Re: FETs and MOSFETs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rob-at-pythonemproject-dot-com>

Tesla list wrote:
> 
> Original poster: "Justin Hays by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<pyrotrons2000-at-yahoo-dot-com>
> 
> Hi All,
> 
> FET is short for Field Effect Transistor, they are generally used for
> small-signal (tiny signals from an antenna) amplification purposes in
> radio circuits. There are many other uses, however. An example of a
> common FET is an MPF102, which Radio Shack stocks.
> 
> A MOSFET is a different kind of FET, and stands for Metal Oxide
> Semiconductor Field Effect Transistor. Generally speaking, MOSFET's
> can handle more power than FET's or JFET's ("J" for Junction), and
> are commonly used in linear amplifier circuits and switching
> applications. If you need a bounceless switc that will go from "open
> circuit" to "short circuit" in less than 20 nanoseconds, a MOSFET
> will do it. Common MOSFET's are the IRF840 (500V 8amps) and the
> IRF740 (400V 10amps).
> 
> All types of field effect transistor have at least 3 leads, one is
> gate (the "control" pin), source, and drain. Some RF (high frequency)
> transistors have two gate pins to ease circuit design.
> 
> But it does go deeper, there are different types of FET's, depending
> on their internal silicon structure. They are N-channel, P-channel,
> enhancement mode, and depletion mode types....it all depends on what
> your circuit requires as to what FET type to use. For instance, if
> you want to turn the transistor ON with some positive voltage (most
> common application), you use an N-channel enhancement mode FET.
> 
> It's very important to note that all types of FET are controlled by
> voltage, and not current.
> 
> The input impedance (gate to source) of a FET is extremely high,
> making them very easy to work with because their input won't load
> (pull current from) your circuit.
> 

Only true at DC.  The typical SSTC FET gate requires amps of current to
drive it to anything closely resembling a square wave at high
frequency.  Thats because most power MOSFETs have 100's to 1000's of pF
capacitance from gate to source.  However, the reason FETs are used
rather than bipolar transistors, it that the gates can be driven
symetrically positive and negative, making on/off switching easy. 
Bipolar transistors require a much more elaborate base drive to
accomplish the same since the base emitter junction usually starts to
break down at -4V, and is clamped at 0.6V or so positive.  Rob.



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