Quote:
Originally Posted by astral highway
Quote:
Originally Posted by Philips210
I does seem the 2N5109 was used for good reason as it's decent fT figure is important
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The transition frequency of many jelly bean types is way over 100 MHz!
I think this is posted as a domestic receiver antenna, not particularly for comms receivers?
I would need a lot of convincing to believe that any specialist RF types are needed for this circuit. |
OK, here's a little bit of convincing....
Ft is the frequency at which the current gain of a transistor has fallen to unity. If you want some gain from the wee beastie, then you need an Ft appreciably above your top operating frequency.
Even if power gain isn't wanted, or achieved in a circuit, you may still want current gain in order to achieve wanted input and output impedances. These circuits try to create both a moderate amount of gain and suitable impedances.
If that's not enough, these antennae are untuned and wide open to all signals in the environment, every last one. You want a transistor with high linearity at these frequencies otherwise intermodulation products of strong signals (even ones outside the tuning range of your radio can create sproggies all over the place and Murphy dictates one will be smack on top of that quiet station you want to hear. 2N5109 was designed for low intermod products and intended as a CATV amplifier device. The 2N3866 is a moderate power RF transistor, good for a QRP PA and it's pretty linear too.
You could make such an antenna work with jellybean transistors, but good RF ones should give significant advantages in several respects.
David