Question

What type of amplifier is a BJT?

Answer 1

A **BJT (Bipolar Junction Transistor)** can be used in various types of amplifiers, but it is typically used in **current amplifiers**. The specific type of amplifier depends on how the BJT is configured in the circuit.

Here are the main types of amplifiers in which a BJT is commonly used:

1. **Common Emitter (CE) Amplifier:**
   - This is the most widely used configuration for a BJT amplifier.
   - In this configuration, the input signal is applied to the base, and the output is taken from the collector. The emitter is typically grounded or connected to a reference voltage.
   - It provides **voltage amplification** and **inverts** the phase of the signal (i.e., 180 degrees out of phase).
   - The CE amplifier has high voltage gain and is used for general amplification purposes.

2. **Common Base (CB) Amplifier:**
   - In this configuration, the input signal is applied to the emitter, and the output is taken from the collector, with the base being grounded or connected to a reference voltage.
   - It provides **current amplification** but has a **low input impedance** and **high output impedance**.
   - The CB amplifier is less common in audio amplification but is used in high-frequency applications.

3. **Common Collector (CC) Amplifier (also called an Emitter Follower):**
   - In this configuration, the input is applied to the base, and the output is taken from the emitter.
   - It provides **voltage gain close to 1** (unity gain) but offers **high current gain**.
   - The CC amplifier is often used for **impedance matching** since it has a **high input impedance** and a **low output impedance**.

### Summary:
- **BJT amplifiers** can be **voltage amplifiers** (e.g., Common Emitter), **current amplifiers** (e.g., Common Base), or **impedance matching amplifiers** (e.g., Common Collector). The choice depends on the specific needs of the application.