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LF: Field effect versus bipolar transistors

To: "LF-Group" <[email protected]>
Subject: LF: Field effect versus bipolar transistors
From: "Dick Rollema" <[email protected]>
Date: Tue, 29 May 2001 11:27:09 +0200
Reply-to: [email protected]
Sender: <[email protected]>
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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