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So when manufacturers build transistors they always have to trade-off certain parameters for others.

RF transistors often have a high gain-bandwith product and low capacitance to make the gbw usable. But what is usually traded for this higher frequency?

E.g. in which cases would a general purpose transistor do a better job at audio frequencies than a RF part of comparable cost and size.

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    \$\begingroup\$ Have you even glanced at the datasheets for a couple of typical RF vs. jellybean transistors? A few things should jump out at you... \$\endgroup\$ – Spehro Pefhany Apr 30 '16 at 13:51
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Partial answer :

1) High gain at high frequencies can be embarrassingly difficult to stabilise. Even conventional BC214s, used as emitter followers, can become UHF oscillators thanks to parasitic inductances in an audio PCB layout, resulting in relatively poor noise, distortion and interference due to RF signals mixing down into the audio band.

More gain at higher frequency just makes this more difficult.

2) Low capacitance often implies small device features. That brings several disadvantages, including poor power handling.

3) Another disadvantage of small device features is increased noise; larger devices, like thicker conductors, tend to involve lower resistances (base spreading resistance is a key parameter) which allows lower thermal noise.

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