Been designing a transistor matcher.
Its attached by USB to a pcb so you can plot the transistor collector voltages from an inverting transistor stage on the screen to compare.
Got the npn part working fine. Currents look fine and hfe was ok but varies a bit depending on where on the curve you are.
However, the pnp side I couldnt get working.
The collector current was flowing 0v4 before the base voltage turned on !
Impossible ?
I flogged the hardware to death trying to find a problem but couldnt.
All voltages looked fine.
Eventually hit me that the microcontroller should invert the PWM output and that flipped curve over to align base and collector currents.
Then the pc end should flip it again for the screen so its same phase as npn.
So wasted a day on searching for a none existent hardware fault.
Its attached by USB to a pcb so you can plot the transistor collector voltages from an inverting transistor stage on the screen to compare.
Got the npn part working fine. Currents look fine and hfe was ok but varies a bit depending on where on the curve you are.
However, the pnp side I couldnt get working.
The collector current was flowing 0v4 before the base voltage turned on !
Impossible ?
I flogged the hardware to death trying to find a problem but couldnt.
All voltages looked fine.
Eventually hit me that the microcontroller should invert the PWM output and that flipped curve over to align base and collector currents.
Then the pc end should flip it again for the screen so its same phase as npn.
So wasted a day on searching for a none existent hardware fault.