Where did friggen beta come from?!!

[duckless]

Hey guys,

I'm new to this thing. Just have a quick question about Common Emitter Transistor Amplifier circuit design! We're given a bunch of values to include in the design at the start for instance supply voltage, gain etc. I was given 3dB frequency of 100Hz in a particular question. This may be really silly but I don't know how to calculate Beta. I have no idea where it comes from and its essential for my calculations. Any help at all is very much appreciated and if you need any more information just let me know.

 

[MrAl]

Hi,

Usually you look for the beta on the data sheet for that transistor.
Many times it says HFE or hFE or hfe instead of 'beta'.
You then think about variations with temperature to see if your
design will fly.

Typical values are 10, 50, 100, 1000, and vary with temperature and
even current flow.

 

[duckless]

Thank you so much. Your a genius!!! My lecturer was using both Beta and Hfe and I thought they were two different things!

 

[MrAl]

Thank you so much. Your a genius!!! My lecturer was using both Beta and Hfe and I thought they were two different things!

Just to note, usually when nothing else is specified "Beta" is used
to refer to the "Beta DC", which is dc forward current gain, but
once in a while the phrase "Beta AC" is used to talk about the
ac gain. You only have to make sure that they mean Beta DC
and that is what hFE will be on the data sheet.

For example, if hFE is 100 with a base current of 0.001 amps we would
get roughly a collector current of 100 times 0.001 which is 0.1 amps.

 

[crashsite]

Friggin' Beta

Hey guys,

I'm new to this thing. Just have a quick question about Common Emitter Transistor Amplifier circuit design!

Actually, there's regular beta and it's pesky counterpart, "friggin' beta" (which gets even nmore explicit as the design fails to work as expected).

For some reason, engineers are always trying to complicate things. Back in the early days of transistors, they were trying to define current gain by, alpha. Basically, alpha is the ratio of emitter current to collector current so they were trying to define base current in a kind of bass-akward manner (as engineers are often wont to do). Beta is a more direct measure.

If you can find the el cheapo digital multimeter on sale from Harbor Freight Tools (sometimes as low as about $3), it has a function for transistor gain (directly displayed as beta).

But, a transistor beta tester is super easy to make. You just put a resistor in series with the BE junction of a known value (I like 10k or 100k), tied between Vcc and common and then put an ammeter in series with the CE path (also Vcc to common). Set your power supply to 10.6V (the .6V to account for the BE voltage drop), and the collector current vs base current is easy to calculate.