16-05-2017, 03:21 PM
Beta is the ratio of collector and base currents. Simple.
The emitter current is the sum of the base and collector currents. Also simple.
Given that, can you work out beta from just emitter and collector currents? Perhaps. Subtract Ic from Ie, the difference is Ib. Then go to line 1 above.
What about emitter and base currents? Subtract Ib from Ie, the difference is Ic. Then go to line 1 above.
But that assumes that your measurements are accurate, and given that Ib is usually measured in microamps in a typical small-signal circuit, that's by no means certain. Remember, beta is often much greater than 100 for small-signal transistors.
Ultimately, who cares? Beta is an irrelevance for most properly designed circuits. It varies so much from sample to sample, with temperature, with collector current, with planetary alignment, etc. A designer might assume a certain minimum value, or a (wide!) range of values, but would never assume a certain value. That would be commercial suicide.
If you cast your mind back to that transistor amplifier exercise, you'll remember that we didn't use beta at all. Which reminds me, you still haven't done the second exercise. Hmm...
In over 30 years of using transistors, I've never knowingly used alpha.
Unfortunately, old text books place the wrong emphasis on transistor theory (and plenty else besides). Also unfortunately, you seem to be unwilling to accept this. As a teacher, it's really painful for me to watch...
The emitter current is the sum of the base and collector currents. Also simple.
Given that, can you work out beta from just emitter and collector currents? Perhaps. Subtract Ic from Ie, the difference is Ib. Then go to line 1 above.
What about emitter and base currents? Subtract Ib from Ie, the difference is Ic. Then go to line 1 above.
But that assumes that your measurements are accurate, and given that Ib is usually measured in microamps in a typical small-signal circuit, that's by no means certain. Remember, beta is often much greater than 100 for small-signal transistors.
Ultimately, who cares? Beta is an irrelevance for most properly designed circuits. It varies so much from sample to sample, with temperature, with collector current, with planetary alignment, etc. A designer might assume a certain minimum value, or a (wide!) range of values, but would never assume a certain value. That would be commercial suicide.
If you cast your mind back to that transistor amplifier exercise, you'll remember that we didn't use beta at all. Which reminds me, you still haven't done the second exercise. Hmm...
In over 30 years of using transistors, I've never knowingly used alpha.
Unfortunately, old text books place the wrong emphasis on transistor theory (and plenty else besides). Also unfortunately, you seem to be unwilling to accept this. As a teacher, it's really painful for me to watch...
