Nate,
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Some additional details:
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In that old thread I pointed to, I suggest L502 is causing larger voltages across the BS170's than there should be:
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?That file with four different LTSpice simulations shows why I think this is the case:
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The first of the four simulations is a model of the QMX as it arrives from QRP Labs, the maximum voltage across the BS170's is 40v.
When looking at the QMX with a scope, we can see even higher voltages across the BS170's, sometimes more than 60v.
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In the second simulation, L502 has been removed.? The maximum voltage across the BS170's is now the 24v we would expect.
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In the third simulation, a simple series LC circuit is used instead of the multi-band LPF of the QMX.
Now the voltage across the BS170's is a nice square wave, not the sharp peaks of the second simulation.
The current through the BS170's is a sinusoid.
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I feel that L502 is not appropriate here, making the QMX amplifier a weird cross between a Voltage Mode Class D amp
and a Current Mode Class D amp.? In the fourth simulation is the equivalent Current Mode Class D amp.
Here it is the current through the BS170's that is a square wave, and the voltage that is a sinusoid.
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For those that don't have the free (and highly recommended) program LTSpice installed
to simulate these things, I have added a pdf of the LTSpice schematic to the files section:
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Jerry, KE7ER
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On Mon, Mar 24, 2025 at 06:46 AM, Jerry Gaffke wrote:
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:
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Hide quoted text
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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
