I was measuring the bias at different source resistors on the output stage of my BA-3 amp, and noticed a variation of ~5% between any two MOSFETs. This makes sense, because my source resistors are 5% tolerance.
I wondered how much this variation affects the overall output, and how it relates to MOSFET matching? I feel like I'm just ignorant enough that I'm probably asking the wrong question, but hopefully this makes enough sense that someone can point me in the right direction 😛
I wondered how much this variation affects the overall output, and how it relates to MOSFET matching? I feel like I'm just ignorant enough that I'm probably asking the wrong question, but hopefully this makes enough sense that someone can point me in the right direction 😛
it is always best to think in terms of specific project, because tolerance demands are not the same for different constructions
for BA3, you're safe in 10% bracket
safe meaning - it'll work and you'll not hear difference
for BA3, you're safe in 10% bracket
safe meaning - it'll work and you'll not hear difference
I matched MOSFET's and source resistors (from a bag of 50) when I was building Aleph J. The voltage drops across all 4 0.47ohm resistors are within + and - 5mV worst case... I am sure it doesn't mean much, but it's nice to see the mV meter showing a very small deviation between all 4 measurements.
OP - The variation may be from the MOSFETs themselves. I wouldn't necessarily assume the resistors, but you're correct, it could be.
As for effect, you've gotten some great answers.
For clarity, what do you mean by "any two MOSFETS"? Was the difference between the highest and lowest measurement ~5%? ~5% difference between any two seems odd, and then you'd likely have a problem.
As for effect, you've gotten some great answers.
For clarity, what do you mean by "any two MOSFETS"? Was the difference between the highest and lowest measurement ~5%? ~5% difference between any two seems odd, and then you'd likely have a problem.
The spread between the minimum and maximum value is within 5% +/-, or roughly 0.238 to 0.262. Most are tighter than that, ~0.242-0.256, but I have a couple outliers.
Your maximum value of .262 is roughly 10% higher than your lowest value of .238.
Either way... I wouldn't sweat it, but I now understand what you mean. (pun intended)
Either way... I wouldn't sweat it, but I now understand what you mean. (pun intended)