That¡¯s very wise of you to accept the possibility that it could be done! ?
I still don¡¯t like adding more noise to that +12 line.?
I¡¯m swamped as it is but eventually I¡¯ll try the little experiment with the zeners. ? The idea that Asher had would certainly work as well but the negative zener might be easier for the downside.?
I don¡¯t understand these avalanche diode¡¯s but that might be another way as well. ?
The more bulletproof we can make this the fewer difficulties will be expressed on the blog.?
Gordon Kx4z?
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On Nov 3, 2023, at 20:27, Ashhar Farhan <farhanbox@...> wrote:
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Steve and gang,
Interesting. I have fallen for the classic mistake of generalization from just one experiments data point.
There is an better way to do this with a slightly more elaborate scheme.
Let us say we want to limit the upper swing of the voltage to 12v. We add a 1N4148 diode from the gate to the 12v line. Now, the diode will conduct only when the gate voltage exceeds 12.6v. The 12v needs to be heavily bypassed, but it will work very fast without adding more than 1 or 2 pf to then gate capacitance.
Clamping the negative swing is trickier. As the gaye bias is in the region of 3 to 4v and you want the swing to continue to at least -9v or thereabouts. You will need a -9v voltage source, so that a reverse diode from the gate to -9v conducts at those swings. One dirty way is to add a 9v pp4 battery to the circuit (that is what I did to try this out), the other way is to?generate this voltage from the RF, pass it through a 79L09 or a zener to get a stable clamping voltage.
I havent tried it out but given that it is push pull, is it enough to clamp the positive swing on both transistors to clamp to the other side? Someone should LTSpice it.
Yet another way is to clamp the wave form a little upstream. A series of 1n4148s across the input winding of the driver transformer can limit the swing too. You will need a pair of them to limit it both ways.
- f
Steve, that is perfect information. THANK YOU ?I believe I bought exactly the zeners you recommended and I should be able to repeat that test!!? I have a 100 MHz Siglent scope and decent probes. ? Whatever the outcome maybe I can get a photo?
Out of an abundance of caution I put them (ie zeners) on my development edition and I am still using the original MOSFETs although I don¡¯t use it that much. When I do use it, it is often driving an SB-200 amplifier ¡.which doesn¡¯t give the best input impedance.? No problems so far. ? I get pretty close to full break in on CW and it is just wonderful.?
Gordon
?.. And now i did find my previous write-up - message # 99738. Search for "91" as-in 91 ohm resistor.? N3SB
