JFETs, or Junction Field Effect Transistors, are three terminal devices that can be used as switches and amplifiers (amongst other things). They are not commonly used anymore but can still be found in some applications. Studying them, though, can give a good understanding of the effects of electrical fields in semiconductor devices.
The set of videos below provides a good introduction to the analysis of JFETs and circuits that use JFETs. These videos include:
- JFET Biasing
- JFET Small Signal Model
- Common Source Amplifier Configuration
- Common Drain Amplifier Configuration
Biasing is the process of configuring the circuit around the JFET to set voltages and currents to specific values to put the JFET into a particular state (e.g., open the channel between the source and the drain). These two videos describe the different states that JFETs can be in and provide some example biasing circuits.
JFET Small Signal Models
A small signal model is a model that can be used to model only the behaviour of the AC portion of a signal applied to a circuit. The small signal model ignores biasing but assumes that the biasing is putting the device into its proper state. This video describes the AC model of a single stage JFET amplifier (including transconductance). This is a rather long video, but it shows both a graphical method and a numberic method for determining characteristics of the JFET small signal model.
JFET Common Source Amplifier
The common source amplifier configuration has the input AC voltage applied at the gate and the output taken at the drain. It is analogous to the common emitter BJT amplifier. This video shows how to analyze a couple of different common source configurations (i.e., one with the source resistor bypassed, one without a bypass) to determine the amplifier gain, input impedance, and output impedance.
JFET Common Drain Amplifier
The common drain amplifier configuration of a JFET is analogous to the common collector (emitter follower) configuration of a BJT. It has the input applied at the gate and the output at the source. This video shows how to determine the input impedance, the output impedance and the voltage gain of three different common drain circuits (each has a different biasing configuration).