BJT Amplifier Calculator

A BJT (Bipolar Junction Transistor) amplifier uses a three-terminal semiconductor device to amplify electrical signals. It operates by using a small input current at the base to control a larger collector current, providing current and voltage gain.

Common emitter provides voltage and current gain with phase inversion, common base offers voltage gain with no phase inversion but low input impedance, and common collector (emitter follower) provides current gain with unity voltage gain and high input impedance.

Bias resistors should be chosen to establish proper operating point (Q-point) in the active region. R1 and R2 form a voltage divider that sets base voltage, while ensuring the base current doesn't significantly load the divider network.

Voltage gain is primarily determined by the ratio of collector resistance to emitter resistance (Rc/Re), transistor beta, and load resistance. Higher Rc or lower Re increases gain, but stability may be compromised.

The emitter resistor provides negative feedback that stabilizes the operating point against temperature variations and transistor parameter variations. It reduces gain but improves stability and linearity.

Higher supply voltage allows larger voltage swings and potentially higher gain, but also increases power dissipation. The supply voltage must be sufficient to maintain the transistor in the active region throughout the signal swing.

Beta values typically range from 20 to 500, with most general-purpose transistors having beta between 50-200. Higher beta transistors require less base current but may be less stable and more expensive.