A JFET has three terminals: the gate, source, and drain.
JFETs are voltage-controlled devices which means that the drain current, ID, is controlled by the amount of gate-source voltage, VGS.
The input impedance of a JFET is normally very high, usually of the order of several megohms (MΩ).
A JFET is a "normally on" device because drain current flows when VGS = 0 V.
The pinch-off voltage, VP, is the drain-source voltage at which ID levels off.
IDSS represents the drain-source current that flows when VGS = 0 V AND VDS › VPIDSS is the maximum possible drain current for a JFET.
The gate-source cutoff voltage, designated VGS(off), is the amount of VGS required to reduce the drain current, ID, to zero. VP = -(-VGS(off))
The transconductance curve is a graph of ID versus VGS. The transconductance curve is nonlinear. The ratio of the change in drain current, ΔID, to the change in gate-source voltage, ΔVGS is the transconductance, gm. The unit of transconductance is the siemens (S).
The transconductance, gm, tells us how effective VGS is in controlling ID.
Gate bias, self-bias, voltage-divider bias, and current-source bias are common ways of biasing a JFET. Gate bias is seldom used because its Q point is so unpredictable.
JFET amplifiers provide less voltage gain than bipolar transistor amplifiers. The input impedance of a JFET amplifier is much higher, however.
When the source resistor is bypassed, a CS amplifier has a voltage gain of gmrL. In a CS amplifier, Vin and Vout are 180° out of phase.
A common-drain amplifier is more commonly known as a source follower. A source follower has high input impedance, low output impedance, and a voltage gain less than one.
A CG amplifier has a voltage gain equal to gmrL just like the CS amplifier. However, Vin and Vout are in phase. The major drawback of a CG amplifier is its low input impedance.
There are two basic types of MOSFETs: E-MOSFETs and D-MOSFETs. Like JFETs, MOSFETs are voltage-controlled devices.
The main difference between a JFET and a MOSFET is that the gate in the MOSFET is insulated from the channel by a thin layer of silicon dioxide (Si02). This makes the input impedance of a MOSFET many times higher than that of a JFET.
D-MOSFETs are "normally on" devices because drain current flows when VGS = 0 V. For D-MOSFETs, IDSS is not the maximum possible drain current.
A D-MOSFET can operate in either the enhancement or depletion mode.
An E-MOSFET is a "normally off" device because there is no drain current when VGS = 0 V. For an E-MOSFET, VGS(th) is the minimum gate-source voltage that produces drain current.
One disadvantage of MOSFETs is their extreme sensitivity to damage by electrostatic discharge (ESD). When handling MOSFETs, extreme care must be taken to ensure that static electricity does not puncture the thin layer of Si02 separating the gate and channel.
When handling MOSFETs, use the grounded wrist strap!
