Self Bias, Source Bias, Voltage-divider Bias, and Current Divider Bias Explained

 

JFET Biasing Fundamentals

Let's find the fundamental concept you need is Kirchhoff's Voltage Law (KVL) applied to the Gate-Source input loop of the JFET.

In any JFET biasing circuit, the goal is to determine the relationship between the Drain current (I_D) and the Gate-Source voltage (V_GS). The general KVL equation for the input loop is:

V_G − V_GS − V_S = 0
→ V_GS = V_G − V_S

Here is how that fundamental logic applies to each specific biasing method:

1. Self-bias (A)

• The Circuit: The gate is grounded through a resistor (V_G = 0), and the source is connected to ground through a resistor R_S.
• The Math: Since V_G = 0 and the voltage at the source V_S = I_D ⋅ R_S, the equation becomes:
  V_GS = 0 − I_D ⋅ R_S = −I_DR_S

2. Voltage-divider bias (B)

• The Circuit: A resistive divider sets a constant voltage V_G at the gate. The source is connected to ground through R_S.
• The Math: V_GS = V_G − I_D ⋅ R_S. If you rearrange this to solve for I_D:
  I_D ⋅ R_S = V_G − V_GS
  → I_D = (V_G − V_GS) / R_S

3. Source bias (C)

• The Circuit: This typically uses two power supplies (+V_DD and −V_SS). The gate is grounded (V_G = 0), and the source resistor R_S is connected to the negative supply −V_SS.
• The Math: The voltage drop across the source resistor is (V_S − (−V_SS)) = I_D ⋅ R_S, so V_S = I_D ⋅ R_S − V_SS.
  V_GS = V_G − V_S = 0 − (I_D ⋅ R_S − V_SS) = V_SS − I_D ⋅ R_S
  Rearranging for I_D:
  I_D = (V_SS − V_GS) / R_S

4. Current-source bias (D)

• The Circuit: An active component, like a Bipolar Junction Transistor (BJT), is used to create a constant current source at the source lead of the JFET.
• The Math: You can tell a formula belongs to a BJT if it contains V_BE (base-emitter voltage) and R_E (emitter resistor). The current provided by the BJT is approximately:
  I_D ≈ I_E = (V_EE − V_BE) / R_E

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