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.

1. 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.

1. 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:

1. 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.