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I stopped using BJTs for a while and now I can't figure out why this configuration does not give me a 12 V PWM at the gate of the MOSFET? I am only getting slightly more than my PWM voltage about 4.2 V.

I have tried resistors at the gate to limit current and different resistors at the PWM for some input impedance but nothing. If I am not mistaken, when the top transistor is on it should effectively close that "switch" and I should get my 12 V at the gate. Maybe I've spent too much time playing with MOSFETs and I have forgotten how BJTs work.

schematic

simulate this circuit – Schematic created using CircuitLab

Null
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Edwin Fairchild
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1 Answers1

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Your BJTs are configured as followers so the gate signal will be smaller than the drive by a diode drop each way. Perhaps add a common emitter amplifier between the drive signal and Q1/Q2 to boost the amplitude to near 12 V peak to peak.

Steve Hubbard
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    oh I see, I remember that term from a couple of semesters ago. I think I need to hit the textbooks for a refresher to see how that is so. – Edwin Fairchild Nov 02 '18 at 04:38