Thread: Transistor gain
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Old 16th May 2019, 5:44 pm   #3
Radio Wrangler
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Default Re: Transistor gain

The hfe figure is the current gain of a transistor, and as Paul says it's under specific conditions. When looking at hfe figures, pay close attention to the collector current and voltage it was specified under. It can change quite a lot, especially with collector current. Really good datasheets give graphs of hfe versus current.

hfe tells you nothing about the frequency capabilities. ft is the frequency at which the falling current gain with frequency is projected to hit unity.

hfe tells you nothing about the noise behaviour

hfe tells you nothing about optimum source and load impedances the device should be presented with for best gain and lowest noise (sadly usually different impedances!) These figures become rather important in RF circuits.

hfe itself varies a lot from device to device even of the same type number. Good designers make circuits which will tolerate this. Plenty of devices have open ended specs with no maximum quoted. So them both working similarly isn't a surprise, just an indication of good design.

Note that your little hfe meter measured the two transistors at different currents.

Too many people get too precious over the closeness of equivalents. Quite often you can get away with murder, but proving it from design calculations is a lot of work.

David
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