I'm a bit new to circuitry, and recently I have been experimenting with transistors, attempting to create some logic gates. I realized however, that basically all of my logic gates output a positive voltage (the emitter receives current from a 5V battery), and if I were to connect two gates, I needed the outputs of the first gate to be used as inputs for the second gate (meaning that the positive current would have to flow into the base of the second gate). My question is, how can you do that? I have spent a lot of time looking into transistors, and it seems to me that only current coming from ground can go through the base of a PNP transistor.

I'm sorry if the problem is due to a major misunderstanding on my part, like I said, I am pretty new to circuitry.

  • \$\begingroup\$ The collector/emitter current of first transistor goes to the second transistor's base. review working of transistors again. it will help. en.wikipedia.org/wiki/Transistor#Transistor_as_a_switch \$\endgroup\$
    – User323693
    Jul 3, 2015 at 3:14
  • \$\begingroup\$ In TTL, the connection is not between emitter and base. \$\endgroup\$
    – CL.
    Jul 3, 2015 at 11:08
  • \$\begingroup\$ You need to be more specific. Put schematics and define your question better. \$\endgroup\$
    – Leo
    Jul 3, 2015 at 22:39
  • \$\begingroup\$ @CL. what do you mean by that? \$\endgroup\$
    – Nathan Yan
    Jul 6, 2015 at 2:08
  • \$\begingroup\$ As shown on the page I linked to, a TTL output is the collector (or collector and emitter of two transistors for a totem pole), and a TTL input is an emitter. In the logic low state, current flows backwards. However, I am not implying that you should use TTL. \$\endgroup\$
    – CL.
    Jul 6, 2015 at 7:41

1 Answer 1


I think what you are looking for is cascoding two transistors as transistors can be CASCADED and CASCODED. that totally depends on the application here is a cascade example, as the output of the first transistor connected to the base of the second transistor "The cascade of a Common Emitter amplifier stage followed by a Common Collector (emitter-follower) amplifier stage can provide a good overall voltage amplifier"

enter image description here

and here is the CASCODE example:

enter image description here

the advantage of using this configuration is to have higher input-output isolation, higher input impedance, higher output impedance, higher gain or higher bandwidth

as I can see from your question, you are dealing with logic gates, or you want to design logic gate. I guess this website is a good start



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