*Question E6A05:
How does DC input impedance at the gate of a field-effect transistor (FET) compare with that of a bipolar transistor?*  

*Correct Answer is  C: An FET has higher input impedance*  

### AI Response: (may contain errors)  

The correct answer is that an FET has higher input impedance compared to a bipolar transistor (BJT). Here's why:

1. **Why the correct answer is correct:**
   - Field-effect transistors (FETs) are designed to have very high input impedance, typically in the range of megaohms (MΩ). This is because the gate of an FET is insulated from the channel, which means that very little current flows into the gate. This high input impedance makes FETs ideal for applications where minimal loading on the previous stage is desired.

2. **Why the other choices are incorrect:**
   - **A: They are both low impedance**: This is incorrect because FETs have high input impedance, while BJTs have lower input impedance, typically in the range of hundreds to thousands of ohms.
   - **B: An FET has lower input impedance**: This is incorrect as it contradicts the fundamental design of FETs, which are specifically built to have higher input impedance.
   - **D: They are both high impedance**: This is misleading because while FETs are high impedance, BJTs do not have high input impedance. BJTs are considered low to moderate impedance devices.

3. **Useful formulas or rules of thumb:**
   - A common rule of thumb is that the input impedance of a BJT can be approximated as:
     $$ Z_{in} \approx \beta \times R_E $$
     where:
     - $\beta$ = current gain of the BJT
     - $R_E$ = emitter resistor (if present)
   - For FETs, the input impedance is often considered to be very high, typically:
     $$ Z_{in} \gg 1 \, \text{M}\Omega $$

In summary, FETs are characterized by their high input impedance, making them suitable for high-impedance applications, while BJTs have lower input impedance.