I've been delving into analog a bit more and am a little confused by a configuration I saw of two PNP BJTs.

I saw two emitters tied together and then their collectors tied together through a resistor. Their bases had different drives. What does this configuration accomplish.


simulate this circuit – Schematic created using CircuitLab

EDIT: I'm not 100%, but I believe in the circuit that I have found the inputs. Apologies if it looks vague. I'm attempting to keep the question simple.


simulate this circuit

  • \$\begingroup\$ Most people would be confused by this circuit. I do not know what it is for. If I put something like that in a schematic, I would add a note explaining what it is supposed to do. Just trying to validate your confusion. \$\endgroup\$
    – user57037
    Apr 13, 2016 at 17:42
  • \$\begingroup\$ It seems like some kind of comparator circuit to me. \$\endgroup\$ Apr 13, 2016 at 17:57
  • 2
    \$\begingroup\$ Can you add some details explaining where you saw this circuit? A book? You reverse engineered a PCB? Photo / link? Put the info in the question. \$\endgroup\$
    – Transistor
    Apr 13, 2016 at 18:11
  • 1
    \$\begingroup\$ Is it a badly drawn long-tailed pair? With missing emitter load? \$\endgroup\$
    – user207421
    Apr 13, 2016 at 20:49
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    \$\begingroup\$ I think I agree with @EJP- the circuit looks incomplete. Is it from a schematic, or was it recreated from an existing PCB tracing the circuit? \$\endgroup\$
    – jrive
    Apr 14, 2016 at 13:31

2 Answers 2


If there are no other power supply connections, then they might be some sort of protection circuit, using the back-to-back BE junctions as crude zener diodes.


From the question I am assuming that you are decoding an existing PCB, which means some elements may be missing.

This looks a lot like a schmitt trigger, designed to threshold an analog signal into a binary one.

The full design and explanation can be seen on http://www.johnhearfield.com/Eng/Schmitt.htm under the A simpler alternative heading towards the bottom. Schmitt Trigger


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