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Old 6th Sep 2010, 12:58 pm   #11
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,488
Default Re: Transistor Choice in radio project

Originally Posted by mhennessy View Post
Um, what has hfe got to do with the gain of the amplifier?...You only need to consider hfe when establishing the biasing circuits.
Voltage gain is sensibly independent of hfe, agreed, but the input impedance of a common-emitter stage is highly dependent on hfe.

So, if you are using your transistor to amplify the output of a diode detector, having an output impedance of a few kilohms, you want an amplifier with a highish input impedance. Otherwise, it'll load the detector, the AF voltage will drop, the transistor will see a smaller input, and give a smaller output.
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