Transistor Amplifier Overview

Overview

This lecture explains how an NPN transistor behaves as an amplifier, the required operating conditions, and the different transistor states (cutoff, active/linear, and saturation).

Practical Transistor Amplifier Circuit

• Real amplifier circuits use resistors to limit current and prevent transistor damage.
• Circuit diagram includes a supply voltage (example: 3V), a collector resistor, and an NPN transistor.
• Voltage at the collector (V_CE) is measured across the collector and emitter; it serves as the output voltage.

Cutoff State

• When base-emitter voltage (V_BE) is less than 0.7V, the transistor is in cutoff state.
• In cutoff, base current (I_B) and collector current (I_C) are approximately zero.
• No voltage drop across the collector resistor, so V_CE equals supply voltage (maximum output voltage).
• The transistor acts as an open switch in this region.

Active (Linear) Region and Amplification

• When V_BE ≈ 0.7V, the transistor starts to conduct; small increases in I_B cause large changes in I_C.
• Collector current (I_C) is β times the base current (I_C = β × I_B), where β is the current gain (e.g., β = 100).
• Output voltage (V_CE) decreases as collector current increases due to the voltage drop across the collector resistor (Ohm’s law: V = IR).
• For example, if I_B = 10μA, then I_C = 1mA, voltage drop across 1kΩ resistor = 1V, so V_CE = 2V (from 3V supply).
• Linear amplification occurs while I_B is within a specific range (not too high).

Saturation State

• When I_B increases beyond a certain limit (e.g., I_B > 30μA), the collector current (I_C) stops increasing—it saturates.
• Maximum collector current is set by supply voltage and collector resistor (I_C(max) = V_supply / R_C).
• Output voltage (V_CE) drops to nearly zero (minimum value).
• The transistor acts as a closed switch (short circuit) in this region.

Applications of Cutoff and Saturation

• In cutoff, the transistor behaves like an open switch (no current flow).
• In saturation, the transistor behaves like a closed switch (maximum current flow).
• These modes are useful for switching applications (e.g., digital logic, on/off control), not amplification.

Key Terms & Definitions

• NPN Transistor — A type of bipolar junction transistor with current flowing from collector to emitter when base-emitter junction is forward biased.
• Cutoff State — Transistor state where V_BE < 0.7V; I_B and I_C ≈ 0; acts as open switch.
• Active (Linear) Region — Region where transistor amplifies; I_C = β × I_B; V_BE ≈ 0.7V.
• Saturation State — State where increasing I_B does not increase I_C; V_CE ≈ 0; acts as closed switch.
• β (Beta) — Current gain; ratio of collector current to base current (I_C/I_B).
• V_CE — Collector-Emitter voltage; output voltage of the transistor circuit.

Action Items / Next Steps

• Review the input-output relationships for each transistor state.
• Practice calculating I_C and V_CE for different I_B values and resistor choices.
• Think about real-world applications for transistor switching behavior.