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What is this circuit symbol? Answered

I'm wondering what Q22 is, I have never seen that before.

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Quercus austrina

Best Answer 6 years ago

That does help to know it is the LM555 schematic. Q22 , Q5 and Q6 are all the same. The base is just pointing below instead above. The PNP device has 1 base, 1 emitter and 2 collectors. This is easy to do on a die in a factory. The base is tied to one of the collectors to form a type of constant current source while having a separate output to control something else.

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T_T_Quercus austrina

Answer 6 years ago

so if I were to have the two center leads connected together, and have those be the base, would it still work?

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Quercus austrinaT_T_

Answer 6 years ago

The reason this works is because the gain is exactly the same for both of the collector leads. If you were to try to use 2 PNP transistors tied together to approximate the structure, the differences between them could have an affect on the proper operation. Emitters tied together, Bases tied together, one of the Collectors tied back to the Bases and R6, the other collector out to the junction of R7/Q20. You will probably get lucky, but might not. Give it a shot.

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Quercus austrinaT_T_

Answer 6 years ago

Pretty much, that's it. The top (emitters) is connected to VCC (pin 8), lower left (base/collector leads connected together) connects to the base of Q11 and R6, lower right (open collector lead) goes to the junction of R7/collector of Q20/base of Q23 as shown here.
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steveastrouk

6 years ago

Its a special transistor, the kind you can only make if you are a semiconductor fab.

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Jack A Lopez

6 years ago

Not sure, but I think it is a current mirror, and the part that programs it (the missing base) has been omitted for brevity.  Or maybe there is a note somewhere that says what the value of its mirrored current outputs are supposed to be.

I mean the symbol looks like a PNP transistor with one emitter, three collectors,and no base, and I think we are supposed to assume those three wires that look like collectors are all sourcing identical currents.

Compare to this picture:
http://sub.allaboutcircuits.com/images/03129.png
in this article about current mirrors:
http://www.allaboutcircuits.com/vol_3/chpt_4/14.html

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T_T_

6 years ago

if it helps, this is the internal diagram of the 555

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The Skinnerz

6 years ago

From this, it looks as though it could either be a dual emitter transistor, connected with one of the emitters feeding back into the base, or one of the additional leads represents the case or die of the transistor.

However, looking through the results for current and obsolete transistors/MOSFETS, nothing like that turned up.