transistor equivalents, what to look for?

Thread Starter

Evanguy

Joined Dec 21, 2014
81
i wondering about using different transistors then a circuit shows and what to look for in doing so, other then the obvious things.
in this case I'm looking to build a small fm radio and it calls for a BF494 (datasheet at bottom), but i do not have one of those in my parts pile, ive searched through about 25 different transistors i have on hand, checking their data sheets and i believe i have found two that will work.
the C3203 also the C3199, them seem like they can handle the job but i have been reading a bit but still dont understand. in this case i believe switching speed is the factor, and near as i can figure that has to do with the FT and FR, they are both around 120mhz same as the BF494

but i have a few transistors that are over 300mhz FR, would they work as well?

basically im asking two things here, one being, well these work for me in the circuit linked at the bottom, and the other is what do you look at to determine the switching speed and also other things you may consider in looking for a transistor not used in circuit diagram.

BF494 datasheet http://www.hestore.hu/files/BF494,495.pdf

C3203 datasheet http://pdf1.alldatasheet.com/datasheet-pdf/view/550190/KISEMICONDUCTOR/C3203.html

C3199 datasheet http://www.fgxdz.com/technicData/TO-92/C3199.pdf

Circuit in question and its "T1" http://3.bp.blogspot.com/-_etP0u4gSRE/Ul4mPwlBvJI/AAAAAAAAFgA/2guwEdAF9ao/s1600/single+BF494+transistor+FM+radio+circuit.png
 
Last edited:

dl324

Joined Mar 30, 2015
10,984
the C3203 also the C3199, them seem like they can handle the job but i have been reading a bit but still dont understand. in this case i believe switching speed is the factor, and near as i can figure that has to do with the FT and FR, and the lower the mhz the faster it is?
Higher ft means faster transistor. C3203 seems to be the closer match.
 

Thread Starter

Evanguy

Joined Dec 21, 2014
81
Thanks for that, so the BC547 work with ft of 300mhz,
its seems the BF494 is kind of slow, there is a lot of transistors with alot higher FT that are not listed at all as fast or for med-high freq, most times listed as general purpose
could even the pile of 2n2222's i have work then at 300mhz FT for the fm receiver?
 

dl324

Joined Mar 30, 2015
10,984
Aside from slightly lower beta, 2N2222 could be an appropriate substitution, but the circuit was designed for BF494, so transistor speed must not be critical. The BC547 is also faster.
 
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