Nate,
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When Allison says a max of 16 volts, she means the power supply into the rig.
Transmitting on 80m with the QMX from a 12v supply into a 50 ohm dummy load can cause
peak voltages of more than 60 volts from drain to ground at those BS170's.?
And 60v is the absolute max listed in the BS170 datasheet.
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In addition to reading the datasheet for the FET's, you also need to know how the circuit works.
That is not a trivial task, unless like Allison you have spent a career doing this stuff.
Here's a tutorial on a Voltage Mode Class D push-pull amp such as used on the QMX:
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Why such high voltages?
As described in that tutorial, the action of transformer T501 causes twice the power supply voltage to appear
across each BS170 when it is not conducting.? In addition, the LPF in the QMX blocks
the undesired harmonic currents from going out to the load, which raises voltages further.
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For those wishing to see what's going on, I'd recommend using
the LTSpice simulator on a model of the QMX final.? Such a model can be found
in the left-most of the four simulations found in this file:
The discussion is in this thread:??
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Jerry, KE7ER
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On Mon, Mar 24, 2025 at 05:20 AM, Nate Reik wrote:
On Sun, Apr 25, 2021 at 10:53 AM, ajparent1/kb1gmx wrote:
There is some magical thinking with regard to RD15hhf parts.
RTFM, in this case, the data sheet.
First its a 16V max part.
Can someone explain this? I'm trying to understand maximum values of RF transistors...and the RD16HHF1 datasheets I see don't say 16v anywhere. I see 20v and 50v ratings...and I see things about rules of thumb elsewhere...but how do I determine from the datasheet of an RF transistor what the maximum DC supply voltage is?
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Thanks for any help with my learning!?
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-Nate
N8BTR
