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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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