Hello Dick,
The power FET's that are used in LF-Pa's are less vulnerable than most
bipolar power transistors. These FET's are designed for switched power
supplies and can withstand quite a lot of mistreatment if some basic rules
are observed.
I am still using the same FET's for the past 3 years in a class D PA
(fingers crossed). I believe that most problems with these FET's occur on
linear PA's, as you try to make a linear amp with devices that are
optimized for switching.
Main advantages of these power FET's are :
- fast switching (= high efficiency in class D or E)
- low 'on voltage' (= low heating of the device)
- far less sensitive to 'thermal breakdown' (compared to bipolar)
- withstand rather high voltages (400V or more for most devices)
- (almost) powerless drive
- cheap
73, Rik ON7YD
At 11:27 29/05/01 +0200, you wrote:
To All from PA0SE
Most of the designs for high power transmitters for the LF bands I have
seen use power-FETs in the final. Examples are the designs by G0MRF and
G3YXM in The low frequency experimenter's handbook. But we all know from
experience that FETs are very vulnerable (extremely fast fuses). Would not
power bipolar transistors be more robust? What are the advantages of FETs
over bipolars?
FETs do not require real power to drive them. But for stability and maybe
other reasons rather low value resistors are often connected from gate to
common; this together with the higher voltage to drive a FET than a bipolar
transistor could well mean that the actual power to drive a FET amplifier
is higher than for a bipolar design.
The Handbook features a design by DJ1ZB using complementary
darlington-pairs (the emitter arrows in some of the transistor symbols
point the wrong way!). But they produce 100W only.
I think I have seen higher power bipolar transistor transmitters in
Break-In. Perhaps the ZL boys can comment.
If anyone has evaluated the pros and cons of field effect and bipolar
transistors for high power LF transmitters I would be grateful to hear them.
73, Dick, PA0SE
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