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[Mgt280y]

I am sourcing some parts locally but they have multiple manufacturers for the same component specifically 2n3904 they are all the same In terms of temps voltages etc only difference is the hFE ratings 100 hFE or 300hFE can anyone tell me how this will affect anything positively or negatively
Thanks dan

[slacker]

The hFE won't make any difference, any 2n3904 will do the job.

[Mgt280y]

Hoping that was the case, did read up on what hFE is but tbh didn't really understand it lol

[PRR]

> 2n3904 ...hFE ratings 100 hFE or 300hFE

hFE of 2n3904 at 10mA is 100 minimum 300 maximum. So it sounds like someone plucked different numbers for the same spec on the same transistors.

It's like buying "1 pound of assorted buns, 6 to 12 count". One store may say "at least 6". Another may say "up to 12". Actually, many bun-buyers can count, and complain, so stores don't usually quote the max.

> read up on what hFE is but tbh didn't really understand it

You run a bus company.

Each bus needs a driver.

Buses can be 50 passenger, 100 pass., 300 pass., 900 passengers. (Maybe not real busses on real roads....)

So one driver can move 50, 100, even 900 passengers per driver.

In a transistor, one electron to Base will move 50, 100, even 900 electrons through the collector.

So it is a measure of "gain". Usually NOT "signal gain" which is other factors. How well the transistor does what its bias supply tells it to do.

When they make buses, they measure the frame rails and count the seats. But imagine they "grew" buses like trees. Some will be more seats than others. When they make transistors they shine some shadows on the Silicon and then bake it. Very small variation in fabrication make significant variations in hFE.

hFE also varies with current (and almost everything). Like a bus that carries 100 passengers per driver at 60MPH but needs 2 drivers per 100 passengers at 15MPH or 120MPH.

With clever conservative biasing, hFE may not be very critical. I was taught to "assume hFE is over 50 (even infinity)" when doing EE101 homework.

Pedal-World is full of less conservative designs where hFE may have to be picked for the circuit, or the circuit may want to be trimmed for the specific hFE you plucked out of the jar.

[ElectricDruid]

Nice, Paul. That's almost certainly the strangest analogy I've come across. But it works, kind of 

T.