finding the subsitute of a Transistor... !!!

[muddasserniaz]

bfr91 sustitute

What are the salient features of a transistor, that need to be compared from datasheet to find its subsitute.. ???

for example here is the datasheet of BFR91 Silicon NPN Planar RF Transistor. how can i find its subsitute.. plz give detailed description or links... not just subsitute transistor numbers... !!

also any online resources/databases which can help do this.. !!

 

[aryajur]

subsitute bfr91

It would mainly depend on the usage of the transistor in the circuit. The way you use it would determine which specification is most important and that specification should be sought in other transistors to find a substitute.

 

[muddasserniaz]

bfr91 substitute

It would mainly depend on the usage of the transistor in the circuit. The way you use it would determine which specification is most important and that specification should be sought in other transistors to find a substitute.

thanks "aryajur" for clarification !!!

but as i am just a hobbyist can someone offer simple tips and tricks???

BTW the author of the circuit (Frequency Counter) says [although i used BFR91 ... Any "fast" NPN transistor should be suitable]... so what should i search in BFR91 datasheet and look for it in other transistors.. !!

article link is .. http://ironbark.bendigo.latrobe.edu.au/~rice/newfm/newfm.html
i am also attaching the schematic.

 

[aryajur]

For this case the transistor is used to act as a switch. So you should see the transistor with fast switching speed. The turn on and turn off times. Also high Beta at higher frequencies. Also look for the capacitances. The substitute transistor should have similar capacitances or maybe less if you use it for higher frequencies.

 

[louisnells]

BFR91 is a transistor used in upper UHF region (we use for even up to many GHz). So if you are not going to use signals upto that level you can substitute a low frequency range transistor.