ \text{mV}}{I_E}$ at room temperature.

Why the other choices are incorrect:
- A: Low input impedance and phase inversion from input to output  
  Incorrect. Emitter followers have high input impedance, low output impedance, and no phase inversion. Phase inversion is characteristic of a common-emitter stage.
- B: Differential inputs and single output  
  Incorrect. That describes a differential pair (long-tailed pair), not a common-collector stage.
- C: Acts as an OR circuit if one input is grounded  
  Incorrect. That’s digital logic behavior (e.g., diode OR or wired-OR), not an analog amplifier’s characteristic.

Useful formulas and rules of thumb:
- In-phase output, unity voltage gain (slightly less than 1):
  $$A_v \approx 0.9\text{–}1.0$$
- High input impedance (good buffer):
  $$Z_{in} \approx (\beta + 1)\,(R_E \parallel R_L)$$
- Low output impedance:
  $$Z_{out} \approx r_e \parallel R_E$$
- Strong current gain and power gain:
  $$A_i \approx \beta + 1$$

Key idea: Common-collector/emitter-follower = buffer stage with high $Z_{in}$, low $Z_{out}$, $A_v \approx +1$, and input/output in-phase.