Whats Wrong with my BJT Buffer?

Started by KerryF, December 16, 2006, 05:56:17 PM

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KerryF

I built the BJT Discrete Buffer from General Guitar Gadgets and for some reason it wont work!  I have tried it with an effect after it, by itself, etc.

Heres the schematic:
http://www.generalguitargadgets.com/pdf/ggg_buffer_bjt_sc.pdf
I used a 2N5089 like they said.  Heres my readings:
Battery: 8.24V
C: 8.24V (This is connected straight to the battery +)
B: 7.05V
E: 8.16V

Maybe a misbiasing, but I dont know about BJTs and biasing...  Whats going on?

R.G.

It is indeed misbiased.

"Bias" is just a fancy word for setting things to be ready for the expected signal - not too much current or too little, not too much voltage or too little. Just right.

In this case, just right means half of the battery voltage on the base. The two 220K resistors are intended to divide the battery voltage in half and apply that to the base. Your battery is 8.24V, so the base should be at 4.12V. Instead, it's at 7.05V. That puts too much voltage on the base, too much current through the emitter, and so there is no voltage swing left to do any signal you stick in.

I think that you have a soldering or component placement problem. Measure the voltage at the junction of the two 220K resistors - is it half the battery? If not, you have to correct that first.

It may be that there's a bad connection to the base, or the emitter may not be connected through its resistor to ground.

R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

KerryF

Fixed it.  Input and transistor were connected to the wrong places  :icon_mrgreen: