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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Fri, 19 Aug 2011 18:51:37 +0100
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Subject: LF: Re: ztx
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Dear Mal, LF Group,

G3KEV wrote:
>I have a couple of ZTX 450 transistors, would these be a substitute for the 
>ZTX690B that you use for loop amps
>Gain looks about the same, don't know about noise figure

Not really. I used the ZTX690B because it has very high hFE when biased to 
several milliamps (roughly 1000) - this results in less shot noise due to 
lower base current. Also, being a quite high current device (2A continuous 
Ic in a T092-sized "E-line" package), it has low parasitic resistances, 
which also helps to minimise noise with a low (50R) source impedance. The 
ZTX450 is somewhat inferior, especially in terms of current gain. It will 
"work", but expect slightly reduced gain, and noise level increased by some 
dBs. If you want to use other devices, look for high hFe and high current 
rating / low Vce(sat), together with a reasonably high fT (>100MHz), when 
biased to 10mA or thereabouts. Typically this means transistors for 
"switch/driver" applications.

Cheers, Jim Moritz
73 de M0BMU