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Prototyp_V1.0
Joined: 03 Apr 2007
Posts: 48
Helped: 6
Location: Norway
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 03 Apr 2008 15:51 transistor with two emitters |
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I guess this doesn't require much explanation. I found a transistor drawing where the transistor have two emitters. Actually, I know about drawings where transistors have many emitters.
Question is: Is this realy just a transistor with diodes on its emitter (see atachment)? Would it be possible to replace a multi-emitter transistor with a regular transistor with sereral diodes attached on it's emitter?
My guess is that the multi-emitter transistor (met.) realy have two or more emitters and that it'l work if I replace it as suggested above. I also guess that the met would have a base-emitter voltage of 0,6 volts when saturated (that's what I'm guessing) while the replacement transistor supplied with several diodes must have at least 1.2 volts to get saturated (common knowledge - Vbe + Vdiode = 1.2 volts).
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melc
Joined: 08 Apr 2005
Posts: 392
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| 04 Apr 2008 18:08 Re: transistor with two emitters |
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No you can't replace succesfully a multi-emiter transistor with one emiter transistor OR-ed logic with diodes.
The key of this design is egual output impedances on emiters outputs (matched emiter currents on all emiter paths).
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FvM
Joined: 22 Jan 2008
Posts: 2315
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Location: Bochum, Germany
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| 04 Apr 2008 18:34 Re: transistor with two emitters |
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| The key of this design is egual output impedances on emiters outputs |
I don't see this point. I think it's just a matter of area minimization in TTL designs. As these inputs transistors are operated digitally with only one of many emitters "on", there's nothing to match. Multi-emitter transistors are of no particular use in analog design, where matching would count, as far as I know.
However, the additional voltage drop is reason enough, why the sugested multi-diode wouldn't operate. But you can use LSTTL structure, it has multiple schottky diodes instead of multi-emitter transistor. By the way, the multi-emitter transistor in TTL input is working as a diode in static operation. Transistor effect is only used to speed-up switching.
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Prototyp_V1.0
Joined: 03 Apr 2007
Posts: 48
Helped: 6
Location: Norway
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| 07 Apr 2008 16:27 Re: transistor with two emitters |
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Reason why I ask is that I have a little private project where I'm trying to assemble some basic logic ports using regular transistors, diodes and resistors.
Most of the easy schematics actually works for simple tasks that doesn't require high speed nor low power. Example schematic:
Then I got the idea of replace the multi-emitter transistors as described in first post. The image below shows a simple nand circuit: 
And here it would be interesting to know if the dual-emitter transistor could be replaced by a single transistor with two diodes instead - and if it still would be a nand circuit 
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FvM
Joined: 22 Jan 2008
Posts: 2315
Helped: 385
Location: Bochum, Germany
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| 07 Apr 2008 20:29 Re: transistor with two emitters |
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| TTL + diodes would need a compensation for the additional voltage drop, LSTTL (actually DTL input with schottky diodes) is more simple.
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