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Beggar 31st August 2002 07:46 PM

Confused over hfe variation in BC559 transistors
 
Hi everyone, im trying to put together a mic preamp P93 of Mr Elliott's and have found a large variation in the hfe of 3 of my transistors, 70, 100 and 140, all apparently BC559. Im not very good (at all) at transistor electronics, and have no idea what the implications of this are (if any), does this actually make any difference?

Sorry for this lame question, any responses would be cool.

Petter 31st August 2002 10:51 PM

Due to the physics involved you will find very large variations in HFE and (Hfe which is the small signal equivalent we usually hope is the same as the DC version which you likely measured) in bipolar junction transistors.

For the same reason, the Vbe is without significant variation.

Conversely you will find that for FET's, the Vgs (Vbe equivalent) is extremely variable (usually say 3.5-4.5V).

Tubes are usually pretty damn close in tolerance.

Petter

kiwi_abroad 1st September 2002 01:08 AM

I havn't looked at the circuit you mentioned, but most, if not all circuits using BJT's are designed to not be influenced by hfe variations.
If you think about it, even if you did get a pair with identical hfe values, those values change with collector current, Vce voltage, temperature, frequency, time, lunar cycles etc. and they will diverge.

Tube_Dude 1st September 2002 01:09 AM

beggar
 
The BC559 may have a a hfe betwen 125-475.

But if you need nmore acuracy in hfe you can use:

BC 559 A------hfe-125-250

BC 559 B------hfe-220-475

choosing the A or the B you have less variation of hfe...


Regards


Jorge

Beggar 1st September 2002 03:54 PM

ok thanks for your replies everyone, i can get on with building my preamp now!

peranders 29th September 2002 06:13 AM

Re: Confused over hfe variation in BC559 transistors
 
Quote:

Originally posted by Beggar
Hi everyone, im trying to put together a mic preamp P93 of Mr Elliott's and have found a large variation...

Sorry for this lame question, any responses would be cool.

Note also that BCxxx have different data from different manufacturers. I noticed that when I did some research about noise and caps in BC560C and BC550C. Frankly everything is different, speed also! At the moment ON semiconductors seems to have the best BCxxx types.


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