Question E7B11:
In Figure E7-1, what is the purpose of R3?  

Choices:  
A: Fixed bias  
B: Emitter bypass  
C: Output load resistor  
D: Self bias    

Correct Answer is  D: Self bias  

Correct answer: D — Self bias

Why D is correct:
- In the Figure E7-1 amplifier, R3 is the resistor in the source/emitter lead. The device’s current flowing through R3 creates a voltage drop that raises the source/emitter potential, thereby making the gate/base negative (for a JFET) or reducing base-emitter voltage (for a BJT) automatically. This is called self-bias (or cathode/emitter/source bias) and provides DC operating point stabilization via negative feedback.
- For a JFET with gate at 0 V, the self-bias relationship is:
  - $V_S = I_D R_3$, so $V_{GS} = V_G - V_S = -I_D R_3$
  - Using the JFET transfer curve: $I_D = I_{DSS}\left(1 - \frac{V_{GS}}{V_P}\right)^2$
- For a BJT, with an emitter resistor: $V_E = I_E R_3$ and approximately $I_C \approx \frac{V_B - V_{BE}}{R_3}$, showing how the emitter resistor sets the bias and improves thermal stability.

Why the other choices are incorrect:
- A: Fixed bias — That would use a set voltage applied to the gate/base (often via a divider) independent of device current. R3’s voltage depends on the device current, so it’s not fixed bias.
- B: Emitter bypass — That’s a capacitor placed in parallel with the emitter/source resistor to increase AC gain by bypassing AC around R3. R3 is a resistor, not the bypass capacitor.
- C: Output load resistor — The load resistor is typically in the collector/drain circuit to develop the output voltage. R3 is in the source/emitter leg, not the output load.

Useful rules of thumb:
- Self-bias improves thermal stability due to negative feedback: if current rises, $V_S$ rises, making $V_{GS}$ more negative (JFET) or reducing $V_{BE}$ (BJT), which pushes current back down.
- If R3 is not bypassed for AC, gain is reduced by local feedback. For a FET stage, a rough small-signal gain with unbypassed source resistor is:
  $$A_v \approx -\frac{g_m (R_D \parallel R_L)}{1 + g_m R_3}$$
- Adding a capacitor in parallel with R3 restores AC gain while keeping DC self-bias.