I built a triple half H-bridge to experiment with a sensorless BLDC. I can make the motor spin but I can't make it run closed loop yet. One of the problems I'm having is that the BEMF from the three phases don't match. In closed loop the BEMF from the floating phase is fed to a comparator and is compared to a common virtual ground.
At the moment I'm trying to match NPN collector current and match PNP collector current by swapping resistors individually. For example, I'm using pn4401 small-signal transistors to drive the bases of three TIP31's and three TIP42g's. One pn4401 has a gain of 139. Another pn4401 has a gain of 226.
I don't have a lot of experience biasing transistors so that's why I'm asking if variation in transistor Hfe is common. Are mosfets easier to balance?