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Simple NPN Questions (emitter diode, pulldown resistor)

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D'Crex

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Hello,

I want to make a simple NPN transistor based adder, but have a problem with how to begin. There are many circuits around, but some are using diodes on the emitters (sink current prevention), some don't. Some are using pull down resistors on the base other's don't. Then there is another thing when some are using resistor on collector.

What would be the best way to connect an npn switch? Should I always use diodes and pull down resistor? (on each of the transistors)

Thank you
 

SunnySkyguy

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Diodes clamp reverse kickback voltages on inductive wires or loads.
Bias resistors speed up turn off.

But not in small circuits where junction capacitance limits speed.

Google images is your friend.
https://www.google.ca/search?q=tran...ChMI9Kfg-qzBxQIVxn6SCh0PDQAJ&biw=1024&bih=644

- - - Updated - - -

Transistor can be wired as OR And or XOR ... and NOT from gate or base to drain or collector

CmOS switches are often used in FF registers for lower chip count than gates.

just remember than ALL LOGIC is fundamentally analog.
 

D'Crex

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Yes, believe me, I looked, but look at this one for an example. It does not use pull down resistors at all. Also no Diodes. Plus there is that resistor on the collector (2.2K).



Thanks for your quick reply!
 
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SunnySkyguy

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Note that most EE engineers (& Profs) still dont know that basic BJT switches use Ic/Ib =10 and special ultralow Vsat ones are rated with lower capacitance and high ratios up to 50:1 for the Rb:Rc or Ic/Ib ratios. So this wont work on typical general purpose types of 10:1 or 20:1

Thats why this works with 100k:2.2k only. See Diodes Inc for their >50 patents on such devices that of course...cost more...

this saturated gain sometimes is related to hFE around 10%.
 

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