Thursday, December 27, 2018

Reversing a transistor hookup - will it work

A subject that comes up now and then is "can the circuit work as connected?" The circuit will likely be a Germanium transistor and then the discussion goes kind of mystic and implies the transistors are just beyond understanding. Oddly the board expert will throw out terms like Ebers-Moll.  Then we hear it is a Germanium being leaky and some other rubbish. So is it possible the circuits work are do we we have a few hundred circuits presented in the old books and magazines that just were untested and will not work?
Oddly when the Ebers-Moll enters the discussion it is with no explanation and is just to impress the readers so let us look at the model.

Ebers-Moll sees the transistor as 2 diodes connected back to back. One is forward biased and one is reversed biased. Would this model represent a real world transistor? Let us look at a couple of them.
This example is 2 diodes sharing the base connection. They are made with the same doping so would be represented by the model quite well. (the exception being the common connection rather than 2 cathodes.) This would be an early transistor such as found in the old books.
A later transistor is the epitaxial shown here. This time we see n- , n+ and p-. What they tell us is the doping levels are not the same. + is heavy doping while - is lightly doped. The 2N2222 is a planar epitaxial transistor so I will use it for the test. (The 2N2222 is the negistor I used in my simple oscillator.)
A quick look at this circuit will reveal I have installed it upside down yet with 10mv in I get 600mv out. A gain of 60 with a backward connected circuit!
I rotated the transistor to the 'proper' position and now have a loss in signal.
I adjusted the bias and now have the gain of 60 with the transistor installed properly. Why did I need to adjust the bias? Remember the p+ and n-? The doping is so a small base emitter current can control a larger emitter collector current. The heavily doped emitter injects carriers more efficiently.

Some food for thought,eh?

EDIT: I should point out the power handling ability of the transistor will need to be watched if using the circuit. The question would be ,'what is the base current?' Using a higher base current could burn the base out. It might be worth a test board.
EDIT 2: I think a 2N5550 will work backwards too.
Another circuit to test.


  1. Hi, I'm building a guitar pedal using a Germanium AC125 transistor. When I properly orient the transistor, there is no gain, but when I put it in backwards, I get a nice slightly distorted gain tone! Is is okay to just use it "backwards"? Will this cause the transistor to eventually fail? How can I get it to work properly oriented? Should I increase R1?


  2. Now let us talk about doubling! Guitar tracks no matter how well performed can often times seem thin in the mix, even when using good guitar pedal effects. Digi Pedal I have to admit that I have tried some multi effect pedals through my tube amp and they sounded very bad (so do some single pedals).