Noob Alert: How to tell the ..

[mat]

..gain of the transistor from the datasheet ?

Is 'hfe' same as the gain ?

:oops:

mat

[smashinator]

Yes, hfe is a measurement of transistor gain.

[Peter Snowberg]

Sometimes it's also called "beta", or "DC current gain".

[R.G.]

Be careful that you know what you're talking about when saying "transistor gain".

Transistors are current mode devices. That is, they let X times more current through the collector than comes in the base. The datasheet calls "X" HFE, the DC current gain.

Current gain is NOT voltage gain, which is what we talk about most. Most transistor circuits do not give anywhere near the voltage gain that they have in current gain. The extra current gain is used up in feedback setups to get stable, predictable operation.

Most straightforward transistor circuits have voltage gains under 20.

Also, HFE is only the gain at DC at the specified conditions on the datasheets. Current gain in bipolars varies with collector current, temperature, and phase of the moon...

[mat]

Thanks for the further clarification mates. And R.G. I have to 'melt' your answer for some time  :oops: (like a couple of years or so.. :oops: )

m :oops: a :oops: t