Wednesday, August 13, 2008

Field-effect transistors (FET)

Family Tree:
JFET: n-channel, p-channel
MOSFET: depletion, enhancement
depletion: n-channel
enhancement: n-channel, p-channel

In FET, the gate draws no current, which resulting high input impedance.

Different between BJT and JFET:

  • Bipolar transistors are normally-off devices: no current through the base, no current through the collector or the emitter. JFET is normally-on devices: no voltage applied to the gate allows maximum current through the source and drain.
  • the amount of current allowed through a JFET is determined by a voltage signal rather than a current signal as with bipolar transistors. with the gate-source PN junction reverse-biased, there should be nearly zero current through the gate connection.
  • For this reason, we classify the JFET as a voltage-controlled device (normally reverse-biased), and the bipolar transistor as a current-controlled device.

JFETs regulate drain current according to the amount of reverse-bias voltage applied between gate and source, much like a BJT regulates collector current according to base current. The relationship between gate-source (control) voltage and drain (controlled) current is nonlinear: as gate-source voltage is decreased, drain current increases exponentially. That is to say, the transconductance of a JFET is not constant over its range of operation.

transconductance (g) = change in drain current / change in gate-source voltage.

FET's nonexistent gate current is its most important character. The resulting high input impedance is essential in may applications.

n-channel enhancement MOSFET: no current flows from drain to source unless the gate is brought positive with respect to source. So when the gate is 'forward bias', there will be drain current. The V-I curve is very similar with BJT.

enhancement, depletion mode: (MOSFET can be made in either, JFET only in depletion mode coz JEFT permit only reverse gate bias)

  • In depletion mode, even with zero gate bias, there is plenty of conduction from drain to source, and gate must be 'reverse-biased' a few volts to cut off the drain current (default on).
  • in enhancement mode, with zero gate bias, there were nonconducting.. Bring the gate positive with respect to the source to drive into conduction.

FET charateristics:

   ^ output (+)

2 | 1
--o----------- input (+)
3 | 4

1. npn BJT, n-channel enhancement-mode MOSFET
2. n-JFET, n-depletion MOSFET
3. pnp BJT, p-enhancement-mode MOSFET
4. p-JFET,

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