Transistor Operating Modes

 

Transistors, specifically Bipolar Junction Transistors (BJTs), operate in four distinct modes: cutoff, active, saturation, and reverse active. To explain these modes in an easy-to-understand way, we can use the "beta man" concept. In this analogy, "beta" represents the current gain of the transistor, or how effectively it amplifies current.

1. Cutoff Mode

Beta man is asleep.

In cutoff mode, the transistor is off, and no current flows from the collector to the emitter.

Mathematical Conditions:

• Base-emitter voltage: V_BE < 0.7V (for silicon transistors).
• Collector current: I_C ≈ 0 because there is minimal base current, I_B ≈ 0.

Example:

• Suppose V_BE = 0.3V. Since V_BE < 0.7V, the transistor remains off.
• If I_B = 0.1 μA, then I_C ≈ 0, illustrating the lack of significant current flow.

2. Active Mode

Beta man is working efficiently.

In active mode, the transistor is on, amplifying the current from the base to the emitter.

Mathematical Conditions:

• Base-emitter voltage: V_BE ≈ 0.7V.
• The collector current is given by I_C = β I_B, where β is the current gain.

Example:

• Let β = 100 (typical for many BJTs).
• Suppose I_B = 1 μA. Then, I_C = β · I_B = 100 · 1 μA = 100 μA.
• Here, V_BE ≈ 0.7V, and the transistor is in its active region, amplifying the base current.

3. Saturation Mode

Beta man is overworked and struggling.

In saturation mode, the transistor is fully on, and the collector-emitter voltage V_CE is low.

Mathematical Conditions:

• V_BE ≈ 0.7V.
• Collector-emitter voltage: V_CE ≈ 0.2V or lower.

Example:

• Let V_BE = 0.7V, and for saturation, V_CE ≈ 0.2V.
• With I_B = 1 μA and β = 100, I_C = β · I_B = 100 · 1 μA = 100 μA.
• In saturation, V_CE ≈ 0.2V, showing minimal voltage drop across the transistor.

4. Reverse Active Mode

Beta man is working in reverse.

In reverse active mode, the transistor behaves differently, typically inefficiently, with the base-emitter junction reverse-biased.

Mathematical Conditions:

• Base-emitter voltage: V_BE < 0.
• Collector current is typically much smaller than in the forward active mode.

Example:

• Suppose V_BE = -0.7V (reverse bias). The transistor is not designed for efficient operation here.
• With I_B = 1 μA, the collector current I_C is generally much lower, I_C = β' · I_B, where β' is much smaller than β.
• If β' = 10, then I_C = 10 · 1 μA = 10 μA, which is significantly smaller compared to the forward active mode.

Summary with Examples:

• Cutoff Mode: V_BE < 0.7V, I_C ≈ 0.
• Active Mode: V_BE ≈ 0.7V, I_C = β · I_B.
  • Example: I_B = 1 μA, β = 100 → I_C = 100 μA.
• Saturation Mode: V_BE ≈ 0.7V, V_CE ≈ 0.2V.
  • Example: I_B = 1 μA, β = 100 → I_C = 100 μA.
• Reverse Active Mode: V_BE < 0, I_C ≈ β' · I_B with β' much smaller.
  • Example: V_BE = -0.7V, I_B = 1 μA, β' = 10 → I_C = 10 μA.