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

*Correct Answer is  C: Common emitter*  

### AI Response: (may contain errors)  

In Figure E7-1, the amplifier circuit is a **common emitter** configuration. Here's why:

1. **Why the correct answer is correct (Common emitter):**
   - In a common emitter amplifier, the emitter terminal of the transistor is common to both the input and output circuits. This means the input signal is applied to the base-emitter junction, and the output is taken from the collector-emitter junction.
   - The common emitter configuration is known for providing voltage gain and is widely used in amplification applications.

2. **Why the other choices are incorrect:**
   - **Common base (A):** In a common base amplifier, the base terminal is common to both the input and output. The input is applied to the emitter, and the output is taken from the collector. This configuration is less common for general amplification because it does not provide voltage gain.
   - **Common collector (B):** Also known as an emitter follower, in this configuration, the collector is common to both the input and output. The input is applied to the base, and the output is taken from the emitter. This configuration provides current gain but no voltage gain.
   - **Emitter follower (D):** This is another name for the common collector configuration, which, as mentioned, provides current gain and is used for impedance matching rather than voltage amplification.

3. **Useful formulas or rules of thumb:**
   - **Voltage Gain ($A_V$) for Common Emitter:**  
     $$A_V \approx -\frac{R_C}{R_E}$$  
     where $R_C$ is the collector resistor and $R_E$ is the emitter resistor. The negative sign indicates a phase inversion.
   - **Current Gain ($\beta$):**  
     The current gain in a common emitter configuration is approximately equal to the transistor's beta ($\beta$), which is the ratio of the collector current ($I_C$) to the base current ($I_B$).

Understanding these configurations and their characteristics is crucial for identifying and analyzing amplifier circuits in the exam.