FET biasing concepts

Related Concepts: FET bias part 2 note

Based on Chapter 7: FET Biasing from your textbook, the three primary methods for biasing Field-Effect Transistors (JFETs) are the Fixed-Bias, Self-Bias, and Voltage-Divider Bias configurations.

Unlike BJT amplifiers, where the controlling variable is a current ($I_B$), FETs are voltage-controlled devices. The goal of these biasing circuits is to establish a stable quiescent point ($Q$-point) defined by a specific drain current ($I_{DQ}$) and gate-to-source voltage ($V_{GSQ}$).

Here is an explanation of the three types to help you prepare for your lecture:

1. Fixed-Bias Configuration

This is the simplest configuration, but it requires two separate DC voltage sources, which makes it less practical for many commercial applications.

• How it works: A fixed DC voltage source ($V_{GG}$) is connected to the gate terminal through a resistor ($R_G$). Since the input impedance of a JFET is extremely high, the gate current ($I_G$) is effectively 0 A. Because no current flows through $R_G$, there is no voltage drop across it, and the negative terminal of the battery is applied directly to the gate.
• Key Characteristic: The gate-to-source voltage ($V_{GS}$) is fixed and determined solely by the external battery $V_{GG}$. It does not change regardless of the current flowing through the device.
• Equation: $V_{GS}=-V_{GG}$

2. Self-Bias Configuration

This is a more common configuration because it eliminates the need for the second battery ($V_{GG}$). Instead, it uses the current flowing through the transistor to create its own bias voltage.

• How it works: A resistor ($R_S$) is placed at the Source ($S$) terminal. When drain current ($I_D$) flows through the device, it also flows through this source resistor, creating a voltage drop ($V_S=I_D{R}_S$). Since the gate is connected to ground (0 V) via $R_G$, and the source is now at a positive potential due to the resistor, the gate becomes negative relative to the source.
• Key Characteristic: The bias voltage $V_{GS}$ is not fixed; it is a function of the output current $I_D$. This creates a feedback mechanism that helps stabilize the operating point.
• Equation: $V_{GS}=-I_D{R}_S$
• Visualizing the Solution: When plotting this mathematically, the equation defines a straight line that passes through the origin (0,0). The intersection of this line with the JFET’s transfer curve (Shockley’s equation) defines the $Q$-point.

3. Voltage-Divider Bias Configuration

This configuration is very similar to the BJT voltage-divider circuit you may have already studied. It provides a fixed voltage at the gate using a resistive divider from the main supply voltage ($V_{DD}$).

• How it works: Two resistors, $R_1$ and $R_2$, are connected in series across the supply voltage. The gate is connected between them. Because $I_G=0$ A, the voltage at the gate ($V_G$) is determined entirely by the voltage divider rule.
• Key Characteristic: Unlike the self-bias configuration, the voltage at the gate is positive relative to ground. However, the source voltage ($V_S$) rises due to the source resistor ($R_S$) until the gate-to-source voltage ($V_{GS}$) becomes the necessary negative value to operate the JFET.
• Equation: $V_{GS}=V_G-I_D{R}_S$ (Where $V_G$ is a constant determined by $R_1$, $R_2$, and $V_{DD}$).

Summary Comparison Table

The text provides a summary of these equations which effectively highlights the differences:

Configuration	DC Bias Equation ($V_{GS}$)	Distinctive Feature
Fixed-Bias	$V_{GS}=-V_{GG}$	Requires two power supplies/batteries.
Self-Bias	$V_{GS}=-I_D{R}_S$	Single supply; bias line passes through origin.
Voltage-Divider	$V_{GS}=V_G-I_D{R}_S$	Single supply; most stable; gate voltage is fixed positive relative to ground.

Analogy: To visualize the difference between Fixed and Self-Bias:

• Fixed Bias is like setting a car’s cruise control to a specific speed (voltage) and locking it there regardless of the road conditions.
• Self-Bias is like the suspension system of the car; the harder the road pushes up (higher current $I_D$), the harder the springs push back (higher voltage drop $V_S$), automatically adjusting to find a stable riding height ($Q$-point).