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Originally Posted by kalee20
Voltage gain is sensibly independent of hfe, agreed, but the input impedance of a common-emitter stage is highly dependent on hfe.
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Yes, hie is hfe/gm. For a typical silicon small-signal device at 1mA, you're talking about 5-8K. The bias network normally contributes as well. But of course, this is the case when there's no emitter resistor (normally a bad plan) or there's a decoupled emitter resistance (not great either - for audio at least - unless there's global NFB elsewhere); otherwise the input impedance is raised by the feedback factor. Of course, this is at LF where the "Millered" collector-base capacitance isn't bringing this down even further, and while I haven't done the sums, I would suspect that for a typical CE IF/RF amplifier, this might well dominate.
Obviously we've no idea what the OP is doing...
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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.
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Having recently studied lots of 1960s radios, I was surprised that the detectors where typically driving into ~5K - I certainly expected higher than that, and couldn't see any other reason for not using higher values. This leads me to conclude that other factors are at play, and I assume that the Hacker brothers et al knew far more than I ever will
Mark