RE: repeat of question

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From: laurahal@microsoft.com
Date: Fri Dec 10 1993 - 16:31:00 EST


Ed Guinn asks:

> A question. I am about to build a 4 to 6 watt amp,
> the design of which comes from page 61 of the Solid State Design book
> by the ARRL. They call for a GE D446C transistor, and suggest a 2n5321
> as a replacement worth experimenting with. Well being very new to this
> hobby I am not sure if the design of the amp is highly dependent on
> the characteristics of those particular devices or can I substitute
> another device...

Things aren't as critical at HF as they are at VHF and UHF.
Buy/beg/borrow/steal a transistor data book and have a look. You want to
match or exceed the D446C's dissipation, get reasonably close on its beta
(Hfe), get the capacitances reasonably close, and match or exceed (but not
by too much) ft. Make sure the various breakdown and bias voltages are
reasonably close, and try it and see.

Try not to spend too much for the transistor you use, so you won't be out
too much if you blow it up.

Matching or exceeding the dissipation is self-explanatory. Matching beta and
ft makes sure that you have the right gain at the right frequency, so that
the amplifier remains as stable as the original. Ditto the capacitances to
get impedances and feedback right.

W1FB's Design Notebook is another good reference on this and many other
issues.

While we're talking about books, any suggestions on a good place to buy
goodies like those Motorola RF devices data books everybody keeps referring
to?

73 from Burnaby,
laura VE7LDH


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