Question E7B10:
In Figure E7-1, what is the purpose of R1 and R2?  

Choices:  
A: Load resistors  
B: Voltage divider bias  
C: Self bias  
D: Feedback    

Correct Answer is  B: Voltage divider bias  

To set the transistor’s operating point, R1 and R2 form a two-resistor network from the supply to ground that establishes a fixed DC bias at the control terminal (base or gate). This is called voltage-divider bias.

Why B (Voltage divider bias) is correct
- R1 is tied to the supply, R2 to ground, and their junction feeds the base/gate, creating a stable DC bias voltage to set the Q-point for linear operation.
- The divider output is:
  $$V_{B/G} = V_{CC}\,\frac{R_2}{R_1 + R_2}$$
- A low Thevenin resistance improves bias stability against device parameter variations:
  $$R_{\text{th}} = R_1 \parallel R_2$$

Why the other choices are incorrect
- A: Load resistors — A load resistor is typically in the collector/drain path carrying signal current. R1 and R2 do not serve as the load; they supply DC bias only.
- C: Self bias — Self-bias (emitter/source/cathode bias) uses a resistor in the emitter/source path to develop the bias from the device’s own current, not a two-resistor divider from the supply.
- D: Feedback — Feedback networks return a portion of the output to the input (often via a resistor or capacitor from collector/drain/plate to base/gate/grid). R1 and R2 connect to the supply and ground, not the output node.

Useful formulas and rules of thumb
- For a BJT:
  - $$V_E \approx V_B - 0.7\ \text{V}$$
  - $$I_E \approx \frac{V_E}{R_E},\quad I_B \approx \frac{I_C}{\beta}$$
- Divider current sizing for stability:
  - Choose $$I_{\text{div}} \approx 5\text{ to }10 \times I_B$$ so that base current has minimal effect on the bias voltage.
- The divider provides DC bias; AC signals are typically coupled via capacitors so the divider doesn’t load the signal path significantly.