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Transistor Question ...

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IronMan79

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Mix two Signals with a Transistor ?

I would like to see if it is possible to mix two signals with one transistor: I don't have equipment to test it but would like to know at least theoretically what could I expect: I want to mix two signals and get some kind of mixed output, it doesn't matter how they are combined, just a random mix of the two signals.

So I though about injecting a signal into the Collector, a different signal into the Emitter and see what could come out of the Base terminal: I expect a very weak signal obviously since I am using the transistor not really as an amplifier but as some kind of mixing signal system, but the weak signal should be some kind of mix between the two signals injected into the Collector and Emitter, using the Base as an Output.

Would it work ? Could I expect to see some kind of mixing of the Signals ? Thanks for any advice or ideas...

I am not trying to solve any "problem", I am not trying to perform any "function", etc. I was just curious to know what happens if a signal is injected into the Collector and another in the Emitter and the base is used as an Output: I know that this is not "the way transistors are used", I know that you can mix signals in many other ways, etc (like injecting a signal in the Emitter and a different one in the Base and getting the Output from the Collector, etc. ), etc.

I was just wondering how a transistor would behave if you made it work "upside down" in a sense, just for "fun" and "curiosity".

I thing that you could expect a very weak signal out through the Base because of "leakage currents" and signals and such ?


[quote name='Xittenn' timestamp='1329206685' post='658294']
Obviously not, because they are, and it is called a common base configuration and it finds special application.
[/quote]


Wrong Answer: tell me what could come out of the Base if it is used as the Output...
 
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Base as output? Possible in theory and impossible in real world.
Two or more signal currents applied to base produce a different signal mix in collector current. Possible from simple signals addition to more complex mix products depending of signals amplitude etc.
 
usually the base and emitter are used as mixer inputs and the collector is the output. did you want linear or nonlinear mixing? with linear mixing, the output consists only of the sum of the inputs. with nonlinear mixing, the output includes intermodulation products of the two inputs, and is usually used for intermediate frequency converters (i.e. an RF frequency of 1Mhz on one input, a local oscillator at 1.455Mhz as the other input, and difference frequencies of 455khz and 2.455Mhz exist on the collector, which is filtered, leaving only the 455khz intermediate frequency).
 
What are you mixing?
Two audio signals?
Two radio signals?
Modulating a radio frequency with an audio frequency?
 
No, I am not trying to solve any problem or trying to perform any function: I just wanted to know what could I expect if I use the transistor "upside down", just for fun, for curiosity, so the idea is to inject a signal in the Emitter and another different one in the Collector and see what would come out of the Base: of course this is not the way a transistor is configured, but that is exactly why I would like to know what could possibly come out of the Base. I imagine a very weak mixed (non linear ?) signal.. but I am just guessing...

It is just an experiment not an application....
 
Keeping it very basic and somewhat crude attached is an example of what mixing two audio signals of 500 Hz. and 1 KHz. would look like. Note in this scenario both signals are fed in at the emitter. The results can be seen in the lower plot image. So without defining Audio, RF or whatever, can two signals be mixed using a single transistor? Yes, they can. The circuit is a slightly modified version of something I saw out there on the web. As drawn it is not to be taken as something anyone would really use but illustrates I believe what you asked.

Ron
 
A transistor connected "upside down" has avalanche breakdown (like some zener diodes) when its backwards emitter to collector voltage exceeds about 5.6V because its reverse-biased emitter-base junction has a max voltage rating of 5V (but most breakdown at about 6V) and its collector-base junction is forward-biased and is in series.
Avalanche breakdown slowly damages the base-emitter junction because it is small and cannot dissipate the heat.
 
"backwards" configurations are possible fir most 3-terminal devices (transistors, tubes,FETs), but not very useful, since their gain is much less than 1, and their outputs are extremely nonlinear. and as AG mentioned, these configurations can damage the device. these are also the reasons "backwards" configurations aren't included in textbooks.

years ago i saw a "backwards" tube amp that was able to directly drive a speaker using the grids as an output terminal. the problem was that the tubes operated at high current, and the grid output was due to grid current, and the high currents stripped the cathodes and melted the grids, so even though it worked, it wasn't very practical.
 
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